DC Motor Control Question

[johnnyw]

We have a DC motor (Nameplate: GE Kinamatic DC Motor, 20 Hp, 1750 rpm, 240 volts, Shunt Wound, Field volts 150, type CD366AT, TENV) and an old controller of unknown origin with no documentation found as of yet - likely a one-off custom. The load is a much larger motor (~100 times DC motor rated Hp) with no other connections that will be coupled via a reduction gear/belt. We need to drive the load motor at rated speed (about 5 times less than the DC motor rated speed) in order to get BEMF data and perform other tests. It is anticipated that the load motor will need relatively high starting torque to get it spinning due to bearing friction and high inertia.

In looking at the DC motor terminal box, I assume it is separately excited since there are both field and armature winding leads and the nameplate gives different voltages for them. What is the maximum breakaway (starting) torque this type of motor can provide? Assuming we cannot operate the motor from the controller, what is the best way to control motor speed below base speed? We have a 150 vdc, 7 amp power supply that could be used for the field winding and a 3 phase fully controlled SCR based rectifier that could be used for the armature winding. If we apply rated field voltage and vary the armature voltage, will this provide good speed control below base speed and high starting torque? Could the field voltage then be used as a fine speed adjustment? Any suggestions would be appreciated. Thanks.

 

[bigamp]

The best way to speed control a separately excited DC motor is by varying the armature voltage. You can "trim" the speed by field control if you need to. Reducing the field current increases the speed but reduces the torque the motor can develop. You may not need to field control if you can get good enough control by armature voltage.

Sorry but I can't say if your motor will be able to develop sufficient torque for your application. For really high torque applications (traction drives) they used to use series excited motors. I believe they also had compound motors where part of the field current was series excited and another part was shunt or separately excited.

Have a go with your motor connected as you suggest.

 

[RadarRay]

First, here is some data you didn't mention. Your 20HP motor has a rated armature current of about 70 amps. The torque output at rated current and rated field, is 60 Ft-lbs. I would gear the system in so the 20HP motor is going at rated speed when the load motor is going at the desired speed.
Now I am a little confused about your 3 Phase controlled rectifier. Does it have a current loop or does it only control output voltage. If it has a current loop, with a little bit of effort you can create a speed controlled DC drive out of it. Then you can use your 7 amp supply for the separate shunt field and use thedrive to control speed. You then have the option of using a tach on the back of the 20HP motor or controlling speed with armature voltage feedback. You don't specify how accurate you have to set speed or maintain it during your tests. This would determine the type of feedback to use.
BTW, when properly protected your 20HP motor should be capable of 150% Current for 1 minute.
One last thing which is very important. Since you state the armature voltage is 240, most likely the motor is 40 or 50 years old. Make sure it was designed to operate off of a Rectified source. You could have a commutation problem if it was designed to operate off of a MG set.