2ND 8 Flashcards
The variable placed in the rotor circuit of the wound rotor AC motor is for the purpose of:
a) speed control
b) frequency control
c) voltage control
d) starting torque control
d) starting torque control
The more common method of starting a synchronous motor is:
a) with a separate DC motor
b) with a squirrel-cage type of winding on the outer edge of the field poles
c) manually
d) by applying DC to the rotor
b) with a squirrel-cage type of winding on the outer edge of the field poles
The synchronous speed of an induction motor is the:
a) speed at which the rotor turns
b) speed of the revolving field
c) frequency of the rotor current
d) slip in percent of rotor RPM
b) speed of the revolving field
What factor determines the rotor speed of an induction motor?
a) frequency
b) slip
c) number of poles
d) any one or all of the above
d) any one or all of the above
A synchronous motor differs from an induction motor in that it:
a) is not self-starting
b) requires an AC and DC power supply
c) may be used for power factor correction
d) all of the above
d) all of the above
The direction of rotation of an induction motor is:
a) opposite the revolving field direction
b) same as direction of revolving field
c) determined by the number of poles
d) determined by the location of it brushes
b) same as direction of revolving field
The principle parts of an induction motor are the:
a) stator and rotor
b) stator, rotor, armature, and sliprings
c) armature, field magnet, sliprings, and brushes
d) armature and field magnets
a) stator and rotor
The type of connection used with a three-phase squirrel cage motor where 58 percent of the voltage is impressed on the field coils for starting, and 100 percent for full load operation is called:
a) across-the-line
b) compensator start
c) wye-delta
d) none of these
c) wye-delta
When large induction motors are started across-the-line under loaded conditions:
a) both the motor amperes and induced rotor currents are very high
b) motor starts must be limited with respect to time
c) sufficient time must be allowed for motor to cool before reclosing the breaker after a shutdown
d) all of the above are true statements
d) all of the above are true statements
Under what conditions can a three-phase synchronous motor be used to correct a lagging power factor?
a) when the counter EMF is caused to become greater than the terminal voltage
b) by decreasing the field strength
c) by under-exciting the motor
d) by causing the terminal voltage of the motor to lead the current
a) when the counter EMF is caused to become greater than the terminal voltage
The speed of a synchronous motor is governed by the:
a) percent of slip
b) pull-out torque
c) field excitation
d) line frequency and number of field poles in the motor
d) line frequency and number of field poles in the motor
The difference between the speed of the speed of the revolving field of the stator and the actual speed of the rotor of an induction motor is called the:
a) motor efficiency
b) pull-out torque
c) slip
d) synchronous speed
c) slip
Why does the capacitor motor have a higher starting torque than the inductively displaced split-phase motor?
a) a higher power factor
b) greater displacement between its start and main winding currents
c) no displacement between start and main winding currents
d) less displacement between start and main winding currents
b) greater displacement between its start and main winding currents
In a split-phase starting arrangement to allow a single-phase AC induction motor to start automatically, the starting winding may be placed in series with a capacitor. The effect of this capacitor is to:
a) produce a revolving field by shifting the phase relationship between the starting winding and the running winding
b) create a counter EMF capable of increasing the torque to a point where the motor will start
c) develop the armature current to a point where the torque is sufficiently high to overcome the inertia of the armature
d) reverse the flow of the AC in the field coils so that opposite poles appear on the armature and field poles simultaneously
a) produce a revolving field by shifting the phase relationship between the starting winding and the running winding
A motor is rated at 10 HP. It is drawing 10,000 watts from the line. Its efficiency is therefore:
a) 75 percent
b) 80 percent
c) 85 percent
d) 90 percent
a) 75 percent
