Motor Speed Control

[JonathanEngr]

I'm trying to automate the motion of some props in a haunted house, and noticed that many halloween sites sell motors to achieve this. I don't want to buy any of the motor kits online since there is no funding for the project (school Fall Festival), and wanted to piece this together using mostly parts I have.

We already have a pulley assembly where a volunteer pulls a cord up-and-down to give motion to the props. Rigging it for a motor should be a snap. The motor I have is a 12V DC motor, 1/30 HP, 20 in-lb torque and 100 RPM speed. 100 RPM is waaaay too fast so I need to step down the speed. I've seen DC speed controllers online for 12V motors, but the highest amperage I've seen is 30A. The power curve on this motor shows a continuous 6A draw at 100 RPM, with the amperage increasing to 60A for very slow rotation. Does this mean I need a 12V/60A speed controller for this motor, and if so, any idea where I can find one (an inexpensive one, mind you!). Or would the 12V/30A be okay? I definitely don't want to start a fire here....!

Less important... I'm trying to find a 12V motion sensor to use to further automate this setup. The MAIN reason I'm doing this is because the motor states it is an "intermittent duty" motor. I know that a continuous duty motor is made to run 24/7 if needed, but how long can an intermittent duty motor run before it needs to "rest"? All of this is greek to me. If you think it would be okay to run for 4 hours straight, I won't need to integrate a motion sensor (although it would be a cool thing to add some fright to the setup!). Thanks so much!

 

[Resqueline]

1/30hp is only about 25W of power, barely over 2A at 12V. If the motor spec' says it draws 6A at full load then it's only 33% efficient.
At a light load, and also at reduced voltage (= reduced speed), it will draw less current - not more - and can thus run continously.
So I'd say that you won't need anything more than a 6A speed controller. A pic' of the motor spec's or a link to it would be useful though.
The intermittence periods would depend on the physical size of the motor and how hard it's overdriven.
A small headlamp washer pump could be rated for 15 seconds on and 45 seconds rest for example.

 

[JonathanEngr]

Here's a link to the site with my motor's information:

http://www.alliedmotion.com/Products/Series.aspx?s=9

At the bottom below the product description you'll see 5 tabs--click on the "performance" tab and you'll see the graph I was talking about. On the torque vs RPM curve they have superimposed the current, and at low RPM the torque is much greater and current draw is also higher. Even in the product description is says the same--6 amps continuous and 60 amps peak.

Anyway--this is why I'm posting here... I have no idea! It would be great if I could use a 15A controller. Those seem to be very common and inexpensive.

 

[JonathanEngr]

By the way, we also have a much more powerful motor availalbe but have no idea if it even runs. It says is it a 90 Volt DC motor. I've never seen a converter for 90V DC... I'm assuming this is industrial...? Can 115 AC even be converted to 90V DC?

 

[JonathanEngr]

Any thoughts on the link I posted?

Also--another problem. I have a gazillion 12v power supply "soap on a rope" cords, and I was thinking of using one to power the motor. However... most are rated for milliamps--only one is rated for a full amp. Thus, is this going to require a dedicated power supply? They have 30a power supplies on Amazon for $20 or so... I assume I'll need one of these to supply power?

With that said, would I be better trying to salvage a 115v motor? It seems 115v gearmotors are pricey, and I was told that varying the speed on a 115v motor is quite a bit trickier (that it can damage the motor). Please help! Running out of time here

 

[JonathanEngr]

BTW--talked to a rep who makes the motor. The motor is rated for continuous duty at full rpms (shaded area on the graph on the website). Only when the RPMs are LOWERED does it become intermittent duty. He said that even at lower RPMs it could be run for hours at a time.

 

[Resqueline]

Yes, when the motor is (over)loaded, running at 12V, the RPM's drop below 100 and it can then only be run intermittently. At no load it seems to draw "only" 2A.
So you can safely use a 15A controller, provided you don't stall the motor. I'm also sure the 90V motor will run on 12V but I have no idea on the current it'll draw.
I wouldn't trust anything less than 1A would run the motor(s). Only one way to find out. Any voltage AC or DC can be converted to any voltage AC or DC btw..

 

[davelectronic]

Motor control

Hi there. I dont know what Resqueline's opinion is on this idea, what about a step down transformer for the motor and a square package bridge rectifier, control the transformers primary via a dimmer triac circuit, simple to set up not very expensive, ok i know its an inductive load, and simple bridge rectifier, but DC motors dont need total full smooth regulator DC to run, DC motors run quit happy on rectified DC, select the apropriate size transformer, dimmer and bridge rectifier.
Dave.

 

[davelectronic]

Motor control

A little practice run of the load to the motor this way, so as you can see the amount of power to apply by the dimmer circuit should prove affective. Its an idea, not written in stone, rather a cheap usable option, it does not need to be over engineered to be functional.
DC motors happily run on straight DC rectification, a square package of 15 plus amps is peanuts, and the transformer and dimmer not a lot more, total cost called be modest, source of parts totals four or so fuse included.
Dave.

 

[JonathanEngr]

Dave--electronics are not my thing *at all*. As I kid I got the @#$^$^%# shocked out of me by a 115V outlet (was plugging in a fan, and the blades of the plug were too far apart... thus I pushed them in as I was plugging it in)... been nervous about it ever since. When I see little electronic components with tiny wires connected to a 115V connection it makes me nervous. I even cringe when I'm on the jobsite and electricians are using a tiny little tester to check voltage on a huge 480v panel... I wait for a "boom!" and sparks to fly.

How do you know what size tranwsformer and rectifier? How are they wired together? Most importantly, is it safe? Sorry to ask so many questions, but electrical just isn't my thing.

BUT... I really want to press through this thing and hopefully learn a bit out of this.

 

[davelectronic]

Motor control

Hi JonathanEngr.
I completely understand your fear of electricity, we all have a fear of it to some extent, its a healthy thing, no fear of electricity kills people.

But having said that can you draft a colleague or friend in to help you, this could help with the worry of the task.

Yes implemented properly my proposed solution to drive your motor is safe, what happens is a dimmer circuit like those for lighting controls the primary voltage and power in to the transformer, in turn the step down voltage is lowered or hired, depending on your movement of the dimmer control, a bridge rectifier converts the AC from the low voltage side of the transformer in to DC voltage, appropriate for the motor to drive it.

This type of control is simple basic, and will do the job, it would need building in a small box, as mains voltages are involved, the input mains needs to be fused, as does the low voltage output for the motor.

Control this way is not critical in it actions, IE precision is not top of the list, but for your application it would almost certainly work fine, a little rehearsal to get the feel for what level of control would give the desired results is best.

So yes done in this way with a colleagues help could work ok. And is safe.

Your other options are to purchase a motor control unit for DC use of the appropriate power rating and voltage, simple, but more expensive, other control circuits can be built of different types, but your electronic knowledge would need to be proficient.

No one else has posted on your question yet today, unless a post has gone on while ive been writing this reply,so i dont know other members response to my proposal yet.
Dave.

 

[Resqueline]

Have you made any progress on the project? Dave's suggestion is adequate, cheap, simple, and perfectly safe.