PWM Controller for giant antique 20+ HP DC motors?

Discussion in 'The Projects Forum' started by DMahalko, Oct 5, 2008.

  1. DMahalko

    Thread Starter AAC Fanatic!

    Oct 5, 2008
    Has anyone ever heard of a PWM controller for museum operation of huge 3 to 6 foot diameter DC motors and rotary converters?

    It would be nice to put some of these old antique motors back into service in museums, rather than having them just sit around collecting dust.



    This is probably an impossible request but I'd like to know if anyone has developed a DC motor controller capable of safely operating historic DC commutated motors and rotary converters from the early 1870 to 1930 period.

    Usually these giant DC motors were started by hand using a huge resistance box starter that put lots of resistance in series with the motor, and then slowly the resistance is reduced by sliding a contactor handle from one terminal to the next over a period of about 10 seconds as the rotor increases to running speed. If supply power is cut off, the starting control handle immediately snaps to the Off position so the motor cannot start in the fully powered state.


    This is because when a DC motor is not spinning it has almost no resistance to current flow and applying full power to a huge DC motor before it is at full RPM can burn up and destroy the motor windings. As the rotor speed increases the rotor produces back-EMF which fights against the input power. This then allows the starting resistance to be reduced until the DC motor has reached running speed.

    These back-EMF resistance effects are generally unnoticed for modern small 5v and 12v DC motors but can be seriously destructive for huge old motors that ran on 250 volt, 25+ amp DC service.

    Even though the hardware tends to be 100 years old I would expect they would work just as well with PWM in place of the manual starting resistance box, plus it seems that a PWM controller could provide for safety measures like detecting sudden unexpected brush sparking, field-coil shorting detection and overspeed detection (using a shaft speed sensor) that could protect an old motor just spinning unattended and on display in a museum.

    The controller design would not necessarily have to be rated for 50+ amp DC output since a museum piece would just spin unloaded, with the only load coming from the rotor bearings and the commutator brush gear. Though, I do not know specifically what sort of amperage would be needed to gently spin up a large antique motor like this.

    - Dale Mahalko

    My Wikipedia user page:
  2. Wendy


    Mar 24, 2008
    Are you wanting something off the shelf, or are you willing to make it yourself?

    You seem like you have a technical hardware background, could you wire something like this safely if you set your mind to it?
  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    That is a giant motor, it would definitely be neat to see running.

    I had no luck trying to find something ready-made for you. I found a 144V 500A driver off of ebay for 1300$, which is too low voltage and rather expensive.

    This FET could for you, since you can mount it easily on a large heatsink and it can handle 70A @ 500V. I like to go a bit higher on the voltage specs, because the inductive load will kick back some EMF. If you are to use something like this, you need about 10A gate drivers to turn them on and off fast enough.

    If you can find a commercial product, you can probably either get a donation or some funding from the museum you intend on donating the motor to. I am far too scared to mess with power like that.. I like to keep my definition of a catastropic failure as a small puff of smoke :p

  4. DMahalko

    Thread Starter AAC Fanatic!

    Oct 5, 2008
    I am not an electrical engineer by trade, I'm just interested in this as a hobby. So I don't have direct knowledge of the equations or PLC programming to build a power controller from scratch. I'm hoping to find an off-the-shelf solution someone else has already developed for this purpose, though most likely this has never been done before.

    There is no particular driving voltage for this hardware. Voltages of 125v, 250v and 500v DC were common, with power systems providing hundreds of amps at those voltages.

    1917 Hawkins Electrical Guide, on Google Books
    Chart of common power system voltages and amperages
    Volume 3, Chapter 37: Wires and Wire Calculations, Page 760,M1

    Such DC power distribution system are obviously nearly impossible to find now, and a 500v DC PWM controller with 150A output to actually drive an old 90 HP DC motor to power an actual load would likely be ridiculously expensive.

    I think I'd settle for something with up to 2,500W output @ 125/250/500 volts DC, which would be about 3 HP, and should be enough to just turn over a huge unloaded motor. Though we're likely talking about a project costing a few thousand bucks now.


    I'm probably going to need to find a PLC hobbyist up for a challenge, though this is a chicken-egg project since I do not yet have a motor like this of my own to work on, and I am hoping to get a historical museum that already has these motors interested in this project.

    - Dale Mahalko
  5. SgtWookie


    Jul 17, 2007
    Wonderful tools of the Industrial Age :)

    While the idea of getting a few of those ancient motors running is appealing to me, putting motors that've been stored for so many years would require a lot of restoration work, even if they were in reasonably good running condition when retired. Just sitting there, they are slowly corroding away.

    Notice all of the corrosion on the commutator; that would have to be machined down to a smooth finish. Removing material from a museum piece is an idea that would make a docent cringe. The windings would need to be hi-pot tested for safety; even that process could cause damage. The brushes use braided or other flexible stranded copper conductors; those would likely be highly corroded and unfit for service. Obtaining identical replacements in good condition would likely be quite a chore.

    It would be far less risky to leave the motor itself non-powered, back the brushes away from the commutator, and rotate the rotor very slowly using a hidden modern fractional HP motor linked via a belt drive. The motor's bearings would likely have to be lubricated at least daily, or large lubrication cups fitted and wired to automatically shut off the display drive motor if empty as a fail-safe.

    A visitor would be hard-pressed to tell if the motor was actually self-powered, or being powered. The modern motor would be far more cost-effective, safer and efficient than trying to build a custom high-power controller for the ancient motor.

    If a fire were caused by someone making modifications to an ancient motor that was supposed to be on static display, I believe the insurance company would have a good case to balk at payment for the losses.

    Is it worth the risk?
  6. haytham

    New Member

    Oct 6, 2008
    thank u for this information
  7. Bailey45


    Oct 27, 2008
    The Keep It Simple Solution here says why not try driving the motor with a 12-24 volt DC source. You ancient starter / controller would have cause a very small starting voltage for the motor. Therefore 12-24 volts may be enough to turn this antique for demonstration purposes.
  8. SgtWookie


    Jul 17, 2007
    Far too risky.

    What you're suggesting will probably result in a meltdown. It's plain to see that the insulation is severely degraded (practically ghost white), the braided brush interconnecting wires are severely corroded, and the commutator has a healthy layer of oxide.

    Attempting to apply power to a motor in this condition would be certain to burn something out in very short order. That would be a shame.

    I'd much rather leave it as-is for a youngster to be able to gaze at it in wonder 20, 30, or 100 years from now - than to destroy such a treasure today.