Electric Power System.docx

May 5

Electrical Machines

2014

_Why three-phase system ? _What is the difference between AC and DC Z motor in terms of rotor and stator ?

---------------------------A paper presented to Dr. Khaled AlAwasa by. Hussein mohammed AlYafe'i

Electric Power System The principal elements of an electric power system are the generating stations, the transmission lines, the substations, and the distribution networks. The generators produce the electricity, the transmission lines move it to regions where it is consumed, and the substations transform it for industrial, commercial, and residential use. Finally, the distribution networks carry the electricity to the customers. Most AC power is generated as three-phase power. Both three-phase and single-phase devices can be powered from a three-phase supply. A three-phase circuit is a combination of three single-phase circuits. The current, voltage, and power relations of balanced three-phase AC circuits can be studied by applying the rules that apply to single-phase circuits. The sine waves of three-phase voltage are separated by 120 electrical degrees because they are generated by three separate sets of armature coils in an AC generator. These three sets of coils are mounted 120 electrical degrees apart on the generator’s armature. The coil ends could all be brought out of the generator to form three separate single-phase circuits, but they are conventionally interconnected so that only three or four wires are actually brought out of the generator.

Single-phase AC voltage with zero power factor angle has both voltage and current sine waves in phase, so they cross the zero line together twice in each cycle. Similarly, a plot of three-phase voltage sine waves, also with zero power factors angle as shown in Fig.1, has all three voltage and current waves crossing the zero line twice each cycle together. Each of its three phases, V1, V2, and V3, is separated by 120 electrical degrees. Power supplied to each of the three phases of a three-phase circuit also has a sinusoidal waveform, and the total three-phase power supplied to a balanced three-phase circuit remains constant.

Figure 1 - Three-phase voltage waveforms are separated by 120 electrical degrees.

Ok, let’s conclude something… As a result, there are two practical reasons why three-phase power is superior to single-phase power for many applications:

1st reason Three-phase machines and controls can be smaller, lighter in weight, and more efficient than comparable single-phase equipment. More power is supplied to them in the same period than can be supplied by a single-phase power circuit. However, the trade-off for this advantage is that three-phase machines and controls are more complex and expensive.

2nd reason Only about 75 percent as much copper wire is required for distributing three-phase power as is required for distributing the same amount of single-phase power .

For further clarification: The main reason is a three phase system is cheaper than a single phase system because less wire is used. Consider a 120V single phase system providing power to three 60ohm resistors. Total resistance is 60ohm/3 = 20ohms. Current is 120V/20ohms = 6A or 2A for each resistor. So a single phase system requires two wires (Line and neutral) carrying 6A each. In a 120V three-phase system, each resistor is connected to each phase creating three separate single phase circuits. Phase current becomes 120V/60ohms = 2A. So we have three wires carrying 2A each. Summary: Single phase - 2 wires carrying 6A. Three phase - 3 wires carrying 2A. So the physical size of the three phase wires can be smaller since the current is smaller (2A vs 6A) and 50% of the copper is saved (no neutral), since for balanced (identical) loads, the three phase voltages are shifted by 120 degrees, which allows current to flow out on one phase and return on the other phases.

Let's explain it by another example : would be a 10 HP motor which uses 7460 watts of power. On a 220v single phase system the motor would pull almost 34 amps. On a 230v 3 phase system it would pull about 20 amps. On a 3 phase motor circuit you could use a number 12 wire on a 30 amp breaker. On the single phase system you would have to use about a number 6 wire with a 50 amp breaker. Two number 6 wires cost a lot more than 3 # 12's. The conduit size may have to be increased to 3/4" where as the 3# 12's could be in a 1/2.

The above was a Comparison for the three -phase system vs .single-phase .

Now what about the 4-phase, 5-phase, 6-phase ...et ? There is no inherent advantage of 4 or 5 phases over three, and they have the disadvantage of requiring more conductors and a greater volume of copper, making them more expensive. So are used because they are the best balance of cost and amount of power flow. Three phase systems provide stability for transmitting smoother power. There could be a higher number of poly phase systems but the cost more to implement without giving the benefit of an equal amount of extra power flow. Three Phase power systems are used because the current is less per phase than what would be needed in a single phase system. Because the current is lower the size of the wire is also smaller which makes the cost of installation and maintenance lower. Other than that the cost is the same. Watts (Power) is power regardless of the type of system.

It is possible to add : The three-phase system is more stability than other system. Increase the number of faces and thus increase harmonic or distortion.

What is the difference between AC and DC motor in terms of rotor and stator ?

By definition, an electric motor is a device that converts electrical energy into mechanical energy. An electrical signal is applied to the input of the motor, and the output of the motor produces a defined amount of torque related to the characteristics of the motor. If you think about the attraction and repulsion of the north and south poles of a bar magnet, you're on your way to understanding what has to happen inside the motor yoke. To achieve rotation, there has to be some interaction between magnetic flux produced by electromagnetism within the motor. DC motors and AC motors accomplish this task in different ways. DC machines can be classified as self-excited, separately excited, permanent magnet (PM), and brushless. Self-excited machines can be further classified as shunt, series, and compound. Compound machines can be further classified as cumulative and differential. Cumulative and differential machines can be further classified as long shunt and short shunt. As you can see, there are a variety of electrical configurations for a DC machine. For the purpose of this article, we will stick with the series- and shunt-wound DC motor. Please note that the interconnection of the field (stationary winding) and armature (rotating winding) determine the machine's operating characteristics.

Fig. 1. The shunt DC motor has the field winding in parallel with the armature.

The shunt DC motor has the field winding in parallel with the armature (Fig. 1). In a parallel circuit, the magnitude of voltage drop across each parallel element is the same, while the magnitude of current through each parallel branch is a function of the impedance of the element. Please note: In a purely resistive circuit, the impedance will equal the resistance as there is no reactive component present. Shunt motors are also called constant speed motors, as they provide relatively stable speed and torque characteristics under load.

Fig. 2. The series DC motor has the field winding in series with the armature.

The series DC motor has the field winding in series with the armature (Fig. 2). In a series circuit, the magnitude of current is the same through all series elements, while the magnitude of voltage drop across each series element is a function of the impedance of the element. Series motors can develop very high starting torque and provide excellent torque characteristics under load. The drawback is speed regulation. As such, never operate a series motor without mechanical load present. Common terms you will hear discussed with DC motors include commutator, brushes, counter electromotive force (EMF), torque, speed regulation, and speedtorque characteristic curves. When used in a motor application, the commutator is a mechanical device that properly directs current flow to the armature.

By contrast, when a commutator is used in a generator application, it acts like a rectifier to convert the generated AC voltage of the machine into DC voltage. Brushes, which are usually made of carbon, are used to transition from a stationary element to a rotating element. EMF is “the difference in potential that exists between two dissimilar electrodes immersed in the same electrolyte or otherwise connected by ionic conductors.” The terms EMF and voltage are often used interchangeably. Remember Faraday's law of magnetic induction where a magnetic field can generate an electric current? As it turns out in the case of a DC motor, when the armature rotates through the magnetic field, an induced voltage opposite in polarity to the applied voltage is created — called counter EMF. Torque is a rotational force that — in simple terms — is the algebraic product of force multiplied by distance. Speed regulation is a measure of how the speed of a DC motor decrease as more mechanical load is applied. It is a function of the armature resistance. Speed-torque characteristic curves are graphs that show the relationship between speed , as a percent of rated speed, and load torque as a percent of full rating. These are very helpful because they illustrate how applied mechanical load affects the speed and torque of series, shunt, or compound DC machines.

Fig. 3. Split-phase is one classification of single-phase motors.

AC machines can be classified as induction, wound rotor, and synchronous. Induction motors can be further classified as 3-phase and single-phase. A 3-phase induction motor can be further classified as delta wound or wye wound. Singlephase motors can be further classified as split phase (Fig. 3), capacitor start, capacitor start/capacitor run, shaded pole, repulsion start, and universal. As you can see, there are several varieties of AC motors. For the purpose of this article, we will stick with an overview of the induction motor.

The induction motor is commonly referred to as a “squirrel cage” induction motor. This is due to the fact that the rotor is constructed in a manner resembling a squirrel's cage. An induction motor has a rotor (rotating part) and a stator (stationary part) within the motor housing. When an AC signal is applied to the stator winding, a rotating magnetic field is produced. This rotating magnetic field induces a signal in the rotor, which also generates a rotating magnetic field. The interaction of these rotating magnetic fields causes rotation. This is an important principle to keep in mind because in the case of a DC motor, the magnetic field is not time varying due to the applied signal. Common terms you will hear discussed with AC motors include frequency, synchronous speed, and slip. An AC waveform is time varying or oscillatory. This means its amplitude starts at zero, rises to some maximum value, returns to zero, falls to some minimum value, and then returns to zero. The number of times this occurs per unit of time is referred to as frequency. In the United States, this frequency is 60 Hertz or 60 cycles per second. Referring to the speed of the rotating magnetic field, synchronous speed is a function of the applied frequency and the number of stator poles in the machine. Slip is a measure of the difference between the synchronous speed of the stator field rotation and the rotor field rotation. Please note that the rotor field rotation is always slower than the stator field rotation.

includes power equations for AC motors. It also includes a graphical aid called the power triangle, which uses trigonometric identities to help with the analysis of power factor. Finally, some practical equations are included to calculate torque, horsepower, and efficiency.