Single-Phase Motors 030202e

Question 1

What is the horsepower range for fractional horsepower motors?

Answer 1

1/100 to 3/4 hp

Question 2

What is the horsepower range for integral horsepower motors?

Answer 2

1hp to 10 hp. With 15 hp being special order.

Question 3

What are two popular motor types?

Answer 3

Resistance-start split-phase motor (split-phase motor).

Two-value capacitor motor (cap-start/cap-run motor).

Question 4

What is counter torque?

Answer 4

Amount of opposition the load presents to being moved.

Opposition to twisting/turning.

Question 5

What is torque?

Answer 5

Twisting force

Question 6

What is running torque?

Answer 6

Amount of twisting force needed to keep a load rotating.

Question 7

What is starting current?

Answer 7

Also known as inrush current is the amount of amps the motor will draw while starting from no speed to full speed.

Question 8

What is starting torque?

Answer 8

Amount of twisting force required to start a load to move.

Question 9

What are the basic components of a split-phase motor?

Answer 9

Frame.
Stator.
Rotor.
Starting switch (cut-out switch)

Question 10

What is the purpose of the frame?

Answer 10

Encloses the stator windings.
Supports the motor’s end bells.
Helps to maintain a air gap between the rotor and stator.

Question 11

What is the stator?

Answer 11

Consists of a laminated steel core with slots to hold the insulated windings.
Has 2 sets of windings, one larger main(run) winding and a smaller start winding.

Question 12

What is the difference between the windings in the stator of a split-phase motor?

Answer 12

Main winding is made of larger insulated copper conductors and mounted on the bottom of the stator.

Start windings are smaller conductors with fewer turns to have greater resistance and less reactance than the main winding.

In a 2 pole motor the start windings are placed 90 degrees from the centre of the main winding and 45 degrees away in a 4 pole motor.

Question 13

What are the rotor-bar conductors made of?

Answer 13

Older ones were made of copper.

Modern ones are made of cast aluminum.

Question 14

What is the purpose of a centrifugal type switch mounted inside the motor?

Answer 14

Serves to disconnect the start winding just before the rotor reaches full-load speed.

Question 15

What are the components of a centrifugal switch?

Answer 15

A stationary component attached inside the end bell with normally closed contacts.

A Rotating component mounted on the rotor shaft known as an actuator.

Question 16

At what point do the stationary contacts open?

Answer 16

Approximately 75% of the rated speed.

Question 17

What is the most common cause for single-phase motor failure?

Answer 17

Faulty start switch.

Question 18

What is another method for a start winding cut-out other than the centrifugal switch?

Answer 18

Current-sensitive relays.

Electronic switching devices like triacs.

Question 19

How does a current-sensitive relay work for a start winding cut out?

Answer 19

Has a solenoid connected in series with the main winding with normally open contacts.

The high inrush current during motor start is enough to activate the solenoid coil and close the contacts to energize the start winding.

When current decreases to rated value the solenoid drops out and start winding is removed from the circuit.

Question 20

How must current-sensitive relays be mounted?

Answer 20

Upright position so that the gravity-controlled contacts reopen successfully.

Question 21

What is the advantage of current-sensitive relays over a centrifugal switch?

Answer 21

Can be remotely mounted as well as are not prone to issues due to dirt and debris

Question 22

What is the advantage of electronic switching devices?

Answer 22

Does not have contacts that may wear out.

Can be located some distance from the motor.

Question 23

What degree of phase displacement between the main and start winding creates the greatest torque?

Answer 23

90 degrees.

Question 24

How much electrical phase displacement between the main and start windings is typical in a resistance-start split-phase motor?

Answer 24

Approximately 25 to 30 degrees.

Question 25

What can happen if the start winding is kept in the circuit too long?

Answer 25

Small-diameter conductors will overheat and likely burn out.

Question 26

What is cross-field effect?

Answer 26

Rotors magnetic field is physically oriented approximately 90 degrees behind the stator. This causes the magnetic fields to build to maximum at different times. The rotors magnetic field is attracted to the stators but since both fields are alternating they can never catch up and creates a continuous rotational torque. Rotors magnetic field lags the stators magnetic field.

Question 27

What is rotor-slip?

Answer 27

The difference between synchronous speed and rotor speed. (The synchronous speed is always higher than the rotor speed.)

Question 28

What is the most common size of resistance-start motor?

Answer 28

1/4 hp

Question 29

How much is the starting current compared to the running current?

Answer 29

5 to 6 times the amount.

Question 30

How much more starting torque is there compared to the running torque?

Answer 30

125% to 150%

Question 31

How are the run windings connected in dual voltage split-phase motors for low-voltage and high-voltage connections?

Answer 31

Low-voltage - run windings and start windings must be parallel connected. High-voltage - The run windings must be series connected and the start winding connected in parallel with only one run winding portion.

Question 32

What are the basic components of a two-value capacitor motor also known as the cap-start/cap-run motor?

Answer 32

An electrolytic capacitor (start-cap). An oil-filled continuous-duty capacitor (run-cap). Cut-out switch (centrifugal or relay type). Main winding. Auxiliary Winding.

Question 33

Why is the start winding called the auxiliary winding in cap-start/cap-run motors?

Answer 33

Continuously energized.

Question 34

What are some characteristics of the electrolytic capacitor (start-cap)?

Answer 34

Rated for short duty time of only three seconds. Readily accept AC without degradation as long as duty time is not exceeded. Should not endure any more than 20 start ups per hour. Common failure point in faulty cap-start/cap-run motors. Microfarad ratings range between 100 to 1000 uF. Most common 150 to 350 uF.

Question 35

What are some characteristics of the Oil-filled capacitor (Cap-run)?

Answer 35

Designed for continuous duty. Oil inside acts as a coolant and outside container is made of tin. Voltage ranges from 370 Vac to 416 Vac. Can withstand voltage potentials higher than RMS values. Common microfarad rating of 10 uF to 50 uF.

Question 36

What is the advantage of the cap-start/cap-run motor of the split-phase?

Answer 36

Capacitance produces an increased phase displacement of 90 degrees between the run winding and start winding. This produces greater starting torque and lower inrush current.

Question 37

What is the starting torque percentage compared to the running torque in a cap-start/cap-run motor?

Answer 37

250% to 450%.

Question 38

What are the general horsepower ratings of a cap-start/cap-run motor?

Answer 38

1/4 hp to 10 hp with 15 hp available as special order.

Question 39

What part of a split-phase motor holds the rotor in place?

Answer 39

The end-bells.