Operating Principles of Three-Phase Induction Motors Flashcards
The flux created by any winding will be at a maximum when the current through that winding is at a _____________
maximum
When the current through the winding reverses, the flux direction created by that winding _________
reverses
What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field?
Frequency increases, speed of rotating field also increases
What effected does increasing the number of poles have on the speed of the rotating magnetic field?
Decreases
Define synchronous speed
Speed at which the field rotates
Since there is no electrical connection between the stator and the rotor, how is current made to flow in the rotor of a squirrel-cage induction motor?
Electromagnetic induction
Define slip speed
Difference between synchronous speed and rotor speed
Define percent slip
Percentage of synchronous speed
Define speed regulation
Difference between no load speed and full load speed of the rotor
How is the direction of rotation reversed in a squirrel-cage induction motor?
Reversing direction of rotating magnetic field. Done by interchanging any two of the stator line connections to the source
Define full load torque
Torque necessary to produce rated horsepower at full-load speed
Define breakdown torque
Maximum torque motor can produce before slowing down and stalling
Define locked rotor torque
Torque produced at the moment power is applied to the stator
Define starting torque
Maximum torque motor will develop at rest for all angular positions of rotor with rated voltage applied at rated frequency
At what rotor speed will the stator current be the highest?
When rotor is at stand still (100% slip)
What type of loads is a NEMA design class C motor suitable for?
High inertia starts. Has locked rotor torque equal to or slightly larger than its breakdown torque
What types of loads is a NEMA design class B motor suitable for?
Low inertia starts. General purpose motor with starting torque typically lower than breakdown torque
What type of loads is a NEMA design class D motor suitable for?
Punch presses, shears, cranes, hoists, elevators and oil well pumping jacks. Very high breakdown torque
AC induction motors are normally designed to develop full-load torque at about __ to __ slip
2-5%
Breakdown torque occurs at approximately __ slip
25%
Breakdown torque has a magnitude of between ___% and ___% of the full load torque
175%, 300%
On standard squirrel cage induction motors, the starting torque has a magnitude of between ___ and ___ of full-load torque, depending on speed and power ratings of motor
70%, 275%
What part of a linear-induction motor moves?
Stator
In what units is the synchronous speed of a linear-induction motor measured?
metres per second
