what is field current in a motor?

in article snipped-for-privacy@private.com, bb at snipped-for-privacy@private.com wrote on

These are very basic questions. Your best bet is to go to the library to get a basic knowledge of fundamentals. You appear to be at a stage of understanding where a few words will not help.

Field current can mean many different things. For a d-c moter, many of which are being used, it is the current used to set up the magnetic field that provides motor torque.

A synchronous motor is one which rotates at the same rate as the line frequency. Multipole motors are often used wher motor speed is an even submultiple of the line frequency.

Bill

Basically the field current provides the magnetic field against which the rotor can 'push'.

It is a type of motor which has to run at a fixed speed which is determined by the frequency of the supply. If overloaded and serious slip takes place, such a motor will burn out.

You need a good text book to explain these things properly.

Don't know what you're talking about. These are specific question about motors which you just have happened to to learned.

Repeat> in article snipped-for-privacy@private.com, bb at snipped-for-privacy@private.com wrote on

This is by no means everything there is to know about the subject but if you are talking about field current vs. line (stator) current for a synchronous motor then here's a start ......

As others have mentioned, a synchronous motor is one which will run at exactly the synchronous speed which is the frequency of the line voltage. There is no slip here as there must be with an induction motor to operate. How is this accomplished? The synchronous motor is started just as an induction motor is, with the exception that when it gets close to synchronous speed (usually about 5% slip or 57Hz on a 60Hz motor), the control will switch in a field supply which is usually 120 or 240 VDC. The field current is also quite low as compared to the ac line current. Having the dc field winding energized will lock the rotor in to line frequency in the stator.

During the starting of a synchronous controller, high voltages can be generated in the field winding as a result of the accelerating motor. This energy must be dissipated to avoid failure of the field supply components and the field winding itself. A carefully sized "field discharge resistor" is connected across the motor field terminals to do just this. The induced field current through the discharge resistor makes for a high voltage on the resistor and the field contactor. The discharge resistor is taken out of the circuit when the motor comes up to speed and the field is applied.

There are basically two types of synchronous motors, brushless (most commonly made today) and brush type which are older. The brush type requires separate field control components while on the brushless type, most of the components are mounted right on the motor.

Synchronous motors are commonly found in applications where a constant speed is required.

peace, FG