SESSION:28

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1)       What is meant by energy ratio?

Energy ratio = Wm/[Wm + R] = 0.455

Wm = mechanical energy transformed

This energy ratio cannot be called as efficiency. As the stored energy R is not wasted as a loss but it is fed back to the source through feedback diodes.

2)       What are the major difference between SRM and stepper motor?

SRM	Stepper motor
SRM is designed for continuous rotation	Stepper motor is designed to rotate in step by step rotation
SRM requires a rotor position sensor	It does not require rotor position sensor.

3)       What is hysteresis current control?

This type of current controller maintains a more or less constant current throughout the conduction period in each phase. This controller is called hysteresis type controller.

4)       When do you go for chopping mode of control in switched reluctance motor?

The chopping mode of control in SRM is applied only low-speed. Here each phase winding gets excited for a period which is sufficiently long.

5)       What is the working principle of Switching Reluctance Motor?

The switched reluctance motor basic principle is based on Faraday’s law of electromagnetic interaction.

·         Whenever a current carrying conductor is placed in a magnetic field, it experiences a force. The direction of force given by Fleming’s left hand rule.

·         SRM is a single excited and double salient pole electric motor. Due to the variation of reluctance the motor should be operated.

·         The SRM develops a torque due to variable reluctance.

·         When air gap is minimum, the reluctance will be minimum; hence inductance will be maximum. So the rate of change of inductance is zero.

·         When the reluctance various in the motor, there will be a change in inductance so when a particular stator winding of SRM is energized,the rotor pole comes in alignment with that stator pole and thus the rotor rotates.

6)       Why rotor position sensor is essential for the operation of Switched Reluctance Motor?

It is normally necessary to use a rotor position sensor for commutation and speed feedback. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

7)       List the disadvantages of a Switched Reluctance Motor?

·         Stator phase winding should be capable of carrying magnetizing current.

·         For high-speed operation developed torque has undesirable ripples is a result develops undesirable noises or acoustic noises.

·         For high-speed current waveform has undesirable harmonics to suppress this effect large size capacitor is to be connected.

·         It requires position sensors.

8)       What are the advantages of switched reluctance motor?

·         Construction is simple and robust.

·         Rotor carries no windings, no slip rings, no brushes, less maintenance.

·         There are no permanent magnets.

·         Ventilating system is simpler as losses takes place mostly in the stator.

·         Power semiconductor switching circuitry is simpler.

·         No shoot through fault likely to happen power short circuits.

·         Developed torque does not depends upon the polarity of current in the phase winding.

·         The operation of the machine can be easily change from motoring mode to generating mode by varying the region of conduction.

·         It is possible to get very high-speed.

·         Depending upon the requirement T-w characteristics can be achieved.

·         It is the self-starting machine.

·         Energy stored in the phase winding is fed back to the supply through the feedback diodes during off period.

9)       What are the applications of SRM?

·         Washing machines

·         Vacuum cleaners

·         Fans

·         Future auto mobile applications

·         Robotics control applications

10)    What is the difference between Synchronous Reluctance Motor and PM (Permanent Magnet) Synchronous Motor?

SRM	PMSM
Rotor has no permanent magnet	Rotor has permanent magnet
Less cost	High cost
Low efficiency	High efficiency

11)     Distinguish between self-control and vector control of PMSM.

Self control	Vector control
Dynamic performance is poor	Dynamic performance is better
Control circuit is simple	Control circuit is complex

12)    What is meant by slotless motor?

In slotless motor, the stator teeth are removed and resulting space is partially filled with addition of copper.

13)    What are the advantages and disadvantages of PMSM?

Advantages

·         It runs at constant speed.

·         No field winding, no field loss, better efficiency.

·         No sliding contacts, so it requires less maintenance.

Disadvantages

·         Power factor of operation cannot be controlled as field winding cannot be controlled.

·         It leads to losses and decreases efficiency

14)    Write the EMF equation of PMSM.

E_ph = 4.44 f. Φm. Kp. Kb. Nph volts

This is the RMS value of induced EMF per phase
where
f – frequency in Hz
N_ph – Turns per phase
Φ_m – Flux per pole
K_w – Winding factor

15)    What are the assumptions made in derivation of EMF equation for PMSM?

·         Flux density distribution in the air gap is sinusoidal.

·         Rotor rotates with a uniform angular velocity of ωm(r/sec).

·         Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns.