0o -
120 -120o
during the lab. What is the rms value of this waveform? The peak amplitude
value? The peak-to-peak value?
Objectives
Background
There are many applications where variable speed motor operation is desired. Even in applications where fixed speed motors have traditionally been used, such as driving compressors and large fans, there are advantages to variable speed operation. This is a very effective way of meeting cooling and ventilating requirements with both good performance and good efficiency. The next generation of auto air conditioners will probably use sealed variable speed motors to drive the compressors rather than the present mechanical drive with a fan belt. This will meet the cooling requirements of the car more precisely and will also not have the leakage of freon around the rotating mechanical seal. (The cars will probably also have 24 V electrical systems to keep down the copper losses.)
There are basically three ways of getting variable speed operation:
The DC motor has been the first choice for variable speed operation until recent years. It has problems regarding size, weight, cost, and maintenance, but the other choices were generally worse. If less than a two to one speed difference was required, then the wound rotor motor would be competitive. Until the last 20 years or so, the variable frequency source would have been a DC machine driving a three-phase generator driving in turn an induction motor. Using three machines to get the output of one would have been justified only in critical cases in explosive atmospheres where the variable frequency source could be located some distance away from the induction motor.
In recent years, however, solid state rectifier-inverter systems have been developed which rectify 60 Hz into DC and then invert the DC into the desired AC frequency. The speed controllers have the same sort of reliability as the induction motors and also have high efficiencies. These are rapidly becoming the standard way of getting variable speed motor operation. Up until the early 1990s these controllers were mostly used for motors of 1 hp and up in size, but now controllers are readily available for the fractional hp motors of this lab.
The basic concept of most variable speed drives is to rectify the input single-phase or three-phase voltage to get dc, and then use solid state switches (usually SCRs) to rapidly switch this voltage across the motor leads. The basic waveform looks something like the wave shown in Fig 1. The variable width pulses are a part of what is called pulse-width modulation. The negative going pulses are obtained by inverting the dc voltage. The motor acts as a low pass filter because of its inductance and inertia, and hence ``sees" only the fundamental component of this wave.
Discussion and Calculations
0o -
120 -120o
during the lab. What is the rms value of this waveform? The peak amplitude
value? The peak-to-peak value?
Instructional Activity in Class
Note that there are several features of this drive which prevent full use of the bench instrumentation. The experiment does not call for the use of these instruments, but it is tempting to use them anyhow, so a word of explanation is in order.
The line-to-line voltage output of the drive is about 240 V rms, but the bench was designed for line-to-line voltages of 150 V rms or less. Some of the wattmeters are rated at 120 V rms line-to-line, hence cannot be used to measure power input to this motor. The analog voltmeter is also not usable since its maximum value is 150 V rms.
Turn drive on by switching to `RUN'. Motor should be turning at a slow rate. Current and voltage readings should be well within range of the instruments. Leave the dynamometer 120 VAC supply off while you are getting acquainted with the drive.
Turn the speed control knob slowly up to maximum. Observe and record in your notebook the sound made by the drive and motor as speed is increasing.
With the motor operating at maximum speed, and with one student designated to watch the ammeter and another to watch the motor, flip the `FWD/REV' switch to `REV'. Describe what happened in your notebook. Flip the switch back to `FWD'. Do the same things happen?
Attach x10 probes to both CH1 and CH2. Check probe compensation by using the calibrator loop on the front of the scope. If the square wave does not appear square, adjust the probe until it does. Check with the instructor if you are unsure how to do this.
Set both channels to 50 Volts/div. Push the "Channel Math" (+-) button to bring up the corresponding menu and adjust the scope to display CH1 - CH2. Use DC coupling, and set the sweep rate at 5 ms/div.
Plug probes into phases A and B of the 208 VAC/3 PHASE at the left of the bench. Turn three-phase breaker on. Sketch the waveform. Record the number of divisions from peak to peak. Turn the three-phase breaker off.
Plug probes into two of the phases at the motor. Switch RUN/STOP to RUN and turn FREQ. SET to MAX. For triggering, try line source. If the frequency output of the drive is not exactly 60 Hz, the waveform will drift but should be stable otherwise.
Sketch the voltage waveform with the sweep speed set at 5 ms/div. Do not try to record all the fine detail. Note the peak to peak value of the waveform. Compare with the 60 Hz AC waveform you just finished measuring.
Set the sweep speed to 0.2 ms/div and triggering to line, if it is not already there. The wave form should be drifting past you on the screen like a scroll. You may need to adjust FREQ. SET slightly if it is going too slow or too fast. After a few sweeps across the screen you should be able to note a repeating pattern. Pick a distinctive transition during the cycle and try to count the number of times the waveform is at its maximum value during one cycle of the approximately 60 Hz fundamental.
Now watch the current waveform scroll past the screen. Why does the current look nearly continuous, more like a saw tooth, when the voltage is a series of square pulses?
Set sweep speed at 5 ms/div. Make a very quick and crude sketch of the waveform. How would you describe the difference between this waveform and a sinusoid?
Connect a banana lead from the scope ground to the green banana jack on the drive. Connect another lead from CH1 to the red banana jack. There is no need for a x10 probe. Try 0.5 V/div on CH1.
Set the scope in the storage mode and so that the trace is continually sweeping. Set the time base to 0.2 sec/div. Adjust the scope intensity for a reasonably narrow trace. Switch the RUN/STOP knob to RUN and adjust the FREQ. SET knob to MAX. Adjust the scope so the maximum voltage fits nicely on the screen.
Use the scope storage buttons (Run, Stop, Auto-store, etc.) on the scope to capture a sample waveform. With the scope set up to store a waveform, switch RUN/STOP knob to STOP. Record the time required for deceleration of the motor. Reset the scope to store another trace and switch the RUN/STOP knob to RUN. Record the time required for acceleration of the motor.
Now use the toggle switch to record the acceleration and deceleration times from 60 Hz to 30 Hz, and from 30 Hz to 60 Hz. (Note: acceleration and deceleration times are internally switch selectable from 0.35 seconds to 26 seconds.)
Record the voltage between the red and green banana jacks for 30 and 60 Hz, as read from the scope. Also measure the motor speed with a Strobotac for both frequencies. Does the voltage seem to vary linearly with applied frequency? With the motor speed?
Conclusion
