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Electrical Objective Questions - MCQs
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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Control of Electrical Drives
Synchronous Motor DrivesStepper Motor Drive
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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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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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