Power Electronics Interview Questions Part-1

Why thyristors are not preferred now a days for chopper fed DC drives? 

Since it requires commutation circuit, thyristors are not preferred now a days for chopper fed DC drives. The commutation circuit is complex for chopper circuits.

What is function of a free wheeling diode in a phase controlled rectifier?

1. Input power factor is improved
2. Make the load current continuous

What causes poor input power factor in phase controlled DC drives?

The input power factor in the phase controlled rectifier is low when the output voltage is less than the maximum, that is when the firing angle is large. When the firing angle increases, the converter draws more lagging reactive power. So the input power is low.

What are the advantages of three-phase drives over single phase drives?

1. Reducing the armature ripple current
2. The filtering requirement is less
3. Armature current is mostly continuous
4. Motor performance is better

What are the various control strategies for varying duty cycle of the chopper

1. Time Ratio Control (TRC)
2. Current Limit Control (CLC)

What are the different methods of speed control of induction motors?

1. Stator voltage control
2. Supply frequency control
3. Rotor resistance control
4. Slip power recovery control

What are the advantages in operation of choppers at high frequency?

The operation at a high frequency improves motor performance by reducing current ripple and eliminating discontinuous conduction.

What is step angle in stepper motor?

Step angle is defined as the angle through which the stepper motor shaft rotates for each command pulse. It is denoted beta.

What is stepper motor?

A stepper motor is a digital actuator whose input is in the form of programmed energization of the stator windings and whose output is in the form of discrete angular rotation.

What is slewing in stepper motor?

A stepper motor operates at high speeds is called as slewing.

Define resolution in stepper motor?

It is defined as the number of steps needed to complete one revolution of the rotor shaft.
Resolution = No of steps/ Revolution

State some applications of stepper motor?

• Floppy disk drives
• Camera shutter operation
• Machine tool application
• Robotics
• Dot matrix and line printers
• Quartz watches

What are the advantages and disadvantages of the stepper motor?

Advantages

1. It is mechanically simple.
2. It requires less maintenance.
3. It can be driven in open loop without feedback
4. As it responds directly to digital control signals, stepper motors are natural choice for digital computer controls.

Disadvantages:

1. Low efficiency with ordinary controller.
2. Limited power output and sizes available
3. Low efficiency with ordinary controller.
4. Fixed step angle

What are the different types of stepping motors?

• Variable reluctance stepper motor
• Permanent magnet stepper motor
• Hybrid stepper motor

What are the different modes of excitation in a stepper motor?

• 1-Phase on or full-step operation
• 2-phase on mode
• Half step operation ( alternate 1-phase on and 2-phase on mode)
• Micro stepping operation

What is meant by half-step operation?

It is the alternate one-phase on and 2-phase on mode operation.
Here, the rotor rotate an each angle is half of the full step angle.

What is meant by full-step operation?

• It is one-phase on mode operation.
• It means, at that time only one winding is energized.
• By energizing one stator winding, the rotor rotates some angle.
• It is the full-step operation.

What is meant by micro stepping in stepper motor?

• Micro stepping means, the step angle of the VR stepper motor is very small.
• It is also called mini-stepping.
• It can be achieved by two phases simultaneously as in 2-phase on mode but with the two currents deliberately made unequal.

What is the main application of the micro stepping VR stepper motor?

• The micro stepping is mainly used where very fine resolution is required.
• The applications are printing and photo type setting.
• A VR stepper motor with micro stepping provides very smooth low-speed operation and high-resolution.

What are the advantages and disadvantages of VR stepper motor?

Advantages:

1. Low rotor inertia
2. Less weight
3. Freewheeling capability
4. High stepping rate capability
5. High torque to inertia ratio

Disadvantages

1. Normally available in 3.6deg to 30 step angles
2. No detente torque available with windings de-energized.

What is a multi-stack VR stepper motor?

Micro stepping of VR stepper motor can be achieved by using multistack VR stepper motor.
It has 3 separate magnetically isolated sections or stacks. Here the stator and rotor teeths are equal.

What are the advantages and disadvantages of permanent magnet stepper motor?

Advantages

1. Requires low power
2. High detente torque as compared to VR motor.
3. Rotor do not require external exciting current.
4. It produces more torque per ampere stator current.