6 replies to this topic

[gerryroxas]

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

[marke]

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,

[gerryroxas]

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

[marke]

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,

[gerryroxas]

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

[themotorman]

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!

The idea of connecting motors mechanically and arranging the drive to make it work efficiently and reliably is complex. The crane system really needs two controllers one for each motor with a controller between them and the operator. This is needed because it seems that they are connected together but not to be used as single larger motor . If this were true then you could simply connect them in series and adjust the field to make sure that they have the same RPM/Volt . If they are identical motors you could probably use the same voltage across each field winding. It might be better to connect the field windings in series since the field is a function of current and this would make sure that the fields had the same current and hence the same magnetic field ( field = number of turns x current ) .
The idea of connecting the armature winding in series is about the same as has already been poste din this discussion. However a series connection only works if the motors are identical in characteristics or can be made to be so by adjustment of field winding current ...hence my first statement is true it is a complex subject..

themotorman..

Best Regards,

Gerry

[Bysikmode]

Thank you informed me.