Synchronous Motor Starter 1

[LAParrotman]

I have a Westinghouse 400Hp synchronous motor attached to a XLE air compressor. Currently I’m using a No. 6 motor starter with a DC power supply for the field excitation and the proper field discharge resistors and relays. The DC field voltage relay kicks in 8 sec after start up to allow the motor to get up to speed. Is this an acceptable way to start this motor or do I need a synchronous motor controller like the GE SPM which senses the rotor speed and the rotor angle before applying the DC field voltage? Is applying the DC field voltage at the proper rotor angle really necessary or can I get by without it?

Thanks very much,
Richard

 

[oldfieldguy]

You missed a part of the equation: Is your motor brushless or brush-type? If it is brushless, the shaft-mounted excitation circuitry will turn on the current to synch at the proper time. You provide excitation voltage.

Since you refer to field discharge resistors then you may well have a brush-type motor. In this case you need some manner of determining the proper time to apply field current to the rotor. This is done on older motors through some clever electromechanical devices, but I have a particular affinity for the GE SPM.

Applying DC at the wrong time can cause some serious mechanical and electrical stresses.

old field guy

 

[LAParrotman]

The motor is a brush type motor. As far as the proper time to turn on the field current, I know that the speed is important but I'm wondering about the rotor angle. Is the proper rotor angle that important? I've asked a couple of people who build motor starters and I'm getting conflicting answers from them as some say yes and others say it's not important at all.

Thanks,
Richard

 

[waross]

The proper rotor angle may be more important than the time, even if he time is zero seconds.
The job of the "Polarized field frequency relay" is to apply the field at the proper time and rotor position. A patent dispute over the original polarized field frequency relay determined that the relay was a new invention and not just a reapplication of existing relays. Evidence was given that energizing the field at a random rotor position (as was done before the invention of the polarized field frequency relay) could and at times did result in severe damage to the synchronous motor being started. If you have any generator experience, a random application of the field is very similar to a random closing of the breaker paralleling two generators.
Compare the capacity of your supply system with the capacity of your motor and guess which will be the "Large" generator and which will be the "Small" generator. Which one will lose??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lansford]

In the mining industry where I worked we had Draglines that had MG sets with manually started syncronous motors. After the motor came up to speed, the operator would apply the field voltage. There did not seem to be any problem.
JIM

 

[LionelHutz]

As the others have posted, if you apply the DC at the wrong time you can cause a torque transient.

However, the level of this torque transient seems to depend on the motor load and the rate the DC is applied. The motor has a very large inductance and the DC current will not suddenly rise to rated field. Rather, it will take time to rise up to the rated current. So, if the motor has little load then the motor may slip a pole while the field current is low causing a smaller torque transient that won't damage the motor. An unloaded motor can even reluctance synchronize due to the residual magnetism in the rotor so it will be synchronized before you even apply the DC.

There is another big problem that can happen though. If you mistime the synchronizing then the motor can slip a pole. This pole slip will cause a large positive voltage to be generated on the field winding. If you have disconnected the resistor before the motor is synchronized and this happens you can damage the rectifier. The same problem can occur during a pole slip when running. The rectifier needs a decently sized protective device, such as a big MOV, if you are going to be operating with contactors.