Dear Friends,

We have 2 identical -5hp, 220Vdc Separately Excited Dc motor. Each motor is connected to individual- identical gear boxes but sharing a common load via shaft linkage for Crane Traversing application.

The 2 motor armatures are in series connection powered by dc variable speed drive, each field winding is in series connection feed with fix voltage from the same VSD. Basically the system is Base Speed only, Variable Armature Voltage at constant field current. The System is running well with this arrangement.

During our Periodic Maintenance, I’ve tried to release the load of both motors by disconnecting the coupling from gear boxes, the idea is to check the individual motor speed rotation without any load on it. To my surprise, motor rotation is not balance, one Motor Armature voltage is higher than its counter part.

Particularly, at low supply voltage of 60 Vdc drive output (low speed reference to the drive), one motor is notably rotating faster than the other one, I measure each armature voltage, Motor A takes 60 Volts while Motor B is almost Zero volt, stalled and humming! By increasing the speed reference to VSD, Motor B starts to rotate but the speed/voltage is not synchronized with respect to Motor A.

Both motor is sharing the same load via mechanical shaft linkages, if the Armature Voltage or Speed of this 2 motors are not equal, Motor A tends to overload and may fails sooner? Motor B with slow speed may become additional load of Motor A?

For 2 identical motor impedances connected in series, do I expect to get a balance Armature Voltage? Comments are very much appreciated.

I apologize if the question is appropriate.


Best Regards,

Gerry

Hello gerryroxas

When you connect two DC motors in series, and drive the same load, they will share the torque.
Once you remove the mechanical coupling, the two motors become independant.
If the mechanical losses on one motor are higher than the other, then one motor will spin faster than the other, resulting in an imbalance in the voltage across the two motors.

To check the speeds, you really need to have control over the voltage across each motor. This is not possible while the two motors are in series.
When the two motors are connected in series, and mechanically coupled, the voltage across each motor should be almost the same.

Best regards,

Hi Marke,

Thank you for prompt reply.I appreciate your concern and willingness to share your knowledge and expertise, this will help us a lot in understanding the behavior of the stuff we are working with.

On behalf of my colleagues who are also viewing this site...Cheers and more power to this valuable site!


Regards,

Gerry

Connecting two DC motors in series is a way of making a differential operation. If you apply more load to one motor, it will slow down and the other motor will speed up.
If you want them to run the same speed, put them in parallel, but you will have to change the controller.

Best regards,

Hi Marke,

I’ve got an idea for that “differential operation” you mentioned. It might be the cause of intermittent fault we are experiencing with one of our Gantry Crane Traversing System. It seems we have mechanical problem with one side of the rail which lower down the speed of one Motor. The other motor maintain or tends to increase the speed which further create unavoidable “steering effect” on the traversing system.

This will result to instability of Armature current which creates shaking motion of the Traverse mechanical system prior to drive fault due to Armature over current. Since both field current is fix, there is no way to compensate for speed synchronization. We will focus then for mechanical setback of our Traversing system.

. Thank you for your follow-up reply. It’s reMarkable how you analyze and synthesize my query. I can not compose a well understood writing, nevertheless I received a well explained reply!

Best Regards,

Gerry