FIGURE 9-1Figure 9-2FIGURE 9-3Voltage Selector Knob (EMS 8821)RPMRPMSecSecFIGURE 9-5Voltage Selector Knob (EMS 8821)CLOSED-LOOP CONTROLFIGURE 9-6Exciter Control (EMS 8241, Synch. Motor)ECE 4501 Power Systems Laboratory Manual Rev 1.19.0 MOTOR DRIVES USING MOSFETS9.1 BUCK CHOPPER DRIVE9.1.1 OBJECTIVETo study the use of Power MOSFETs in a Buck Chopper to control a DC Motor.9.1.2 DISCUSSIONThe speed of a DC Motor is directly proportional to the line voltage applied to it. Given a fixed DC source, VS, and a Power MOSFET to act as a switch, it is possible to control the average voltage applied to the motor using a technique called Pulse-Width Modulation (PWM). In the circuit shown in Figure 9-1 below, the source voltage, Vs, is “chopped” to produce an average voltage somewhere between 0% and 100% of Vs. Thus the average value of the voltage applied to the Motor, Vm, is controlled by closing and opening the “switch”, Q1. To close the switch, a firing signal is delivered to the gate of the MOSFET, causing it to conduct between source and drain. To open the switch, the firing signal is removed and the MOSFET is self-biased to stop conducting. In PWM, the switch is closed and opened every modulation period.FIGURE 9-1In discussing the period of modulation, let time be divided into uniform periods of one millisecondeach and let a period be called T, the modulation period. During T, there is a time, t0 to t1, during which the MOSFET Q1 is on, and a time, t1 to t2, during which it is off, as indicated in the Figure 9-2 below. This is true for each period and therefore Q1 turns on and off 1000 times every secondwhen T = 1 ms.-1 -ECE 4501 Power Systems Laboratory Manual Rev 1.1FIGURE 9-2When Q1 is on, Vs volts are applied to the motor for t1 milliseconds. When Q1 is off, zero volts are applied to the motor. However, the armature current, Ia, is still allowed to circulate through the diode. The magnitude of the armature current will diminish between t1 and t2 as losses in the motor dissipate energy. The voltage, Vm, seen by the motor can be expressed in terms of the source voltage, Vs, and the “ON” time, t1, and the period of modulation, T. The equation is:Vm =  Vs where  = t1 / TThe symbol  is called the Duty Cycle. As duty cycle is increased from 0% to 100%, the average voltage applied to the motor increases from 0 to Vs volts and the motor speeds up.It is sometimes more useful to think of the frequency of modulation as opposed to its period. Modulation frequency is just the inverse of the period of modulation, fm = 1/T. State of the art PWM converters have modulation frequencies as high as 200kHz, although some people argue that frequencies above 20 kHz lead to unnecessary expense. 9.1.3 INSTRUMENTS AND COMPONENTSPower Supply Module EMS 8821DC Metering Module EMS 8412DC Motor/Generator Module EMS 8211Smoothing Inductor Module EMS 8325Power MOSFET Module EMS 8837Enclosure/Power Supply EMS 8840Chopper/Inverter Control EMS 9029-2 -ECE 4501 Power Systems Laboratory Manual Rev 1.1Voltage/Current Isolator EMS 9056 (or 9050 and 9051)Inertia Wheel EMS 9126Hand Tachometer EMS 89209.1.4 PROCEDURECAUTION! – High voltages are present in this Experiment. DO NOT make any connections with the power supply ON. Get in the habit of turning OFF the power supply after every measurement.1. Verify that all power switches are OFF, then connect the circuit shown in Figure 9-3 below: NOTE: In smoothing inductor terminals 2-3 and likewise terminals 6-7 should be connected. FIGURE 9-32. Install the Inertia Wheel, EMS 9126, on the shaft of the DC Motor/Generator and verify that it is securely fastened.-3 -ECE 4501 Power Systems Laboratory Manual Rev 1.13. Turn on the Enclosure/Power Supply (EMS 8840) by setting its rocker switch to the “1” position. DO NOT turn on any other power supply. Adjust controls to the following settings:LabVolt Equipment OscilloscopeDevice Setting Option SettingVoltage Selector Knob (EMS 8821)7-N Chan. 1 Sensitivity: 5 V / Div.DC CoupledInterconnection Switch (EMS 8837)0 (open) Chan. 2 Sensitivity: 2 V / Div.DC CoupledDC Source 1 (EMS 9029)Minimum( full ccw)Vertical Mode:Display Mode:ChoppedA/BDC Source 2 (EMS 9029)Maximum(full cw)Time Base: 0.2 ms / Div.MODE (EMS 9029)CHOP. PWM Trigger Source:Trigger Slope:ExternalPositive (+)Rheostat (EMS 8211)Minimum(full ccw)Trigger Coupling: HF Rejection4. Verify that the voltage control knob on the main Power Supply is at zero percent, then turn on the 24 Vac power switch and the main power switch.5. Adjust the voltage control knob for 100% (120 Volts) operation. Turn the Field Rheostat on the DC Motor clockwise until the field current reaches its rated amps (0.4 A). It is OK if the motor is not turning. Note that the DC Motor is “separately excited”. In other words, the shunt field in the stator is created by a constant source. This will make the motor behave much like a Permanent Magnet DC Motor.6. Use the channel position knobs on the Oscilloscope to place the Channel 1 signal in the top half of the screen and Channel 2 in the bottom half.7. The DC Source 1 knob controls the duty cycle, , of the PWM driver. It is currently set at minimum (0% duty). Turn the knob clockwise and set the motor speed to 300 RPM.8. Observe the waveforms on the Oscilloscope and compare them to the waveforms in Figure 9-2. Note any differences:____________________________________________________________________________________________________________________________________________________________________________________________________9. Continue to adjust the DC Source 1 knob to determine first the minimum and then the maximum speed for the motor. Leave the motor at maximum speed while you record your values.-4 -ECE 4501 Power Systems Laboratory Manual Rev 1.1Minimum Speed Maximum SpeedRPM RPM10. Try to determine the time required for the motor to decelerate by quickly changing the duty cycle to minimum and measuring the number of seconds until the motor reaches minimum speed. Record the time below.11. Now quickly change the duty cycle to maximum and measure the number of secondsfor the motor to achieve maximum speed. Maintain top speed and record the resultDeceleration Time Acceleration TimeSec Sec12. Quickly turn the DC Source 1 knob to about 30% of maximum and observe the armature voltage and current waveforms on the oscilloscope as the motor decelerates. Sketch them below.FIGURE 9-4: Vm and Ia