[Ricka]

I'd like to reduce the voltage going into a motor from 120 to 95. It's an 18 amp DC treadmill motor running on rectified house current. Could anyone tell me what size resistor (ohms and watts) to use? Thank you.

Rick A

[donf]

Rick,

To begin with a unit this size (120V /18A) would be required to be placed on a dedicated 20 Amp circuit. Is that what you have it on? If you read the manufacturer's label, you should see that requirement.

If it is on a 15A general use circuit, your amperage is already exceeded. The same would be true if the treadmill was placed on a general use 20A circuit. A general use circuit would have several receptacles on the circuit. If this is true, then you are limited to 80% of amperage. In this case 20A X .8 = 16A. Again, the 18A required by the treadmill is exceeded.

On a dedicated 20A circuit the full 20A is available.

Just curious, why would you want to reduce the voltage anyway? Reducing the voltage will adversely affect the amperage needed by the treadmill.

[Ricka]

Don:

The circuit is adequate. The motor originally ran off a PWM controller, which, I'm pretty certain, reduced the voltage to 95 because that's what the motor is rated at. I'm using the motor to run a vertical milling attachment. The controller failed and rather than shell out the money for a new one, I want to run the motor at full speed and achieve different speeds via step pulleys. The motor will run on rectified 120v but it is overspeeding and I don't want to damage it.

[donf]

First, this motor is not designed to be used as a milling motor. So I do not know what type of sealing you would have to put around it to protect it from explosion or fire due to dust getting into the motor. But logically I do not see a reason why it could not be adapted. However, I would not recommend the modifying of the input voltage.

If you try to use the motor as a stand-alone motor, then you have to do the motor calcs to make sure the supply circuit is properly installed.

What is the HP of the motor?

Since it runs on household voltage and current, we know it is a single phase motor. You have no choice but to get the Full Load Current so you can properly set up the supply voltage and breaker size. You also have to properly size the motor protection breaker.

Are you capable of designing such a circuit and submiting it to the LAHJ for approval?

[tkrussell]

Rick, you would be better off getting a new PWM controller,

Motor Controllers - All Kits & Modules - Cana Kit


Don is not understanding that your trying to control a DC motor, not an AC single phase motor

Yes, the unit you plugs into an AC wall receptacle, but inside there is a rectifier that feeds the PWM contoller that then controls your DC motor.

To determine what fixed resistor is needed, as your looking for, you need to know the wattage of the motor at the desired voltage and speed. You may need to do some experimenting to learn this.

[Ricka]

TK:

I wish I could troubleshoot and fix electronics. When the controller fails all I can do is buy another. I figured I'd be ahead to simply run it at full speed. Also, as I understand it, the on/off action of a PWM controller means you get less speed, but also less torque. The bigger the endmill, the lower the speed and the more torque needed, so I think I'm better off with the motor putting out it's full rated power. It's rated at 2hp, 18a and 95v, so I figure it's 1710w. I'm running the motor through a stand-alone bridge rectifier rated at about double the amps of the motor. I think I could experiment with a variable resistor, like you suggest, and come up with an ohm rating, but I still wouldn't know the wattage the resistor needs to handle. Hoping someone here could figure that out. Thanks.

Rick A

[donf]

TK,

I'm familiar with PWM, but at the chipset design and programming.

You were correct, I did not pick up that this is a DC motor. I also should have picked it up when he said he wanted to use it on a rectifier. My error.

However, since the motor is planned for a milling room, aren't the Class 2, Division 1 rules required to be adhered to?

A motor deigned for use in a treadmill isn't the same as a motor designed for use in a woodshop, correct?

[Ricka]

Donf:

It's a milling machine- a metal working machine tool, and it's a home shop, not a commercial shop that is governed by any rules except not to burn my house down. It won't get the kind of hard use it would in a machine shop. Adapting treadmill motors to lathes, mills, etc. is commonly done by hobbiests because of the variable speed and availability of used teadmills.

Someone elswhere suggested that a resistor big enough to do what I want would be unrealistically large, expensive, and heat producing. It may be that I'm barking up the wrong tree and need to consider a tranformer ahead of the rectifier. That's why I turn to forums like this- to learn about stuff I don't know.

Rick

[donf]

I was thinking of milling with respect to a wood shop. Thanks for the clarification.

Personally, this is not something I would recommend. If the motor you have is still working, I would replace the PWM as TK suggested, assuming that you PWM is not an intrinsic part of the motor.

My only concern is that this motor may need to be cooled by air flow. If it is, then you have the concern that fumes or fluids will be pulled into the motor and cause problems.

I would make absolutely sure that this motor is suited for the kind of work you are going to do.

By the way, it does not have to be a commercial environment. It just has to fit into the definition the type of zone. It is a fire safety issue.