Unit 35 Electric Motors

Questions and Answers

What is meant by motor torque?

The power required for a motor to drive a fan, compressor, or any other piece of equipment.

What are the two principal parts of a motor?

The Stator and the Rotor.

How is the speed of an inductive motor determined?

By the number of poles and the frequency of the alternating current.

Why are capacitors rated for a higher voltage than the motor they are used with?

It is the result of the charging and discharging of the capacitor in series with an inductive load. Signup and view all the answers

List the common types of single-phase motors.

Shaded Pole, Split-Phase, Capacitor Start, Permanent Split Capacitor, Capacitor Start/Capacitor Run. Signup and view all the answers

Why would a start capacitor have a bleed resistor connected between the two capacitor terminals?

To drain the capacitor's voltage during the motor's off cycle. Signup and view all the answers

What advantages do three-phase motors have over single-phase motors?

Easily reversible, less running torque pulsation, higher starting torque, higher efficiency. Signup and view all the answers

Can a shaded pole motor be electrically reversed? Why or why not?

No, since the shaded pole is fixed. Signup and view all the answers

What happens if there is single phasing of a three-phase motor?

It will quickly overheat and damage the windings. Signup and view all the answers

How did split phase motors get their name?

Because the electrical characteristics of the two windings essentially split the single phase into two phases that are out of phase with each other. Signup and view all the answers

Can a split-phase motor be operated at more than one voltage?

True Signup and view all the answers

How may a capacitor start motor be distinguished from a split phase motor?

Capacitor start motors can be distinguished by the start motor mounted to the unit. Signup and view all the answers

What type of motor is most commonly used in HVACR applications today? Why?

PSC motors. They have a 65% efficiency rating and a low starting torque. Signup and view all the answers

What is the function of the centrifugal switch found on split phase motors?

The start winding is dropped out of the circuit after the motor has reached appropriate speeds. Signup and view all the answers

What is the advantage of a CSCR as compared to a PSC?

The CSCR motor comes with the addition of a start capacitor, providing extra phase shift to get the motor started. Signup and view all the answers

What are ECM motors?

Electronically Communicated Motors. Signup and view all the answers

What are the principal advantages of ECM motors?

High efficiency, Low maintenance, Speed control, Constant CFM. Signup and view all the answers

What is a VFD, and what is it used for?

Variable-frequency drive. It controls the rotational speed of an electric motor. Signup and view all the answers

How many degrees apart are the windings of a three-phase motor?

120 degrees apart. Signup and view all the answers

Study Notes

Electric Motors Overview

Motor torque is defined as the power required for a motor to drive equipment like fans or compressors.
A motor consists of two principal parts: the Stator (stationary part) and the Rotor or armature (moving part).

Motor Speed Determination

The speed of an inductive motor is determined by the number of poles and the frequency of the applied alternating current.

Capacitors in Motors

Capacitors are rated for higher voltage than the motor to accommodate charging and discharging in relation to an inductive load.
A start capacitor may include a bleed resistor to drain voltage during the motor's off cycle, preventing discharge across contactor points.

Types of Motors

Common types of single-phase motors include:
- Shaded Pole
- Split-Phase
- Capacitor Start (CS)
- Permanent Split Capacitor (PSC)
- Capacitor Start/Capacitor Run (CSCR)
PSC motors are favored in HVACR applications due to their 65% efficiency and low starting torque.

Three-Phase Motors

Three-phase motors offer several advantages over single-phase motors, such as:
- Easier reversal
- Reduced running torque pulsation
- Higher starting torque
- Increased efficiency
Single phasing of a three-phase motor can lead to quick overheating and damage to windings.

Operation and Characteristics of Split-Phase Motors

Split-phase motors derive their name from the splitting of a single phase into two phases that are out of phase.
These motors can be operated at multiple voltages via series or parallel circuits.
The centrifugal switch in split-phase motors disconnects the start winding after the motor reaches appropriate speeds.

Comparison of CSCR and PSC Motors

CSCR motors feature an additional start capacitor, providing extra phase shift for easier starting compared to PSC motors.

Advanced Motor Technology

Electronically Communicated Motors (ECM) are known for high efficiency, low maintenance, speed control, and constant CFM (cubic feet per minute).
Variable-frequency drives (VFD) are used to control the rotational speed of electric motors by adjusting the frequency of supplied electrical power.

Design Considerations

In three-phase motors, the windings are spaced 120 degrees apart, facilitating balanced operation.

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Description

Test your knowledge on electric motors with this quiz. Explore essential concepts such as motor torque, the main parts of a motor, and how speed is determined. Ideal for students studying electrical engineering or related fields.