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Do PMDC motors and SWDC motors require different motor controllers? Answered

I'm looking to run two Etek-R PMDC motors from one 700 amp controller. Will a GE series 700 amp controller work? Upon calling the company that offered it today I received-- through proxy-- the response that the compnay carried no permanent magnet controllers. Is there a difference between pm and sw controllers? I was under the impression that both required the same type of control: varying voltages.

could the company have confused permanent magnet with pulse width modulation?

any explanation is greatly appreciated!



Best Answer 7 years ago

GE has too many people, I know of a salvage section,
that cut up a Brand New electro-locomotive that was left in their yard
by another screw-up !

PMDC motors are controlled only by armature power alone.
Shunt Wound motors have an armature but the main speed control is
  achieved by varying of the Shunt Field
  ( Far Less Current then an armature ).
So I would expect a difference of ten in controlled power.
The SWDC motor still has a fixed or stepped armature to power.
How big are your PM machines ?


+1. We oldtimers DID electric machine theory....

I appreciate the wisdom of the oldtimers! Unfortunately my highschool doesn't really offer any resources in the realm of what we're talking about here so I'm relying on you.

iceng, We are working with two Etek R motors. So the machines are not locomotives, but will still draw 660 amps peak. Will we be able to effectively control power with a beefy, yet average golf-cart controller?

I don't understand what you mean by a difference of ten in controlled power.

If your golf cart is 24V that makes it a 20 HP motor.
My reference to GE was to point out you should not to trust
  the robot advice you received.

Regrettably, I don;t know what a beefy golf cart controller can do.
You are sure to be controlling the sum of the motor currents 660 x 2 amps.

Unless like a some vehicles I know the motors are wired in series.
Giving the cart the electric equivalent of a differential gear allowing for inherent automatic drive wheel speed differences in turns.
Then only 660 amps.

To control a 600 amp PMDC motor speed
the controller has to be able to vary the full 600 amps.

To control a 600 amp Shunt Wound DC motor speed
the controller has to be able to only vary the
Shunt Field Windings less then 60 amps.
The remaining current is fed straight to the armature.

So in one case, you need a controller for 600 amps
In the other case, you only need a controller for 60 amps.
That's a 10 to 1 power current level
makes for lower cost control in one case.


very very interesting stuff.

In fact we're not making a golf cart at all but rather a solar powered boat. We're running at 36 volts so supposedly we're going to have something like 30 horsepower.

I've decided upon the Alltrax AXE-4865 650 amp programmable controller. I just saw a package deal where a smaller 300 amp version of the same controller was utilized with my Etek R. The description says it works with permanent magnet motors so I should be covered.

thanks you very much for your answers!

If you only need single quadrant drive (ie you aren't trying to regenerate the brakes, this Instructable is EXCELLENT. 


If you already have a controller, post some details of it here. It strikes me that GE support doesn't know what its talking about......


The Alltrax AXE4855 is a 500 Amp programmable controller which works for brushed permanent magnet motors (what i've got). I've copied and pasted the product features from the Alltrax website below:

"Type: DC "SERIES WOUND" motor controller
Undervoltage cutback: adjustable, 16-30 VDC
Overvoltage shutdown: adjustable,

30-60 VDC (48V models) (60VDC MAX)
60-90 VDC (72V models) (90VDC MAX)

Operating Frequency: 18kHz
Control voltage range for Key Switch (KSI), Throttle and Reverse inputs:

24-48 VDC Nom, 60 VDC Max

Reverse Horn Output: 50mA sink max
Standby Current (Powered Up): < 35mA
Throttle Input:

ITS (inductive)
Resistive 0-5K ohm (+/-10%) (2-wire and 3-Wire)
Resistive 5K-0 ohm (+/-10%)

Operating Temperature: -25C to 75C, 95C shutdown
Adjustments via Controller Pro software:

Throttle acceleration / deceleration rate and map profile
Armature current limit
Brake current limit
Under / Over voltage shutdown
Half Speed Reverse
High Pedal Disable
Plug Brake"

This unit has ample protection for itself, sounds like a fine motor drive.
The one thing you and I want to see, is how many amperes it can control.

There has to be a place where the field and armature get attached
( Big Screw Downs )
Is there any text printed there ??


It wasn't GE support but rather an online golf cart website. When I called them they called an expert at a warehouse that stocks the controllers. I think something must have gotten lost in translation because their website has many permanent magnet controllers.

I think i'm going to go with the Alltrax AXE4855 but that's not for certain yet. I'm also looking at some Kelley products.

Right after I posted that PWM comment I came across about a million PWM controllers for larger applications. Boy did I feel silly. In my 17 years of like I have never seen them used for anything bigger than remote control cars! Anyway, I've definitely got my eyes out for a PWM that I can use in my set up now.

Thanks for your help Steveastrouk and Frollard!

Really for a pm motor you want to drive it with full voltage, but variable pulse width modulation. That way, it generates full torque (better efficiency) an average portion of the time. Running a dc motor at lower than spec voltage causes a loss in power dysporportionate to how much power is applied in that case.

Anyhoo - as to your question I dont know if that particular controller will work. What IS a swdc? All I can think of are pmdc, brushed, and brushless AC (in various phases)

PWM doesn't get any more power out of the motor. Area under the curve and all athat.


A SWDC is a shunt wound direct current motor. From some of my research online I've discovered that they are really only used for large power applications (above 1 horsepower).

If I'm reading you correctly, you more commonly work with small hobby type motors in which PWM is (I think?) more common?

Anyway, somehow we have a brushed PMDC motor on our hands that draws 330 amps peak. We're going to run it at 36 volts. We basically need a way to control power so that we don't blow through turns and kill ourselves.

No, PWM is applied across the entire range of motors !
I was commenting to Frollard that PWM isn't a magic solution to motor control that extracts more energy out of the drive than any other approach.