Induction Motor and Drive Performance 1 Induction Motor Drivee Performance Introduction Over the past few years there have been great improvements in power electronics and their uses in motor drives. Today, these motor drives are dependable and can provide highly accurate speed and voltage control. Induction motors and drives can be found in many applications such as electric and hybrid vehicles, or plant process applicationss where variable speed is necessary. Induction Motor Drives In the previous lab, it was shown that the normal operating region of the induction motor is over a small speed range (when operated from a constant frequency, constant voltage source) ). In some application, such as electric vehicle drives, a much wider speed range is necessary. In these applications, a motor drive can apply variable frequency and variable voltage to the motor. Figure 1 shows the schematic of the power section of a typical three-phase induction motor drive. The six switches ( S 1 through S 6 ) convert the dc voltage V o to ac voltages which are applied to the motor. Figure 1. Schematic diagram of a three-phase motor drive Figure 2 shows the motor voltage and current (as labeled in Figure 2) waveforms of the motor drive system. The voltage is made from high switching frequency switching between V o, 0, and Vo. The inductance of the motor filters out the high frequency harmonics and the current is fairly sinusoidal.
Induction Motor and Drive Performance 2 Figure 2. Motor drive voltage and current waveforms The advantage of the drive system is the ability to control the magnitude and frequency of the applied voltages. In most motor drive systems, the frequency is varied from 0 to 100-Hz (slightly above the motors rated frequency of 60-Hz) ). The voltage is usually increased in proportion to the frequency in order to maintain constant flux in the motor. The drives used in this lab will respond to an increase in load by automatically increasing the line-to-line current supplied to the motor. Laboratory Software Figure 3 shows a screen-shot of the software interface for this experiment. The waveforms show the motor voltage and current. As can be seen, the voltage source is not perfect and contains harmonics. The current also contains harmonics due to the motor tooth saturation. The inductionn motor input voltage, current, real power, reactive power, and frequency are displayed. Also displayed are the armature and field current of a DC generator used to apply a load to the inductionn motor. The software allows control of the IM speed, DC torque, and DC speed. This allows the user to vary the load applied to the IM. In this experiment, several load points will be taken. As the load is increased (through the armature current), the IM will slow down but the motor drive will increase the current frequency in order to control the mechanical speed. Figure 3. Induction motor load test software.
Induction Motor and Drive Performance 3 Laboratory Work Induction Motor Load Test Connect the induction and DC motors as shown in Figure 4. Have your TA double check your wiring before you turn on the drive. Once your wiring has been okayed by your TA, turn on the motor drive. Then start the Labview software titled Induction Motor Day 3. Adjust the IM speed slider to 75% of rated speed (or 1350 RPM) the induction motor should start and operate near the commanded speed. Measure the mechanical speed and add this data to the software then click the Add button. For speed measurement, use the hand-held tachometer. It is necessary to press the button once and wait for the meter to awake, then hold the button and wait for the reading to settle. Now it is time to apply a load to the induction machine. Leave the speed command at 0, but apply a 10% torque command. You will hear the DC drive and the IM will start to strain. The motor will slow down and at the same time, the current applied to the induction motor by the drive should increase. Measure the speed for this load point, type it into the software program, and click Add. Continue to increase the DC torque command by 10% increments up to 100%. At each point, measure the speed and log the data. Repeat this procedure for an IM speed of 50%. Decrease the DC torque to zero, then decrease the IM speed to zero. Switch off the drives once the motors have stopped. Wait until the text on the LED screen goes away before removing the cables. Save the logged data by clicking Save.
Induction Motor and Drive Performance 4 Figure 4. Laboratory setup for the load test.
Induction Motor and Drive Performance 5 Calculations and Questions 1. Drive Performance: Using the induction motor data found in this lab, and data found in previous induction motor labs; calculate the Torque of the IM at the various load points. Create a plot of torque versus IM current for each IM speed command (75% and 50%). Describe each plot. 2. Drive Performance: Using the speed measurements, plot speed vs IM current for each speed command. Describe how well the drive regulates the mechanical speed.