# Question for Motor experts



## Qmavam (Aug 17, 2008)

I'm looking for info on how a shunt field type motor works and some advice
If you know of any URLs about shunt field type motors, please post them.
I have a shunt type motor, the shunt is 40 ohms.
I have a series type motor controller (Alltrax NPX4834).
I know your supposed to use a different controller for shunt
type motors, but I have what I have.

I have three thoughts about how to connect it.

Just connect the shunt to the battery voltage and leave it. 

Connect the shunt to the battery voltage with a reostat 
so the current can be adjusted. (Where to have the current
at what time would be the question)

Connect the shunt to the motor plus and let the controller
PWM the parallel shunt/armature connection.

This is on a gokart and I'm using the contoller at about 1/3
of max current.

Can you help?
Thanks, Qmavam


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## major (Apr 4, 2008)

Qmavam said:


> I have a shunt type motor, the shunt is 40 ohms.
> I have a series type motor controller (Alltrax NPX4834).
> Can you help?
> Thanks, Qmavam


Hi Qmavam,

A PWM controller designed for use with a series motor may not do so well on the armature only. There is quite a bit less inductance. However, some PWM controller will work on PM motors, which have no series inductance except for the armature. Those PM motors typically have a higher inductance than the armature in a series motor. With your controller, it's a crap shoot. Give it a try.

Hooking the shunt field to the armature and running the "shunt motor" from the PWM would be trouble. When starting (from zero RPM) the voltage to the motor is low so the voltage across the shunt field is low and field amps will be low. This condition results in a very weak field meaning high armature current, low torque and lots of brush arcing.

So the best bet is to power the field separately from the correct voltage supply. If it is a 36 volt motor, likely the shunt field is designed and rated for 36 volts. If your system is higher voltage, you can over excite the shunt field for short periods, like 10 to 20 seconds for stronger accelerations. But too high a voltage on the shunt field risks overheat in the coils.

You should apply full shunt field for accelerations and heavy loads. You can weaken the field (use a lower voltage and current in the field) for some degree of speed control when cruising. This should be done only when the PWM control is full on.

Hope that helps,

major


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## Qmavam (Aug 17, 2008)

Hi major,
I'm wondering about your statement below;

"When starting (from zero RPM) the voltage to the motor is low so the voltage across the shunt field is low and field amps will be low. This condition results in a very weak field meaning high armature current, low torque and lots of brush arcing."

As you know with PWM the voltage on the motor is full battery voltage, but pulsed on and off. So, I would have, full voltage on the shunt during each pulse, now, it is an inductor, and I don't know the rise time to get full current.

I'm getting quite confused at this point, I thought I could make this work until I got a different motor. Now I'm not so sure.
Qmavam


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## major (Apr 4, 2008)

Qmavam said:


> Hi major,
> As you know with PWM the voltage on the motor is full battery voltage, but pulsed on and off. So, I would have, full voltage on the shunt during each pulse, now, it is an inductor, and I don't know the rise time to get full current.
> Qmavam


Hi Q,

Yeah, both the armature and field see the same PMW. Each has a different inductance, so the ripple is bound to be different. But, say at start, you have a 10% duty cycle PWM. On a 36 volt battery, that would be 3.6 volts average to each, armature and field. They would have different peak currents, but would effectively have the current as if you applied a 3.6 volt source to them. O.K. for the armature, it is what you want to limit current. But gives you way too low a field current.

Just keep the field powered at rated voltage and you can get it to work.

Regards,

major


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## Qmavam (Aug 17, 2008)

major said:


> Hi Q,
> 
> Yeah, both the armature and field see the same PMW. Each has a different inductance, so the ripple is bound to be different. But, say at start, you have a 10% duty cycle PWM. On a 36 volt battery, that would be 3.6 volts average to each, armature and field. They would have different peak currents, but would effectively have the current as if you applied a 3.6 volt source to them. O.K. for the armature, it is what you want to limit current. But gives you way too low a field current.
> 
> ...


 I'd have to see the waveforms on a scope. I'd suggest that during the pulse you would have a high voltage on the shunt. When it's off you have zero. I guess the question is, how high does the current get during the 10% pulse length.
We should actually be talking current. During the current pulse the shunt current would rise from zero to maximum (.7 amps for this motor) in some unknown amount of time.

I'll look at powering the shunt continuously.
Thanks, Qmavam


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## major (Apr 4, 2008)

Qmavam said:


> During the current pulse the shunt current would rise from zero to maximum (.7 amps for this motor) in some unknown amount of time.
> Thanks, Qmavam


Hey Q,

Yeah, current rises during the "on" pulse, but then decays during the "off" period. So the average current is equal to the average voltage divided by the resistance. If the duration of the "on" pulse was long enough to allow it to reach its maximum, then it would decay back to zero in the same amount of time during the off pulse. For a 10 % duty cycle, this would leave over 80% at zero current. This would be a discontinuous case. I doubt if the frequency and inductance would support this case. But by all means, look at it on a scope. You can use a power supply, or the motor controller, by just connecting the motor field. Put a true RMS meter, or analog meter on it and you'll see Ohm's law still applies.

Regards,

major


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