Machines; Single Phase Motors Flashcards
Between single phase and three phase, which performs better?
3 phase
What hp are most single phase motors?
Fractional (1/6 to 3/4 hp)
Typical efficiency ratings of single phase motors?
60%
Newer energy efficient models can reach ____ % efficiency
68
What type of technology has made significant improvements to single phase motor design?
Written-pole
How many sets of stator windings are commonly found in single phase induction motors?
2
What produces the rotating magnetic field required to start the motor from one single-phase voltage?
Current in one stator winding is caused to be out-of-phase with the other
How are split phase motors classified?
By the way the two stator windings are used to start and run the motor.
What are the 4 classifications of split phase motors?
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)
How does a resistance split-phase motor create a phase angle between the two currents in the stator?
Uses windings with different values of resistance and inductance
What are the basic components of a resistance split-phase motor?
Stator, frame, rotor, centrifugal switch
What are the run windings?
Also called main windings, are large insulated copper conductors mounted in the bottom of the stator slots.
What are start windings of a resistance split-phase motor?
Also called auxiliary winding, made of smaller conductors with fewer turns placed on top of the stator slots
Positioning of start windings and main windings on a two-pole vs. 4-pole motor.
2 pole = center of start @ 90° from the main
4 pole = 45°
What type of rotor is used in a resistance split phase motor?
Squirrel cage type
What are most squirrel cages made of?
Cast aluminum
Where is a fan mounted for cooling a resistance split phase motor?
Rotor
What is a centrifugal switch used for?
Disconnecting the start winding when the motor reaches full load speed
What components make up a centrifugal switch?
Stationary component (N/C contact) on end bell Rotating component on rotor
How is the switch of a centrifugal switch actuated?
By centrifugal force produced by the spinning rotor
When does the centrifugal switch contact open?
75% of the rated speed of the motor
How is the contact of a centrifugal switch connected?
Series with start winding
What other methods (besides centrifugal switch) are available to cut out the start winding?
Current sensitive relay
Electronic switching devices (triacs)
What type of start winding cut-out does not have contacts and can be located some distance from the motor?
Triacs with solid state controls
How does a current-sensitive relay work to cut out a start winding?
High inrush current pulls in the relay (closing N/O contact). Decreased running current drops out relay.
T/F a current sensitive relay can be remotely mounted.
True
What kind of field does a run winding produce?
Pulsating magnetic field
What kind of magnetic field is required to start a motor?
Rotating magnetic field
How is the run winding and start winding of a resistance split phase motor connected with respect to eachother?
In parallel
What are the resistive and inductive reactive characteristics of a start winding compared to a run winding?
Higher resistance
Lower inductive reactance
Which winding has current that is more in Phase with the applied single-phase voltage?
The start winding
Describe how a rotating magnetic field is produced in a split phase motor
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
What is the starting torque characteristics of a split phase motor?
Starting torque is poor
In a resistance split phase motor what is the displacement of the main and start winding currents?
25 to 30 degrees
What would happen if the start winding was kept in the circuit too long?
Heat created in this coil which is made of smaller diameter conductors would burn out the winding
What is the relationship between the currents flowing in the rotor and the induced rotor voltage of a resistive split phase motor?
Rotor currents lag induced rotor voltage by about 80°
