Block K - Single-Phase Induction Motors

Objectives

1. Measure starting currents of capacitor start and split phase induction motors.

2. Measure running speed and determine slip of induction motors.

Background

Single-phase induction motors are widely used in sizes of 1 hp and less, less commonly in sizes up to 10 hp, and rarely above 10 hp. Compared with three-phase motors, they are hard to start, they run rough (noisy), and they are more expensive, at least in sizes above 1 hp. We use them wherever three-phase is not available, but would normally choose a three- phase motor if three-phase power is available.

There are three major types of single-phase motors, the capacitor start, the split phase, and the shaded pole. The first two require a start winding in addition to the main or run winding. The shaded pole gets its starting torque by placing a shorted single turn of copper wire around half of each stator pole. The reason for these additional parts is that a squirrel cage rotor in a single coil has no starting torque. It needs something like a second coil rotated in space, and driven by a voltage with a phase shift, to get a rotating flux to establish a starting torque. The three-phase motor has such a rotating flux automatically, hence has excellent starting characteristics without any additional hardware.

It is easy to build a start winding rotated 90^o in space from the run winding. It is more difficult to get the necessary phase shift in current flowing through this winding, since we have only one voltage available. The capacitor start does this by placing a capacitor in series with the start winding. Depending on the relative values of capacitance, and resistance and inductance of the start winding, it might be possible to get a phase shift of 60^o or so between the run and start windings. The split phase motor uses only different relative values of resistance and inductance to get a phase shift between the run and start windings of probably not more than 20^o. This means that the capacitor start motor will have better starting characteristics than the split phase motor. The shaded pole motor has even poorer starting characteristics.

Because of the poor starting characteristics, the shaded pole motor will be used only in applications where low starting torque is acceptable, such as household fans. The shaded pole is also the cheapest motor since it does not need a second coil or a switch to switch it in and out of the circuit. This means that it is the motor of choice in consumer products where its performance is acceptable.

In applications where starting torque is critical, such as motors driving compressors in refrigerators and air conditioners, the capacitor start motor is usually selected. The split phase motor is then selected for those intermediate applications where its starting torque is acceptable and the extra cost of the capacitor puts the capacitor start motor at an economic disadvantage.

Once the motor is running the start winding is unnecessary and can be taken out of the circuit. Since the start winding only needs to function a few seconds at most, it can be sized to draw a relatively large current for a short time, thus improving the starting performance. If the start winding is reversed, the direction of starting torque also reverses so the motor rotates in the opposite direction. This can be convenient in applications like garage door openers.

Discussion and Calculations

1. Describe the necessary switch arrangement which will allow garage door motor start winding to be reversed each time it operates. You will need a double pole, double throw arrangement rather than the single pole, single throw switch shown in Fig. 1. Write down your ideas on how it might be done mechanically. The switch must be closed when the motor is stopped, open when the motor reaches about half speed, and close again after the motor is turned off but in the opposite position so the motor will start in the opposite direction the next time it is turned on. Everything must happen internal to the motor. No external actuators are allowed.

Instructional Activity in Class

1. There should be three single phase induction motors per bench, machines C (1/4 hp, split-phase), E (1/12 hp, capacitor-start), and I (1/12 hp, split-phase). Locate these machines, and connect machine C to the dynamometer on the baseplate. Connect machine C in the circuit of Fig. 2. Note that to use the starting relay, you must connect the 120 VAC lead to the center connection on the machine and the ground lead to the bottom of the MAIN and START windings. The top of the MAIN and START windings are not used. Set the storage oscilloscope in the storage mode, adjust the single phase variable autotransformer to 100% of 120 volts, turn it on and record peak starting current, the number of cycles required to go to steady state and the rms running current. If the motor doesn't start immediately, turn off the variable autotransformer and consult the instructor. With the motor running, adjust the small variable autotransformer on the dynamometer to get rated torque. Measure the speed and record the current. How does this compare with nameplate rating? Turn the small variable autotransformer on the dynamometer back to zero, but leave the belt on. Reduce the voltage to, say, 90% and 80% of 120 volts and determine the peak starting current, the number of cycles to steady state and the rms running current for these reduced voltage conditions.

   Compare the starting current, both amplitude and time to reach steady state, for the single-phase motor to the starting current of the three- phase induction motor measured in Block I. If you have turned in your notebook for the previous lab, get it back for a few minutes to check these numbers.

2. We want to determine the minimum voltage at which the motor will start. If the motor doesn't start immediately, the current will be quite high and the motor will overheat, fuses will burn out, the instructor will get uptight, etc. So turn the switch on and if the motor doesn't start quickly, turn the switch back off. Adjust the variable autotransformer voltage down until the motor doesn't start. Unplug the motor and measure the output voltage of the variable autotransformer at this setting.

   One popular way of saving power is for the electric utility to reduce voltage on its system. What would you consider the maximum safe reduction?

3. Connect the circuit of Fig. 3 using machine C. Remove the belt. Do not leave rated voltage applied to the MAIN winding more than a few seconds without the rotor turning. Close S₁. If the motor does not start immediately, give the pulley a spin. Note the direction of rotation. Open S₁ and allow the motor to stop. Give the pulley a good spin in the opposite direction and close S₁. Does the motor run equally well either direction? Record your observations.

4. Set up the circuit in Fig. 4. Close S₁ and S₂ at the same time and open S₂ as soon as the motor is running. Observe direction of rotation. Change the circuit to Fig. 5 and repeat. Did the direction of rotation change?

5. Repeat Lab Activity 1 for machines E and I. Discuss the differences between the starting currents for the 1/4 HP and 1/12 HP splitphase induction motors (C and I) and between the 1/12 HP splitphase and the 1/12 HP capacitor start induction motors (I and E). See if you can agree within your group why the nameplate on machine E shows a rated current of 3.70 amps when the actual current you measured was closer to 2.3 amps.

Conclusion

1. Compare the noise level in the lab between the single-phase and three-phase motor experiments.

2. Compare single-phase and three-phase motors in regard to starting current and starting times.

3. Compare the capacitor start with the split phase motor in regard to starting current and starting times.