# Quick Answer: Synchronous Motor Questions For Interview?

## What are the main characteristics of synchronous motor?

Characteristics of Synchronous Motor are constant speed motors. The speed of the motor is decided by the number of poles and frequency. Compared to an induction motor, it is very sensitive to sudden changes of load. This causes a hunting of the rotor and finally leads to stability problems.

## What are the difficulties in starting a synchronous motor?

Slight torsional vibration causing problems in machines attached to a synchronous motor, trouble starting, cracked amortisseur windings, broken couplings and the inability to maintain synchronous speed should signify potential problems in a rotor circuit.

## What techniques are available to start a synchronous motor?

Damper Windings is the most widely used methods to start a synchronous motor. A Damper Winding consists of heavy copper bars inserted in the slots of the pole faces of the rotor as shown in the figure below. These copper bars are short-circuited by end rings at both ends of the rotor.

## What is the power factor of a synchronous motor?

The usual synchronous motor power factors are unity (1.0) or 0.8 leading. Values of 0.7 or 0.6 leading will give more leading correction to an otherwise lagging system.

## What is synchronous speed?

Synchronous speed is a significant parameter for the rotating magnetic field-type AC motor. It is determined by the frequency and the number of magnetic poles. Synchronous speed No = [rps, revolutions per second] f = Frequency [Hz] p = Number of magnetic poles.

## What is synchronous speed and it’s formula?

This rotating magnetic field moves with a speed called synchronous speed. The Synchronous speed can be calculated as follows: 120 times the frequency (F), divided by the number of poles (P): The synchronous speed decreases as the number of poles increases.

## Which type of starter is used for synchronous motor?

Controllers for synchronous motors have four components: a three-pole starter for the ac stator circuit, a contactor for the dc field circuit, an automatic synchronizing device to control the dc field contactor, and a cage-winding protective relay to open the ac circuit if the motor operates too long without

## How synchronous motor works as a power factor correction?

The power factor of a synchronous motor is changed with a change in the excitation. When the excitation of the motor is increased, the power factor changes from lagging to unity and then to a leading power factor.

## Why the synchronous motor is not self started?

Above a certain size, synchronous motors are not self – starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

## Which method is used for magnetic locking?

2. Which method is/are used to perform magnetic locking? Explanation: A sub motor is needed to make the synchronous machine run till the speed of synchronous speed and to achieve that we need additional motor.

## What are the applications of synchronous motor?

Synchronous motors are normally used in applications in which a constant and precise speed is required. Typical applications of these low power motors are positioning machines. They are also used in robot actuators. Synchronous motors are also used in ball mills, watches, record players, and turntables.

## How do you control the speed of a synchronous motor?

Changing the number of poles is not easy, so we do not use that method. However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied. We can control the speed of the synchronous motor by changing the frequency of the supply to the motor.

## What is true synchronous mode?

In true synchronous mode, the stator supply frequency is controlled from an independent oscillator. This allows rotor speed to track the changes in synchronous speed. When the desired synchronous speed (or frequency) is reached, the rotor pulls into step, after hunting oscillations.

## Why DC is used for excitation?

In the alternator, as the rotor conductor changes its position as the rotor rotating so constant flux is to be produced in the rotor winding and it is possible only when the DC supply is given to the rotor winding. As we need constant flux that is why DC is used instead of AC for excitation of Synchronous Alternator.