Re: RTC?

Don Vollrath

You ARE overthinking. With motors wired in parallel: Wheel contact
to the rail and same gearing will force each motor to be at the same
rpm. Any slight differences in BEMF (magnet strength) may cause a
slight amount of current draw difference between motors. During the
decoder PWM Off period to measure motor BEMF there may be a minor
flow of current between motors, but the BEMF signal read by the
decoder will be the average of the two motors. That BEMF voltage
will be what the decoder will regulate. With two motors in series:
Again, the motors will operate at the same rpm (no wheel slipping).
The total BEMF read back to and regulated by the decoder will be the
sum of BEMF from motor 1 + BEMF of motor 2. In this case the motors
will be running about 1/2 the rpm as above for the same total applied
voltage. The current through each motor will be identical, but there
may be minor differences in individual BEMF (not speed, but BEMF).
The decoder should have no problem coping with either connection when
properly adjusted.

What DOES change is that the series connection generates less torque
per change in applied motor volts. This is a reduction in 'gain'
that affects motor BEMF/RTC/Speed regulator response. One needs to
increase the sensitivity of the BEMF/RTC regulator to compensate for
that effect.

--- In QSIndustries@..., "shoggoth43" <shoggoth43@...>


I'm not sure how any of the dual motored units would work with
Unless you can sync both motors together somehow, each time you drop
the power to the motor ( PWM ) there's no real way to predict how
BEMF the decoder will read. One motor might have a much larger
voltage than the other and the next time maybe both put out the same
and then the next it could be reversed. This would be the same in
parallel or serial motors. Maybe it all averages out somehow and
overthinking this?

Did this used to work fine in RTC before you rewired it in serial?


--- In QSIndustries@..., "Clark Thorp" <llyffant2000@>

Like many O scalers I usually rewire my dual motor diesels. The
rewiring converts the parallel motor wiring to serial, thus
the current load and the top speed of the loco. But....

I have noticed that my QSI Atlas GP9 locos are now behaving a bit
wobbily in RTC mode, especially at low speeds. They run OK in
I'm wondering if perhaps the series motor connection is
with the RTC mode feedback.

Can anyone shed some light on this?

Clark Thorp

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