I recently did this by uncoupling the motor from the load, disconnecting the shunt field, set the armature current limit at zero to begin with, for safety then after asking for about 5% of speed I increased the current limit until the motor began to turn. Noted the direction. Re-connected the shunt field and ran it again with the presumption that if it now turned the other way then the series field was hooked up the wrong way. (The motor went in the correct direction when I got onsite and their shunt field current was not too weak but they pulled excessive armature Amps.)

I would like to know if there is a method to check this -safely- without uncoupling the motor.

 

Checking Relative Polarity of DC Motor Fields

Motor speed is unstable if speed increases due to an increase in load current. As a result of instability, motor speed may hunt or overspeed. These are unsatisfactory, possibly dangerous, drive conditions. One of the possible causes of unstable performance of shunt wound DC motors is incorrect series field polarity relative to the shunt field due to improper connection. Relative polarity of the shunt and series fields can be
checked as follows:

1. Connect a low scale (3 volt) DC voltmeter across the shunt field terminals (F1, F2) with F1 connected to the positive (+) terminal of the meter. At least one of the shunt field leads must be disconnected from the controller.

2. Use two flashlight batteries as a source of low voltage (3 volts). Hold or connect the negative, to contact the S-2 series field terminal. Hold one end of a wire conductor to the positive (+) center terminal so that the
other end of the wire can be used to make and break contact with the S-I series field terminal.

3. Watch the deflection of the voltmeter needle when contact is made with S-I and when contact is broken.

4. When contact is made, the needle will first deflect in either the up scale or down scale direction and then return to zero. Deflection will be in the opposite direction when contact is broken.

5. Relative polarities of the shunt and series fields are correct (ampere-turns are cumulative) if the voltmeter needle deflects up scale when contact is made and down scale when contact is broken.

6. Relative polarities of the shunt and series fields are incorrect (ampere-turns are differential) if the voltmeter needle deflects down scale when contact is made and up scale when contact is broken. The motor connections must be changed so that relative polarity is correct. Relative polarity can be incorrect because of an error in the connections to the control or because the series fields leads are improperly marked. In any case, the motor connections must be changed so that relative polarity is
correct.

If only one series field terminal is available at the controller, use it and the available armature terminal for the test. For example, use S-2 and A-l, if S-1 and A-2 arc connected together at the motor and not brought to the
controller.

 

If you have an oscilloscope, you can use it in place of an analog meter. I have done the same test procedure using a 9v battery and my scope. This procedure also works to determine armature wires that are not labeled. Typically A1 lead will tie directly to one of the brush racks and the A2 lead will come from a special winding which is tied in series to the opposing brush rack of A1. This special winding is called an interpole.

It is very important that S2 and F2 are always tied to the negative outputs of the controller. If the motor is running the wrong direction, swap armature leads.

If the motor controller (DC drive) is regenerative then tie S1 and S2 together or tape off each lead separately and don't use them at all. A good example of a typical DC motor connection diagram is the Reliance diagram 406770-2; type that number into your favorite search engine and you should be able to find it. Good luck.