ELEC 242 Lab

Experiment 1.3

Tachogenerator

Equipment

Components

Although the strobotach can give us an accurate indication of speed, it's not very useful for building control systems. With the stroboscope, we put in a number (the function generator frequency) and it tells us whether or not the shaft is turning at that speed. If it isn't, we adjust the frequency until it is. What we need is something that directly gives us a signal whose value corresponds to the rotational speed. Fortunately, we can get just that with another DC motor. It doesn't matter whether a DC motor is turning itself or being turned by an external torque, the back emf will be the same. In other words, if we connect the shaft of a DC motor to another rotating shaft, it will produce a voltage which is proportional to the speed of rotation. This is the principle of the tachogenerator.


Step 1:

 Get a second 18 V motor from the cart. If it has a test disk on it, remove it. Leaving your original disk mounted to your original motor, press the shaft of the new motor into the hole in the large end of the disk.


Step 2:

 Remove the banana plug cables from the power supply and the DMM. Plug your lab partner's BNC-banana adapter into the COMMON and 0 TO +20V terminals of the power supply, with the ground bump in the COMMON terminal.


Step 3:

 Plug a BNC clip lead into this adapter and connect the clips to the second motor.

Step 4:

 Turn on the power supply and set the voltage to about 10 V. The motors should be spinning, and there should be a reading on the DMM. The original motor is now acting as a generator, being driven by the new motor.

Step 5:

 Increase the power supply voltage until the first motor is putting out 16 V. Use the strobe to find the speed. Record the generator output voltage and the speed.

Step 6:

 By decreasing the power supply voltage, reduce the generator output voltage in steps of 2 V until the motors stop turning. At each step, record the generator voltage and the speed.

Step 7:

 Plot the generator speed vs. voltage. Compute the slope and intercept of this line.

Question 3:

 The slope of this line should be the same as that of the line in the previous experiment, and the intercept of this line should be zero. Is this the case? Based on the material in the Background section, explain both the similarities and the differences between these two curves. (Hint: assume that the load torque does not vary with speed.)

Step 8:

 Adjust the power supply until the DMM reads 10 V.

Step 9:

 Disconnect the BNC clip lead from the DMM and connect it to CH 1 of the oscilloscope. Sketch the waveform, indicating the peak voltages and the period. The ripples in this waveform are caused by the motor's commutator as it switches from one set of armature windings to another. What is the relationship between the frequency of this waveform and the speed of the motor?

Step 10:

 Disconnect the BNC clip lead from the power supply and connect it to the function generator.

Step 11:

 Turn the DUTY control fully counterclockwise and set the other controls to produce a 1 Hz square wave. Turn the AMPLITUDE control to maximum.

Step 12:

 Sketch the waveform displayed on the oscilloscope.