SPECIAL ELECTRICAL MACHINES

Part-A

1. Define Reluctance motor.

A reluctance motor that utilizes an ac rotating field, which allows for the possibility of extremely smooth torque and good operation to low speeds.

2. What are the types of reluctance motor?

1. Salient rotor2. Radially laminated rotor3. Axially laminated rotor

3. Write the features of SRM?

i. Synchronous reluctance machine rotor can be constructed entirely from high strength, low cost materials.

ii. Synchronous reluctance motors are more reliable.iii. There need not be any excitation field thus eliminating electromagnetic spinning

losses. 4. What is the principle of reluctance motor?

Working principle of reluctance motor:

The stator consists of two windings namely main winding and auxiliary winding. The auxiliary winding consists of capacitor in series with it. Thus there exists a phase difference between the currents carried by the two windings and corresponding fluxes. Such two fluxes to produce the rotating magnetic field. The technique is called split phase technique of production of rotating magnetic field. The speed of this field is synchronous speed which is decided by the number of poles for which stator winding is wound.

5. Draw the speed-torque characteristic of SRM?

6. Define vernier motor.

Vernier motor is a permanent magnet motor and has a high torque at low speed. It operates on the principles of a Vernier.

7. What are the applications of SRM?

1. Fiber-spinning mills2. Industrial process equipment3. Metering pumps4. Wrapping and folding machines

8. Differentiate synchronous reluctance motor and switched reluctance motor?

S.no Synchronous reluctance motor Switched reluctance motor

1.

2.

Synchronous reluctance motor does not requires a rotor position sensor.

It requires less maintenance.

Switched reluctance motor requires a rotor position sensor..

It requires more maintenance.

9. What are the applications of vernier motor?

The Vernier motor is mainly used where require low speed and high torque.

10. Write the features of vernier motor

The peculiar feature of this kind of motor is that a small displacement of the rotor produces a large displacement of the axes of maximum and minimum permeance.

UNIT-II- STEPPING MOTORPart-A1. Define step angle

2. What are the different types of stepper motors?

1. Variable reluctance stepper motor2. Permanent magnet stepper motor3. Hybrid stepper motor

3. Define variable reluctance motor

It is the most basic type of stepper motor. It works on the principle of variable reluctance. The motor has a stator which is usually wound for three phases. The stator has six salient poles with concentrated exciting windings around each one of them.

4. Write few applications of stepper motor

1. Floppy disk drives2. Quartz watches3. Camera shutter operation4, Dot matrix and line printers5. Machine tool applications6. Robotics

5. What is meant by single and multi stack configuration?

In single stack we can make a single step only at a time. But in the case of multi stack configuration we can obtain smaller step sizes, typically in the range of 2º to 15º.

6. Define hybrid motor.

A hybrid stepper motor combines the features of both PM and VR stepping motors.

7. How the stepper motors differ from other motors?

Advantages1. It can driven open loop without feedback.2. Responds directly to digital control signals, so stepper motors are natural choice for digital computer controls.3. It is mechanically simple.4, It requires little or no maintenance.Disadvantages1. Low efficiency with ordinary controller.2. Fixed step angle.3. Limited ability to handle large inertia load4. Limited power output and sizes available.

8. Write the advantages of hybrid motors

I. Less tendency to resonate2. Provide detent torque with windings de-energized3. Higher holding torque capability4. High stepping rate capability

9. What is meant by drive circuit?

The output from the logic sequence generator signals are low level signals which are too weak to energize stepper motor windings. To increase the voltage, current and power levels of the logic sequence output by using power semiconductor switching circuit. This circuit is called power drive circuit.

10. List the disadvantages of variable reluctance motor

1. Normally available in 3.6° to 30 step angles.2. No detente torque available with windings de – energized.3. Exhibit mid-range response at some stepping rate under some drive condition.4. Low efficiency at low voltages and stepping rate.

UNIT III- SWITCHED RELUCTANCE MOTORSPart-A

1. What is meant by rotating electrical machine?

A machine which is working in the basic principle of electromagnetic induction to produce an angular displacement as motor (Electrical energy to mechanical energy) or to generate electrical energy as generator (Mechanical energy to electrical energy) is known as rotating electrical machine.

2. Write the principle and operation of SRM?Switched reluctance motor (SRM) is an electromagnetic and electro dynamic equipment which converts the electrical energy into mechanical energy. The electromagnetic torque is produced on variable reluctance principle.. SRM makes use of

Power semiconductor switching circuitry and Rotor position sensor.

SRM is singly excited and doubly salient electrical motor. This means that it has salient poles on both the rotor and stator but only one member carries windings. The rotor has no windings, magnets and cage winding but it is build from a stack of salient pole laminations.

3. Define torque prediction

4. What are the type of power controllers used in SRM?

a) Two power semiconductor devices and two diodes per phase.b) (n+1) power switching devices and (n+1) diodes.c) Phase winding with bifilar wires.d) Split link circuit used with even phase number.e) C-dump circuit.

5. In which type of controller used to convert D.C to three phase A.C?

In Microprocessor or computer based controller used to convert DC to Three phase AC.

6. What are the functional blocks used in microprocessor control?

DC supply unit Power semiconductor switching circuit. Microprocessor control unit. RPS unit and reference signal.

7. Write the advantages of microprocessor based control

More flexibility Ease of operation Repeatability of parameters for many drive applications High reliability Low cost

8. Compare microprocessor control and computer control

9. What are the functional blocks used in computer control?

DC supply unitPower semiconductor switching circuit.Computer interfacing and control unit.RPS unit and reference signal.

10. What is non linear analysis?