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4th Year Electrical > Machines; Single Phase Motors > Flashcards

Machines; Single Phase Motors Flashcards

1
Q

Between single phase and three phase, which performs better?

A

3 phase

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2
Q

What hp are most single phase motors?

A

Fractional (1/6 to 3/4 hp)

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3
Q

Typical efficiency ratings of single phase motors?

A

60%

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4
Q

Newer energy efficient models can reach ____ % efficiency

A

68

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5
Q

What type of technology has made significant improvements to single phase motor design?

A

Written-pole

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6
Q

How many sets of stator windings are commonly found in single phase induction motors?

A

2

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7
Q

What produces the rotating magnetic field required to start the motor from one single-phase voltage?

A

Current in one stator winding is caused to be out-of-phase with the other

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8
Q

How are split phase motors classified?

A

By the way the two stator windings are used to start and run the motor.

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9
Q

What are the 4 classifications of split phase motors?

A

1) resistance split phase motor
2) capacitor-start motor
3) permanent-split capacitor motor
4) two-value capacitor motor (ie. Capacitor start capacitor run motor)

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10
Q

How does a resistance split-phase motor create a phase angle between the two currents in the stator?

A

Uses windings with different values of resistance and inductance

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11
Q

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

A

Stator, frame, rotor, centrifugal switch

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12
Q

What are the run windings?

A

Also called main windings, are large insulated copper conductors mounted in the bottom of the stator slots.

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13
Q

What are start windings of a resistance split-phase motor?

A

Also called auxiliary winding, made of smaller conductors with fewer turns placed on top of the stator slots

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14
Q

Positioning of start windings and main windings on a two-pole vs. 4-pole motor.

A

2 pole = center of start @ 90° from the main

4 pole = 45°

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15
Q

What type of rotor is used in a resistance split phase motor?

A

Squirrel cage type

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16
Q

What are most squirrel cages made of?

A

Cast aluminum

17
Q

Where is a fan mounted for cooling a resistance split phase motor?

A

Rotor

18
Q

What is a centrifugal switch used for?

A

Disconnecting the start winding when the motor reaches full load speed

19
Q

What components make up a centrifugal switch?

A 
Stationary component (N/C contact) on end bell
Rotating component on rotor
20
Q

How is the switch of a centrifugal switch actuated?

A

By centrifugal force produced by the spinning rotor

21
Q

When does the centrifugal switch contact open?

A

75% of the rated speed of the motor

22
Q

How is the contact of a centrifugal switch connected?

A

Series with start winding

23
Q

What other methods (besides centrifugal switch) are available to cut out the start winding?

A

Current sensitive relay

Electronic switching devices (triacs)

24
Q

What type of start winding cut-out does not have contacts and can be located some distance from the motor?

A

Triacs with solid state controls

25
Q

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

A

High inrush current pulls in the relay (closing N/O contact). Decreased running current drops out relay.

26
Q

T/F a current sensitive relay can be remotely mounted.

A

True

27
Q

What kind of field does a run winding produce?

A

Pulsating magnetic field

28
Q

What kind of magnetic field is required to start a motor?

A

Rotating magnetic field

29
Q

How is the run winding and start winding of a resistance split phase motor connected with respect to eachother?

A

In parallel

30
Q

What are the resistive and inductive reactive characteristics of a start winding compared to a run winding?

A

Higher resistance

Lower inductive reactance

31
Q

Which winding has current that is more in Phase with the applied single-phase voltage?

A

The start winding

32
Q

Describe how a rotating magnetic field is produced in a split phase motor

A

Much like the rotating magnetic field produced by a 3-phase ac voltage this rotating magnetic field is produced because the out of phase currents on the start and run windings build to a maximum value at different times

33
Q

What is the starting torque characteristics of a split phase motor?

A

Starting torque is poor

34
Q

In a resistance split phase motor what is the displacement of the main and start winding currents?

A

25 to 30 degrees

35
Q

What would happen if the start winding was kept in the circuit too long?

A

Heat created in this coil which is made of smaller diameter conductors would burn out the winding

36
Q

What is the relationship between the currents flowing in the rotor and the induced rotor voltage of a resistive split phase motor?

A

Rotor currents lag induced rotor voltage by about 80°

4th Year Electrical (24 decks)

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