Working Principle of Hysteresis Motor
The operation of a hysteresis motor is based on the hysteresis effect.
The hysteresis effect refers to the lag between the magnetization of a ferromagnetic material (with relative permeability > 1) and the applied magnetizing force when subjected to a varying magnetic field. This lag results in energy loss and is utilized in hysteresis motors to produce torque.
Construction
1. Stator
- The stator is designed to produce a synchronous revolving magnetic field from a single-phase AC supply.
- This is typically achieved using a permanent-split capacitor (PSC) type construction.
- The main winding and starting winding are connected in such a way that they operate in the same configuration during both starting and running conditions.
2. Rotor
- The rotor is made of a smooth cylindrical core constructed from magnetically hard steel.
- It has no windings or teeth and relies entirely on the hysteresis and eddy current effects for torque production.
Operation
- When the stator is energized by a single-phase supply, it produces a synchronously revolving magnetic field due to the phase-splitting capacitor arrangement.
- Owing to the hysteresis effect, the magnetization axis of the rotor lags behind the stator's rotating magnetic field axis by a certain hysteresis lag angle. This causes the rotor and stator poles to become magnetically locked.
- If the rotor is stationary, the starting torque developed is:
- Starting torque ∝ (Stator flux × Rotor flux × sin θ)
- where θ is the angle between rotor and stator flux axes.
- As the rotor accelerates, it eventually reaches synchronous speed and continues to run at that speed. Once synchronized, the motor maintains synchronism and adjusts the torque angle as required.
Characteristics
- Self-starting and synchronizes under any load, regardless of the moment of inertia.
- Operates smoothly and quietly, making it ideal for applications requiring silent and vibration-free performance.
- Provides constant speed operation under varying loads.
Hysteresis Motor MCQs with Detailed Explanations | SSC JE Electrical
Prepare effectively for SSC JE, RRB JE, UPPCL JE, and other electrical competitive exams with these well-curated MCQs on Hysteresis Motors.
❓ 1. A hysteresis motor operates on the principle of:
A) Electromagnetic induction
B) Hysteresis loss ✅
C) Eddy current loss
D) Magnetisation of rotor
📝 Explanation:
Hysteresis motors rely on hysteresis loss in the rotor, which is made of a magnetically hard material with high retentivity. This allows the motor to operate without any DC excitation.
❓ 2. In a hysteresis motor, the hysteresis torque remains practically:
A) Variable
B) Fluctuating
C) Constant ✅
D) Zero
📝 Explanation:
The hysteresis torque remains nearly constant from standstill to synchronous speed, ensuring smooth acceleration and performance.
❓ 3. Which of the following statements about hysteresis motors is NOT correct?
A) By changing stator poles, different synchronous speeds can be achieved.
B) Hysteresis motors have uniform torque from standstill to synchronous speed.
C) The number of rotor poles is greater than the stator poles. ❌
D) The number of rotor poles is equal to the stator poles. ✅
📝 Explanation:
The number of rotor poles is equal to stator poles for proper synchronous operation. Option C is incorrect.
❓ 4. The rotor of a hysteresis motor is made of:
A) Magnetically hard cobalt steel
B) Magnetically smooth chrome steel ✅
C) Steel laminations
D) Solid steel
📝 Explanation:
Magnetically smooth chrome steel offers high retentivity and wide hysteresis loop, ideal for hysteresis motor rotors.
❓ 5. Which type of motor runs at synchronous speed without DC excitation and is not dependent on load torque?
A) Reluctance motor
B) Servo motor
C) Shaded pole induction motor
D) Hysteresis motor ✅
📝 Explanation:
Hysteresis motors operate at synchronous speed regardless of the load and without any external excitation.
❓ 6. The area of the hysteresis loop for the rotor material in a hysteresis motor should be:
A) Medium
B) Very large ✅
C) Infinite
D) Very small
📝 Explanation:
A larger hysteresis loop means higher hysteresis loss, which is essential for torque development in hysteresis motors.
❓ 7. Which motor is relatively free from mechanical and magnetic vibrations?
A) Reluctance motor
B) Shaded-pole motor
C) Universal motor
D) Hysteresis motor ✅
📝 Explanation:
Due to its smooth cylindrical rotor and lack of teeth, a hysteresis motor produces minimal vibration and noise.
❓ 8. The rotor of which motor does not have winding on it?
A) Repulsion motor
B) Hysteresis motor ✅
C) Universal motor
D) Reluctance motor
📝 Explanation:
Hysteresis motor rotors are solid cylinders made of hard magnetic material without windings.
❓ 9. In a single-phase hysteresis motor, the rotor cylinder is made of:
A) Magnetically hard cobalt steel
B) Magnetically smooth chrome steel ✅
C) Steel laminations
D) Solid steel
📝 Explanation:
The rotor is crafted from smooth chrome steel to ensure the required magnetic properties for torque generation.
❓ 10. The rotor of a hysteresis motor made of ceramic permanent magnet material causes:
A) No hysteresis losses
B) Stationary flux
C) No eddy current losses ✅
D) High eddy current losses
📝 Explanation:
Ceramic materials have high resistivity, significantly reducing eddy current losses in the rotor.
❓ 11. The direction of rotation of a hysteresis motor is determined by:
A) The resistivity of rotor material
B) The amount of hysteresis loss
C) The permeability of rotor material
D) Position of the shaded pole with respect to the main pole ✅
📝 Explanation:
The shaded pole in the stator creates a phase difference, which sets the direction of starting torque and thus rotation.
❓ 12. Hysteresis motors are best suited for:
A) High starting torque applications
B) Variable speed drives
C) Quiet and smooth operation ✅
D) Heavy-duty industrial machines
📝 Explanation:
These motors are ideal for applications like timers, clocks, and record players due to their silent and smooth operation.
❓ 13. The rotor of a hysteresis motor is typically made of:
A) Magnetically hard steel ✅
B) Magnetically soft iron
C) Aluminum
D) Copper
📝 Explanation:
Magnetically hard steel retains magnetism longer, crucial for maintaining torque in hysteresis motors.
❓ 14. In a hysteresis motor, the torque is produced due to:
A) Only hysteresis losses
B) Only eddy current losses
C) Both hysteresis and eddy current losses ✅
D) Neither hysteresis nor eddy current losses
📝 Explanation:
While hysteresis losses ensure steady torque, eddy current losses also contribute to torque generation, especially during startup.
❓ 15. Hysteresis motors are commonly used in:
A) Electric trains
B) Vacuum cleaners
C) Electric clocks ✅
D) Water pumps
📝 Explanation:
Electric clocks require quiet, constant-speed operation, making hysteresis motors an ideal fit.