MadSci Network: Engineering

Re: What is the difference between a synchronous and asynchronous generator?

Date: Thu Jul 8 18:03:25 1999
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Engineering
ID: 927822769.Eg

Let me set up a thought experiment with some definitions.

A "machine" has a shaft for rotational energy input/output and electric 
terminals for electric energy input/output.  If it takes electric energy, it 
is "motoring;"  If it gives electric energy, it is "generating."

A synchronous generator rotates an electromagnet within coils generating three-
phase alternating current.  All by itself, it is not "synchronized" with 

So let's have two steam-turbine synchronous generators put in parallel, with 
both turning at 3600 revolutions per minute (rpm).  Now, the generators must 
both turn at the same speed.  If one loses steam completely, it will motor at 
3600 rpm pulled along by the other generator working harder.  But let's put the 
steam back, and have the two turning as generators.  The point is,  either 
machine can only generate or motor at 3600 rpm.

Picture a third synchronous generator with a diesel engine attached as a 
potential prime mover.  I must start the engine and have it turning at 3600 rpm 
before I parallel this third machine.  If I feed enough fuel, it will join the 
other steam-powered generators and share the load at 3600 rpm.  This is the 
synchronization aspect.  This "doubly-excited" machine (DC on the rotor; AC on 
the stator) can only participate in average energy conversion at 3600 rpm.  It 
must be synchronized.

Can we parallel a fourth machine that does not have to be synchronized?  Yes.  
I attach a two-pole squirrel-cage induction motor to another prime mover -- 
this time, a windmill propeller.

I parallel the four machines, and the induction motor begins spinning the 
propeller at a speed less than synchronous speed -- say, 3200 rpm.  The 
induction motor is acting as an asynchronous motor.  Suddenly, the wind picks 
up big time, and the propeller, instead of taking energy,  begins pumping 
mechanical energy into the motor.  Now the speed is 4000 rpm, and the induction 
motor is now acting as an asynchronous generator.  Any speed above 3600 rpm 
will result in generation.

Being singly excited (AC on the stator) allows the induction machine to 
generate or motor at any speed.  Above 3600, it generates.  Below, it motors.  
There's your asynchronous generator.  But it must work with other machines, at 
least one of them one that is doubly excited.

This is not an easy concept to describe in words.  I'm sure good animations 
from those with the talent could do it better.

Larry Skarin 

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