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Tech Forum

November 2017

Variable DC Motor Control

I’d like to have continuous variable control of the motor in my bench vibrator/polisher. It has a DC (not AC!) motor that is rated at 90 VDC with a power supply that delivers 10 amps. I prefer to build something myself. Anyone have some design tips, or better yet, a schematic?

Robert Browning
Boston, MA


What about using a variac with a standard full wave rectifier and large filter capacitors? The schematic lists several commercial components.

Several things to mention before any attempt at construction.

  • Be aware that most variacs are not fully isolated, meaning the neutral on the mains supply is connected to the neutral on the output.
  • Mechanical stops will need to be implemented on the control dial as not to exceed a 90 volt DC output and most likely on the low end to prevent the motor from stalling.

Steve Ghioto
Atlantoc Beach, FL

Here is the "wrong" answer to the problem: Harbor Freight sells a Router Speed Control for $20. Plug it in between the wall outlet and your tool. See if it does what you want. If not, take it back and they will give you your money back.

Chip Veres
Miami, FL