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A Cogging DC Motor
Copyright © 2001 Francis J. Martino


A 300 HP DC motor was cogging while driving a take-up reel in the paper industry. Cogging
is defined to be a continuous oscillation of motor speed. In the absence of a control problem
that would cause poor speed regulation, cogging will be caused by excessive heat build-up
within the motor which results in excessive internal losses within the motor windings and
a subsequent regulation instability.


Data

* Motor nameplate data was not available.

* The nominal rating of a 300 HP DC motor is 478 Amps @ 500 armature VDC.

* Nominal overcurrent allowance for a 300 HP motor is 478 FLA @ 150% = 717 A allowed
for one minute.

* Nominal rating of a DC motor without a blower requires derating below 60% base speed.

* Measured running amps under full load varied between 350 to 425 amps.

* Each start or jog would cause the motor to draw a measured 720 amps for one to one
and a half minutes. The duty cycle for the application included a total of eight starts and
jogs per hour. Thus the total time of inrush current of twelve minutes per hour yielded a
20% duty cycle for inrush current.

* 480 VAC power input would sag to 465 VAC on start or jog and coil voltage would sag
to 115 VAC.

* The controller was designed for an output of six pulses per cycle.

* The armature contactor was rated at 535 amps, but had excessive pitting on the
contacts and a short lifetime of nine months.


Recommendations

1) The first problem to be addressed was the contactor. The operation of the contactor
was proper in that it would close prior to the SCRs phasing up, and the SCRs would be
phased back prior to the opening of the contactor. Thus, the contactor would neither
make nor break a current flow during normal operation.

The contactor was rated properly for a nominal 300 HP motor full load amps and for
the inrush of 720 amps. Although there was no manufactuer's rating available on the
duty cycle of inrush current, the 20% duty cycle may have been a contributing factor
to the low contactor life-time.

The wiring of the polarity of the two normally open poles of the output contactor
was not in accordance with the manufacturer's recommendation for connection of
the armature leads. Polarity is important because the incorrect polarity would cause
the two internal magnetic fields of the blow-outs to be interferring with current flow.

The purpose of the magnetic blow-outs is to suppress arcing across the contacts
upon opening if the contactor opened during power conduction. However, an incorrect
polarity could cause the magnetic field of the normal running current to interact with
the magnetic blow-out fields and cause the current to flow through the contacts in an
uneven manner, thus causing heating and pitting on portions of the contacts.

The short life-time of the contacts was therefore attributed to the uneven flow of
current in the contacts due to improper polarity and the excessive duty cycle of high
current flow during jog and run.

Replacement with an eight-hundred amp contactor was recommended. A larger size
contactor would reduce contact pitting due to inrush and, therefore, reduce current
surges into the motor armature that would be caused by the pitting. However, it could
not be assured that any associated reduction in motor heating by the reduction of the
current surges would be significant enough to keep the motor from cogging.

2) A significant portion of motor heating could be attributed to a lack of proper
cooling due to low speed operation during starts, jogs and normal running speeds.
To cool a DC motor that is running for long lengths of time under 60% base speed,
a blower must be used to cool the motor. If the motor has no blower and has an open
enclosure, a blower or separate ventillation must be added to move air into the
motor enclosure.

If the motor is totally enclosed and fan cooled, the motor fan will not cool the
motor at low speeds. Remove the fan and fan cover and provide continuous ventilation
directly over the surface of the motor.

3) A significant portion of motor heating could be attributed to the high duty cycle
of the starting current occurring eight times per hour. In the absence of test
equipment, it was reasonable to assume that the 720 amps as previously measured
by the user indicated that the controller was driven into current limiting with
each start and jog.

In an effort to lower current on start and jog, set the acceleration to the
maximum acceleration time possible. On a large reel of material, a logarithmic
acceleration which provides an "S" curve on start-up will be preferrable to a linear
acceleration. The slower acceleration on start would serve to reduce the effects of
the break-away inertia of the load, thus lowering the starting current.

4) If a motor was designed for use with a motor-generator set, it will be be able
to accept only a DC current that has a low or negligible ripple. When a motor that
is rated for use with a motor-generator set is powered from a six pulse per
cycle SCR (Silicon Controlled Rectifier) controller, it will overheat due to the high
ripple content of the current that is produced by the SCRs.

That problem is normally dealt with by using a SCR controller that has a twelve
pulse per cycle output rather than the common six pulse per cycle controller.
The twelve pulses will create a higher average level of current that will be closer
to the peak level and, thus, heating from the ripple will be greatly reduced.

Another method which is more preferred is to add a DC choke in the DC bus. The
choke will reduce the ripple on the bus and smooth the current being delivered to
the drive.

5) The addition of a field economy feature will reduce the field voltage during
times when the motor is not operating.

6) Use an oscilloscope to determine if the DC armature voltage ripple is
symmetrical. Any significant asymmetry will cause heating. It will also indicate
a malfunction in the bus capacitor filter, firing circuits, or output SCR devices.
Also check the field voltage to determine if all the field diodes are operational.

7) When in armature control, the DC field must be energized with maximum rated
field current.

8) Check for incoming AC power line imbalance.

9) Check the brushes for wear and proper alignment. Check the commutator segments
for wear and for carbon build- up between the segments.

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