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Questions and Answers
Which of the following is NOT an advantage of three-phase induction motors?
Which of the following is NOT an advantage of three-phase induction motors?
Squirrel cage induction motors have external resistance that can be added in the rotor circuit.
Squirrel cage induction motors have external resistance that can be added in the rotor circuit.
False
What type of construction do slip ring induction motors have?
What type of construction do slip ring induction motors have?
Three-phase winding like stator
Three-phase induction motors have a power range from fractional H.P. to ______.
Three-phase induction motors have a power range from fractional H.P. to ______.
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Match the following features of Squirrel Cage and Slip Ring induction motors:
Match the following features of Squirrel Cage and Slip Ring induction motors:
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What is the main reason why three-phase induction motors are commonly used in industries?
What is the main reason why three-phase induction motors are commonly used in industries?
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The speed of a three-phase induction motor is dependent on the frequency of the supply.
The speed of a three-phase induction motor is dependent on the frequency of the supply.
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What is a key difference in efficiency between Squirrel Cage and Slip Ring motors?
What is a key difference in efficiency between Squirrel Cage and Slip Ring motors?
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Study Notes
Three-Phase Induction Motor Advantages
- Three-phase induction motors are the most common type used industrially.
- They are simple in design, durable, inexpensive, and easy to maintain.
- Power ratings range from fractional horsepower (H.P.) to 10 megawatts (MW).
- Run at a nearly constant speed from zero to full load.
- Robust construction with no brushes or rotor shaft contacts (squirrel cage).
- High power-to-weight ratio compared to DC motors.
- Lower cost per unit of power.
- Speed is directly dependent on frequency.
Stator and Rotor Components
- Stator: The stationary part of the motor, typically with a laminated iron core and windings arranged in a specific configuration (e.g., star-connected).
-
Rotor: The rotating part of the motor. Different rotor types exist (e.g., squirrel cage or slip ring).
- Squirrel-cage rotor: Has short-circuited bars (conductors) embedded in an iron core, providing a path for induced currents.
- Slip ring rotor: Has rings on the shaft that allow external resistance to be connected to adjust starting torque and speed control.
Working Principle
- Rotating Magnetic Field (RMF): Alternating currents in the stator windings create a rotating magnetic field.
- Induced Currents: This rotating field induces currents in the rotor conductors, creating a magnetic field in the rotor.
- Electromagnetic Force: The interaction between the stator's rotating magnetic field and the induced rotor magnetic field produces a torque, causing the rotor to spin.
- Slip: The difference between the synchronous speed and rotor speed, represented as a percentage.
Squirrel Cage vs. Slip Ring Induction Motor
| Feature | Squirrel Cage | Slip Ring |
|---|---|---|
| Construction | Simple | Complicated |
| Rotor | Rotor bars are short-circuited | Rotor windings connected to slip rings |
| End/Slip Rings | Not present | Present |
| External Resistance | Not possible / Permanent short-circuited | Possible / Variable starting torque |
| Starting Torque | Lower | Higher |
| Speed Control | Difficult | Easier (Variable frequency control) |
| Efficiency | Generally higher (Normal operation) | Lower, due to external resistance |
| Maintenance | Generally maintenance-free | Requires periodic maintenance (slip rings) |
| Cost | Lower | Higher |
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