8 - 20 Flashcards
The variable resistance 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
pg 11
The more common method of starting a synchronous motor is?
A) With a seperate 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
pg 4
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 the rotor RPM
B) Speed of the revolving field
pg 3
What factors determine 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
pg 8
A synchronous motor differs from an induction motor in that it:
A) have no built-in starting torque
B) require a Direct Current excited field
C) may be used for power factor correction & voltage regulation
D) All of the above
D) All of the Above
pg 3
The direction of rotation of an induction motor is:
A) Oposite the revolving field direction
B) Same as direction of revolving field
C) Determined by the number of poles
D) Determined by the location of its brushes
B) Same as direction of revolving field
pg 12
The principle parts of an induction motor are the:
A) Stator and rotor
B) Stator, rotor, armature and slips rings
C) Armature, field magnet, slip rings and brushes
D) Armature and field magnets
A) Stator and rotor
The type of connection used within a three- phase squirrel cage motor where 58% of the line voltage is impressed on the field coils for starting, and 100% for full load operation is called:
A) Across-the-line
B) Compensator start
C) Wye- delta
D) None of these
C) Wye- delta
pg 9
When large induction motors are started across the line under loaded conditions:
A) both the motor ampres and the induced rotor currents are very high
B) the heating effects of these currents often make it necessary to limit the number of motor starts with respect to time
C) if sufficient time is not allowed for motor to cool to a safe starting temperature before reclosing the breaker, the high starting currents and the resulting temperature rise will damage the insulation, weaken parts and eventually cause motor failure
D) All of the above statements are true
D) All of the above statements are true
pg 6
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
pg 4
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
pg 3
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
pg 5
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
pg 13
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 running winding
B) Create a counter EMF capable of increasing the torque to a point where the motor will start
C) Develope 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 simultameously
A) Produce a revolving field by shifting the phase relationship between the starting winding and running winding
pg 14 & 15
A motor is rated at 10 HP. It is drawing 10,000 watts from the line. It’s efficiency is therefore:
A) 75%
B) 80%
C) 85%
D) 90%
A) 75%
KW input PS = HP x 0.746/efficiency
10 = 10HP x 0.746 / E
10E = 7.46
E = 0.746 or 75%
pg Review pg 5
