# Cheapest Way To Reverse/Vary Speed of DC Motor

1. Feb 17, 2014

### 9986mkoh

Hi,
I'm new here and had a question relating to my senior design project in mechanical engineering. My project ends up involving a small DC electric motor, which needs to be able to be reversed and it would be nice to be able to have some speed variability.

I'm aware of speed controllers and I know how to reverse a DC motor just with polarity, so I'm going to have a 3 position switch for that. However, I'm unaware of exactly how to add some speed variability without actually buying a speed controller, which is an issue because we're on a sort of tight budget. Torque isn't a big issue either. My idea originally was to use a rheostat rated for the max amp load of the motor. For the most part, we are limited to sites like McMaster Carr but we can also use others if the parts are cheap to do this (since we'd be paying for it out of pocket). I've also heard of an H-bridge circuit to avoid the losses of a variable rheostat (potentiometer), but I'm not too familiar with that.

If anyone has any suggestions or comments on this idea, I'd greatly appreciate any help you could offer.

Thanks,
Mike

2. Feb 17, 2014

### meBigGuy

Depends on the power you need to control and the voltage driving it. Solutions can be as simple as a variable voltage regulator and switches or a 555 driving a switching transistor. A series resistor is a vey poor solution since it severly limits startup torque. It is worth understanding how a DC motor works with respect to current draw and speed.

Essentially you can think of a motor as containing a generator creating a back EMF. As a result, when going fast at no load, it creates maximum back EMF and draws the least current. When stalled, it has no back EMF and draws maximum current. A simple motor controller deals with this. For example
http://www.circuitstoday.com/low-voltage-dc-motor-speed-control-circuit OR http://www.electroschematics.com/407/dc-motor-speed-regulator/
Here is a 555 pulse based solution.
http://www.circuitstoday.com/dc-motor-controller

A H bridge can be used as a simple direction switch or can be integrated into a sophisticated motor controller. But you have to be carefull and be sure that when you switch it you don't short the supply.

3. Feb 18, 2014

### 9986mkoh

Ok, thanks. What I'm dealing with is a 1A motor, specifically this one http://www.mcmaster.com/#6409k27/=qqyi1b
So I've been looking for something fairly cheap or a cheap way to control this, and I only need about 5 in-lb of torque for this application.

4. Feb 18, 2014

### meBigGuy

The 555 based pulse solution would work.

I need to look for a simple motor controller IC. This page has a bunch. http://www.vetco.net/catalog/default.php?cPath=114_117_460&osCsid=jq6ck2fnqnb82f3bcp5b0buld6 [Broken]

possibly something like the NTE1748, but I need to look at some application information. The datasheets are a bit terse.

Last edited by a moderator: May 6, 2017
5. Feb 18, 2014

### 9986mkoh

I'm sorry, but I'm a little bit confused by that. Am I supposed to get one of those that'll work with my motor amp load and wire it up as in your previous message? I just really don't have too much of an electrical background, I took circuits one and that's about the extent of it for now other than the research I've been trying to do online.

Thanks,
Mike

6. Feb 19, 2014

### meBigGuy

The NTE datasheets are the most incomplete I have ever seen, and there is no application information. So I guess that is a dead end for me.

The 555 based pulsed driver is the easiest I've found so far. It requires a switch to change directions, but an H bridge could be substituted. http://www.circuitstoday.com/dc-motor-controller

It also requires 2 supplies or a regulator for the 555 if you want to run the motor off 24V.

Last edited: Feb 19, 2014
7. Feb 19, 2014

### meBigGuy

8. Feb 20, 2014

### 9986mkoh

Thank you, that's perfect for what we're doing and I really appreciate it. We just ordered that as well as the transformer to power it.

Thanks again,
Mike

9. Feb 28, 2014

### Mike_In_Plano

I'd mount a 2n3055 transistor on a big aluminum plate with a bit of grease to transfer the heat. Tie the collector to 24 volts and emitter out to the motor. Tie the base to the wiper of a panel mount 2k pot with the ground and 24 volts on either end.
Simple, stupid, dirty...

10. Mar 1, 2014

### Averagesupernova

And probably not enough beta in the 2n3055...

11. Mar 1, 2014

### Mike_In_Plano

Good point, the wiper will not work in an ideal fashion as about 16ma will required for a worst case part supplying 1 amp. Then again, the wiper will present an ever decreasing impedance (from 1k down) as it is turned from midway to full. Thus it will work in any case. An improvement on this would be to use a 1k, 1 watt pot.

- Perfectionist is the enemy of success.

12. Mar 1, 2014

### jim hardy

13. Mar 1, 2014

### mjhilger

Seems I'm late to the party, but a PWM (pulse width modulator circuit) is the best approach. And if you are using a switch such that you don't need direction control in the circuit all the better. So the approach with the 555 is exactly what you need. That said, and I don't know your time frame, but ebay has numerous great solutions from China for very low $such as http://www.ebay.com/itm/12V-24V-3A-...469?pt=LH_DefaultDomain_0&hash=item2ecf36897d . Again, these guys have certainly suggested good working solutions, but if building the stuff and$ are driving factors, ebay is hard to beat if you can tolerate the shipping time from China (usually 14 days or so).

14. Mar 1, 2014

### meBigGuy

hmmm.. The ad says 24V, but the PDF says 12.

15. Mar 2, 2014

### jim hardy

12?

Looks to me like it's got ajumper for 12 or 24 volt operation that just sets current thru zener regulator ZD1..

16. Mar 2, 2014

### meBigGuy

duh.... I saw +12V in two pictures and ignored the rest. oh well.

17. Mar 2, 2014

### jim hardy

well, i do that all the time.
but i'm old enough it's expected.....

18. Mar 3, 2014

### meBigGuy

You're not the only one

19. Mar 3, 2014

### Mike_In_Plano

I'm continually building fixtures for my ME friends as it seems that acquiring electrical materials, soldering, and crimping are all skills that are brushed over in the ME programs.
I'm surprised that additional emphasis hasn't been given to integrating these common functions on ready to use circuit boards with screw-down terminals.