Can anyone explain why it's possible for an induction motor to self start but not a synchronous machine? If they both have a wound rotor, and they both have a rotating magnetic field, why can only self start but not the other?

Thanks for any help!

 

See the section "Starting Methods" in the link ...

http://en.wikipedia.org/wiki/Synchronous_motor

 

Although in principle and practice you are correct. It's possible to start a synchronous machine with the rotor winding [field circuit] initially closed through a resistance - which limits the current & voltage on the field winding.

The motor will normally increase speed to somewhere below synchronous speed at which point the field excitation can be introduced to bring it up to synchronous speed.

I guess it boils down to issues such as torque pulsations and current levels during start-up.

Induction style rotor bars or amortisseur windings are often built into synchronous motors to assist starting and (in the latter) dampen torque pulsations.

 

A synchronous motor without a starting winding has no starting torque. The rotor of a synchronous motor has fixed magnetic poles which exactly follow the rotating field of the stator at normal operating speed, thus are "synchronous" with the field. But at stall the field rotates too fast to have a net starting torque and so the rotor will just slightly oscillate due to the plus and minus torque fluctuations at the mains frequency.

An induction motor has no fixed rotor magnetic poles. They are formed by the current induced into the rotor by transformer action from the rotating field of the stator. The rotor current forms induced poles that rotate at the stator rotation rate so the attraction between the rotor field and the stator field produces a net torque on the rotor. Since the magnitude of the induced rotor current (and thus the rotor magnetic field) is proportion to the slip frequency between the rotor and the rotating field, the rotor torque is highest at stall, giving a high starting torque.

 

Also, with the advent & increasing use of variable frequency solid state power converters/inverters one can indeed start a synchronous motor from standstill.

 

This was very helpful, thanks! The part about the rotor on a synchronous machine being fixed helps.

Why though isn't a current induced in the rotor of the synch machine also like it is in an induction machine? I assume it's because the existing B field is there but I thought that field was small by design so as to limit waste...

 

Why though isn't a current induced in the rotor of the synch machine also like it is in an induction machine? I assume it's because the existing B field is there but I thought that field was small by design so as to limit waste...

A wound rotor induction machine has a spatially distributed rotor winding similar in construction to that on the stator winding. At standstill the rotating stator field induces currents in the rotor winding which produce a rotating field in the rotor of the same rotational frequency as the stator. This leads to the presence of starting torque.

The synchronous motor winding isn't wound the same way and it can't produce a rotating field [or starting torque] in the same manner as the wound rotor induction motor. Hence the need to build into the rotor additional windings if standstill starting is required in the particular synchronous motor application.

 

in the induction motor, your rotor circuit is basically a short, providing for high currents required for starting torque. The synchronous circuit is high impedancde for field control and simple can't develop the torque required.

 

Oooookay, got it, thanks!