EXP. 6. Three Phase Motor-Drive and DC/AC Inverter

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1 EXP. 6. Three Phase Motor-Drive and DC/AC Inverter AIM The aim of this experiment is to investigate a three phase induction machines and motor drives, DC/AC inverter. In this experiment, you will learn the energy convertion techniques, and understand variable frequency inverter-fed induction motor drives. Write a report based on your measurement and calculation results of the motorgenerator set and power measurement of induction machines. Submit your report to course convenor, Prof. Lu s mail box (#19) at N44, Technology Building before 5:00pm Friday. INTRODUCTION A. Three Phase AC Machines and Drive The induction motor can only run efficiently at certain condition, i.e. close to the synchronous speed of the rotating field. The best method of speed control must therefore provide for continuous smooth variation of the synchronous speed, which in turn calls for variation of the supply frequency. This is achieved using an inverter to supply the motor. A complete speed control scheme which includes tacho (speed) feedback is shown in block diagram form in Figure 1. We should recall that the function of the converter (i.e. rectifier and variablefrequency inverter) is to draw power from the fixed-frequency constant-voltage mains, and convert it to variable frequency, variable voltage for driving the induction motor. Both the rectifier and the inverter employ switching strategies, so the power conversions are accomplished efficiently and the converter can be compact. Fig. 1 General arrangement of inverter-fed variable-frequency induction motor speedcontrolled drive. Variable frequency inverter-fed induction motor drives are used in ratings up to hundreds of kilowatts. Standard 50 Hz or 60 Hz motors are often used, and the 3315ENG Efficient Energy System 1

2 inverter output frequency typically covers the range from around 5 10 Hz up to perhaps 120 Hz. This is sufficient to give at least a 10:1 speed range with a top speed of twice the normal (mains frequency) operating speed. The majority of inverters are 3-phase input and 3-phase output, but single-phase input versions are available up to about 5 kw, and some very small inverters (usually less than 1 kw) are specifically intended for use with single-phase motors. A fundamental aspect of any converter, which is often overlooked, is the instantaneous energy balance. In principle, for any balanced three-phase load, the total load power remains constant from instant to instant, so if it was possible to build an ideal 3-phase input, 3-phase output converter, there would be no need for the converter to include any energy storage elements. In practice, all converters require some energy storage (in capacitors or inductors), but these are relatively small when the input is 3-phase because the energy balance is good. However, as mentioned above, many small and medium power converters are supplied from single-phase mains. In this case, the instantaneous input power is zero at least twice per cycle of the mains (because the voltage and current go through zero every half-cycle). If the motor is 3-phase (and thus draws power at a constant rate), it is obviously necessary to store sufficient energy in the converter to supply the motor during the brief intervals when the load power is greater than the input power. This explains why the most bulky components in many small and medium power inverters are electrolytic capacitors. The majority of inverters used in motor drives are voltage source inverters (VSI), in which the output voltage to the motor is controlled to suit the operating conditions of the motor. Current source inverters (CSI) are still used, particularly for large applications, but will not be discussed here. The induction motor is clearly more robust and better suited to hazardous environments, and can run at higher speeds than the d.c. motor, which is limited by the performance of its commutator. Most low and medium power inverters use MOSFET or IGBT devices, and may modulate at ultrasonic frequencies, which naturally result in relatively quiet operation. B. Torque Speed Characteristics-Constant V/F Operation When the voltage at each frequency is adjusted so that the ratio V/f is kept constant up to base speed, and full voltage is applied thereafter, a family of torque speed curves as shown in Figure 2 is obtained. These curves are typical for a standard induction motor of several kw output. As expected, the no-load speeds are directly proportional to the frequency, and if the frequency is held constant, e.g. at 25 Hz in Figure 2, the speed drops only modestly from no-load (point a) to full-load (point b). These are therefore good open-loop characteristics, because the speed is held fairly well from no-load to full-load. If the application calls for the speed to be held precisely, this can clearly be achieved (with the aid of closed-loop speed control) by raising the frequency so that the full-load operating point moves to point (c). We also note that the pull-out torque and the torque stiffness (i.e. the slope of the torque speed curve in the normal operating region) is more or less the same at all points below base speed, except at low frequencies where the effect of stator resistance in reducing the flux becomes very pronounced. (The importance of stator resistance at low frequencies is 3315ENG Efficient Energy System 2

3 explored quantitatively) It is clear from Figure 2 that the starting torque at the minimum frequency is much less than the pullout torque at higher frequencies, and this could be a problem for loads which require a high starting torque. The lowfrequency performance can be improved by increasing the V/f ratio at low frequencies in order to restore full Xux, a technique which is referred to as low-speed voltage boosting. Most drives incorporate provision for some form of voltage boost, either by way of a single adjustment to allow the user to set the desired starting torque, or by means of more complex provision for varying the V/f ratio over a range of frequencies. Fig. 2. Torque speed curves for inverter-fed induction motor with constant voltage frequency ratio C. Open-loop speed control In the smaller sizes the simple constant V/f control is the most popular, and is shown in Figure 3. The output frequency, and hence the no-load speed of the motor, is set by the speed reference signal, which in an analogue scheme is either an analogue voltage (0 10 V) or current (4 20 ma). This set-speed signal may be obtained from a potentiometer on the front panel, or remotely from elsewhere. In the increasingly common digital version the speed reference will be set on the keypad. Some adjustment of the V/f ratio and low-speed voltage boost will be provided. Fig. 3. Schematic diagram of open-loop inverter-fed induction motor speed controlled drive 3315ENG Efficient Energy System 3

4 SPECIFICATIONS AC Drive Model MODEL: FM OC Input Power Rating I/P: AC 1PH, 200 ~ 240V, 50/60Hz Output Rating O/P: AC 3PH, 0 ~ 240V, 1HP, 4.2 Amps Fig. 4. Three phase motor and drive system Fig. 5. Three phase motor drive circuit diagram 3315ENG Efficient Energy System 4

5 Fig. 6. Three phase motor drive installation EQUIPMENT Fig. 7. Voltage and Frequency Variations during Operation AC Motor-DC Generator unit and AC motor Drive system Digital Power Meter: WT ENG Efficient Energy System 5

6 DC/AC Inverter Load (Lamp) or power resistor <100W CRO Multimeter EXPERIMENT A. AC Motor-Drive and DC Generator Test In this experiment, you will use an open-loop AC motor drive system, and measure the voltage, speed and efficiency of AC motor and DC generator unit. The measurement procedure is described as follows (The speed limitation should be noted): 1. Set-up an open-loop AC motor drive system as shown in Fig. 8 and AC Motor/DC generator unit. The open-loop motor drive system can provide different speed by adjusting the motor drive box. 2. Measure input current and voltage of the AC motor drive with AC Motor/DC generator unit, and output voltage of DC generator, and motor speed without a load using manually controlled system. Plot input voltage and frequency vs motor speed characteristics, and motor speed vs output voltage of DC motor characteristics. 3. Repeat the above measurement with a load (use the lamp or power resistor as the load). Similarly plot input voltage and frequency vs motor speed characteristics, and motor speed vs output voltage characteristics, and measure the waveform of DC output voltage and waveform. Fig. 8. Voltage and current measurement on primary side of AC Drive B. AC Motor Drive-DC Generator Unit and DC/AC Inverter with Load Test 1. Connect DC/AC inverter to AC Motor Drive-DC Generator Unit and measure the input power, voltage, current, motor speed, and out put DC voltage of generator and waveform, and AC voltage from DC/AC inverter with and with out load. 3315ENG Efficient Energy System 6

7 Fig. 9 AC Motor Drive-DC Generator Unit and DC/AC Inverter with Load Test 2. Measure the total power efficiency from input AC power (through AC motor and DC generator unit) and out put power (load), and calculate the power efficiency of the whole power conversion system using the following formula, P = out η (1) Pin Reference: [1] P. Schavemaker and L. V. D. Sluis, Electrical Power System Essentials, Wiley, 2009 [2] Tony R. Kuphaldt, Lessons in Electric Circuits, Volume II AC, Sixth Edition, last update July 25, 2007 [3] Austin Hughes, Electric Motors and Drives Fundamentals, Types and Applications Third edition. [4] Australia Pty_ Ltd.htm Appendix. 1. TECO FM50 Manual 2. TECO Three Phase Induction Motor Instruction Manual 3. TECO LV TEFC Catalogue TECOCI Technical Section (download from Australia Pty_ Ltd.htm) 3315ENG Efficient Energy System 7