Rheostat, Potentiometer, or What?

[Tom LaRussa]

I have a couple DC motors that I want to adapt for shop use. I have a couple of bridge rectifiers that I bought from Surplus Center, http://www.surpluscenter.com/item.asp?UID=2004101111253836&item=22-1180&catname=electric , and some heat sink material I picked up at Frye's in Fremont, CA, (before the move to Florida).

Okay, so, now I need something to control the juice, right?

The question is, what?

Do I want a rheostat, or a potentiometer, or are they the same thing, or are they both the wrong thing, or what???????????? There are a whole bunch of both of them listed here: http://www.surpluscenter.com/sort.asp?numrec=20&sort=2&keyword=EMRH&catname=electric&UID=2004101111253836

Why couldn't I just use a wall dimmer switch, like the ones used commonly for ceiling mounted dining room lights? Should be able to handle the amps, no?

If it makes a big difference, the specs for the motors are:

[1] 90-130 VDC, 1.5-2.5 HP

[2] ~110 VDC, ~1.5 HP

TIA for any help.

[Dan Cameron]

Tom, you might want to consider getting a variac, which is a variable transformer. Of course if you don't need to control the speed you can just connect up the bridge to the line voltage. You can get a second speed by just driving the motor with a half-wave rectifier.

Dan

[Ralph Barhorst]

Tom,

I have designed some DC motor controls in my past life. I would not use either a potentiometer or a rheostat.

The potentiometer is a very low current control and could only be used with a true motor control.

The rheostat can handle a higher current but would get hot when used as a motor control. If you did use a rheostat you would have a difficult time maintaining a constant motor speed. The speed would slow down considerably when the motor is loaded.

What I would recommend is this motor control with the 10K potentiometer that is recommended with the control. It should be capable of maintaining a constant speed and should handle the motors that you have. The maximum motor current of this control is 10 Amps. Check the motor current of your motors to see if this is enough for your motors.

http://www.surpluscenter.com/item.asp?UID=2004101111253836&item=11-2434&catname=electric

[Bob Winkler]

How about using a router variable speed control? Available at MCLS as well as lots of other places for a reasonable price.

[Carl Eyman]

Bob, I think not on the router control. It is for AC motors. I believe it does its t6hing by fooling the motor into thinking th frequency has changed. (please someone corredt me if I'm wrong). With a DC motor one's only option I think is varing the voltage. Someone suggested a half wave rectifier, but in effect that is lowering the voltage, at least, the motor thinks so. Hence a variable voltage transformer (Variac) is the logical answer. Trouble is I think they are expensive.

[Tom LaRussa]

Of course if you don't need to control the speed you can just connect up the bridge to the line voltage. You can get a second speed by just driving the motor with a half-wave rectifier.
Dan
Thanks Dan. That's a good idea!

I plan to use one of the motors to make a buffer, which doesn't really need speed control at all.

:)

[Tom LaRussa]

What I would recommend is this motor control with the 10K potentiometer that is recommended with the control. It should be capable of maintaining a constant speed and should handle the motors that you have. The maximum motor current of this control is 10 Amps. Check the motor current of your motors to see if this is enough for your motors.

http://www.surpluscenter.com/item.asp?UID=2004101111253836&item=11-2434&catname=electric (http://www.surpluscenter.com/item.asp?UID=2004101111253836&item=11-2434&catname=electric [/QUOTE)
Thanks Ralph,

The motors have the following specs:

(1) 120 VDC, 11.2 amps, 1.5 HP @ 4800 RPM

(2) second motor has two modes:

intermittant -- 130 VDC, 18.5 amps, 2.5 HP @ 6750 RPM
continuous -- 95 VDC, 1.5 HP, (amps & RPMs not listed)

Two questions:

1. Looks like I need a bigger motor control if I want variable speed? That's a bugger, because I really want to be able to use the bigger motor on variable speeds, but I also don't want to spend any more than is absolutely necessary. Even the 10 amp controller from Surplus Center costs quite a bit more than I paid for either motor. :(

2. Are you sure I can't just use a dimmer switch like this one -- http://www.homedepot.com/prel80/HDUS/EN_US/shop_cart/pg_print_product.jsp?CNTTYPE=PROD_META&MID=9876&prod_id=163180&com.broadvision.session.new=Yes&CNTKEY=shop_cart%2fpg_print_product.jsp ???

That would be so convenient -- not to mention cheap! ;)

[Bob Winkler]

Bob, I think not on the router control. It is for AC motors. I believe it does its t6hing by fooling the motor into thinking th frequency has changed. (please someone corredt me if I'm wrong). With a DC motor one's only option I think is varing the voltage. Someone suggested a half wave rectifier, but in effect that is lowering the voltage, at least, the motor thinks so. Hence a variable voltage transformer (Variac) is the logical answer. Trouble is I think they are expensive.
Carl, the router speed controls I'm familiar with are for DC and "universal" motors usually found in drills, routers, etc. See attached link.
http://www.woodzone.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=WOS&Product_Code=828122&Category_Code=RA

I still think this would work ok.

Bob

[Ralph Barhorst]

Tom,

The dimmer switch will not work. The biggest problem is its current capacity. It is only 600 Watts which means its max current is 5 Amps.

The suggestion of using the router speed control is probably the best since it is designed for over 3 HP and 20 Amps.

For the buffer application which does not need speed control you could just take the 120Volts AC and run it through your rectifier to produce DC. The motor would just run at its maximum speed. Just make sure that the motors max speed it not too high for buffing applications.

[John Gregory]

Can this device be safely used for other power tools, such as lathes, grinders, etc?

[Bob Winkler]

Can this device be safely used for other power tools, such as lathes, grinders, etc?
Probably not. I believe that most lates, grinders, etc. are AC Induction motors. The speed of these are controlled by the motor design and can only be varied by Variable Frequency Drives (big $$$), or by mechanical means (pulley and belts). AC Induction motors are cheap and reliable, but are not easily speed controlled.

Bob

[Kurt Aebi]

Tom,

Make sure your bridge can handle the startup and continuous current draw of the motor. You may want a large heatsink, if the bridge doesn't already have one. You can always add capacitors to the output of the bridge to give you a "Chopper" type drive and limit the ripple a little more giving you a more stable DC to drive the motor.

As to your original question as to the terminology "Potentiometer" and "Rheostat" Basically, they are the same thing - a variable resistor. Generally they refer to the Potentiometer when dealing with small current (< 1 Amp) or DC circuits and the Rheostat for higher current (> 1 Amp) or AC circuits. Generally what surplus houses call rheostats would handle higher currents and disperse the heat generated a little better than what would be called a potentiometer. But in actuality the term can pretty much be interchangeable. A Variac is actually a Rheostat with some other devices incorporated.

I hope this helps you. If it were me, I'd buy the DC motor Drive from the Surplus house even though it is more expensive than your motor. It has been my experience that the motor is usually the cheapest part of the total drive package anyhow.