AC Machines (SCIM) Flashcards
List the 3 main types of 3-Phase AC motors
- Squirrel Cage Induction Motor
- Synchronous motor
- Wound rotor induction motor
What are the main constructional parts of a 3-Phase Induction Motor?
- Stator
- Rotor
- End Bells
- Bearings
How are the two ccts of an induction motor similar to those of a transformer?
Stator = Primary Winding Rotor = Secondary Winding
What is meant by the term Synchronous Speed?
The speed at which the ‘rotating magnetic field’ produced by the stator winding (poles) moves.
What 3 basic factors allow the development of a rotating magnetic field in a 3-phase induction motor?
- stator phase windings placed 120° apart
- phase currents being 120° out of phase
- AC in each phase winding
what factors determine the synchronous speed of an induction motor?
- # of stator poles
- Applied Frequency
60xF/Pairs of poles
What is meant by the term “slip” ?
Rpm difference between rotor speed & synchronous speed of the stator
Why cant the rotor of an induction motor turn at synchronous speed?
There will be no cutting action (relative motion) of stator flux on rotor bars to induce a rotor current necessary for torque
How would you reverse the the direction of rotation for a 3-phase induction motor?
Interchange any two line leads to the stator (phase rotation)
What is the purpose of “skewing” rotor bars in induction motors?
To provide for a more equal distribution of rotor flux allowing for smoother torque development & quieter operation.
Why is the starting current of an induction motor so much higher than its rated running current?
100% slip = maximum cutting rate of stator flux on rotor bars (no Cemf). Large secondary currents results in large primary current
List the 3 main factors that determine the torque developed by an induction motor.
- Stator flux
- Rotor current (flux)
- Rotor PF
T = K x Stator Flux x Rotor Current x Rotor PF
What affect does slip have on rotor reactance?
- Rotor frequency is directly proportional to % slip
- Rotor Reactance is directly proportional to frequency
what is meant by the term “speed regulation” ?
Difference between no-load speed & full-load speed
At starting (rotor at standstill), how much larger is the rotor reactance of a 3-phase induction motor than it is when operating at 5% slip?
20 times
What constitutes the “power losses” for induction motors?
- Copper losses (I²R)
- Iron losses
- Rotational losses (friction & windage)
Why would increasing the rotor circuit resistance of an induction motor increase its starting torque?
Rotor PF is improved
For a given value of slip, the torque output of an induction motor varies as the square of the applied voltage. Explain.
Change in applied voltage to the stator results in the change of stator flux. This also changes rotor current. The combination results in the torque being proportional to the square of the applied voltage.
What is Starting Torque?
Rotational force developed at the instant of start.
What is Full-Load Torque?
Rotational force required to keep rated load turning
What is Breakdown (pullout) Torque?
Amount of overload torque which will stall the motor.
What are the objections to building a SCIM with high enough rotor resistance to provide excellent starting torque?
- Poor speed regulation
- Higher I²R losses
- Poorer efficiency
Give two advantages that a SCIM might have over other types of induction motors?
- Relatively constant speed between no-load & full-load
- Initial purchase cost
- Ruggedness
- low maintenance
- high dependability
What are the constructional differences between a double SCIM and a standard SCIM?
Double has 2 sets of rotor bars embedded within rotor iron
How is the double SCIM operated compared to a standard SCIM?
Double has higher starting torque but slightly poorer speed regulation
What is the advantage of a double SCIM compared to other induction motors?
Relatively good starting torque characteristics combined with good speed regulation.
On what type of driven load would you specify a high resistance type SCIM?
Higher torque loads
compressors, conveyors
Will a 3-phase motor start if one of the supply lines is open?
Usually not - depending on winding configuration, inertia load & HP rating
If a 3-phase motor is running at rated load and one line opens:
- Will it remain running?
- What is this called?
- How may you recognize it?
- At reduced speed, reduced torque & higher current
- Single Phasing
- Slower speed, excessive current, humming noise
If the frequency of the supply increases, what happens to the speed of magnetic rotation?
It increases.
What happens to the speed of rotation if more poles are added?
it is reduced.
Rotor frequency is dependant on ____?
Slip.
Rotor F = Supply F x %slip
Why is rotor frequency significant?
As rotor frequency changes, the rotor reactance changes
What is the value of voltage and current in a SCIM at standstill?
Will be at maximum.
High rotor current cancels _____ in a SCIM
Counter EMF
What is constant in a SCIM?
Resistance
How is torque produced in a SCIM?
Produced by the interaction of stator flux & rotor flux
Rotor flux is proportional to _____, therefore:
Rotor flux is proportional to rotor current, therefore torque is a function of rotor current & frequency
What happens as torque accelerates the rotor?
Torque accelerates the rotor quickly, as this is happening the rotating magnetic field will cut less of the rotor bars & less current will flow in the rotor & stator.
When rotor speed finds a balance of produced torque & load, what happens?
Rotor frequency will decreases.
As rotor frequency goes down, rotor reactance goes down.
Why does a lower rotor impedance not yield more rotor current?
Because the induced voltage has also decreased with less slip.
As load increases in a SCIM, what happens?
Rotor speed decreases and slip increases. Rotor bars are cut by more flux & rotor current cancels more stator CEMF resulting in more input power to the motor.
What percentage slip is full-load torque developed at for most SCIMS?
2-5 %
What percent is breakdown torque compared to full-load torque?
200-300%
When is torque maximized?
Maximized when
Rotor Resistance = Rotor Reactance
what does rotor design affect?
- Inrush current
- Starting torque
- Speed regulation
- Full-load torque
What is the difference between small & large diameter rotor bars?
Small = more resistance
What is the relationship between the rotor bars near the outside of laminated core compared to those deep in the laminations?
Outside rotor bars = low inductive reactance compared to those deep in the laminations
Low resistance rotors starting and running characteristics?
- Poor starting torque
- Better running torque
High resistance rotors starting and running characteristics?
- Good starting torque
- Low running torque
