Chapter 15 Study Guide Electric Motors

How do three-phase motor windings differ from a single-phase windings?

Have 3 sets of stator windings energized by a three-phase power source. Each set of windings connects to one face of the power source.

What are the components in the motor circuit of a CSCR (capacitor-start, capacitor-run) motor and how does the motor work?

It is an induction motor that uses phase splitting to achieve initial rotation. A start capacitor adds more phase splitting which gives the motor extra torque at start-up. At 75% of rated of speed a switch opens disconnecting the start winding & capacitor. The run capacitor remains in the circuit to improve the power factor.

Why does the run winding have a lower Ohm reading than the start winding?

Larger gauge wire, fewer turns than start winding.

What is LRA?

Locked rotor amps — the current the motor draws as power is first applied to start turning the rotor. 2 to 6 times the full load amperage.

What is rated ambient temperature?

Maximum ambient temperature at at which the motor can operate.

What is insulation class on a motor data plate?

Maximum temperature that the insulation on the wires of the motor windings can withstand before breaking down.

What happens if start winding is left in the circuit?

May overheat.

How do you Ohm out a compressor to identify unmarked terminals and troubleshoot?

Measure homes from C-S, C-R, R-S. Ohm out each terminal to case/ground of compressor. Use process of elimination to determine R,S & C terminals.

What form of energy does the rotor convert electrical into?

Mechanical energy

What is motor efficiency?

Mechanical energy produced by shaft divided by power input.

Why does the start winding have a higher inductance value than the run winding?

More coil turns

What can cause LRA (locked rotors amps) besides motor startup?

Motor having to start under a heavy load or mechanical interference like fans with worn bearings, compressors pumping against excessively high head pressure, etc.

What is the stator attached to?

Motor housing

Where are fans used in HVACR?

Motor-driven fans are used an outdoor unit (condenser) and the indoor air handler (evaporator).

What type of capacitor is used on a three-phase motor?

No capacitors are needed for phase splitting.

Does CEMF have the same polarity as the stator?

No, polarity induced by the rotor is opposite.

ECM (Electronically Commutated Motor) motors are designed to _____.

Operate very quietly

What is an ECM (Electronically Commutated Motor) motor.

Programmable dc motor that has a control module to control voltage to the stator windings. Rotor made of a permanent magnet, no induction, rotor is not energized.

What does a run capacitor do?

Provides a signal that is out of phase for added torque during the motors entire operation.

What is synchronous speed?

Refers to the speed of the rotating magnetic field in the stator.

How do you change a three-phase motors rotation?

Rotation can be changed by switching any two power leads.

What are the two main parts of a motor?

Rotor and stator

What is the part of the motor called that rotates?

Rotor or armature is the axle mounted part that rotates.

Why are ohm readings different from terminal to terminal?

Run - larger wire, fewer windings, less resistance. Start - smaller wire, more windings, more resistance. Common - total both R and S.

What are the components in the motor circuit of a PSC (permanent split capacitor) motor?

Run capacitor in series with start winding

How does a shaded pole motor work?

Single phase motor that uses shaded field poles instead of a start winding to produce starting torque.

What controls the speed of the compressor vs. the motor driving it?

Size of the pulley

What is the difference in synchronous and rated full load speed called?

Slip

What are the components in the motor starting circuit of a CSIR (Capacitor Start Induction Run motor)?

Start capacitor and start relay

What is the stationary part of a motor called?

Stator

What is a start winding?

Stator windings that are energized during motor starting only and to add additional torque.

What is a run winding?

Stator windings that are energized during the entire operation of the motor. Provide bulk of magnetic force to drive the rotor.

Where are electrical connections made to control and power the motor?

Terminal box

What is the rated horsepower?

The amount of mechanical work that a motor can perform based on both motor speed and torque.

Motors differ from each other by _____.

The amount of starting torque and running torque that they generate.

What happens when a motor is slowed considerably by a heavy load?

The current induced in the rotor increases greatly. The current increases because the rotor turns more slowly, which means counter emf and resistance to current drops. With such high current the motor will overheat.

What is full-load amperage?

The current level at which the full-load torque and hp are reached.

What is a direct drive compressor?

The motor and compressor shaft are directly connected using a coupling.

What is capacitive reactance?

The opposition to the flow of current that causes voltage to lag behind current and both to alternate out of phase.

What is power factor?

The ratio of apparent power to true power under full load conditions.

How is an internal overload wired on a single phase hermetic compressor? (Not in chapter)

The run and start winding connect to a thermal overload which in turn connects to the common terminal. You get O.L. from S-C & R-C but ohms from S-R.

What is frequency?

The sine wave cycles per a second the electricity is generated at. 60Hz in the U.S. Power supply frequency must match frequency of a motor.

What is rated full load speed?

The speed motors are rated at, which is the operating speed of a motor under full load when both the rated voltage and frequency are supplied.

What is torque?

The work performed by twisting or turning action.

What are the two most common types of motor protection?

Thermally protected and impedance protected.

What are important facts about CSCR (capacitor-start, capacitor-run) motors?

These are very efficient motors that have enough power to start heavy loads.

How do run capacitors dissipate heat?

They are oil filled to dissipate heat compared to start capacitors that are dry electrolytic material that does not dissipate heat. Q15

Are single or three-phase motors more efficient?

Three-phase are more efficient than single phase.

What does a parallel wiring of a compressor and condenser motor allow for?

To be controlled by the thermostat. When the compressor runs, the condenser fan also runs.

What is the purpose of capacitors?

To create a much larger phase displacement to help start the motor?

What is the purpose of phase splitting?

To create a rotating magnetic field to start single phase motors.

What is rated voltage?

Voltage level where the motor will perform best. Also called nameplate voltage.

What is possible in regard to programming ECM( Electronically Commutated Motor) motors?

When powering fans, the motor may be set to maintain a constant airflow that will adjust the speed of the motor to account for closed dampers, obstructions, or high external static pressures. Both speed and torque can be set for specific applications.

How do you wire a three-phase motor for high voltage?

Windings wired in series for high voltage.

How do you wire a three-phase motor for low voltage?

Wired in parallel for low voltage.

What are field windings?

Wires wrapped around the field poles of the stator. Current flowing through the field windings creates a magnetic field around the field poles.

How is torque generated with 3 phases?

Within one cycle, the three separate currents signals reach peak voltage at separate times. As a result, a three-phase motor generates the necessary torque from it's rotating magnetic field in the stator.

What term describes winding out of phase?

Phase splitting

What is service factor?

The overload capacity (or horsepower) at which a motor can operate without overheating or being damaged.

What is an induction motor?

AC motors that operate by using a magnetic field generated in the stator to induce current in the rotor.

What 3 properties help maintain the rotor at a constant speed?

Applied voltage, CEMF, & Current

If an air conditioner runs for 2 hours, how long does the start capacitor run?

Approximately for a second for motor start-up only.

What action do poles with unlike electrical charges produce?

Attract each other.

How can a rotor be slowed by a heavy load?

Bad bearings, belt too tight, high external static pressure.

What parts hold the rotor in an exact position?

Bearings

What are 3 terminals on a hermetic compressor?

C -common, R -run, S -start

What would be the result of that heavy load to the motor?

CEMF drops causing amps to increase.

What are important facts about a PSC (permanent split capacitor) motor?

Can be multi-speed. Sensitive to 5-10% voltage drops. Thermal overloads used. Low torque applications 120V-240V

What does CSIR stand for?

Capacitor Start Induction Run motor

What causes capacitive reactance?

Capacitors

What causes a motor to be less than 100% efficient? and load?

Clearance space, bearing friction, imperfect windings

Can a three-phase motor run on different voltages?

Commonly a "dual voltage" motor, typically 240V or 460V motor due to windings arranged in pairs.

How is start winding wired?

Connected between the start terminal and common terminal of the compressor.

How is the start winding in a compressor wired?

Connected between the start terminal and common terminal of the compressor.

Where are motor leads for condenser fan connected?

Connected to the common terminal and the running terminal of a compressor motor. This connection puts the fan motor in parallel with the compressor motor.

What can a VFD (variable frequency drive) control on a motor?

Controls torque, acceleration and deceleration. Motors can be operated at specific speeds and improve system efficiency.

What are special considerations needed for hermetic compressors?

Cooling accomplished by fins, water in a coil, cool refrigerant from suction line or condenser. Requires leakproof electrical connections. Wire insulation must be resistant to oil and chemicals in the refrigerant.

What causes the run and start winding current to be out of phase with each other?

Different inductance values

How does the CSIR (Capacitor Start Induction Run motor) motor work?

During start-up, current passes through both the start winding and the run winding. At about 75% of the motor's rated speed, a centrifugal switch or a relay disconnects the start winding and start capacitor. The motor continues to run as an induction motor using only the run windings.

What are field poles?

Electromagnets whose polarity changes as current flow alternates in the field windings.

What does ECM stand for?

Electronically Commutated Motor

What are the caps/plates that close the openings at either end of the motor?

End bells

What 2 parts are found on the stator?

Field poles and field windings i.e. stator windings

What are important facts about a shaded pole motor?

Has low torque than any other motor. 1/100th to 1/6th HP

The speed of an AC motor is determined by _____.

Hz, number of stator poles, 120 x 60Hz / # poles N(s) = 120V x (F÷P) F = frequency (Hz) P = # of poles

How do you wire a start capacitor?

In series with the start winding

How do you wire a run capacitor?

In series with the start winding and in *parallel with the run winding.* (not clearly spelled out in the chapter)

What information does Ohming out a compressor give you? (not in chapter)

Indicates if windings are shorted or grounded and identifies terminals.

What is a split phase motor?

Induction motor that operates in the fractional horsepower range and uses phase splitting to achieve initial rotation. Found in capillary tube systems...

How does a PSC (permanent split capacitor) work?

Induction motor that uses phase splitting to achieve initial rotation and a single run capacitor remains in the circuit to improve the power factor. Current flows through both windings during the motors entire operation.

Describe basic motor operation?

1. Current from the source is applied to stator windings creating electromagnetic field. 2. Magnetic field induces current in the rotor. 3. Rotor creates an EMF that interacts with the electromagnetic created by the stator. 4. The interaction between the rotor and stator magnetic fields cause the rotor to rotate.

At what percent of full-load speed is the start winding opened?

60% to 75%

What is an open-drive compressor motor?

A compressor driven by an external motor. The compressor can be directly connected to the motor or connected by a belt.

What is a variable frequency drive?

A device that uses a rectifier circuit to convert ac to dc voltage and an inverter to convert it back to an ac sine wave whose frequency can now be changed to alter the speed of a motor.

What is a hermetic compressor?

A motor and compressor sealed inside a dome.

What is an intermittent duty motor?

A motor that operates at alternate intervals on/off.

What is a continuous duty motor?

A motor that operates at an indefinitely long time.

What is a squirrel cage rotor?

A rotor made of metal bars instead of wires.

What is a single phase motor?

A single-phase motor is an ac motor that runs on a single phase of alternating current.

Does the rotor create an electromagnetic field?

Yes