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Synchronous Motor Drives

Starting Synchronous Motors

Pull in of Synchronous Motors

Braking of Synchronous Motors

As the name suggests the synchronous motorsrotates at synchronous speed. The main advantage of synchr

motorsare that they run on 3 ac supply and dc supply is given to the rotor when they run on synchronous spee

loss is very minimal. We can say that if the synchronous motorsare designed to run only at rated synchronous sp

then what is the use of introducing drives to them. The answer is pretty simple, synchronous motor drivesmak

starting, pull in and braking process smooth and without any problems. We shall discuss about them one by one.

Starting Synchronous Motors

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Electrical Drives

Dynamics of Electrical Drives

Electric Motor Power Rating

Thermal Model of Motor

Motor Duty Class

DCMotor Drives

Induction Motor Braking

Induction Motor Drives

Types of Braking

Control of Electrical Drives

Synchronous Motor DrivesStepper Motor Drive

Variable Frequency Drive

Field Oriented Control
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The problem with synchronous motors are that they are not self- starting. Before discussing the starting method o

motor, we should know about the type of supply and the rotor and the stator of the motor briey. The sta

synchronous motors are similar to that of an induction motorsbut the only dierence lies in the rotor, the rotor

synchronous motors are given dc supply.

Now before knowing, how the synchronous motors are started, we should know why they are not self started

answer can be given as, when 3 phase supply is given to the stator, there is a rotating magnetic uxwhich rota

synchronous speed, and if the rotor is also given dc supply, rotor acts as a magnetic uxwhich rotates at synchrspeed, and if the rotor is also given dc supply, rotor acts as a magnet having two salient poles. As the roto

standstill position, it cannot follow the magnetic eldwhich is rotating at synchronous speed. The rotor stacks

position because the opposite poles move so rapidly that the rotor locks, this is the reason why synchronous m

are not self starting. Now coming to the point how synchronous motors are started. At rst the synchronous m

are started as a normal induction motors, the rotor of the motor is not given dc supply, when the motor reache

rotor and pull in takes place, which is discussed later.

Another method of starting the synchronous motor drives is by external motor. In this method the rotor o

synchronous motoris rotated by an external motor and when the speed of the rotor reaches near synchronous s

the dc-eld is switched on and pull in takes place. In this method, the starting torque is very low and it is no

popular method also.

Pull in of Synchronous Motors

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Se

When the rotor of the synchronous motors reaches near synchronous speed, then the dc eld supply is switch

and the pull in process begins. As during switching on the dc supply due to the phase angle and torque angle the

various disturbances seen in the motor and there are several slip of poles of air gap ux is also seen. As the

process is completed the rotor acquires synchronous speed. The complete pull in as fast as possible the dc s

should be switched on at the most favourable angle. Like when the synchronous motor is running as induction m

the dc supply should be fed when the induction motoris at top speed, this will be the best moment because the

dierence will be

least at that point of time.

Braking of Synchronous Motors

As we know, there are three types of braking i.e, regenerative, dynamic and plugging type braking. Bu

synchronous motor drivesonly dynamic braking can be applied though plugging can be applied theoret

Regenerative braking cannot be applied to them as they need higher speed than synchronous speed. Dynamic br

is done by disconnecting the motor from supply and connecting it across a three phase resistor. At that time the

works as asynchronous generator and energy is dissipated at the resistors. Plugging is not used for synchr

motors as high plugging electric currentcan cause severe disturbance and damage in line.
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