Two mechanically coupled DC motors can run at one shaft speed while developing unequal torque. In this installation, separate SIMOREG armature converters control a BHEL motor and a replacement Kirloskar motor, while one stand-alone SIMOREG DC Master supplies the two series-connected fields. Because the measured open-circuit characteristics (OCCs) differ, matching armature-current commands alone does not establish equal torque.
Correct the field and speed interpretation
Series-connected fields carry the same field current. Different voltage drops across the windings do not establish different field currents, nor do they directly quantify flux. Use each motor's OCC at the same test speed to compare the induced EMF produced by that shared field current. The rigid coupling prevents different steady shaft speeds; it does not force two independently selected speeds to coexist.
At a common speed, different induced EMF indicates different flux. That difference changes the armature voltage required at a given current and can cause unequal torque even when the slave receives the master's current reference.
Relate OCC mismatch to torque sharing
For each motor, DC torque is proportional to flux multiplied by armature current. Therefore, equal armature currents produce equal torque only when the two flux values are equal. At the common OCC test speed, induced EMF can serve as a relative flux indicator.
This relationship supports a current-reference correction derived from the two OCC curves. It does not supply a numeric factor because the evidence contains no OCC points at a common speed and field current.
Optimize the master/slave configuration
The documented arrangement has the master armature converter operating in speed mode and the slave operating in current mode with its speed controller bypassed. Master connector K141 feeds slave parameter P601. Both converters currently use P169 = 1 and P170 = 0; P500 and P503 remain at unspecified default values.
- Record both OCC values at the same test speed over the field-current range used by the mill, including the configured 120 A point.
- Calculate the required slave current-reference ratio from the measured flux or induced-EMF ratio. Apply that ratio to the K141-to-P601 reference path only if the installed SIMOREG configuration provides an appropriate scaling function.
- Keep each armature converter parameterized for its connected motor's nameplate data. Do not change P169 and P170 merely to obtain current balance; the evidence does not define their functions or show that P169 = 0 and P170 = 1 corrects flux mismatch.
- Test at progressively loaded operating points. Compare both armature currents, armature voltages, shaft speed, and available coupling-load indication. Confirm stable sharing during acceleration, steady rolling, deceleration, and any field-weakening operation.
Recognize the hardware limitation
A common series field supply provides no independent field-current adjustment for the two motors. Drive tuning may reduce current or torque imbalance by scaling the slave armature-current reference, but it cannot make the motors' OCCs identical. A single fixed scale factor is adequate only if the flux ratio remains acceptably constant across the operating field-current range.
If the OCC ratio changes materially with field current, especially during field weakening, the correction must vary with the operating point or the fields must become independently controllable. Whether the existing SIMOREG system supports that compensation cannot be established from the supplied evidence; verify the installed drive documentation and the referenced Master/Slave Switchover application before changing the configuration or interlocks.
FAQ
Will equal armature current make tandem DC motor torque equal?
Only if both motors have equal flux. With different OCCs, use T ∝ ΦI and scale the slave current reference by the inverse flux ratio measured at the same speed and field current.
Should SIMOREG P169 and P170 be reversed to fix current imbalance?
Not from the available evidence. The installation currently uses P169 = 1 and P170 = 0, but no supplied parameter definition shows that changing them to P169 = 0 and P170 = 1 compensates for mismatched OCCs.
Can series-connected motor fields be balanced independently?
No. Both fields carry the same current, so the common field controller cannot independently correct their flux; use armature-current-reference compensation or provide independent field control if operating-point compensation is required.