A Science Of Electric Drive.docx

Chapter Four A science of Electric Drive 1. Electric Drive in the frame of other sciences

Electric drive: the speeds of rotation of these machines can be controlled by: -

The applied voltage Frequency of the source current. Implementing the concept of drive.

The main advantage of third concept is, the motion control is easily optimized with the help of drive. In very simple words, the systems which control the motion of the electrical machines are known as electrical drives. A typical drive system is assembled with electric motor (may be several) and a sophisticated control system that controls the rotation of the motor shaft. Now a day, this control can be done easily with the help of software. So, the controlling becomes more and more accurate and this concept of drive also provides the ease of use.

Functional diagram of Electric drives

2. Parts of Electrical Drives The diagram which shows the basic circuit design and components of a drive also shows that, drives have some fixed parts such as, load, motor, Power Converter, control unit and source. These equipments are termed as parts of drive system. Now, loads can be of various types i.e. they can have specific requirements and multiple conditions, which are discussed later, first of all we will discuss about the other four parts of electrical drives i.e. motor, power Converters, sources and control units.

Electric motors are of various types. The dc motors can be divided in four types – shunt wound dc motor, series wound dc motor, compound wound dc motor and permanent magnet dc motor. And AC motors are of two types; induction motors and synchronous motors. Now synchronous motors are of two types; round field and permanent magnet. Induction motors are also of two types; squirrel cage and wound motor. Besides all of these, stepper motors and switched reluctance motors are also considered as the parts of drive system.

So, there are various types of electric motors, and they are used according to their specifications and uses. When the electrical drives were not so popular, induction and synchronous motors were usually implemented only where fixed or constant speed was the only requirement. And for variable speed drive applications, dc motors were used. But as we know that, induction motors of same rating as a dc motors have various advantages like they have lighter weight, lower cost, lower volume and there is less restriction on maximum voltage, speed and power ratings. For these reasons, the induction motors are rapidly replaced the dc motors. Moreover induction motors are mechanical stronger and requires less maintenance. When synchronous motors are considered, wound field and permanent magnet synchronous motors have higher full load efficiency and power factor than induction motors, but the size and cost of synchronous motors are higher than induction motors for the same rating. Brush-less dc motors are similar to permanent magnet synchronous motors. They are used for servo applications and now days used as an efficient alternative to dc servo motors because they don’t have the disadvantages like commutation problem. Beside of these, stepper motors are used for position control and switched reluctance motors are used for speed control. Power Converters - are the devices which alter the nature or frequency as well as changes the intensity of power to control electrical drives. Roughly, power converters can be classified into three types, a) Variable Impedances are used to controlling speed by varying the resistance or impedance of the circuit. But these controlling methods are used in low cost dc and ac drives. There can be two or more steps which can be controlled manually or automatically with the help of contactors. To limit the starting current inductors are used in ac motors. b) Switching circuits in motors and electrical drives are used for running the motor smoothly and they also protect the machine during faults. These circuits are used for changing the quadrant of operations during the running condition of a motor. And these circuits are implemented to operate the motor and drives according to predetermined sequence, to provide interlocking, to disconnect the motor from the main circuit during any abnormal condition or faults. c) Power Electronic Converters, As the name suggests, PE converters are used to convert currents from one type to other type. Depending on the type of function, PE converters can be divided into 4 types.

I. II. III.

AC to DC converters Choppers or DC-DC converters Inverters

IV.

AC-AC converters a. Cyclo-converters b. AC regulators

AC to DC converters are used to obtain fixed dc supply from the ac supply of fixed voltage. Choppers or dc-dc converters are used to get a variable DC voltage. Power transistors, IGBT’s, GPO’s, power MOSFET’s are mainly used for this purpose. Inverters are used to get ac from dc; the operation is just opposite to that of ac to dc converters. PWM semiconductors are used to invert the current. Ac-Ac converters a. Cycloconverters are used to convert the fixed frequency and fixed voltage ac into variable frequency and variable voltage ac. Thyristors are used in these converters to control the firing signals. b. Ac Regulators are used to obtain the regulated ac voltage, mainly auto transformers or tap changer transformers are used in these regulators. Sources may be of 1 phase and 3 phases. 50 Hz ac supply is the most common type of electricity supplied in India, both for domestic and commercial purpose. Synchronous motors which are fed 50 Hz supply have maximum speed up to 3000 rpm, and for getting higher speeds higher frequency supply is needed. Motors of low and medium powers are fed from 400V supply, and higher ratings like 3.3 kv, 6.6 kv, 11 kv etc are provided also. Control unit – choice of control unit depends upon the type of power Converter that is used. These are of many types, like when semiconductor converters are used, and then the control unit consists of firing circuits, which employ linear devices and microprocessors. So, the above discussion provides us a simple concept about the several parts of electrical drive.

3. Classification of Electrical Drives The classification of electrical drives can be done depending upon the various components of the drive system. Now according to the design, the drives can be classified into three types such as single-motor drive, group motor drive and multi motor drive. The single motor types are the very basic type of drive which are mainly used in simple metal working, house hold appliances etc. Group electric drives are used in modern industries because of various complexities. Multi motor drives are used in heavy industries or where multiple motoring units are required such as railway transport. If we divide from another point of view, these drives are of two types: Reversible types and Nonreversible types. This depends mainly on the capability of the drive system to alter the direction of the flux generated. So, several classification of drive is discussed above.

4. Dynamics of electrical drives When a electric motor rotates, it is usually connected to load which has a rotational or translational motion. The speed of the motor may be different from that of the load. To analyze the relation among the drives and loads, the concept of dynamics of electrical drives is introduced.

We can describe the dynamics of electrical drive easily by the following instant.

Here, J = Polar moment of inertia of motor load Wm = Instantaneous angular velocity T = Instantaneous value of developed motor torque T1 = Instantaneous value of load torque referred to motor shaft Now, from the fundamental torque equation

For drives with constant inertia,

So, the above equation states that the motor torque is balanced by load torque and a dynamic torque J(dω m/dt). This torque component is termed as dynamic torque as it is only present during the transient operations. From this equation, we can determine whether the drive is accelerating or decelerating. Such as during accelerating motor supplies load torque and additional torque component essentially. So, the torque, balancing the Dynamics of electrical braking is very helpful. 5. Electric Drive control Electrical drives have become the most essential equipment now a day in the electrical motors and other rotating machines. We know that electrical drives mainly accomplishes three kinds of work, i) ii) iii)

Starting Speed control and Braking.

It can be said that the electrical drives enable us to control the motor in every aspect. But control of electrical drives is also necessary because all the functions accomplished by the drives are mainly transient operations i.e the change in terminal voltage, current, etc are huge which may damage the motor temporarily or permanently. That’s why the need of controlling the drives rises and there are various methods and equipment’s to control different parameters of the drives which are discussed later. 5.1. Closed Loop Control of Drives In control system there are two types of systems, one is open loop and the other is closed loop control system. In open loop control system the output has no effect on the input, i.e the controlling phenomenon is independent of the output, on the other hand closed loop control system is much more advanced and scientific, here the output is fed back to the input terminal which determines the amount of input to the system, for example if the output is more than predetermined value the input is reduced and vice-versa. In electrical drives feedback loops or closed loop control

satisfy the following requirements. i) Protection ii) Enhancement of speed of response iii) To improve steady –state accuracy In the following discussions, we will see through different closed loop configurations which are used in electrical drives irrespective of the type of supply they are fed, i.e dc or ac. a. Current Limit Control During the starting, we know if precautionary measures are not taken there is a chance of huge current flow through the motor circuit. To limit the current and sense the current fed to the motor, current limit controller is installed. The feedback loop does not affect the normal operation of the drive but if the current exceeds the predetermined safe limit, the feedback loop activates and the current is brought down below the safe limit. Once the current is brought down below the safe limit the feedback loop again deactivates and in this way the control of current takes place.

b. Closed Loop Torque Control This type of torque controller is seen mainly in battery operated vehicles like cars, trains etc. the accelerator present in the vehicles is pressed by the driver to set the reference torque T. The actual torque T follows the T * which is

controlled by the driver via accelerator.

c. Closed Loop Speed Control

Speed control loops are perhaps the most widely used feedback loops for drives. If we first see the block diagram of this loop then it will be a lot easier for us to understand. We can see from the diagram that there are two control loops, which can be said as an inner loop and outer loop. The inner current control loop limits the converter and motor current or motor torque below the safe limit. Now we can understand the function of the control loop and drive by practical examples. Suppose the reference speed Wm* increases and there is a positive error ΔWm, which indicates that the speed is needed to be increased. Now the inner loop increases the current keeping it under maximum allowable current. And then the driver accelerates, when the speed reaches the desired speed then the motor torque is equal to the load torque and there is a decrease in the reference speed Wm which indicates that there is no need of any more acceleration but there must be deceleration, and braking is done by the speed controller at maximum allowable current. So, we can say

that during speed controlling the function transfers from motoring to braking and from braking to motoring continuously for the smooth operation and running of the motor. 6. Electric Braking The term braking comes from the term brake. We know that brake is an equipment to reduce the speed of any moving or rotating equipment, like vehicles, locomotives. The process of applying brakes can be termed as braking. Now coming to the term or question what is braking. First of all we can classify the term braking in two parts i) mechanical braking and the ii) electrical braking. Mechanical braking is left out here because as it is an electrical engineering course, we should only focus on electrical braking here. In mechanical braking the speed of the machine is reduced solely by mechanical process but electrical braking is far more interesting than that because the whole process is depended on the flux and torque directions. We will further see through the various types of braking but the main idea behind each type of barking is the reversal of the direction of the flux. So, we can understand that when it is asked that what is braking? We can say that it is the process of reducing speed of any rotating machine. The application of braking is seen at almost every possible area, be it inside the motor used in factories, industrial areas or be it in locomotives or vehicles. Everywhere the use of mechanical and electrical brakes is inevitable. Types of Braking Brakes are used to reduce or cease the speed of motors. We know that there are various types of motors available (DC motors, induction motors, synchronous motors, single phase motors etc.) and the specialty and properties of these motors are different from each other, hence this braking methods also differs from each other. But we can divide braking in to three parts mainly, which are applicable for almost every type of motors. i) Regenerative Braking ii) Plugging type braking iii) dynamic braking. Regenerative Braking Regenerative braking takes place whenever the speed of the motor exceeds the synchronous speed. This baking method is called regenerative braking because here the motor works as generator and supply itself is given power from the load, i.e. motors. The main criteria for regenerative braking is that the rotor has to rotate at a speed higher than synchronous speed, only then the motor will act as a generator and the direction of current flow through the circuit and direction of the torque reverses and braking takes place. The only disadvantage of this type of braking is that the motor has to run at super synchronous speed which may damage the motor mechanically and electrically, but regenerative braking can be done at sub synchronous speed if the variable frequency source is available. Plugging Type Braking Another type of braking is plugging type braking. In this method the terminals of supply are reversed, as a result the generator torque also reverses which resists the normal rotation of the motor and as a result the speed decreases. During plugging external resistance is also introduced into the circuit to limit the flowing current. The main disadvantage of this method is that here power is wasted.

Dynamic Braking Another method of reversing the direction of torque and braking the motor is dynamic braking. In this method of braking the motor which is at a running condition is disconnected from the source and connected across a resistance. When the motor is disconnected from the source, the rotor keeps rotating due to inertia and it works as a self –excited generator. When the motor works as a generator the flow of the current and torque reverses. During braking to maintain the steady torque sectional resistances are cut out one by one.

7. Induction motor Drives Before discussing about the induction motor drives we have to understand and know about induction motors. In very simple words induction motors can be described as a three phase, self starting constant speed ac motors. The reason of describing induction motors as constant speed is because normally these motors have a constant speed depending on the frequency of the supply and the no of windings. In the past it was not possible to control the speed of the induction motors according to the need. That’s why their use was limited and despite having many a motors they advantages over dc motors they could not be used because of this disadvantage. But at the field of drivers have improved due to the availability of thyristors or SCRs, power transistors, IGBTs and GTOs the variable speed induction motor drives have been invented. Though the cost of these drivers are more than dc driver, but still the use of induction motors are increasing and they are replacing dc motors because of their advantages. While discussing about this topic we will look through the starting, braking and speed control of induction motors. Starting of Induction Motors We know that the induction motor is self starting i.e when the supply is given to the motor it starts to rotate without any external help. When an induction motor is started as there is no resistance initially (i.e, during starting), there is a tendency of huge current flow through the rotor circuit which may damage the circuit permanently. To overcome this problem various methods have been introduced to limit the starting current. Some of the methods are 1) Star delta starter 2) Auto- transformers starter 3) Reactor starter 4) Saturable reactor starter 5) Part winding starter 6) ac voltage controller starter 7) Rotor resistance starter is used for starting of wound rotor motor. The induction motor drives are normally designed to run on delta connection, but during starting the supply is given from star connection because then the starter voltage and current reduces by 1/√3 times than the delta connection. When the motor reaches a steady state speed the connection changes from star to delta connection.

Another type of starting method of induction motors is the Auto transformer starting. Since we know that the torque is proportional to square of the voltage. By auto transformers the starting voltage and current are reduced to overcome the problem of overheating due to very high current flow. During starting the ratio of the transformer is set in a way that the starting current does not exceed the safe limit. Once the induction motor starts running and reaches a steady state value, the autotransformer is disconnected from the supply. The circuit diagram is given here with

Another method of starting is softstart using saturable reactor drivers. In this method high reactance is introduced into the circuit so that the starting torque is closed to zero. Now the reactance is reduced smoothly during starting and the

starting current increases and the torque also varies steplessly. In this method the motor starts without any jerk and the acceleration is smooth, that’s why its called soft start also.

Unbalanced starting scheme for soft start is another type of starting method where the impedance is introduced only in one of the supply phases. During starting the impedance is kept very high so that the motor operates as a single phase motor, the speed torque characteristics at that time is similar to curve A in the graph. When the speed attains a steady state value the impedance is removed completely, at that time the curve is similar to B, which is the natural characteristics of the machine. This starting method is also without any jerk and the operation is very smooth.

Part winding starting method is special for squirrel-cage induction motors. In this method two or more starter winding are connected in parallel. When the motor starts, any one of the winding is connected as a result the starter impendence is increased and starting current is reduced. When the steady speed is acquired by the motor both the windings are connected.

Specially for wound rotor motors. Rotor resistance starter is used. In this method external resistors are used in the rotor circuit to limit the starting current. Maximum value of resistance is chosen to limit the current at zero speed within the safe value. As the speed increases, the sectional resistance is the temperature rise is lower than other methods of starting high acceleration, frequent starts and stops starting with heavy loads can be done with this type of starting method.

Braking of Induction Motors When it comes to controlling an electric machine by electric drivers braking is a very important term because it helps to decrease the speed of the motor according to will and necessity. Braking of induction motors can be classified mainly in three types 1) Regenerative braking 2) plugging or reverse voltage braking

3) Dynamic braking which can be further classified as a) ac dynamic braking b) self-excited braking using capacitors c) dc dynamic braking d) zero sequence braking To explain that regeneration braking for induction motor, we can take help of the equation Pin = 3 N Is Cosθs here θs is the phase angle between the stator voltage and stator current, the simple words whenever this phase angle exceeds 90° (i.e θs>90°) regenerative braking can take place. To explain this more clearly and easily we can say that whenever the speed of the rotor exceeds synchronous speed, regeneration braking occurs. That is because whenever the rotor rotates at a speed more than synchronous speed there is a reverse field occurs which opposes the normal rotation of the motor and therefore braking takes place. Main disadvantage of this type of braking is that the speed of the motor has to exceed synchronous speed which may not be possible every time. To acquire regenerative braking at a lower speed than synchronous speed, variable frequency source can be used.

Plugging of induction motors is done by interchanging any two of the supply terminals. When the terminals are reversed the operation of the machine changes from motoring to plugging. From technical point of view and for better understanding it can be said that the slip changes from ‘s’ to (2-s), which indicates that due to reversal of the terminals the torque also changes its direction and braking occurs

The first classification of dynamic braking of induction motors is ac dynamic braking any one of the supply phase is disconnected from the supply and then it is either kept open or connected with the other phase. The first type is known as two lead connections and the second one is known as three lead connection. To understand this braking method clearly we can assume the system to be a single phase system. Now the motor can be considered to be fed by positive and negative sequence voltages. That’s why when the rotor resistance is high the net torque is negative and braking can be acquired.

Sometimes capacitors are kept permanent by connected across the supply terminals of the motor. This is called selfexcited braking using capacitors of induction motors. This type of braking works mainly by the property of the capacitors to store energy. Whenever the motor is disconnected from the supply the motor starts to work as a selfexcited induction generator, the power comes from the capacitors connected across the terminals. The values of the capacitor are so chosen that they are sufficient to make the motor work as an induction generator after being disconnected from the supply. When the motor works as an induction generator the produced torque opposes the normal rotation of the motor and hence braking takes place.

Another type of dynamic braking is dc dynamic braking. In this method the stator of running induction motor drives is connected to dc supply. The consequences of connecting a dc supply to the stator is as follow, the dc current produces a stationary magnetic field, in the rotor keeps rotating and as a result there is a induced voltage in the rotor winding, therefore the machine works as a generator which opposes the motion of the motor and braking is acquired

Speed Control of Induction Motors We have discussed about the starting and braking of induction motors but what about controlling the speed during the running time. Speed control of induction motors can be done in six methods which are i) Pole changing ii) Stator voltage control iii) Supply frequency control iv) Eddy –current coupling v) Rotor resistance control

vi) Slip power recovery We know that the speed of the induction motor is inversely proportional to number of poles. So it is possible to increase or decrease the speed of the induction motor if the number of the poles are decreased or increased respectively. The motor in which the provision of changing the number of poles is present, they are called ‘pole changing motor’ or ‘multi –speed motor’. Another method of controlling the speed of induction motor drives is the stator voltage control. Stator voltage is directly responsible for the rotating speed of the rotor. Torque is proportional to voltage squared and the current is proportional to the voltage. So, if the stator voltage is reduced the speed reduces and similarly if the stator voltage is increased the speed also increases.

The speed of an induction motor is proportional to the product of the supply frequency and air gap flux. But as there is a chance of magnetic saturation while decreasing the supply frequency, that’s why not only the frequency but the v/f ( i. e the ratio of supply voltage and frequency ) is controlled and this ratio is tried to be kept constant. And if the speed is needed to be changed the ratio of v/f is changed accordingly.

The eddy current speed control method is done by placing an eddy current clutch between an induction motor is running at a fixed speed and the variable speed load. Now what is this eddy current clutch? It is nothing but an induction motor drives in which both stator and the rotor are allowed to rotate. The rotor is coupled with the main induction motor. When eddy currents are produced in the rotor drum, their interaction with the stator field and a torque is produced which rotates the main motor. By controlling the dc current through the stator winding the speed of the motor can be controlled. Depending on the rotor resistance, the speed of the rotor falls or increases. The variation of speed torque characteristics with respect to change in rotor resistance is shown in the figures below. This speed controlling method is better than many other methods because of low cost.

8. Dc motor Drives Everywhere DC motors are used in large applications, the use of drives are very necessary for the smooth running and operating of these motors. The DC motor drives are used mainly for good speed regulation, frequent starting, braking and reversing. Before enlisting the practical applications of the drives used for DC motors, we will discuss about the different operation of electric drives for different purposes: We know that, normally the rotor of a DC motor is energized by the commutation process through brushes. So the maximum allowable starting current is determined by the current which can be safely carried out by the brushes without sparking. In general, the motors are such designed that they can carry almost twice of the rated current during starting condition. But for some specially designed motors, this can be almost 3-5 times of the rated current. But why so much current flows through the circuit of DC motors during the starting condition ? This is because when the motor is at stand still condition, there is only a small armature resistance present in the circuit so no back emf is generated. That is why when the motor is started with full supply voltage across its terminal, there is a huge current flow through the motor, which may damage the motor because of heavy sparking across the commutators and a huge amount of heat is generated. That is the reason why some precautionary measures are taken during the starting of the DC motors. The speed of a DC motor drive is usually controlled by variable resistance method, which can be also used to limit the starting current as shown in the figure below. When the motor gains speed and the back emf increases, the resistances are cut out one by one from the circuit and therefore the current is kept within permissible limit.

Braking of DC Motors Braking is a very important operation for DC motor drives. The need of decreasing the speed of a motor or stopping it totally may arise at any moment, that’s when braking is applied. braking of DC motors is basically developing a negative torque while the motor works as generator and as a result the motion of the motor is opposed. There are mainly three types of braking of DC motors : i. Regenerative braking ii. Dynamic or rheostat braking iii. Plugging or reverse voltage braking. Regenerative braking takes place when the generated energy is supplied to the source, or we can show this via this equation : E > V and negative Ia.

As the field flux cannot be increased beyond a rated value, so regenerative braking is possible only when the speed of motor is higher than the rated value. The speed torque characteristics is shown in the graph above. When regenerative braking occurs, the terminal voltage rises and as a result the source is relieved from supplying this amount of power. This is the reason why loads are connected across the circuit. So, it is clear that regenerative braking should be used only when there are enough loads to absorb the regenerative power. Dynamic braking is another type of braking of DC motor drives where the rotation of the armature itself causes the braking. This method is also a widely used DC motor drive system. When braking is desired, then the armature of the motor is disconnected from the source and a series resistance is introduced across the armature. Then the motor acts as a generator and current flows in the opposite direction which indicates that the field connection is reversed. The diagram for separately excited and series DC motor both are shown in figure below.

When braking is required to occur quickly the resistance (RB) is considered to be of some sections. As the braking occurs and the speed of the motor falls, the resistance are cut out one by one section to maintain the light average torque.

Plugging is a type of braking where the supply voltage is reserved when the need of braking arises. A resistance is also introduced in the circuit while braking takes place. When the direction of the supply voltage is reserved, then the armature current also reserves forcing the back enf to a very high value and hence braking the motor. For series motor only armature is reversed for plugging. The diagram of separately excited and series excited motors are shown in the figure.

Speed Control of DC Motor Drives The main application of electric drives can be said as the need of braking of DC motors . We know the equation to describe the speed of a rotating dc motor drives is as

Now, according to this equation, the speed of a motor can be controlled by the following methods i. Armature voltage control ii. Field flux control iii. Armature resistance control Among all of these, armature voltage control is preferred because of high efficiency and good speed regulation and good transient response. But the only disadvantage of this method is that it can only operate under the rated speed,

because the armature voltage cannot be allowed to exceed rated value. The speed torque curve for armature voltage control is shown below.

When speed control is required above the rated speed, field flux control is used. Normally in ordinary machines, the maximum speed can be allowed up to twice of the rated speed and for specially designed machines this can be allowed up to six times of the rated speed. The torque speed characteristics for field flux control are shown in the figure below.

How the armature voltage control and field flux control is made to operate below and above the rated speed is shown in the figure below.

Now, finally coming to resistance control method. Here speed is varied by wasting power in an external resistor that is connected in series with the armature. This method is not used very much because it is an inefficient method of controlling speed and it is only used in the places where the speed control time forms only a fraction of the total running time, such as traction. The speed torque curve of dc motor drives is given below.

So, the application and types of dc motor drives have been discussed very easily. 9. Synchronous Motor Drives As the name suggests the synchronous motorsrotates at synchronous speed. The main advantage of synchronous motors are that they run on 3 ac supply and dc supply is given to the rotor when they run on synchronous speed the loss is very minimal. We can say that if the synchronous motors are designed to run only at rated synchronous speeds, then what is the use of introducing drives to them. The answer is pretty simple, synchronous motor drives makes the starting, pull in and braking process smooth and without any problems. We shall discuss about them one by one. Starting Synchronous Motors

The problem with synchronous motors are that they are not self- starting. Before discussing the starting method of this motor, we should know about the type of supply and the rotor and the stator of the motor briefly. The stator of synchronous motors are similar to that of an induction motors but the only difference lies in the rotor, the rotor of the synchronous motors are given dc supply.

Now before knowing, how the synchronous motors are started, we should know why they are not self started? The answer can be given as, when 3 phase supply is given to the stator, there is a rotating magnetic flux which rotates at synchronous speed, and if the rotor is also given dc supply, rotor acts as a magnetic flux which rotates at synchronous speed, and if the rotor is also given dc supply, rotor acts as a magnet having two salient poles. As the rotor is at standstill position, it cannot follow the magnetic field which is rotating at synchronous speed. The rotor stacks at its position because the opposite poles move so rapidly that the rotor locks, this is the reason why synchronous motors are not self starting. Now coming to the point how synchronous motors are started. At first the synchronous motors are started as a normal induction motors, the rotor of the motor is not given dc supply, when the motor reaches the rotor and pull in takes place, which is discussed later. Another method of starting the synchronous motor drives is by external motor. In this method the rotor of the synchronous motor is rotated by an external motor and when the speed of the rotor reaches near synchronous speed, the dc-field is switched on and pull in takes place. In this method, the starting torque is very low and it is not very popular method also. Pull in of Synchronous Motors When the rotor of the synchronous motors reaches near synchronous speed, then the dc field supply is switched on and the pull in process begins. As during switching on the dc supply due to the phase angle and torque angle there are various disturbances seen in the motor and there are several slip of poles of air – gap flux is also seen. As the pull in process is completed the rotor acquires synchronous speed. The complete pull in as fast as possible the dc supply should be switched on at the most favourable angle. Like when the synchronous motor is running as induction motor, the dc supply should be fed when the induction motor is at top speed, this will be the best moment because the speed

difference will be least at that point of time. Braking of Synchronous Motors As we know, there are three types of braking i.e, regenerative, dynamic and plugging type braking. But for synchronous motor drives only dynamic braking can be applied though plugging can be applied theoretically. Regenerative braking cannot be applied to them as they need higher speed than synchronous speed. Dynamic braking is done by disconnecting the motor from supply and connecting it across a three phase resistor. At that time the motor works as asynchronous generator and energy is dissipated at the resistors. Plugging is not used for synchronous motors as high plugging current can cause severe disturbance and damage in line. 10. Variable Frequency Drives Variable Frequency Drive(VFD) is a power electronics based device which converts a basic fixed frequency, fixed voltage sine wave power (line power) to a variable frequency, variable output voltage used to control speed of induction motor(s). It regulates the speed of a three phase induction motor by controlling the frequency and voltage of the power supplied to the motor.

Since the number of pole is constant the speed Ns can be varied by continuously changing frequency. It is interesting to know that the first A.C. drive (400 HP) based on thyratron cycloconverter-fed WRIM was installed in 1932 by F.E. Alexanderson of General Electric in the Logan Power Station of Pacific Gas and Electric Company. From then industrial drives have evolved rapidly by dedicated effort of many scientists and engineers all over the world resulting in development of advanced drive technology such as Variable Frequency Drive(VFD). VFD is a power electronics based device which converts a basic fixed frequency, fixed voltage sine wave power (line power) to a variable frequency, variable output voltage used to control speed of induction motor(s). It regulates the speed of a three phase induction motor by controlling the frequency and voltage of the power supplied to the motor. Ns=120f/P Since the number of pole is constant the speed Ns can be varied by continuously changing frequency.

Working of Variable Frequency Drive

Any Variable Frequency Drive or VFD incorporates following three stages for controlling a three phase induction motor. Rectifier Stage

A full-wave power diode based solid-state rectifier converts three-phase 50 Hz power from a standard 220, 440 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers for high voltage system. Inverter Stage

Power electronic switches such as IGBT, GTO or SCR switch the DC power from rectifier on and off to produce a current or voltage waveform at the required new frequency. Presently most of the voltage source inverters (VSI) use pulse width modulation (PWM) because the current and voltage waveform at output in this scheme is approximately a sine wave. Power Electronic switches such as IGBT; GTO etc. switch DC voltage at high speed, producing a series of short-width pulses of constant amplitude. Output voltage is varied by varying the gain of the inverter. Output frequency is adjusted by changing the number of pulses per half cycle or by varying the period for each time cycle. The resulting current in an induction motor simulates a sine wave of the desired output frequency. The high speed switching action of a PWM inverter results in less waveform distortion and hence decreases harmonic losses. Control System

Its function is to control output voltage i.e. voltage vector of inverter being fed to motor and maintain a constant ratio of voltage to frequency (V/Hz). It consists of an electronic circuit which receives feedback information from the driven motor and adjusts the output voltage or frequency to the desired values. Control system may be based on SPWM (Sine Wave PWM), SVPWM (Space Vector modulated PWM) or some soft computing based algorithm. Induction Motor Characteristic under Variable Frequency Drive

In an induction motor voltage induced in stator, E is proportional to the product of the slip frequency and the air gap flux. The terminal voltage can be considered proportional to the product of the slip frequency and flux, if stator drop is neglected. Any reduction in the supply frequency without a change in the terminal voltage causes an increase in the air gap flux which will cause magnetic saturation of motor. Also the torque capability of motor is decreased. Hence while controlling a motor with the help of VFD or Variable Frequency Drive we always keep the V/f ratio constant. Now define variable ‘K’ as, K=1/f rated For operation below K < 1 i.e. below rated frequency we have constant flux operation. For this we maintain constant magnetization current Im for all operating points. For K > 1 i.e. above rated frequency we maintain terminal voltage Vrated constant. In this field is weakened in the inverse ratio of per unit frequency ‘K’.

For values of K = 1 we have constant torque operation and above that we have constant power application.

Merits of using Variable Frequency Drives Energy Saving

Primary function of VFD in industry is to provide smooth control along with energy savings. The variable speed motor drive system is more efficient than all other flow control methods including valves, turbines, hydraulic transmissions, dampers, etc. Energy cost savings becomes more pronounced in variable-torque ID fan and pump applications, where the load’s torque and power is directly proportional to the square and cube of the speed respectively. Increased Reliability

Adjustable speed motor-drive systems are more reliable than traditional mechanical approaches such as using valves, gears, louvers or turbines to control speed and flow. Unlike mechanical control system they don’t have any moving parts hence they are highly reliable. Speed Variations

Beyond energy saving, applications such as crushers, conveyors and grinding mills can use the motor and VFD’s packages to provide optimal speed variations. In some crucial applications, the operating speed range can be wide, which a motor supplied with a constant frequency power source cannot provide. In the case of conveyors and mills, a VFD and motor system can even provide a “crawl” speed foe maintenance purposes eliminating the need for additional drives. Soft Starting

When Variable Frequency Drives start large motors, the drawbacks associated with large inrush current i.e. starting current (winding stress, winding overheating and voltage dip on connected bus) is eliminated. This reduces chances of insulation or winding damage and provides extended motor life.

Extended Machine Life and Less Maintenance

The VFD’s greatly reduce wear to the motor, increase life of the equipment and decrease maintenance costs. Due to optimal voltage and frequency control it offers better protection to the motor from issues such as electro thermal overloads, phase faults, over voltage, under voltage etc. When we start a motor (on load) with help of a VFD, the motor is not subjected to “instant shock” hence there is less wear and tear of belt, gear and pulley system. High Power Factor

Power converted to rotation, heat, sound, etc. is called active power and is measured in kilowatts (kW). Power that charges builds magnetic fields or charges capacitor is called reactive power and is measured in kVAR. The vector sum of the kW and the kVAR is the Apparent Power and is measured in KVA. Power factor is the ratio of kW/KVA. Typical AC motors may have a full load power factor ranging from 0.7 to 0.8. As the motor load is reduced, the power factor becomes low. The advantage of using VFD’s is that it includes capacitors in the DC Bus itself which maintains high power factor on the line side of the Variable Frequency Drive. This eliminates the need of additional expensive capacitor banks. Slip Power Recovery

The fundamental power given to rotor by stator is called air gap power Pg. The mechanical power developed is given by Pm=Pg-sPg

The term ‘sP’ is called slip power.

If the slip is very large i.e. speed is low then there is ample waste of power, a common example is kiln drives of cement industry. This power can be saved through slip recovery scheme.In this scheme slip power is first collected through brushes of WRIM. This slip power recovered is then rectified and inverted back to line frequency and is injected into supply through coupling transformer.The scheme is shown in figure below.

Applications of Variable Frequency Drive 1. They are mostly used in industries for large induction motor (dealing with variable load) whose power rating ranges from few kW to few MW. 2. Variable Frequency Drive is used in traction system. In India it is being used by Delhi Metro Rail Corporation. 3. They are also used in modern lifts, escalators and pumping systems. 4. Nowadays they are being also used in energy efficient refrigerators, AC’s and Outside-air Economizers.

11. Advantages of Electrical Drives Electrical drives are readily used these days for controlling purpose but this is not the only the advantage of Electrical drives. There are several other advantages which are listed below: 1) these drives are available in wide range of torque, speed and power. 2) The control characteristics of these drives are flexible. According to load requirements these can be shaped to steady state and dynamic characteristics. As well as speed control, electric braking, gearing, starting and many things can be accomplished. 3) They are adaptable to any type of operating conditions, no matter how much vigorous or rough it is. 4) They can operate in all the four quadrants of speed torque plane, which is not applicable for other prime movers. 5) They do not pollute the environment. 6) They do not need refueling or preheating, they can be started instantly and can be loaded immediately. 7) They are powered by electrical energy which is atmosphere friendly and cheap source of power. Because of the above mentioned advantages of electrical drives, they are getting more and more popular and are used in a wider range of applications.

Chapter Five Electric motor Power Rating Power rating for electrical machines indicates the required supply voltage for smooth running of that machine; it also shows the permissible maximum amount of current which can easily flow through the machine. And there will be a chance of breakdown in the machine if those parameters go beyond this limit. Similarly when we discuss about motor power rating, we are looking for the suitable conditions where maximum efficiency is obtained from the electric motor. When the motor have insufficient rating, there will be frequent damages and shut downs due to over loading, and this is not intended. On the other hand, if the power rating of a motor is decided liberally, the extra initial cost and then loss of energy due to operation below rated power makes this choice totally uneconomical. Another essential criterion of electrical motor power rating is that, during operation of motor, heat is produced and it is inevitable due to I2R loss in the circuit and friction within the motor. So, the ventilation system of the motor should be designed very carefully, to dissipate the generated heat as quickly as possible. The output power of the motor is directly related with the temperature rise, that’s why it is also called thermal loading. The thermal dissipation will be ideal when the ventilation system is designed in such a way that the heat generated during the operation is equal to or less then heat dissipated by the motor to the surrounding. Now, due to the design of motors, temperature is not same everywhere inside the motor. There is a high amount of heat produced in the windings because, windings cause

higher heat generation. The insulating materials used in the winding are also chosen depending on the amount of heat generated inside the motor during operation. So in the end it can be said that the main objectives of selecting and finding out motor power rating are: i. obtaining the suitable thermal model of motor and design the machine properly ii. Finding out motor duty class iii. Calculating motor ratings for various classes of duty a. Thermal model We know that, when an electrical motor and drive operates, there is a generation of heat inside the motor. The amount of heat generated inside the motor should be known as accurately as possible. That’s why thermal modeling of motor is necessary. The material of the motors and the shapes and size of the motors are not unique but the generation of heat does not alter very much depending on these characteristics. So, a simple thermal model of any motor can be obtained assuming it to be a homogeneous body. The main aim of this modeling is to choose the appropriate rating of a motor so that the electric motor does not exceed its safe limit during operation.

At time‘t’, let the motor has following parameters p1 = Heat developed, watts p2 = Heat dissipated to the cooling medium, watts W = Weight of the active parts of the machine. h = Specific heat, Joules per Kg per 0C. A = Cooling Surface, m2 d = Co-efficient of heat transfer, w/m2/0C θ = Mean temperature rise oC Now, if time dt, let the temperature rise of the machine be dθ, Therefore, Heat absorbed in the machine = (Heat generated inside the machine – Heat dissipated to the surrounding cooling medicine) Where, dθ = p1dt – p2dt…………….(i) Since, p2 = θdA…………….(ii) Substituting (ii) in (i), we get

Here, C is called the thermal capacity of the machine in watts/oC and D is the heat dissipation constant in watts/oC. When we acquire the first order differential equation of the equation –

We obtain the value of K by putting t = 0 in equation (iii) and get the solution as

So, from the above equation we can find out the rise in temperature inside a working machine, which is very near to being accurate and if we plot a graph for the variation of temperature risk with time during heating and cooling and thus the thermal modeling of a motor gets completed.

b. Motor Duty Class and its Classification Now days, in almost every applications, electric motors are used, and to control them electrical drives are employed. But the operating time for all motors is not the same. Some of the motors run all the time, and some of the motor’s run time is shorter than the rest period. Depending on this, concept of motor duty class is introduced and on the basis of this duty cycles of the motor can be divided in eight categories such as: i. Continuous duty ii. Short time duty

iii. Intermittent periodic duty iv. Intermittent periodic duty with starting v. Intermittent periodic duty with starting and braking vi. Continuous duty with intermittent periodic loading vii. Continuous duty with starting and braking viii. Continuous duty with periodic speed changes Continuous duty: This duty denotes that, the motor is running long enough & the electric motor temperature reaches the steady state value. These motors are used in paper mill drives, compressors, conveyors etc.

Short time duty: In these motors, the time of operation is very low and the heating time is much lower than the cooling time. So, the motor cooks off to ambient temperature before operating again. These motors are used in crane drives, drives for house hold appliances, valve drives etc.

Intermittent periodic duty: Here the motor operates for some time and then there is rest period. In both cases, the time is insufficient to raise the temperature to steady state value or cool it off to ambient temperature. This is seen at press and drilling machine drives.

Intermittent period duty with starting: In this type of duty, there is a period of starting, which cannot be ignored and there is a heat loss at that time. After that there is running period and rest period which are not adequate to attain the steady state temperatures. This motor duty class is widely used in metal cutting and drilling tool drives, mine hoist etc.

Intermittent periodic duty with starting and braking: In this type of drives, heat loss during starting and braking cannot be ignored. So, the corresponding periods are starting period, operating period, braking period and resting period, but all the periods are too short to attain the respective steady state temperatures, these techniques are used in billet mill drive, manipulator drive, mine hoist etc.

Continuous duty with intermittent periodic loading : In this type of motor duty, everything is same as the periodic duty but here a no load running period is occured instead of the rest period. Pressing, cutting are the examples of this system. Continuous duty with starting and braking: f this system. Continuous duty with periodic speed changes: In this type of motor duty, there are different running periods at different loads and speeds. But there is no rest period and all the periods are too short to attain the steady state temperatures.