Unit 17 Electric Motors

Rotor

The rotating or moving component of a motor, including the shaft

Stator

The component in a motor that contains the windings: it does not turn

Bearings

A device that surrounds a rotating shaft and provides a low-friction contact surface to reduce wear from the rotating shaft

Run Windings

The electrical winding in a motor that draws current during the entire running cycle

Squirrel Cage Rotor

Describes the construction of a motor rotor

End Bells

The end structure of an electric motor that normally contains the bearings and lubrication system

Housing

Term used to describe the casing or the shell of a motor

Torque

The twisting force often applied to the starting power of a motor

Slip

The difference in the rated rpm of a motor and the actual operating rpm when under a load. Also, a device used to connect sections of square or rectangular ducts. See S Fastener

Induction Motor

An alternating current motor where the rotor turns from induced magnetism from the field windings

Centrifugal Switch

A switch that uses a centrifugal action to disconnect the start windings from the circuit

Split-Phase Motors

A motor with run and start windings

Electronic relay

A solid-state relay with semiconductors used to stop, start, or modulate power in a circuit

Single-Phase

The electrical power supplied to equipment or small motors, normally under 7 1/2 hp

Back Electromotive Force (BEMF)

This is the voltage-generating effect of an electric motor's rotor turning within the motor

Potential Relays

A switching device used with hermetic motors that breaks the circuit to the start capacitor and/or start windings after the motor has reached approximately 75% of its running speed

Leads

Extended surfaces inside a heat exchanger used to enhance the heat transfer qualities of the heat exchanger

Continuous Coil Voltage

This is the maximum back electromotive force (BEMF) that the relay's coil can tolerate "continuously" without overheating and opening circuit

Pick-Up Voltage

The back electromotive force (BEMF) voltage that must be generated across the potential relay's coil to cause the relay armature to move and open the relay contacts between terminals 1 and 2

Drop-Out Voltage

The minimum back electromotive force (BEMF) that the potential relay coil must experience or its armature will drop out and the contacts between terminals 1 and 2 will close

Current Relays

An electrical device activated by a change in current flow

Switch Frequency

The speed at which the variable frequency drive's power device switches the waveform's positive and negative halves on and off is referred to as the switch frequency or carrier frequency

Pulse-Width Modulator

An electronic device in a motor circuit that is used to control motor speed for variable-speed motors

Inverter

The switching or transistor section of the variable frequency drive (VFD) that produces an AC voltage at just the right frequency for motor speed control. This section converts the DC voltage back to AC voltage

Carrier Frequency

The speed at which the variable frequency drive's power device switches the waveform's positive and negative halves on and off. Also referred to as the switch frequency

fans, pumps, compressors

Uses of Electric Motors

Used to turn _________, _________, _________________

air, water, refrigerant, fluids

Uses of Electric Motors

Facilitate the circulation of _______, __________, _______________, and other _____________

Motors

Uses of Electric Motors

_____________ are designed for particular applications

correct

Uses of Electric Motors

The _________ motor must always be used

similar

Uses of Electric Motors

Most motors operate on ______________ principles

Stator with motor windings:

Parts of an Electric Motor

generates a magnetic field when energized

Rotor

Parts of an Electric Motor

rotating portion of the motor

Bearings

Parts of an Electric Motor

Allow low-friction rotation of the shaft

End bells:

Parts of an Electric Motor

support bearings and/or shaft

Housing:

Parts of an Electric Motor

holds all motor components together and facilitates motor mounting

Electricity, magnetism

Electric Motors and Magnetism

____________________ and ________________ are used to create rotation

run

Electric Motors and Magnetism

Stator has insulated windings called ________ windings

Rotor, run windings

Electric Motors and Magnetism

__________ may be constructed of bars

Squirrel cage rotor; positioned between the _____________________

magnetic field

Electric Motors and Magnetism

Rotor turns within the _________________________

decreases, 60

Determining a Motor's Speed: Formula

As the number of poles increases, the motor speed __________________

The U.S Frequency is _____ Hz

(Frequency x 120) / number of poles

Determining a Motor's Speed: Formula

Motorspeed (RPM) =

60, 120, 3600

Determining a Motor's Speed: Formula

Ex: A 2 pole motor will turn at a speed of (____ x _______) / 2, 7200 / 2 = ______________ rpm

Motor Slip:

Determining a Motor's Speed: Slip

Internal motor component friction and resistance of connected mechanisms

Often expressed as a percentage

(Calculated motor speed - Actual motor speed) / Calculated motor speed x 100

Determining a Motor's Speed: Slip

Formula for calculating slip: ________________________________

Ex: (3600-3450)/3600 x 100 = motor slip - 4.17%

start, higher, smaller, active circuit

Start Windings

Enables the motor to __________ and in the right direction

Start winding has ____________ resistance than the run winding

Wound with ___________ diameter wire

Removed from the_______________ once the motor starts

high, starts

Starting and Running Characteristics

Refrigeration compressors have _________ starting torque

Twisting force that _________ the motor

Locked Rotor Amperage (LRA), Full Load Amperage (FLA), Rated Load Amperage

Starting and Running Characteristics

Two different current ratings:

_______________________ or _________________________

__________________________ (RLA)

unequal

Starting and Running Characteristics

Motor may start with ______________ pressures across it

single, Transformer, circuit breaker panel, fuse box, 115, 230

Electrical Power Supplies

Residences are furnished with ___________-phase power

__________________ may supply multiple houses

Power feeds through the ________________________ or ________________ to circuits in the house

Typical residential panels provide _______ and ________ volts

three

Electrical Power Supplies

Commercial and Industrial facilities require _________-phase power

115, 208, 230

Single-Phase Open Motors

Residential Motors operate at ________ V or _______ to ________ V

460

Single-Phase Open Motors

Commercial motors operate at voltages up to _________ V

one, two, dual voltage motors, differently

Single-Phase Open Motors

Some motors designed to operate at _________ of _________ different voltage (_________________)

Dual voltage motors are wired ______________ for each voltage

reversible

Single-Phase Open Motors

Some motors have __________________ rotations

Two

Split-Phase Motors

_________ separate motor windings

efficiency

Split-Phase Motors

Good running _________________

Medium

Split-Phase Motors

____________ amount of starting torque

1800, 3600

Split-Phase Motors

Normal operating ranges are _________ rpm and ___________ rpm

Motor speed

Split-Phase Motors

_________________ is determined by the number of poles and the method of wiring the poles

de-energize

The Centrifugal Switch

Commonly used on open motors to ______________ the start winding

75%

The Centrifugal Switch

Opens its contacts when the motor reaches about _________ of its rated speed

9 spark

The Centrifugal Switch

When the contacts open and close, ______________ is created (arcing)

refrigerant atmosphere

The Centrifugal Switch

Not used in a __________________________

start

The Electronic Relay

Used to open the ___________ windings after the motor has started

Solid-state device

The Electronic Relay

______________________ designed to open the start winding circuit when the design speed has been obtained

Split phase, increasing, series, start, phase angle

Capacitor-Start Motors

___________________ motor with distinctly different start and run windings

Start capacitor assists the motor starting by _________________ the starting torque

Start capacitor is wired in ___________ with the motor's start winding

Start capacitor is removed from the circuit when the ___________ winding is removed

Start capacitor increases the _______________

fans, compressors

Capacitor-Start, Capacitor - Run Motors

Efficient single-phase motor often used with belt-driven _______ and _______________

Run

Capacitor-Start, Capacitor - Run Motors

________ capacitor improves running efficiency

series, parallel

Capacitor-Start, Capacitor - Run Motors

Start and Run capacitors are wired in ________, but are ______________ to one another

energized

Capacitor-Start, Capacitor - Run Motors

Run capacitor is in the circuit whenever the motor is _____________________

rise

Capacitor-Start, Capacitor - Run Motors

Motor amperage will _______ if run capacitor goes bad

Simplest, run, starting torque, efficiency, single, multispeed, multispeeds, increases, decreases

Permanent Split Capacitor (PSC) Motors

_______________ split-phase motor

Only a ______ capacitor is used

Low __________________ and good running _______________

Can be ____________ or ____________________ motors, and __________________ have many wires at the motor electrical connections

As resistance ________________, motor speed decreases

As resistance ________________, motor speed increases

Small starting, shading, induced current, rotating field, rotation

Shaded-Pole Motors

____________________ windings or _____________ coils at the corner of each pole

Help the motor start by providing an __________________ and a ________________

Location of the shading coil determines the direction of _______________

low, efficient

Shaded-Pole Motors

Very _______ starting torque and not as _____________ as the PSC motor

horsepower

Shaded-Pole Motors

Manufactured in the fractional __________________ range

commercial

Three-Phase Motors

Normally used on _________________ applications

power supply

Three-Phase Motors

Must have a three-phase __________________

start, capacitors

Three-Phase Motors

Has no ___________ winding or _________________

high

Three-Phase Motors

Very ________ starting torque

two motor leads

Three-Phase Motors

Rotation of motor can be changed by switching any ___________________

metal shell

Hermetically Sealed Motors

Sealed inside a ______________

Split phase, capacitors-start, capacitor-run, permanent split-capacitor

Hermetically Sealed Motors

Most commonly used motors, ________________; _________________; _________________; and _______________________

vapor

Hermetically Sealed Motors

Resigned to operate in a refrigerant ____________ atmosphere

more

Hermetically Sealed Motors

Start winding has _________ resistance than the run winding

insulated

Hermetically Sealed Motors

All motor leads are ______________

high

Potential Relay

Used on motors requiring ________ starting torque

high

Potential Relay

Coil with very ________ resistance

closed

Potential Relay

Normally ___________ contacts

induced voltage

Potential Relay

Relay operates on the ____________________ across the start winding

open, close

Potential Relay

The contacts ________ when the induced voltage rises, when the induced voltage drops, the relay contacts __________

ohmmeter, connecting wires, 1, 2, 2, 5

Troubleshooting

Use a simple __________________

Take ________________ off relay

Measure resistance across:

____ and ___ terminals

___ and ____ terminals

1, 2, zero

Troubleshooting

Use a voltmeter to measure the voltage across terminals ____ and ____

Voltage reading of _______ indicates contacts are not opening

fractional horsepower motors

The Current Relay

Used on _________________________________

fixed orifice

The Current Relay

Used with __________________ metering devices

Low

The Current Relay

________ resistance coil in series with the run winding

series

The Current Relay

Normally open contacts in ________ with start winding

run

The Current Relay

Upon start up, only the _______ winding is energized

locked rotor amperage

The Current Relay and Amperage

The motor draws _________________________

closes

The Current Relay and Amperage

The increased amperage ___________ the relay contacts

starts

The Current Relay and Amperage

The start winding is energized and the motor __________