Operation of Three-Phase Induction Motors Flashcards
Formula:
n_sync =
120 f / N
–> 120 frequency / Number Poles
Formula
%slip =
n_sync - n_rotor / n_sync
At no-load condition, the rotor speed of a three-phase induction motor is the same as the synchronous speed because there is no load, so no torque is required.
True or False?
False; The rotor speed always lags the magnetic field speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque.
The speed difference between the actual rotor speed and synchronous speed is called ___________.
Slip Speed
The slip speed of a rotor (increases, decreases, stays the same) as load is added tot the shaft of a three-phase induction motor.
increases
A motor with low percent speed regulation is more efficient than one with high percent speed regulation.
True or False?
True
Rotating magnetic fields have a constant magnitude and rotate at a constant speed determined by the ________ of _______ and the __________ of the power supply.
number of poles;
frequency
The rotor resistance of SCIM is treated as a ________.
constant
The equation to calculate rotor frequency is
Fr = s x Fbr
The equation to calculate rotor voltage is
Er = s x Ebr
The equation to calculate rotor reactance is
Xr = s x Xbr
Once a motor is at running speed, if load counter torque becomes greater than __________ _______ the rotor stalls.
Breakdown torque
State three quantities that are directly proportional to slip
Rotor Voltage
Rotor Frequency
Rotor Reactance
As a SCIM accelerates from a standstill to normal running speed, active rotor current:
a) decreases continuously
b) increases continuously
c) increases and then decreases
d) decreases and then increases
c) increases and then decreases
In North America, a motor running at 90% of syncronous speed has a rotor frequency of:
a) 0 Hz
b) 6 Hz
c) 54 Hz
d) 60 Hz
b) 6 Hz
Maximum torque and maximum active current are produced when:
Rotor reactance equals rotor resistance
Rotor power factor angle is 45 degrees
Rotor power factor is 0.707
NEMA design A or B rotors produce their maximum torque at about ___% to ___% of synchronous speed.
75% to 80%
Mechanical power is the product of what two (2) mechanical quantities?
Torque
Speed (rotational frequency)
The NEMA rotor design that produces the highest starting torque is:
a) design A
b) design B
c) design C
d) design D
d) design D
The NEMA rotor design that produces the highest starting current is:
a) design A
b) design B
c) design C
d) design D
a) design A
The NEMA rotor design that produces the highest breakdown torque is:
a) design A
b) design B
c) design C
d) design D
a) design A
The NEMA rotor design that produces the lowest full load slip is:
a) design A
b) design B
c) design C
d) design D
a) design A
The NEMA rotor design that is the most common is:
a) design A
b) design B
c) design C
d) design D
b) design B
What are four disadvantages of wound-rotor motors?
Higher Cost of the wound rotor
Larger frame needed to accommodate brushes and slip rings
Required auxiliary equipment (the secondary resistors and their controller)
Extra maintenance to monitor and replace the brushes
Speed control of a wound-rotor motor is suitable from 0 r/min to half of syncronous speed.
True or False?
False; Speed control of a wound-rotor motor is most suitable from 0 r/min to full-load speed.
The rotor that performs best in terms of producing high starting torque, low starting current and full-load efficiency is the:
a) NEMA design A
b) NEMA design B
c) NEMA design C
d) wound-rotor
d) wound-rotor
The main disadvantage of a wound-rotor motor is its:
a) higher starting current
b) excessive starting torque
c) inadequate starting torque
d) greater cost cost compared to SCIM and higher maintenance
d) greater cost cost compared to SCIM and higher maintenance
The starting torque of a wound-rotor motor can be adjusted by the machine operator.
True or False?
True
A wound-rotor motor should be started with the resistors disconnected from the rotor circuit (bypassed).
True or False?
False; the resistance is switched into the rotor circuit for starting
Efficiency is the ratio of ______ energy (power) to ______ energy (power).
output energy to input energy
The losses in an induction motor are _________ and __________.
electrical and mechanical
Electrical losses have two sources:
Iron Losses
Copper Losses
Mechanical losses (which are ultimately converted to heat) are caused by:
Friction
Windage
Speed regulation is most related to the percentage of:
a) slip the motor has at full speed
b) rotational frequency change from no-load to full-load speeds
c) motor efficiency at full-load
d) motor efficiency as compared to torque output
b) rotational frequency change from no-load to full-load speeds
Rotor resistance (Rr) is a constant in the rotor of a SCIM.
True or False?
True
As the rotor of a SCIM accelerates, the rotor resistance decreases.
True or False.
False
Once the rotor of a SCIM starts to accelerate, the rotor reactance increases.
True or False?
False
At full load, the power factor of the rotor in a SCIM is normally (lower than, higher than, about the same as) its power factor at no-load.
lower than
As load is added to a SCIM, current will (increase, decrease, stay the same)
Increase
Increasing the number of poles in an induction motor will (increase / decrease) the synchronous speed.
decrease
120 x f / Number of poles
The torque developed in a WRIM is directly proportional to ______ active current.
rotor
The rotor active current of a SCIM can be found by
Multiplying rotor current by rotor power factor.
Design A rotors produce a starting current of about _____ % of full-load current
600%
Design A rotors produce a starting torque of about _____ % of full-load torque
150%
Design A rotors produce a breakdown (maximum) torque of about _____ % of full-load torque
200%
Design ___ and ___ rotors are suitable for most loads, but not high inertia loads or other hard to start loads
Design A and B
Design B rotors produce a starting current of about _____ % of full-load current
500%
Design B rotors produce a starting torque of about _____ % of full-load torque
150%
Design B rotors produce a breakdown (maximum) torque of about _____ % of full-load torque
200%
Design C rotors produce a starting torque of about _____ % of full-load torque
225%
Design ___ and ___ rotors produce a breakdown (maximum) torque at the instant of startup (maximum torque is equal to starting torque)
Design C and D
Design C rotors produce a starting current of about _____ % of full-load current
500%
Design D rotors produce a starting current of about _____ % of full-load current
400%
Design D rotors produce a starting torque of about _____ % of full-load torque
275%
Design ___ rotors are most suitable for high inertia loads and for frequently started, hard to start loads that overheat design C rotors.
Design D
