Test Bed System to Evaluate the Efficiency of Variable Speed Drives ...

Test Bed System to Evaluate the Efficiency of Variable Speed Drives Under Varying Load Conditions M L Walker, Student Member, IEEE ,G Diana*, Member, IEEE Department of Electrical Engineering, University of Natal, Dalbridge, 404 1, Durban e-mail: Walkerm1@,eng.und .ac.za ,* Gdiana(ii,eng.und.ac.zg

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Abstract This paper describes the hardware requirements for a user friendly, easy to use test bed system to evaluate the efficiency of Variable Speed Drives under variable load conditions. The test bed will allow Variable Speed Drives to be loaded with any desired load characteristic to enable measurement and comparison of the savings that can be achieved when replacing fixed speed drives with variable speed drives.

Fig. 3 shows the flow rate of the air or liquid for a specific speed of the pump (no head) or fan, from which it can be concluded that at low flow rates it would be beneficial to reduce the speed of the pump or fan, resulting in large savings. 1.2 " I I

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Zndpx terms - Variable Speed Drive, Configureable load, DC drive, VSI

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I. INTRODUCTION The control of air or liquid flow is used frequently in industry for the production of many products. When using motors running at fixed speed the control of the air or liquid flow (oil, water, etc.) has been done mechanically by restricting the air or liquid flow, which becomes very inefficient at low flow rates. A test bed system to measure the power savings that can be made in those applications where Variable Speed Drives (VSDs) can be used is being developed. The test bed system will allow the VSD to be loaded with out requiring an actual pump or fan, thus any load could be placed on the VSD through the use of a configureable load. The efficiency of the VSD can then be evaluated at various speeds under the specified load condition. The hardware is the key to the correct operation of the test bed and will affect the validity of the results if not implemented correctly. Section I1 gives a brief overview of why VSDs are being used to replace the conventional methods of controlling liquid or air flow. Section I11 describes some of the hardware developed for the test bed system and presents results to show their use. 11. OBJECTIVES

Most fixed speed industrial applicationsinvolving pumps and fans have the motor stimy coupled to the load and fed directly from the mains as shown in Fig. 1.

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Fig. 2

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Speed Versus power required by pump or fan

---0.05 0.15 0.25~0.35~0.k5~0.b5~0.~5~0.~5~0.85~0.~5~ Speed

Fig. 3

Speed versus Flow rate for Pump or Fan

The main objective in developing a test bed system is to evaluate the savings thiat can be made by using a VSD to control the flow rate of liquids or gasses compared to that of a fixed speed drive as shown in Fig. 1.

3 phase Fig 1

Typical fixed speed system with an induction motor

When low flow rates are required, (as the motor runs at a fixed speed) the flow has to be mechanically retarded causing the efficiency of the system to drop. Fig. 2 shows the power required from the induction motor as a function of the speed of the pump (no head) or fan.

0-7803-4756-0/98/$10.00 1998 IEEE

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Fig. 4

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Typical System using a VSD

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111. SYSTEM DESCRIPTION n

If the induction motor of Fig. 1 is supplied by a Voltage Source Inverter VSI, it will enable the speed of the motor to be varied according to the flow rate required, as shown in Fig. 4.

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DC Link

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DC link Representation

The power being drawn by the voltage source inverter is given by eq. 3

Fig. 5

System Block Diagram

The system shown in Fig. 4will be simulatedon a test bed whose block diagram is shown in Fig. 5. The industrial load will be simulated by the DC motor which is stiffly coupled to the induction motor, which will allow the AC VSD to be loaded with any desired load characteristic or duty cycle. The required load torque produced by the DC motor can be controlled either in accordancewith the induction motor speed (from the torque speed curve entered by the user) or independently of speed. The AC VSI and the DC Drive are electrically connected via a common DC link, allowing the DC motor to generate power back onto the DC link, saving power, only the power lost in the test bed system will be drawn from the mains supply thus keeping the efficiency of the test bed high (lower running costs ). A 30kW test bed system is being built, to ensure that the switching losses in the H-Bridge and mechanical losses are small and have no significant impact on the accuracy of the results gained.

A. Common DC Link For the efficiency of the AC VSD to be calculated the power flowing in the system has to be determined Fig. 6 shows the various currents (which can be physically measured) flowing into or out of the DC link. The current drawn by the VSI

(Ivsd)

can be calculated

The power lost in the DC machine can be calculated from known parameters which can be measured by preforming a number of tests on the machine. The power lost inside the induction motor and the VSD drive can thus be calculatedby using eq. 4

The efficiency of the VSD can now be calculated using eq. 5.

B. PC Controller The PC and it’s related components are used to control the test bed system as well as to capture the required results, and consist of three main components. 1) RIDE 4.0 software 2) PC32 Card 3) PWM and tacho card

1) RIDE 4.0 software : The users interface to the system is via RIDE 4.0 software which is a visual SimulationiRealtime software package developed by Hypersignal@[l]. It allows a user to implement block diagrams which are then compiled, downloaded and run on a Digital Signal Processor (DSP) card. The desired load characteristic (speed torque

curve) will be loaded into RIDE 4.0 software, and used to control the operation of the DC machine. The software also enables data to evaluated on line or to be saved to disk for recordingresults which can then be evaluated at a later stage.

using eq. 1.

For the power flow in the test bed system to be known only the three parameters I, , I, ,V , , have to be physically measured. The total power lost in the system is given by eq. 2 and is equal to the power supplied to the system by the mains.

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2) PC32 Card : The PC32 card is a standard DSP card manufactured by Innovative Integration [2]. The PC32 card contains a DSP processor (TMS320C32), which communicates with RIDE 4.0 software running on the PC via dual port RAM [2]. The Card contains 4 Digital to

Analog (D/A) converters of which one is used to control the speed of the VSD, and 4 Analog to Digital (AID) converters which are used to measure the current and voltage values in the system. 3) PWM and Tacho Card : A PWM and Tacho card has been developed and built to allow the PC32 card and Hypersignalto directly control power electronicdrives. The PWM and Tacho card connects directly to the memory expansion header on the PC32 card. allowing one to easily and efficiently implement different control methods for AC and DC drives with little or no extra hardware. Fig. 7 shows the block diagram of PWM and Tacho card

PWM and Tacho Card

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Fig. 9

Connection of PWM IC to H bridge inverter

C. DC Machine Control IN I

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Fig. 7

Block Diagram of PWM and Tacho Card

The address decoding allows the DSP card to communicate with the various peripherals on the PWM and Tacho cards as well as allowing more than one card to be operated from the DSP card simultaneously. The PWM and Tacho Card contains a three phase PWM ASIC (PBM 1/87) [3] which is connected to an inverter via a fiber optic interface, to provide isolation between PC and the power electronics and eliminate any possible ground loops or possible corruption of the PWM signals due to noise as shown in Fig. 8.

Armature current control of the DC motor has been opted for as it will allow the DC motor to operate over a large speed range. To test the operation of the PWM and tacho card and the RIDE 4.0 software a small 12Vpermanent magnet DC motor was used as a load and an armature current controller was implemented in the RIDE 4.0 software, the block diagram for the armature current controller is shown in Fig. 10

Fig. 10

The armature Current is measured using a LEM module, and read via channel 1 of the A/D converter on the PC32 card. The armature current reference (Iref) (square wave of 20 Hz)and the actual armature current are shown in Fig 11.The PWM switching frequtmcywas set to 20kHz.

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PBM 1107

IV. Conclusion

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Fig. 8

block diag" for DC motor current controller

Connection of PWM IC to Inverter

By using the trigger pulses from one of the three phase legs, a DC drive can be implemented by using the PWM signals to control a H Bridge as shown in Fig. 10. The PWM and Tacho card also has a tacho controller ASIC (TC 3005H) [4] which can return the position or count (in a register) from an incremental tacho. Due to the versatility of the ASIC it can control either one or two incremental encoders.

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The system will prove useful in demonstratingthe power savings that can be made by replacing older methods of controllingair or liquid flow (restrictingthe flow) by varying the speed of the pump or fan which moves the air or liquid. The test bed system will be able to measure the energy saved under any load conditions imposed on Induction motors as well as enabling the efficiency of loads with any specified duty cycle to be evaluarted. The PWM and Tacho card has successfullybeen used by a number of students in implementing designs involving three phase inverters and motor drives. There is further work which can be performed on the test

Manual” Hypersignal, 1996 [2]

Innovative integration, ‘‘ PC32 Hardware Users Manual “ Innovative Integration, 1995

[3]

Hanning Electro-Werke, “ Pulse width Modulator

bed systemwhen it is complete this includes the study of the effects of quality of supply on either the DC or AC VSD and the testing of new control strategies for variable speed drives.

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V. NOMENCLATURE

TI K

4 I, Is I,, Vdc Pt, Pvsd P, Pvl

Hypersignal”, “Block Diagram/RIDE User’s

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Connectionof PWM IC to H bridge inverter

PBM 1/87 . PBM 1/89 Data sheet 1993

Load torque from DC machine (N.m) DC machine constant DC machine flux (wb) DC machine armature current (A) Supply current to DC link (A) Current drawn by VSI (A) DC link Voltage (V) Total power lost (W) Power drawn by VSD (W) Power loss in DC motor (W) Power lost in VSD and IM (W) VI. REFERENCES

[I]

PWM and Tacho Card

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Revision 2.0,

[4]

Hanning Electro-Werke, “ Tacho-Controller TC 3005H Data sheet “Revision 1.1, 1994

[5]

M L Walker and G Diana “Test Bed System to Evaluate the Efficiency of Variable Speed Drives Under Varying Load conditions” Seventh South African Power Engineering Conference 1998, pp 223-226