Motors Test 1
% Slip Formula
% Slip = (No load speed - Full load speed)/ Full load speed
A variable AC motor drive is able to operate the motor
-Below base speed -Above base speed -At base speed
Shunt field coil windings of DC motors
-High resistance -Constructed with many turns of small diameter -Are connected across the supply voltage source
The direction of rotation of a shunt wound DC motor is changed by
-Reversing the current flow through the armature -Reversing the current flow through the field
The direction of rotation on a split-phase motor can be reversed by interchanging the connections to
-The run winding -The start winding
The synchronous speed of a 2-pole, 60 hertz AC motor is
3600 RPM
A________ is an example of constant HP load
A lathe
On a 3-phase motor, the stator windings are arranged to create
A rotating magnetic field
The two basic categories of motors are
AC and DC
The voltage induced into the armature of a DC motor is know as the
CEMF
The _____ motor has the highest starting torque of all single phase motors
Capacitor start/capacitor run
The ________ motor has the highest starting torque of all the single phase motors
Capacitor start/capacitor run
When a load is applied to a 3 phase squirrel cage induction motor the rotor speed will
Decrease slightly
An electric motor uses what to operate
Electric current and magnetism
A current-carrying conductor , placed in and at right angles to a magnetic field, tends to move parallel to the field
False
Direct current motors can normally operate directly from the power supplied by the electric utility system
False
Direct current permanent magnet servo motors are used in cranes to start heavy loads
False
Single phase operation of a three phase motor will reduce the current in the two remaining phases
False
The locked-rotor starting current of an induction motor is several times less than the full load current
False
Unlike three phase induction motors, single phase types are self starting
False
The speed of as squirrel cage motor can be varied by varying the supply
Frequency
A wound-rotor induction motor is normally started with
Full external rotor resistance
The shunt type DC motor is known to have
Good speed reevaluation
The series type DC motor has
High starting torque
Armature Current Formula
Ia = (motor terminal voltage - CEMF)/(armature circuit resistance)
A polyphase synchronous motor is frequently used to
Improve power factor
As the load of an induction increases the power factor will
Increase
Which of the following NEMA insulation classes had the highest safe operating temperature?
NEMA H
The direction of rotation of 3 phase induction motor is determined by
Phase sequence
DC compound motors are used to operate loads that require
Relative constant speed under varying loads
The synchronous speed of an induction motor refers to the speed of the
Rotating magnetic field
Synchronous Speed Formula
S = 120f/ # of poles
The universal motor is constructed similar to that of a ______ type DC motor
Series
A DC series motor
Should never be operated without a load
A squirrel cage rotor consists of
Single bars short circuited by end rings
Which is not a requirement for motor action
Source of mechanical energy
If the field current of a shunt-type DC motor is held constant and the armature voltage is increased, as a result
The speed of the motor will decrease
Torque can best defined as
The turning force produced by a motor
One advantage DC type motors have over AC types is that
Their speed can be easily varied
The purpose of a commutator on a DC motor is
To maintain the proper direction of current flow in the armature conductors
A DC series motor runs fast with a light load
True
A good capacitor will hold a charge even when power is removed
True
A split phase motor relies on the start winding to create a phase shift
True
A three phase synchronous motor can produce a leading power factor
True
Compared to s squirrel cage motor of equivalent HP, a wound rotor motor has a higher starting torque and a lower starting current
True
Overheated bearings may cause motor noise
True
Single phasing refers to the operation of a 3 phase motor on two phases is lost
True
The commutator in a DC motor reverses armature coil current flow to produce continuous armature rotation
True
The direction of rotation for a 3-phase AC motor can be changed by exchanging L1 and L2 of the voltage supply
True
The speed of a 3-phase synchronous motor is the same as that of the rotating magnetic field
True
The speed of a DC shunt motor is inversely proportional to the field current
True
A series type DC motor uses field coils
With low resistance made up of a few turns of large diameter wire
A DC shunt motor should never be operated
With the field circuit open
