unit 20 Troubleshooting Electric Motors

Most electrical problems with an electric motor are ____.

• short circuited windings, closed windings, and grounded windings

• short circuited windings. open windings, and grounded windings

• all of the above

• none of the above

A technician needs to check a single-phase compressor. He labels the three terminals "A," "B," and "C" and takes resistance readings of the windings. He reads 10 ohms from "A" to "B," infinity from "B" to "C," and infinity from "C" to "A." Identify the terminals.

• A: common B: start C: run

• Cannot identify terminals: winding grounded

• A: start B: run C: common

• Cannot identify terminals: open winding

Technician "A" says that while checking the windings of a compressor, a reading of 10 ohms or less from the common terminal to ground indicates that the motor is okay. Technician "B" says that the resistance of the start winding should always be less than the resistance of the run winding. Which of the following statements is correct?

• Technician "A" is correct. Technician "B" is correct.

• Technician "A" is correct. Technician "B" is incorrect.

• Technician "A" is incorrect. Technician "B" is correct.

• Technician "A" is incorrect. Technician "B" is incorrect.

An open winding in an electric motor means that ____.

• the winding is making contact with the motor frame

• one winding is making contact with another winding

• a wire in a winding is broken

• the centrifugal switch to the start winding is open

Technician "A" says that a shorted winding will result in an increased current flow through the winding. Technician "B" says that if a motor is to be replaced, a motor with a lower horsepower rating should always be used. Which of the following statements is correct?

• Technician "A" is correct. Technician "B" is correct.

• Technician "A" is correct. Technician "B" is incorrect.

• Technician "A" is incorrect. Technician "B" is correct.

• Technician "A" is incorrect. Technician "B" is incorrect.

If the resistance of the run winding decreases, the ____.

• motor probably will not start

• motor will draw too much current while it is running

• centrifugal starting switch will not start the motor

• load on the motor is probably too great

If a condenser fan motor grounds out, it will usually ____.

• trip a circuit breaker to shut off the unit

• keep running as the current is only going to ground

• run a little faster because the amperage draw will increase

• cause the evaporator to ice up

A megger ____.

• is an ammeter that can measure very small amounts of current

• is a digital voltmeter that measures very high resistances

• measures high voltage direct current

• measures very high resistances

If a motor hums but does not start, there is a potential problem ____.

• with the starting components

• with an open winding

• with insufficient supply voltage

• all of the above

Tags

NGSS.HS-PS3-5

The recommended way to discharge a capacitor is to ____.

• place a screwdriver across the terminals

• assume it will discharge itself over a period of time

• use a 20,000 ohm 5 watt resistor that has had insulated leads soldered to it

• use an ohmmeter