Performance of a variable- speed inverter/motor drive

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by S. A. Tassou and T. Q. Qureshi Brunel University

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This article presents the results of an experimental investigation into the application of inverter-based variable-speed drives to positive displacement rotary vane refrigeration compressors. The investigation considers the effects of the inverter on a number of operating parameters such as the harmonic currents and voltages on both the supply side and motor side, power consumption and power factor, starting current and overall system efficiency. The results indicate that the use of off-the- shelf inverters for variable-speed control of refrigeration compressors can inject harmonics to the supply system which exceed the maximum limits imposed by BS5406. The inverter may also cause a reduction in the power factor and in the overall efficiency of the drive. Variable-speed operation of a rotary vane compressor may not offer efficiency advantages over fixed- speed operation but can provide other benefits such as better temperature control and faster response to sudden changes in load.

Introduction he application of power electronics in the conversion T and control of electrical power

has been considered for many years as a method which offers considerable potential for energy savings and reduction of environmental pollution. It has been estimated that around 65% of the average industrial electricity bill accounts for electric motors used in different applications, costing over €3000 million per annum in electrical energy. At least 10% of the power used by these motors could be conserved through the use of various design and control techniques.’ The majority of these motors are used to drive fans, pumps and compressors, most of which employ some kind of conventional control to regulate the flow of the working fluid in order to match the supply to demand.

Variable-speed drives based on power semiconductor devices are now available specifically for use in constant torque load applications (conveyors, mixers, machine tools etc.) and variable (quadratic) torque loads in flow control applications

(pumps, fans and centrifugal compressors) where considerable mechanical and fluid energy is being wasted by traditional mechanical flow control methods. In flow control applications, variable-speed drives have been successfully employed in machines having continuous flow characteristics (dynamic type) with lower success in machines having intermittent flow characteristics (positive displacement type). The present work is concerned with the investigation of the application of variable-speed drives to positive displacement rotary and reciprocating refrigeration compressors. Better refrigeration system design and compressor control in the UK could result in savings in excess of € 12.5 million per annum.2

Presently available variable-speed drives are based on a variety of power semiconductor devices depending on the application. These devices include bipolar and MOS field-effect transistors, silicon controlled rectifiers (SCRs), triacs, gate turn-off thyristors (GTOs), insulated-gate bipolar transistors (IGBTs), static induction transistors

(SITS), static induction thyristors (SITHs) and MOS-controlled thyristors (MCTs). Among these, the IGBT converter is the most common drive in medium power range applications due to its comparatively high switching speed, low noise and better performance characteristics than other competing drives.

Inverter-based variable-speed drives introduce disturbances, especially harmonics of the supply frequency, into the supply grid to which they are connected. Injection of harmonic frequency currents can distort the supply voltage from its sinusoidal waveform and this may have undesirable effects since the same voltage source is used to supply other equipment in parallel with the drive.3 Harmonics on the motor side can increase slip and motor losses due to a non-sinusoidal voltage waveform, and can introduce speeditorque oscillations which could impose extra stresses on the windings. The aim of this study is to investigate the performance of inverter-based variable-speed drives in refrigeration applications and quantify the effects of the inverter on the overall system efficiency. Specific objectives include:

0 the investigation of the magnitude and effects of current and voltage harmonics at the input and output sides of the PWM inverter

0 the evaluation of the efficiency of the variable-speed drive, over a range of speeds and load conditions

0 the identification of factors which could influence the practical application of variable-speed drives to refrigeration compressors.

Experimental test facility

T facility which was designed

he ex peri men ta I program me was carried out using a test

os VARIABLE SPEED DRIVE 1 input power ! from mains

output power _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ to motor ' DC link

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water lines low pressure

- high pressure refrigerant lines

- -

I water in

I water out

Water In Water Out I I , T 5 i AI c: FD1 FM ~ SGI

condenser ~

ws OIL MEASUREMENT SETUP

DATA ACQUISITION SYSTEM

flEEE-488 bus

interface modules

thermocouples transducers

OS oil separator RCM rotary compressor PCM piston compressor SV solenoid valve TEV thermostatic expansion valve EEV electronic expansion valve SG sight glass FM liquid line flow meter FD filter drier RSP refrigerant sample point RE rotary encoder WFT water flow transducer V flow control valve P pressure transducer T thermocouple SC sample cylinder WS weighing scale NV needle valve

Fig. 1 Schematic diagram of experimental test facility

around a chiller of nominal cooling capacity 25 kW. The chiller was supplied with shell and tube heat exchangers, an externally equalised thermostatic expansion valve, a four- cylinder hermetic compressor designed for use with R22 and appropriate controls. A Danfoss DQ5 electronic expansion valve was mounted in parallel to the thermostatic valve to enable performance comparison of the two valves to be carried out. Modifications carried out on the system enable easy introduction and testing of alternative types of compressor and variable-speed drives. Test conditions on the chiller are achieved through a water loading system and a recirculatory air tunnel which acts as a balancing mechanism between the hot and cold sides of the system. The tunnel can also be used to simulate ambient conditions for air-cooled evaporators and condensers. A schematic diagram of the system is shown in Fig. 1 .

All tests were carried out on an open-type rotary vane compressor which was mounted in parallel to the hermetic reciprocating compressor. The compressor was driven by a standard three-phase induction motor which was directly coupled to the corr;pressor in an 'open type'

configuration. Motor speed control was achieved by a pulse-width- modulated [PWMl inverter

condenser and evaporator, and water and refrigerant flowrates. A PC-based data-acauisition system

consisting of an input diode bridge which rectifies the three-phase AC supply to DC and an IGBT converter which converts the DC voltage into a three-phase PWM output.

comprehensively instrumented to provide the necessary data for control and optimisation studies. A precision universal power analyser was used to measure the characteristics of the inverter with an approximate accuracy of 0.1 %. The power analyser is capable of measuring the fundamental component as well as the harmonic contents of voltage and current on each phase up to the 50th harmonic. A rotary encoder which provided an output in the form of pulses ( 1 00 pulsesirev) was used to measure the compressor shaft speed. An output stage circuit to convert the pulses into a voltage signal was developed to allow automatic speed logging by the data-logging system. Other logged data included refrigerant pressures and temperatures at four points in the refrigeration circuit, before and after each major component, water temperatures before and after the

The test facility is

was used for automatic data logging.

Measurement of inverter characteristics

In evaluating the performance of variable-speed inverters, a preferred arrangement is one that enables the measurements before and after inverter simultaneously for a selected operating condition. This type of arrangement involves duplication of very expensive measuring equipment. An alternative arrangement which was adopted in this investigation was to take measurements first before the inverter and then after the inverter at the same steady-state conditions using the same measuring instruments.

The harmonic content of both the voltage and current for all the components up to the 50th harmonic was determined by carrying out a discrete Fourier analysis at each harmonic frequency. The following data were recorded during this analysis:

0 Frequency 0 Voltage harmonics 0 Current harmonics

194 fnMPl I T I N C rC f n N T R n l F N C I N F F R I N C In1 I R N A I A I IC1 IST 199h

@ Voltage distortion factor 0 Current distortion factor.

Results and discussion Effects of inverter drive on electrical supply system

he most important characteristics of an electric T load is the waveform of the

current drawn from a sinusoidal supply voltage. The current and voltage waveforms were recorded both at direct mains supply and before and after the inverter at the base frequency of 50 Hz and at a low frequency of 30 Hz for comparison purposes. Fig. 2 compares the current and voltage waveforms when the motor was driven directly from mains power and through the inverter. These waveforms were created by synthesising the magnitude and phase of each measured harmonic (up to the 40th).

On direct mains power supply (Fig. 2 (a ) ) both waveforms were nearly sinusoidal with total current harmonic distortion 6.63% of the fundamental, at a fundamental current of 13.54 A, and total voltage harmonic distortion 2.89% of the fundamental, at a fundamental voltage of 236.3 V. The waveforms on VSD power supply measured before the inverter are shown in Fig. 2(b) . The voltage waveform is quite similar to the mains voltage waveform at base frequency and nearly sinusoidal, with a total voltage harmonic distortion of 2.46% of the fundamental, at a fundamental voltage of 235.7 V. The current waveform, however, was highly distorted with a total current harmonic distortion of 1 1 6.3% of the fundamental, at a fundamental current of 10.14 A. The voltage waveform after the inverter (Fig. 2 ( c ) ) was highly non-sinuosidal with voltage distortion up to 56% of the fundamental, at a fundamental voltage of 230.7 V. The current waveform, however, was nearly sinusoidal with very low current distortion, less than 3%.

At 30 Hz, the voltage harmonic distortion before the inverter remained approximately the same as that at 50 Hz, but the current distortion increased to 1329% of the fundamental at a fundamental current of 6.1 6 A. After the inverter, the voltage distortion at 30 Hz increased to 1 1 6% of the fundamental, at a fundamental voltage of 143.8 V. The current waveform at 30 Hz was not significantly different from that at 50 Hz. -

the general requirements and British Standard BS5406 defines

1'5 f

1

0 50 100 150 200 250 300 0 50 100 150 200 250 300

a voltage waveform at mains current waveform at mains

1.5 r 3r

-1.5 I * . I , * , 0 50 100 150 200 250 300 0 50 100 150 200 250 300 voltage waveform at 50 Hz before inverter current waveform at 50 Hz before inverter

1'5 r

-1.5 -1.5

voltage waveform at 50 Hz after inverter 0 50 100 150 200 250 300 0 50 100 150 200 250 300

current waveform at 50 Hz after inverter

Fig. 2 Voltage and current waveforms

2'5 i

channel 1

0 channel 2

=channel 3

-6S5406 limits

2 5 8 11 14 17 20 23 26 29 32 35 38 harmonic number

Fig. 3 Harmonic spectrum at mains power supply

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2

z 1

2

1

a

2

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- BS5406 limits

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harmonic number a

w channel 1

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channel3

- BS5406 limits

14 17 20 23 26 29 32 35 38 harmonic number

b

Fig. 4 Harmonic analysis on supply side: (a) Harmonic spectrum at 50 Hz before inverter; (b) Harmonic spectrum at 30 Hz before inverter

maximum permissible values of harmonic components of the input current which may be produced by a piece of equipment tested individually under specified condition^.^ The standard also gives practical guidance on test procedures and on the practical application of the requirements.

Fig. 3 shows the harmonic analysis on the supply side when the compressor motor is fed directly from the grid. It can be seen that for all three phases only the 3rd, 5th, and 7th current harmonics are significant, but even these are well within the limits imposed by the BS5406 criteria.

The effects of the inverter on the harmonics injected into the grid on one side and the harmonics injected into the motor on the other side are shown in Figs. 4 and 5, respectively, for the frequencies of 50 Hz and 30 Hz. In Fig. 4, which displays the current harmonics on the supply side, it can be seen that the harmonics causing the greatest concern are the 5th, 7th, 1 1 th and 13th at both frequencies. These harmonics exceed considerably the BS5406 requirements and can be seen that the higher the frequency the more these harmonics exceed the standard limits.

From Fig. 5, which shows the

current harmonics on the motor side, it can be seen that the harmonics are well within the limits imposed by BS5406. The total current harmonic distortion at output was nearly the same as that present on mains (sinusoidal) supply. Although the current harmonic distortion of the inverter output was small, it is well known that the peaks and pokes in the chopped output voltage waveform (Fig. 2 ( c ) ) can impose extra stresses in the motor, increase noise and reduce motor life. Although the very small current harm8nic distortion has negligible effect on the average torque developed by the motor, it can give rise to additional iron and copper losses which will lead to an increase in the winding temperature and a reduction in motor eff i~iency.~

Fig. 6 shows the effect of frequency and load on the current distortion on the supply side. All results were taken at a constant evaporating temperature of 6°C and condensing temperatures of 30°C 35°C and 40°C which represent the load variation on the motor. It can be seen that a reduction in the frequency causes a linear increase in the current distortion. The load variation on the motor has a similar effect. An increase in the load causes an almost linear increase in the current distortion.

Effects of inverter on starting current One of the main disadvantages of

induction motor driven compressors is the very high current required by the motor at start-up. The response of the compressor motor when fed directly from the mains during the initial 60 ms from start-up is shown in Fig. 7 ( a ) .

It can be observed that the voltage reaches a peak of about 335 V and the current rises to a peak of about 160 A RMS within 10 ms from start-up before reducing to steady state RMS voltage of about 235 V and current of about 12 A RMS within 60 ms. During this initial period, the voltage distortion factor was very small, about 3.5?/0, whereas the current distortion factor was of the order of 85%.

When the same motor was operated through the inverter (Fig. 7(b)) the current increased gradually from about 4 A RMS at start-up to a peak of around 30 A RMS before decreasing to the steady state running current. The current distortion factor was of the order of 200% but the voltage distortion factor was small, approximately of the same value as that for mains operation. The above shows that when the motor is driven through the

196 frSMPl ITINC F; fnhlTRnI FhlCIMFFRlhlC In1 IRhlAl AI IC1 IKT 1QQh

inverter the latter acts as a soft- starting mechanism and thus eliminates the effects of high currents and mechanical stresses which are associated with electromechanical starting devices.

Effect of harmonics and inverter efficiency on power consumption

The power associated with the energy that is converted into work and is consumed by the system is referred to as ‘true power’ (kW). True power is sometimes referred to as real power or active power and is computed by:

active power = 2n (A J (v i ) dt)’” watts

0

l

I

where v is the instantaneous voltage and i is the instantaneous current.

The active power consumption of the motor at fixed operating conditions increases linearly with speed as shown in Fig. 8. It can also be seen that, for the same load at 50 Hz, the actual power consumption of the rotary compressor motor with PWM inverter was higher compared to mains power supply. The increase in actual power was due to the addition of PWM inverter losses.

Another important characteristic of a variable-speed drive is the power factor. The power factor of the load can be defined as:

true power apparent power true power factor =

The apparent power is measured in kVA and can be computed from:

apparent power = (VRMS x ARMS)

where VRMS is the true RMS input voltage and ARMS is the true RMS current .

With strictly sinusoidal current and voltage waveforms, the power factor can be expressed as the phase displacement between voltage and current waveforms. If the current and/or voltage waveforms are distorted, however, the fundamental components of voltage and current may well be in phase but the true power factor will be less than unity. When harmonics are present, the fundamental power factor, cos 4, is called the displacement power factor. Traditional corrective measures for poor power factor, the addition of shunt capacitors, may actually decrease rather than increase the true power factor. It is important, therefore, to measure both true and

displacement power factor in order to devise suitable corrective measures for the improvement of poor power factor.

The true power factor of the load was measured at different operating conditions, as shown in Fig. 9. It can be seen that the power factor decreased not only with decrease in PWM inverter output frequency and load but also with the addition of harmonics. The Figure also compares the PF achieved at maximum load when the load is driven directly from mains power with that achieved when the load is driven through the inverter. Maximum PF was achieved at maximum load conditions with

mains power supply which was free of harmonics. The performance of the inverter driven rotary vane compressor over a range of frequencies at an evaporating temperature of 6”C, a condensing temperature of 35°C and a refrigerant superheat temperature of 10°C is summarised in Table 1.

From Table 1 it can be seen that the power consumption of the system measured before and after the inverter exhibits a linear variation with operating frequency between 30 and 50 Hz. The inverter efficiency was determined as the ratio of the power consumption measured before and after the

2.5

2

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0

w channel 1

0 channel 2

channel 3

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2.E

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0.5

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harmonic number a

channel 1

0 channel2 w channel 3

- BS5406 limits

2 5 8 11 14 17 20 23 26 29 32 35 38

harmonic number b

Fig. 5 Harmonic analysis on motor side: (a) Harmonic spectrum at 50 Hz after inverter; (b) Harmonic spectrum at 30 Hz after inverter

COMPUTING & CONTROL ENGINEERING JOURNAL AUGUST 1994 197

5 125 a,

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0 35 C condl6 C evap

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100 I I I

25 30 35 40 45 50 55 frequency. Hz

Fig. 6 Effect of frequency and load on current distortion at supply side

inverter. Because of the relatively high efficiency of the inverter it was difficult to ensure high accuracy due to the fact that any errors in the measurement of power which are associated with the complex waveform could be wrongly interpreted as inverter losses. For more accurate measurement of inverter efficiency, major losses in the inverter can be determined as a function of the system operating variables such as voltage, current and frequency using calorimetric

techniques.6 This was beyond the scope of the present investigation which was concerned with the variation of the inverter efficiency with frequency rather than the absolute value of efficiency. The results indicate that the efficiency of the inverter was of the order of 95% and remained fairly constant within the operating frequency range.

The coefficient of performance of the system, which can be defined as the ratio of the cooling capacity of the system over the total power

CHI

signal parameters CH1 = 200 V/division CH2 = 40 Ndivision timebase = 10 ms/division capture time = 60 ms

CH2

input measured before the inverter, does not exhibit a wide variation with operating frequency and in fact decreases slightly as the frequency decreases from 50 to 30 Hz. This indicates that a rotary vane compressor operated at variable frequency through a PWM inverter will not provide steady-state efficiency advantages over fixed- speed operation at mains supply. This may be due to the fact that the additional losses caused by the inverter outweigh the gains from a reduction in the friction losses in the compressor at reduced speed. The application of variable-speed control to rotary vane refrigeration compressors, however, may lead to other benefits such as better temperature control and quicker response to sudden changes in load.

Conclusions

T investigation can be summarised as follows:

1 Harmonic currents generated by the inverter and injected back into the supply system exceed the maximum limits imposed by British Standard BS5406 particularly at harmonic numbers 5, 7, 1 1 and 13. The penetration of these harmonics into the AC system can be attenuated through the use of tuned harmonic filters or the use of a 12-pulse convertor which can eliminate lower-order harmonics. Both these measures, however, will result in higher capital cost for the drive. Harmonics on the motor side fall within British Standard requirements.

he main conclusions that can be drawn from this

CH1 signal parameters CH1 = 100 V/division CH2 = 20 Ndivision timebase = 10 mddivision

CH2

a b

Fig. 7 Voltage and current response at compressor start-up: (a) Voltage and current waveform on mains power supply; (b) Input voltage and current waveform on inverter power supply

198 COMPUTING & CONTROL ENGINEERING JOURNAL AUGUST 1994

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i I

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I i I ~

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0.8

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inverter 40/06 a mains 40/06

inverter 35/06 o mains 35/06

inverter 30/06 0 mains 30/06

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3 25 30 35 40 45 50 55

frequency, Hz

0.7 g 8 & z c

0.65

0.6

Fig. 8 Variation of active power consumption with frequency and load

-

-

-

A mains 40/06

0 mains 35/06

0 mains 30/06

inverter 40/06 inverter 35/06 inverter 30/06

0.55 25 30 35 40 45 50 55

frequency, Hz

Fig. 9 Variation of power factor with frequency and load

Table 1 Performance of variable-speed drive

2 The efficiency of the inverter is high, of the order of 95’0, and remains relatively constant over the range of operating frequencies investigated.

3 The inverter causes a reduction in the power factor of the order of between 5 and 1 O O O depending on the system load. The power factor decreases with a reduction in the operating frequency.

4 The coefficient of performance of the refrigeration system remains fairly constant over the operating frequency range of the system, and thus it can be concluded that operation of rotary vane compressors at reduced speeds offers no steady-state efficiency advantages over full-speed operation. For this type of compressor, increased overall drive losses at reduced speed and load seem to offset any gains in compressor efficiency associated with the reduced friction losses at lower speeds. Variable-speed operation, however, may lead to other benefits such as better temperature control and quicker responses to sudden changes in load.

References 1 McNAUGHT, C.: ‘Making motors cut

current costs’, Professional Engineering, January 1993, pp.14-15

2 ENERGY EFFICIENCY OFFICE: ‘Guidance notes for reducing energy consumption costs of electric motor and drive systems’, Good Practice Guide 2, Good Practice Programme, 1989

3 ELECTRICAL POWER RESEARCH INSTITUTE: ‘Evaluation and comparison of state of the art techniques for energy conservation by reduced losses in AC motors’, EPRl Project Report

4 BRITISH STANDARDS INSTITUTION: ‘Disturbances in supply systems caused by household appliances and similar electrical equiment’, British Standard BS5406, Parts 1 , 2 and 3, 1988

5 MURPHY, J. M. D.: ‘Thyristor control of AC motors’ (Pergamon Press Ltd., New York, 1973)

6 NOVOTNY, D. W.: ‘A comparative study of variable frequency drives for energy conservation applications’, Dept. of Electrical and Computer Engineering, University of Wisconsin, Madison, 1981

RP1201-13, 1991

7 JOHNSON, K., and ZAVADIL, R.: frequency, Hz ‘Assessing the impacts of nonlinear parameter

30 35 40 45 50 loads on Dower Quality in commercial _.

buildings-an overview’, Proceedings

Management, 1991, pp. 1863-1869 6’48 7 ‘17 of the IEEE, Energy Conversion and power consumption, kW 4.4 1 5.08 5.75

(before inverter) power consumption, kW 4.15 4.85 5.51 6.16 6.87 (after inverter) inverter efficiency, YO 94.30 95.60 95,70 95.10 95.80 coefficient of performance 2.69 2.72 2.74 2.76 2.84 lC0Pl

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0 IEE: 1994

The authors are with the Department of Mechanical Engineering, Brunel University, Uxbridge. Middx. U88 3PH, U K.

COMPUTING d CONTROI FNGINFFRING 101 IRNAI AI IC1 IST 1996 199