HVACR ELECTRICAL-CMS, 8 of 14, Capacitors and Capacitance

The life of a motor start capacitor will be shortened if the voltaqe across the capacitor exceeds % of the rated voltage

15%

What is the recommended value (in ohms) of a resistor used to discharge a capacitor?

15,000 ohm

A typical start capacitor is designed for a maximum of starts per hour.

20

. If two 400 uF capacitors are connected in series, what is their total capacitance?

200 uF

. If two capacitors rated at 125 V are connected in series, what is their total voltage?

250V

A capacitor increases the starting torque of a motor to % of its full-load power

300 to 500%

What are the typical ranges of capacitance and voltage for a run capacitor?

5 to 50 uF and 370 to 440 V

What is the total capacitance of a circuit in which a 30 uF capacitor, a 20 uF capacitor, and a 10 uf capacitor are connected in series?

5.46 uF

What are the typical ranges of capacitance and voltage for a start capacitor?

50 to 380 uf and 110 to 330 V

What is a picofarad?

A measure of capacitance equel to a millionth of a millionth of a farad (or one one-millionth of a microfarad).

How can a capacitor's value be increased?

By increasing the size of the plates and decreasing the distance between them.

What is the total capacitance of a circuit in which a 5 uf capacitor, a 13 uf capacitor, and a 12 uf capacitor are connected in parallel?

C = 5 + 13 + 12 = 30 uF

Does a run capacitor increase or decrease current draw?

Decrease

A motor start capacitor is a(n) _____________________ capacitor.

Electrolytic.

A capacitor is a device that stores a quantity of _______________ .

Electrons.

A start capacitor usually has a(n) __________ value of capacitance and a(n) ___________voltage rating than a run capacitor.

High ...lower

In a capacitor-start, capacitor-run motor, are the capacitors connected in series or in parallel with each other during start-up?

In parallel.

How long can a capacitor remain charged?

Indefinitely

A capacitor is made up of two metal plates separated by a(n) ____________ .

Insulator

Does a start capacitor increase or decrease starting current?

It decreases starting current

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.

When capacitors are connected in series, the total capacitance of the circuit is always ________ than the ________ capacitance of any of the individual capacitors.

Less than the smallest

The terminal of a capacitor marked with a dot should always be connected to the _____ side of a circuit.

Line

Does a split-phase motor have a high starting torque or a low starting torque?

Low

The capacitor with the _________voltage rating determines the maximum safe operating voltage that can be applied to a parallel circuit

Lowest

Which is the better insulator, a material with a high dielectric constant or a material with a low dielectric constant?

Material with a high dielectric constant

The unit of measurement normally used in rating the capacitance of a capacitor is the ____________ .

Microfarad (uF)

Capacitance is usually measured in __________

Microfarads

What two factors determine the size of a run capacitor?

Motor horsepower and applied voltage.

Do electrons flow through a capacitor?

No

Does a run capacitor reduce power consumption?

No

What is the dielectric constant of air?

One, ( it is the standard )

When you use a digital capacitance meter, a reading of "00.0" indicates a(n) ______________ capacitor.

Open

Capacitors connected in parallel result in a higher capacitance because the _______________has been increased.

Overall plate area

When capacitors are connected in parallel, the total capacitance of the circuit is the __________of all the individual capacitances.

Sum

The working voltage of a series circuit is the ____________ of the working voltages of the individual capacitors.

Sum of the working voltages of the individual capacitors.

Define capacitance.

The ability to store electrical energy in an electrostatic field

What three factors determine the capacitance of a capacitor?

The area of the plates, The distance between the plates, The dielectric constant of the material between the plates.

What happens to the capacitance of a capacitor if the plate area is doubled, all other factors being equal?

The capacitance doubles.

When you use a neon light tester to test a capacitor, what is the significance of a continuously glowing light?

The capacitor is shorted.

What happens when a capacitor becomes shorted?

The dielectric breaks down and the plates short to each other

What happens if the potential difference across the plates is greater than the dielectric can withstand?

The dielectric will break down and a short circuit will occur.

When you use a digital capacitance meter, what reading is displayed if the capacitor being tested is shorted?

The digital readout will display a "1" or "1x."

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 will happen when you use an analog ohmmeter to test a good capacitor?

The meter's needle will deflect first toward zero, then slowly back toward infinity. This is because when the meter is first attached to the cap. it begins to charge and there for acts like a short. When the cap. becomes charged current stops flowing and acts like an open circuit.

What will happen when you use an analog ohmmeter to test an open capacitor?

The meter's needle will not move. (It will stay on infinity)

On a motor equipped with both a start capacitor and a run capacitor, which capacitor will have the higher capacitance rating?

The start capacitor.

What determines the safe working voltage of a capacitor?

The strength of the dielectric material.

Why should you use a resistor instead of a screwdriver to discharge a capacitor?

There is an internal fusing device inside some capacitors that may blow if the capacitor is discharged with a screwdriver or jumper wire.

Why are split-phase motor compressors usually used on domestic refrigerators and freezers?

These applications use capillary tubes and therefore require a lower starting torque.

What are the four main problems associated with capacitors?

They may be open, shorted, leaking, or undergo a change in value.

Why do start capacitors have a fuse hole?

To allow gases to escape when the capacitor becomes overheated.

A permanent split-capacitor motor operates as a(n) -phase motor.

Two

What should you do before performing the ammeter/voltmeter test?

Use an ohmmeter to make sure that the capacitor is not shorted.

When replacing a start capacitor, a variance in capacitance of can usually be tolerated.

±10%

Start capacitors have typical tolerance ratings of _______________ %

±20 to ±25%