Stab shunt wound motor

Does anyone know what stab shunt wound motor is or why it is special? We have one that is 1150 rpms DC 50 hp

A shunt-wound motor is a DC motor in which the wound electromagnetic rotor field is in parallel (not series) with the armature. The rotor is said to "shunt" the armature. A stab is shaped piece of metal that makes up a certain type of wound rotor. A shunt-wound motor has decreasing torque as speed increases. A shunt-wound motor has good speed regulation, and it is classified as a constant speed motor, even though the speed does decrease slightly as load is increased.

A shunt wound motor is a motor with the field connections parallel to the armature connections. In most cases the field is separately excited, which allows extended speed constant horsepower operation.

If the field current is held constant the motor speed at any given armature voltage will be limited by the back-EMF generated by the armature spinning in the shunt winding magnetic field. This back-EMF is proportional to the speed of the armature and the strength of the field winding magnetic field. Decrease the strength of the field winding magentic field and the armature will spin faster for a given armature voltage.

As the load on a DC motor increases the magnetic field generated by the armature increases. This tends to decrease the effect of the magnetic field field from the field winding. This has the same effect as decreasing the shunt field current and the motor will pick up speed..

A stabilizing shunt (or stab shunt) is an additional field winding that is connected in series with the armature winding. The intent of a stabilizing shunt is to increase the stator magnetic field strength as the armature current rises at the same rate that the stator magnetic field from the shunt field is decreasing due to the same armature current. The net result is that the motor speed will stay much more constant over a much larger torque range than if a stabilizing shunt winding is not used.

The only trick is that the relationship of the stab shunt to the armature connections must be correct or the stab shunt will make matters worse. Check your motor connection diagram carefully.

Keith

Trying to simplify the above a bit, a stab or stabilized shunt wound DC motor is a motor with two fields. One is a separate low current winding that is designed to be excited by a separate power supply in the drive. Unless the motor is to be operated over its nameplate base speed, this power supply will be a constant source of current so the field is held at constant magnetic intensity. The leads in the junction box are labeled F1 and F2.

The second field is wound over the top of the field above and is in series with the armature. It is a high current circuit and consists of only a few large conductor turns. It's purpose, as described in posts above, is to improve the torque-speed characteristic of the motor so, as the motor is loaded heavier and heavier, the torque either holds constant or increases slightly. The junction box leads are marked S1 and S2.

Since the series field is in the armature circuit, if you reverse the motor, the series field is now subtracting from the shunt field instead of adding to it so the motor develops much less torque. Bottom line----Stab Shunt and another variant, Comp Shunt, motors are not symetrical by direction and so, are used mainly for unidirectional applications.

It is a common error to have the S1 and S2 leads wired backward. That's the first thing I check before starting up a DC system.

Dick

So if it has a shunt field, a series field then would it not be a compound motor?

So then can I say the "stab shunt" field is only one of several compound wound motor designs?

Dan Bentler

leit, yes. Compound fields are done to make the torque characteristic rise more than it would under heavy load conditions and a plain shunt field. If you add only a little series field to flatten or slightly increase the torque curve, its a stab shunt motor

If you add somewhat more series field so the torque increases significantly under heavy loads, that would be a comp (compensated) shunt motor.

Both a compound wound and are not symetrical as to rotor direction given a fixed wiring scheme.

I have seen examples of compound motors having their armature connections AND their series field connections reverse simultaneously to acheive higher torques in both directions but this is rare and expensive. Probably cheaper to buy a straight shunt motor a bit oversized for the application and reverse the simple way---reverse the A1 and A2 leads or use a reversing drive.