HVACR ELECTRICAL-CMS, 11 of 14, Motor/Compressorss
What is cross phasing?
A condition in which some of the windings in a motor are trying to turn in one direction, while others are trying to turn in the opposite direction, thus working against each other and causing the motor to overheat.
What is a universel motor?
A motor that can run on either ac or dc.
What two mechanisms are used to provide snap action?
An external spring and a bi-metal disk
When replacing a start capacitor, a variance in capacitance of ________can usually be tolerated.
+- 10%
Split-phase motors generally are used for applications of __________hp and less.
0.75 hp
List the five main components of a typical circuit breaker.
1. Frame 2. Contacts 3. Operating mechanism or Trip bar 4. Terminals 5. Arc extinguishers
A typical motor is designed to operate within a(n)__________ % range of its rated voltage.
10%
The maximum current imbalance of a 3 phase motor is ________ %
10%
The starting torque of a split-phase motor is ____ % of the full-load torque.
175%
The voltage imbalance between any two legs of the supply voltage applied to a three-phase motor must not exceed _____%
2%
What type of motors are generally used in large commercial air conditioning applications?
3 phase
After the first contactor in a PWS motor is energized, there is a time delay of ________ before the second contactor is energized.
3/4 to 1 1/2 seconds
Most motors are designed to operate in an ambient temperature of not more than ____________
40 C or 104 F
In general, an open-type motor is _____ heavier than a hermetic motor of the same horsepower.
50%
What is an acceptable temperature rise for an open-frame motor'?
75 C
If a PSC motor will not start, or draws excessive current when running, what is the most likely cause?
A bad capacitor.
What is the best way to determine the direction of rotation of a PSC motor?
By direct observation of the driven fan. Look for dirt accumulation on the leading edge of the fan blade
A split-phase motor uses either a(n) ___________or a(n) ________to switch the start winding in and out of the circuit.
Centrifugal switch or a current relay
What is the major difference between a fuse and a circuit breaker?
Circuit breaker are resettable
The contacts of a voltage-type starting switch are normally _______________
Closed.
When you must replace a motor rated at 1/4 hp or less, is it more accurate to match horsepower or current draw?
Current draw.
Does a run capacitor increase or decrease current draw?
Decrease.
The windings of a three-phase motor may be connected in a(n) _________ configuration or a(n) __________ configuration.
Delta, wye
What type of fuse allows for a time delay?
Dual element fuses
When a copper wire is passed through a magnetic field, it generates a(n) _______
Electric current.
A motor converts_________ energy into __________ energy.
Electrical, mechanical.
A motor start capacitor is a(n) _________ capacitors.
Electrolytic.
A capacitor is a device that stores a quantity of ____________ .
Electrons
When a magnetic starter overload relay is used, what happens if the voltage drops to less than 90% of the rated voltage?
Excessive current draw through the heater elements causes them to overheat, thus opening the contacts and breaking the circuit.
What is the result of even a small imbalance in 3 phase input voltage?
Excessive heat will be generated in the motor windings.
You must always select a fuse with a voltage rating equal to or ________ than the voltage in the circuit that is to be protected.
Greater
In a capacitor-start, capacitor-run motor, are the capacitors connected in series or in parallel during start-up?
In parallel.
What is a thermistor ?
Solid state temperature sensitive resistors.
What kind of motors are commonly used in residential refrigerator compressor units?
Split phase motors
Capacitor-start motors are used in applications that require a relatively high _____ load, but a relatively low _____ Load.
Starting, running.
What is temperature rise of a motor?
The amount by which a motor's temperature will increase over the ambient air temperature when operating at its rated load and speed.
What happens to the capacitance of a capacitor if the plate area is doubled, all other factors being equal?
The capacitance doubles.
What would happen to the contacts of a bi-metal protector if it did not have snap-action operation?
The contacts would be subject to arcing, and would become welded together.
What does the current rating of a fuse indicate?
The current that the fuse will allow without opening.
Name two types of three-phase squirrel-cage motors that have comparatively low starting currents.
The double squirrel-cage motor and the increment-type motor.
The speed of an ac motor depends on the _____________ and the __________________
The frequency of the current supplied to it and the number of poles of the motor.
Which element of a magnetic overload starter carries the full motor current?
The heater element.
When is it desirable to use "across-the-line" starting?
The inrush or locked rotor amperage is low enough to make the use of only one starting device economical.
What is the working voltage of a capacitor?
The maximum voltage that may be safely applied across the capacitor without breaking down the dielectric between the plates.
What does the voltage rating of a fuse indicate?
The maximum voltage that the open fuse will block.
What happens when thermal-acting bi-metal contacts are heated?
The metal with the high expansion rate expands faster, resulting in a bending action that opens or closes the contacts.
What two factors determine the speed of an induction motor?
The number of poles and the frequency of the current.
Which type of fuse holder is safer, the clip type or the post type?
The post type, it is fully enclosed in a insulated holder.
Is the speed of the rotor greater than or less than the speed of the rotating field? What is the difference called?
The speed is less, it is called slip
On a motor equipped with both a start capacitor and a run capacitor, which capacitor will have the higher capacitance rating?
The start capacitor.
In a split-phase motor, which winding has the greatest amount of resistance?
The start winding.
How does a thermal-only overload differ from a thermal/current overloads
The thermal-only overload as two terminals, the thermal/current overloads has three.
In what respects are PSC motors more versatile than shaded-pole motors?
They are electrically reversible, have multiple speeds, and can run on multiple voltages.
How is the motor heat generated in a hermetic compressor dissipated?
Through radiation, refrigerant vapor, and oil vapor.
A permanent split-capacitor motor operates as a(n)________ -phase motor
Two
When does "over-motoring" occur?
When a larger-horsepower motor is used to replace a smaller-horsepower motor. The result is a waste of electrical energy, overheated wiring, and a loss of torque being supplied to the motor.
When does single phasing occur in a three phase motor?
When one of the lines is open between the motor and the supply.
What is the function of the centrifugal switch attached to the shaft of a split-phase motor?
When the motor attains 75 to 80% of its running speed, the switch opens and disconnects the start winding from the circuit
If a hermetic motor develops a locked-rotor condition, how soon can damage occur if the protector does not operate?
Within 20 seconds.
When you replace a PSC motor, should you also replace the capacitor? Why or why not?
Yes. Because the replacement motor usually will require a different capacitance, and the old capacitor may have changed value from what it was originally
If the average current through the windings of a three-phase motor is 25 A, and the current measured through one of the individual windings is 28 A, is the maximum allowable current imbalance being exceeded?
Yes. The deviation from the average current is 3 A. When divided by the average current of 25 A and multiplied by 100, this amounts to a 12% current imbalance, well over the 10% limit.
Which has a higher starting torque, the capacitor-start, induction-run motor or the capacitor-start, capacitor-run motor?
capacitor-start, capacitor-run motor
When is it permissible to replace one fuse with another fuse that has a higher current rating?
never
A dc motor whose motion consists of discrete increments, instead of continuous rotation, is called a(n) __________motor.
step
The fusible link of a single-element fuse is commonly made of _______________
zinc
A synchronous motor has no________ .
Slip
How does NEMA define the difference between integral-horsepower motors and fractional-horsepower motors?
Integral-horsepower motors are rated at 1 hp and above, fractional-horsepower motors are rated at less than 1 hp.
What is a simple way to reverse the direction of rotation of a three-phase motor?
Interchange any two leads
A protective device designed to be mounted directly on a motor's windings is called a(n) ________ overload.
Internal overload or inherent protection.
What is the defining characteristic of a universal motor?
It can operate on either ac or dc.
If two capacitors are connected in series and one fails due to a short, what may happen to the other one?
It generally will short out also
What is the purpose of a run capacitor?
It improves the power factor of a motor
When two capacitors are connected in parallel, what happens to the total capacitance?
It increases.
If two capacitors with the same voltage rating are connected in parallel, what happens to the total voltage rating of the two?
It remains the same.
Do shaded-pole motors have a high or low starting torque?
Low
What do the abbreviations MCA and MOCP stand for?
MCA = Minimum Circuit Amperes or Ampacity (Smallest allowable current on which the equipment can operate). MOCP = Maximum Overcurrent Protection (the largest allowable fuse or circuit breaker that the manufacturer will allow on the equipment.)
What force operates an electric motor?
Magnetism.
A generator converts __________ energy into __________energy.
Mechanical, electrical.
The unit of measurement normally used in rating the capacitance of a capacitor is the ________________
Microfarad (uF).
What two factors determine the size of a run capacitor?
Motor horsepower and applied voltage
How can you determine the synchronous speed of a motor?
Multiply the frequency of the power supply by 120, and divide the result by the number of poles the motor has.
Does a run capacitor reduce power consumption?
No
Can you use an ohmmeter to check the resistance in variable-speed motor windings?
No.
Is a single-phase ac motor self-starting?
No.
The contacts of a current-type starting switch are normally. ___________
Open
20. The service factor of a motor measures its designed ______________
Overload capacity
How do permanent split-capacitor motor currents compare with shaded-pole motor currents of the same size?
PSC motors usually draw half as much current as shaded-pole motors.
Dual-voltage motors are connected in _______ for low-voltage operation, and in _____for high-voltage operation.
Parallel, series.
What type of starting method is required for large compressors and large current demands?
Part winding start
The type of motor in which the capacitor is never taken out of the circuit is called a(n) ___________motor.
Permanent split capacitor motor
The voltage at which a magnetically operated device starts to operate is called the _______________
Pickup voltage
Name the two basic types of fuses
Plug and cartridge fuses
What is the one positive way to check for an open fuse?
Remove it from the circuit and check it with an ohmmeter
When the north poles of two permanent magnets are brought close together, they tend to _________each other.
Repel.
How can you determine the direction of rotor rotation in a shaded-pole motor?
Rotation is always in the direction of the shaded pole.
The induction motor derives its name from the fact that ac voltages are induced in the ___________ by the rotating magnetic field of the ___________
Rotor, stator
What are the two types of motor windings in a simple ac motor?
Run and Start windings
Calculate the percent of slip for a motor with a synchronous speed of 3,600 rpm and a rotor speed of 3,400 rpm.
S = ((Ss - Sr)/Ss) X 100 (3600 - 3400/3600) X 100 = 200/3600 X 100 = 5.55%
In a capacitor-start, induction-run motor, the capacitor is wired in _____________ with the start winding.
Series
What are the three main types of dc motors?
Series-wound, shunt-wound, and compound wound.
