Section 4 Unit 14- Electric Motors

Air cooled motors must be located in...

a moving airstream.

Motor speed (___)

rpm

As the number of poles increases...

the motor speed decreases.

Induction Motor

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

Current Relay

An electrical device activated by a change in current flow.

Pulse-Width Modulator

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

Rotor turns...

Within the magnetic field.

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.

Rotor

The rotating part of a motor, including the shaft.

Parts of Electric Motors...

- Stator with motor windings - Rotor - Bearings - End bells - Housing

Centrifugal Switch

-Commonly used to open motors to de-energize the start winding. -Opens its contacts when the motor reaches about 75% of its rated speed. -When the contacts open and close, a spark is made. (arcing) -Not used in a refrigeration atmosphere.

Capacitor Start, Capacitor Run Motors

-Efficient single phase motor often used with belt driven fans and compressors. -Run capacitor improves running efficiency. -Run capacitor is in the circuit whenever the motor is energized. -Both are wired in series, but are parallel to each other. -Motor amperage will rise if the run capacitor goes bad.

Three Phase Motors

-Normally used on commercial applications. -Must have a three phase power supply. -Has no start winding or capacitor. -Very high starting torque. -Rotation of motor can be changed by switching any two motor leads.

Hermetically Sealed Motors

-Sealed inside a metal shell. -Most commonly used motors... -Split phase, capacitor start, capacitor start; capacitor run and permanent split capacitor. - Designed to operate in a refrigerant vapor atmosphere. - Start winding has more resistance than the run winding. - All motor leads are insulated.

Permanent Split Capacitor(PSC) Motors

-Simplest split phase motor. -Only a run capacitor is used. -Low starting torque and good running efficiency. -Can be single or multi speed motors. -Multispeed have many wires at the motor electrical connections. -As resistance decreases, motor speed increases and vice versa.

Shaded Pole Motors

-Small starting windings or shading coils at the corner of each pole. -Help the motor start by providing an induced current and a rotating field. -Location of the shading coil determines the direction of rotation. -Very low starting torque and not as efficient as the PSC motor. Manufactured in the fractional horsepower range.

Capacitor Start Motors

-Split phase motor with distinctly different start and run windings. -Start capacitor assists the motor starting by increasing torque. -Start capacitor is wired in series with the motors start winding. -Start capacitor is removed from the circuit when the start winding is removed. -Start capacitor increases the phase angle.

Motor Slip

Internal motor component friction and resistance of connected mechanisms. Often shown as a %.

Start Windings

Lets the motor to start and in the right direction. Higher resistance than the run winding.

Stator

Part in a motor that contains the windings. It does not turn.

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.

Single Phase

The electrical power supplied to equipment or small motors, normally under 7.5 hp.

Electric motors...

Turn things such as fans, compressors and pumps, and facilitates the circulation of air, water, refrigeration and other fluids.

Squirrel Cage Rotors

Construction of a motor rotor.

DC Converters/rectifiers

Converts AC power to DC power.

Split Phase Motors

-Two separate motor windings. -Good running efficiency. -Medium amount of starting torque. -Normal operating ranges are 1800 rpm and 3600 rpm. -Motor speed is determined by the number of poles and the method of wiring the poles.

Electronically Commutated Motors (ECM)

-Used on open drive fans less than 1 hp. -Armature commutator with permanent magnets. -Motors are factory calibrated. -Two piece motor: motor section and controls. -Motor can be checked with an ohmmeter. -Controls can be checked with a test module.

Residential Single Phase Open Motors operate at...

115 V or 208 to 230 V.

Commercial Single Phase Open Motors operate at...

460 V.

Bearings

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

Split Phase

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.

Capacitors smooth out...

DC voltage.

Centrifugal Switch

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

Potential Relay

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.

Housing

Casing or the shell of a motor.

Leads

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

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.

Run Windings

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

End Bells

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

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.

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.

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.

Torque

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

Continuous Coil Voltage

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

Back Electromotive Force (BEMF)

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

Refrigeration compressors have a ______ starting torque.

high

Diodes do NOT...

regulate the DC voltage and are not controllable.