09 - Motors

Question 1

Define the stator

Answer 1

The stationary part of the motor that houses the field poles and windings. When supplied with current, it creates a magnetic field that induces rotation in the rotor

Question 2

Define the rotor

Answer 2

The rotating part of the motor housed inside the stator. It contains windings, and will spin when under the influence of the magnetic field created by the stator.

Question 3

Define field poles

Answer 3

Field poles are solid iron poles built into the stator that the field windings are wrapped around.

Question 4

Define field windings

Answer 4

Field windings are rotations of copper wire wrapped around the field poles built into the stator that when supplied with current, will induce a magnetic field on the rotor

Question 5

What is a squirrel cage rotor

Answer 5

A rotor constructed out of a solid iron core, encased in metal bars that connect at the ends to form a complete circuit. Negates the need for copper windings on the rotor.

Question 6

Define motor end bells

Answer 6

End bells/plates enclose the motor on either end and provide a place for the bearings to sit inside the motor housing. When the motor shaft is mounted in the bearings, the end bells support the rotor.

Question 7

Define motor nameplate

Answer 7

Provides all necessary and relevant information on the motor including rpm, voltage, amperage, hz, hp, bearing no, frame type, and motor size.

Question 8

Define motor terminal box

Answer 8

Installed onto the exterior of the motor housing, it contains all connections used to control and power the motor.

Question 8

Explain the relationship of synchronous speed, rated full load speed, and slip.

Answer 8

Synchronous speed happens when the magnetic fields for the rotor and stator are running at the same speed. Rated full load speed is the rotation speed of the motor when under load. Slip is the difference between the two, which is usually a 4-5% difference.

Question 9

Define torque

Answer 9

The work performed by a twisting or turning action

Question 10

Define the difference between start windings and run windings

Answer 10

Run windings operate continuously throughout the entire runtime of the motor, and perform the bulk of the magnetic drive for the rotor. Start windings only operate temporarily during the motors start to add extra torque.

Question 11

What is the best way to clean a motor?

Answer 11

Remove the motor from the housing and brush / wipe any dust off. Use compressed air or nitrogen to blow any remaining dust out of the inside of the motor housing if the buildup is significant.

Question 12

Between the fan side and back side/rear bearings, which usually tends to wear out faster?

Answer 12

The fan side bearing will fail first, then cause the rear side to deteriorate due to the extra stress

Question 13

How can you determine shaft integrity when replacing bearings?

Answer 13

If the inside surface of the bearing is rotating on the shaft when spun, the shaft surface has been worn down, and bearing replacement is not recommended. Replacement of the shaft/motor is the better alternative.

Question 14

What will happen to the rotor/stator inside the motor in the event of failed bearings?

Answer 14

The surfaces of both the rotor and the stator will be scarred up. This in some cases can lead to an electrical failure. (Rotor cuts through wiring insulation and shorts to ground)

Question 15

What is a quick and easy way to find the shaft size for a motor?

Answer 15

Reference the motor’s frame size number, and compare it to an electric motor frame chart

Question 16

What is the symbol for a PSC motor?

Answer 16

Question 16

What is the purpose of including a capacitor in the circuitry of a single phase PSC motor?

Answer 16

It simulates the third phase

Question 17

What is the difference between a three phase wye and delta configuration?

Question 18

What is the difference between a 4 wire and 3 wire PSC motor?

Answer 18

3 Wire PSC motors connect the start and run wires to the capacitor. The run wire will be on the capacitor terminal that shares the line wire for L1, and the start wire will connect to the load side of the capacitor.

4 wire PSC motors have two dedicated wires for the capacitor. The run wire will connect directly to the contactor.

Question 19

What is motor shaft grounding?

Answer 19

Shaft grounding devices, or shaft earthing devices, redirect extraneous electrical current that accumulates on motor shafts away from more sensitive motor components. By reducing these extraneous currents, shaft grounding devices prevent unnecessary wear and early bearing failure. Unprotected, motor bearings typically provide the lowest path of resistance for circulating currents that have built up on the motor shaft. It’s important to redirect these currents to a more ideal path away from the motor entirely, sparing bearings and internal motor components from these unnecessary and damaging electrical currents.

Question 20

What should any motor that exists on a VFD be protected by?

Answer 20

Shaft grounding

Question 21

What is the difference between internal and external shaft grounding?

Answer 21

Internal shaft grounding is factory installed, and will be indicated on the pecker head. External shaft grounding is almost always aftermarket installed