120V DC Motor speed controller

Atlas Knife Company

Atlas Knife Company

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Feb 16, 2010
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I've got a 1HP 120VDC motor for my grinder. Will a standard PWM dimmer slow it, or do I need something else? It has a diode bridge in a box on the side that converts AC to DC, so I can work with either the AC or DC if need be.
 
Photos please let's see the data plate.

Are there any brushes



120VDC is unusual, at least to me, I lee lots of 90 and 180
 
If you have a full wave rectifier and your controller is rated at 15 amps and your motor is wound right it should work although you will probably get about 90 volts out of the recifier at 120 volts AC. The schematics for a speedmaster SCR is much more complex than a simple full wave recifier so you may have variations in the speed of the motor and trouble maintaining torque at a given speed. This is what I think I understand from the explanation given to me by an electrical tech.
 
If I read Zaph right, he has a regular 1HP 120VAC motor and a pulse width dimmer control switch.

You can not run a standard motor from a PWM dimmer switch. It will burn up the motor. They will run some motors, like routers and flex shafts, but not brushless standard TEFC type motors.
 
I purchased the motor from www.surpluscenter.com last fall for $40. They no longer have them in their catalog, but it was listed as a 1HP 120VDC motor, 1750 RPM, 35 amp full wave bridge rectifier, pre-corded for 120V wall outlet. I remember thinking "This is too good to be true, low RPM, decent power, and only $40." If it didn't work, I wasn't out a ton of money. Yes, 90/180VDC are much more common. I looked a few days ago, but didn't see a plate on the motor. I'll look again when I'm at the shop next.
 
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35 amp?

Way too much

Type and you mean 3.5 ? way too little


My other 1 HP motors are at about 12 amps.
 
There are possibilites...what is the efficiency of the motor? Running a few calculations with the figures that you have provided your motor efficiency would be like 1/5th...that possibly can not be correct?
 
No, the bridge rectifier is rated for 35A, meaning it can easily handle the 12 amps required for the AC/DC conversion. I'll take the covers off and look for brushes, and get some photos.
I found it, it wasn't surplus center, it was burden sales and they still sell them. Which is odd, because Burden Sales is a division of Surplus Center! I would contend that there isn't a better deal on a 1HP DC motor, especially if I can get a speed controller to work on it.
motor.jpg
 
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That won't be of much or any use in the shop. It could run a small micro-lathe, or something light duty that takes a V-belt. Torque will be low, most likely. The 5/16" shaft is the clue that the power and torque is not high, or it would twist the shaft in half.
The 1HP rating is probably very over-optimistic.

To run that as VS, you could do two things:
One is use a variac. The motor speed will be relative to the input voltage. This would require no changes to the motor.
The other is a PWM or other DC drive that is rated for 1HP. You would remove the diode bridge, and connect the DC output from the drive to the motor.
 
"That won't be of much or any use in the shop." We can't all have 3-phase Baldor motors. The fact is that it's a big step up from the 1/2HP 4x36/2x72 that I've been using. To be able to use my EERF grinder now with a $40 motor, and the ability to upgrade the motor in the future is great. Yes, the 5/16" shaft is a bit small, that's why I've implemented a removable axle on the outside of the drive wheel that is easily removed. It supports the tension of the belt and makes the shaft size irrelevant.

The solution that I came up with is to build a couple more sizes of drive wheels. Like a belt setup, it will increase torque as it lowers speed.
 
A modern dimmer switch will clip your AC wave but your bridge rectifier should allow the signal through to drive your DC motor. But, your full wave rectifier will not give you 120 VDC. It will give you @ 90VDC. I think this will lower the power output of a 120VDC motor. I think the motor will give @ 3/4 HP max at full RPM.
 
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