Vsd

InsideENERGY The Continuing Professional Development programme MODULE 004 • MAY 2003

VARIABLE SPEED DRIVES In the fourth of EiBI and the Institute of Energy’s CPD training modules, Debbie Hobbs of Faber Maunsell and Chris Monson of Trend Control Systems examine the basics in VSDs. FOR THIS MODULE GOAL: To introduce building operators/energy managers to the concepts and opportunities in using variable speed drives in buildings & industry related to HVAC. OBJECTIVES: After studying this module readers should be able to: • understand the concept of the VSD; • understand the areas of VSD application in HVAC circuit; • identify opportunities in their building/industry; • check for approximate savings potential by installing a VSD; • talk to suppliers with more details, which can benefit both; • check additional benefits that can be achieved by VSDs other than energy savings (process control); • consider other areas where VSDs can be used. Variable speed drives (VSDs) have now become a common sight in buildings and industry. There are many reasons to explain this growth, but the main ones are cost savings and improved process control. VSDs are not new. They have been in use in critical process industries for several years and have very recently grown to a level that they can find use in smaller industry and buildings. Subsequently, the technology has also developed considerably, with features that look at factors that might affect the installation, e.g. size, harmonics. VSDs are electronic gadgets which can be attached to an induction motor with a control mechanism (like temperature, pressure, etc). They can be incorporated into any motor with a variable load, but the most common applications are pumps and fans. Other applications include air compressors, agitators, etc. VSDs, also called Variable Frequency Drives, can sense the temperature/pressure of a system and reduce/increase frequency (V/f) thereby reducing the rpm of the motor, which in turn reduces the flow. The response is fast, another reason why it is preferred by industry. During the construction phase of an industry/building, the primary focus would be on completing the project successfully. As a result, design engineers build in some extra capacity in order to have some flexibility during initial operation and to cater for future expansion. This normally leads to excess capacities in equipment like pumps and fans. Apart from these some process

operations could be highly variable. Design engineers have to consider equipment for the maximum use (or) the worst situation. This leads to an excess capacity situation when the demands are less. VSDs try to minimise the energy consumption in both these conditions by reducing the flow. VSDs are often chosen for better process control, e.g. temperature and relative humidity control in a building, chemical reactor operation, etc. VSDs are also being considered during design stage resulting in equipment being specified very close to the actual requirement. VSDs have been widely used in pumps and fans. Ten million motors are installed in the UK, responsible for 64 per cent of the nation’s electricity use. Fifty per cent of these are installed on pumps and fans. The following paragraphs speak about the basic principles of induction motor, VSD and then focus on the HVAC application to pumps and fans with a case example. BASIC PRINCIPLES The most commonly used motor is the threephase, asynchronous AC motor, also called an induction motor, which is both inexpensive and very reliable. The two main components of an asynchronous motor are the stator (stationary element) and the rotor (rotating element). The speed at which a motor turns is dependant on the speed the magnetic field generated in the stator rotates, which in turn is dependant upon the a.c supply frequency and the number of phase windings (poles) in the stator housing. Rotation [revs/min] is at: (f x 60)/p where f = a.c. frequency and p = no. of pole pairs. The principles that drive the motor mean that the speed of the rotor does not reach that of the rotating field. The slip is the difference between the speed of the rotating field and the speed of the rotor and is often expressed as a percentage of the synchronous speed. It is normally between 4 and 11 percent of rated speed. For example, the mains supply in most European countries is 400V 50Hz. This gives a four-poled asynchronous AC motor an approximate speed of 1440rpm applying a slip factor of 4 per cent. Centrifugal fans and pumps, as frequently found in HVAC systems are variable torque loads. With a variable torque load, the torque required to drive the load increases in proportion to the square of speed as shown in Figure 1.

100% 90% 80% 70% 60% 50% 40% Torque

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Figure 1: Torque & Power characteristic of variable torque loads

Figure 1. Torque & Power characteristics of variable loads The basic equations governing this application are summarized as: • Flow A Motor speed • Torque A (Motor speed) _ • Fan Power A (Motor speed) _ The load increases from zero torque at zero speed through 25 per cent torque at 50 per cent speed to 100 per cent torque at 100 per cent speed. Since power is proportional to torque multiplied by speed, power is proportional to the cube of speed. At 50 per cent speed, the load requires only 12.5 per cent of maximum power. The power/speed relationship is sometimes known as the ‘power cubed’ law or as the ‘square torque’ law. Hence controlling flow by motor speed reduction means that a relatively small speed change produces a large fall in absorbed power. Obviously, there will be no energy saving if the power of the motor matches exactly that of the load. In constant flow applications one of the most cost-effective options for improving the operating efficiencies of oversized motors is to replace them with adequately sized motors, which essentially deliver a higher efficiency than the former. However, careful consideration has to be given before arriving at any conclusion regarding the loading of the motor. Replacing motors based on incorrect assessment of the motor loading may lead to a motor burn out if the assessed loading is greater then the rated capacity of the replaced motor. In applications where there is variation in flow; speed control is the preferred technique to other means of regulating flow because it provides the most

efficient method of flow management and presents significant energy saving opportunities. A machine will always operate where its pressure/flow characteristics match those of the system. As an example, consider a fan with the characteristics shown in Figure 2. It is operating in a system with the flowdependent frictional loss characteristic shown. The operating point will be at A. The airflow can be regulated by adjusting the position of fan inlet vanes, adjusting the position of discharge dampers, or by adjusting the speed of the fan. Use of dampers increases the system frictional characteristics, giving a new operating point B. Although the flow falls as desired a pressure drop must be effected across the output damper to meet the system requirement and the power reduction is very small (A similar effect is obtained when a throttle is fitted to a pump system). An alternative approach is to vary the fan characteristics by reducing the speed. The new fan characteristics for a reduced speed give a new operating point C. Since the power used by the fan is proportional to the flow pressure, while the flow is the same as for B, the power is much less.

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increasing resistance as damper closes

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Volume flow Figure 2: Typical fan characteristic

Fig. 2 Typical fan characteristic

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Figure 3: Power v's Flow for different methods of control For Pumps Recirculation 100

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Capacity in % Figure 3. Power v’s Flow for different methods of control

Figure 3 illustrates how typically power varies as a function of flow for each of the regulating methods. The important thing to be noted here is that fans are mostly controlled on the suction side and pump always on the delivery side. If a fan is damper controlled on the delivery instead of the suction then the power consumption would go up. It is mainly due to the increase in average density, hence higher power consumption. Any method of speed reduction will give the same effect. However, a variable speed drive permits operation anywhere on the flow curve and is generally the preferred choice for the flexibility it affords. Consequently HVAC system designs are increasingly incorporating variable speed drives to provide control of a.c. motors on fan and pumps to regulate flows of air and water. Clearly, where flow rates can be regulated motor energy can be significantly reduced as the relationship between flow rate and motor power follows a cube-law i.e. reducing the flow rate by 20 per cent reduces power consumed by 50 per cent. However, in-use energy savings are only part of the attraction to deploying variable speed drives. In addition, the variable volume water systems associated with these are inherently more efficient avoiding the wasteful pumping and heating or cooling of water. Furthermore, the reduced wear and tear and mechanical shock due to controlled acceleration & deceleration and through basically operating at lower speeds and loads will extend not only the service intervals and life of the motor but also the machinery that is driven by the motor. Not only are significant operational savings possible, the use of variable speed drives affords specification and installation benefits: • variable speed drives replace a number of switch gear components that are normally fitted and wired within a motor control centre such as contactors and overloads. With increasing skilled labour costs the use of variable speed drives becomes an increasingly simple-to-fit, cost-effective alternative. • in larger applications considerable material and installation savings can be made by having to install only three conductors, instead of six required in Star-Delta starting applications. As starting current is always kept within the nominal value, the size and cost of cabling and fuses is also minimised. • designers are increasingly specifying variable speed drives as they remove a significant sizing risk out of the design process, simplifying their roles; and moving the sizing to an empirical activity at commissioning time. ADVANCED MOTOR CONTROL – VARIABLE SPEED DRIVE Electronic variable speed drives essentially convert the fixed frequency and voltage from the mains to controllable variable frequency and voltage (V/f Ratio). Also known as variable frequency drives or inverters they work by first converting the a.c. mains supply to d.c. using a rectifier. After smoothing by capacitors, the d.c., supply is chopped by six microprocessor-controlled, highfrequency transistors in such a way as to synthesise a variable frequency, variable voltage a.c. source of power for the motor. Most drives are controlled by a technique called pulse width modulation (PWM), in which the motor voltage is made variable by applying the intermediate d.c. voltage to the motor windings for longer or shorter periods of time. The control circuit determines the on and off switching times and sequence of transistors.

VSDS IN BUILDING HEATING AND COOLING

A typical Air Handling Unit (AHU) consists of a supply fan, heating coils, cooling coils, filters, RH controller and a return fan. The controls maintain the set temperatures and RH in the room by various control valves and dampers, which close / open depending on the ambient conditions. Due to the variable nature of the climate outside, the heating and cooling systems are designed for their maximum use and worst conditions. This makes the system more variable in nature and justifies well for VSD. The savings obtained are considerable due to the high degree of climate change. A typical AHU is explained as figure 4.

Figure 4 – Typical AHU System In the above system, a VSD can be incorporated into the following: Supply and return fans: The flow of air to the room can be varied in order to maintain a constant temperature. Apart from this all fixed dampers can be kept open and flow can be controlled by a VSD. Chilled/hot water pumps: keeping the airflow constant, the flow of chilled water / hot water can be varied. The savings depend on the number of AHUs, their location, etc. In most of the cases a combination of both would give additional savings and good process control.

VSD ON A SUPPLY FAN In one process industry a supply fan was providing air to a process room. They had three issues: during frequent product changes, they have to decrease the air changes by 40 per cent, overcome duct damage due to high pressure before the damper, and energy savings. The plant teams were interested in installing a VSD for this fan, but wondered how to calculate the savings potential and the payback. The pressure measurements were taken at different points across the ducts for both the products. Simultaneously, the power measurement was also taken for the motor. The pressure drop across the dampers was found to be 68 per cent. This indicates that only 32 per cent of the fan’s capacity was being used and the rest was getting dropped across the damper.

Pressure drop across damper (prod B) = (((4.33”)-(1”))/((4.33)-(-0.57”))) x 100 = 67.90 per cent Pressure drop across damper (prod A) = 77.30 per cent (was 6.3” on the delivery side) A VSD was installed for this fan with a pressure sensing control mechanism and the plant saved 60 per cent (average) of the existing power consumption. Additional advantages were instant change over during product change and savings on the duct maintenance. Annual savings = £5,750, Investment = £7,500, Payback = 16 months (taking 4.2 P/kWh) USING A VSD WITH A PUMP A pump with an operating point A was being operated by valve control – this would move it to Point B (very small drop in power consumption. If the same is operated with a VSD, then the point moves to Point C (with a considerable reduction in power – 40 per cent). PERFORMANCE IN-USE A number of considerations should be made when considering variable speed drive applications. • since there is rarely any need for rapid acceleration or deceleration variable torque loads such as fans and pumps do not require any momentary overload

or breaking capability; and in normal running there is little chance of experiencing high overloads from shock loading or high breakaway torques. Hence, variable speed drives specified for HVAC applications should be capable of providing 150 per cent starting torque for two seconds with 60 seconds overload capacity at 110 per cent full load torque. • with most fans, one third to one half of full speed is the minimum speed that produces any significant flow. Since the torque requirement drops off as speed is reduced, the motor is not in danger of overheating due to the reduced flow of cooling air at low operating speeds. Drive speed range capability is generally not an issue in variable torque applications. • often in fan applications, the draft inside a duct will cause the fan to rotate even after it has been powered down. If a variable speed drive is started into a “wind-milling” rotating motor, it generates a high surge current. This can cause the drive to trip out on over-current. A useful feature available in some drives is the ability to catch a flying motor. The drive, when first enabled to run, will initially ascertain the running speed of the motor due to the draft. If the motor direction of rotation is opposite to the speed command, the drive will then bring the motor to rest first before accelerating it in the correct direction. If the direction of rotation is the same as that of the speed demand, the drive will very likely accelerate the motor from its running speed. • other factors (including harmonics, etc) that might affect the existing electronic equipment in the building or industry.

Energy = (Flow x Pump head) / (Pump efficiency x motor efficiency A= 4 x 54.6/0.8 = 273W B= 3.2 62.7/(0.8 x 0.8) = 154W C= 3.2 x 34.7/(0.8 x 0.8) = 154W Energy saved = ((B-C)/B) = ((257-154)/257) =40 per cent MODULE 4 Test Questions Please mark your answers on the sheet below by placing a cross in the box next to the correct answer. Only mark one box for each question. You may find it helpful to mark the answers in pencil first before filling in the final answers in ink. Once you have completed the answer sheet in ink, return it to the address below. Photocopies are acceptable.

1. At what speed will a 6 pole motor rotate when connected to a 50hz a.c. supply? 50 r.p.m. 470 r.p.m. 940 r.p.m. 2,250 r.p.m. 2. Which types of load offer the greatest scope for realising energy savings by reducing output to match demand? Variable torque loads such as centrifugal pumps and fans Constant torque loads such as conveyors and hoists Constant power loads such as grindstones and saws Fixed speed loads such as calenders and rolls 3. What is the relationship between absorbed power and the speed of a centrifugal pump? Power alpha(Motor speed) _ (Motor speed) _ alpha Power Motor speed alpha(Power) _ Power alphaMotor speed 4. Approximately how much energy can be saved by reducing the speed of a centrifugal pump by 20 per cent? 20 per cent 64 per cent. 80 per cent 50 per cent 5. At 75 per cent speed approximately what volume .ow is delivered by a centrifugal pump? 75 per cent 55 per cent 42 per cent 90 per cent 6. What would the simple payback period be for a 20 per cent oversized 22kW fan that runs nine hours per day seven days a week; assuming the installation of a variable speed drive to be £4,000 and electricity costs 6p/kWh 2 years 2.5 years 1.5 years 3 years. 7. Which component of a variable speed drive converts the a.c. mains supply to d.c.? transistors capacitors

inverter rectifier 8. Which of the applications listed below will not generally be suitable for variable speed drive control? systems with large variable flows systems incorporating diversion or bypassing arrangements. system operating at close to full flow for the majority of its use. systems with long distribution networks 9. Which of the factors listed is not a benefit when deploying variable speed drives in retrofit applications to regulate flow rates? reduced wear and tear on mechanical components reduced installation costs reduced power consumption reduced heating or cooling of distribution systems 10. Which of the techniques listed is able to infinitely vary flow rates on motor driven fans and pumps? multiple speed motors variable speed drives changing pulley or gearbox ratios use of a different motor 11. Pulse width modulation is a technique that varies the output frequency of the variable speed drive by applying the intermediate d.c. voltage to the motor windings for longer or shorter periods of time varying the intermediate voltage frequency varying the a.c. input voltage to the drive using a voltage-controlled oscillator where the frequency follows the amplitude of the intermediate voltage. Name.......................................................................................... (Mr., Mrs., Ms) Business Address ............................................................................................. ............................................................................................................................ Town................................................................................................................... Post Code .......................................................................................... Email address................................................................... Tel No................................................................................................................. Completed answers should be sent to: Education and Training Officer Energy Institute 61 New Cavendish Street London W1G 7AR Tel: 020 7467 7178 Fax: 020 7255 1472

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