Electric Motor question? Any Electrical Engineers?

I need clarification or possibly education on electric motor P.O.O. I am making a motorized holloween display and using a motor to open a coffin lid and move a knife wielding arm. I have a 110 volt 5000 RPM electric motor. It has a 391:1 gear drive to produce about 12.78 rpm at the shaft. I would like to reduce the RPM to about 6. I believe that amperage is what really drives the motor, so I cannot use a rheostat to control it. I am thinking about using a three speed control from a ceiling fan. Are there any problems with that approach? Would there be any adverse effect on the motor?

First off, I’m not an EE, just a JoAT, MN.

Dropping the current might help but I think most AC motors are a fixed RPM based on the AC frequency (IE, 60Hz vs. 50hz.) A three way switch will not help unless you have the extra wires coming out of the motor to wire to the switch.

If you’re already at 12 RPM, could you just effect a 2:1 reduction? You can get fractional horsepower pulleys at some hardware stores (not Home Depot; I just verified this with my project.) If the drive is only lightly stressed, you might be able to use something from a hobby store or parts from an Erector set (stop laughing; it’s a toy “parts kit” for kids to build all sorts of wierd stuff.)

BTW, what are you building?

I’m building a motorized coffin. The fabricated a eliptical cam that is mounted to the shaft. This lifts the lid about 4.5 inches. Attached to the cam is a connecting rod. This moves a knife wielding arm. The arm comes out of the gap between the lid and the bottom of the coffin. The is powered by a motion detector. As the kiddies walk towards the house, the coffin will be illuminated, the lid will open and the arm comes out. The power will be on for about 1 minute after motion stops. I have considered building a 2:1 gearbox, but I would need to redesign the internals, reposition the motor, etc… Looking for a quick way to slow down the cycle.

You have a 5000 RPM motor? 3600 RPM is the fastest you can get an AC driven motor at 60 Hz, and 3000 RPM is the fastest for 50 Hz. If you have an inverter (VFD) then you should be able to reduce the speed with it.

If you don’t, it has to be a DC motor, and you need to know what type of DC motor it is. Large DC motors (series field, compound, or seperately excited) have really nice speed control, but smaller ones with permanent magnets have no speed control at all.

Your best bet would be to get a motor with the same frame size but half the speed, so half the horsepower. It would have the same torque, so it should work OK. In small sizes they are not that expensive.

Engineer Don, I will get the information from the motor and post when I get home. Here is what I do know: It definitely is fed by AC. Plug it into the wall and it works, swap a couple of wires it spins in the opposite direction. It is 115 V, 60 Hz, with 5,000 RPM and 391:1 gear reduction. The actual size of the motor is roughly 2 ½ inches in diameter, 6 inches in length. The motor attached to the gearbox with screws. The gearbox has the stationary mount. There may be a built in power converter to DC. There are no controls what so ever. The only option is how it is wired and that is only to change the direction. Big question is: will reducing the voltage or amperage to the motor cause harm? Will it generate excess heat or whatever that may damage the motor or cause a fire? Will a ceiling fan control work to reduce the speed? Will a Rheostat work?

As mentioned earlier, instead of worrying to much about frying it, get a couple of very small sheaves (pulleys) say one 2" (if you can get it that small) and the other 4" and get your reduction that way… or you could use small chain/sprocket combo, like 25 or 35 chain, but unless you live in a big city you may have trouble getting sprockets that small out of inventory.

That is what I would do.


Bernse, as mentioned earlier, I have considered it. It comes down to the amount of labor to design and build mounting hardware for a pully system or a gearbox to reduce this to the RPM I want. We are talking about a wooden coffin constructed of ½ plywood and 1 x 6 pine. An old motor, a hand made plywood cam, a few pieces of thin steel, a few bolts, a bunge-cord, a plastic machette, red paint, and a rubber glove. The only things I’m missing are the string and glue. The kids love it. I had a blast scrounging for old parts and building it. Had a couple of beers with the neighbor discussing ideas. I just think it moves to fast, so I am looking for a CHEAP, EASY, and QUICK way to reduce it. I could spend $130.00 at Grainger and buy a variable speed motor, but what fun would that be? :smiley:

Try this,

Take the motor out of an electric drill, one of those that are speed controlled using an electronic trigger module.You should be able to fabricate something from that.

If the motor has brushes on it then it is possible to run it on dc but it does mean working out which connections go to the brushes and which to the windings, since they are usually joined together on this type of motor(universal motor) you would probably need to separate them all and join them to a conector strip.Once that has been done it is an easy matter to to feed them from a dc supply with the rheostat in the brush circuit.I might add that unless you have a good idea of what you are doing you would be best advised not to do this.

Another option would be to use a variac.This is a portable transformer whose secondary can be rotated with respect to its primary so giving a variable ac output.Best place to get such an item is a military surplus equipment store or maybe borrow one from a college.I used to use one when repairing tv’s but they are hardly used in that trade nowadays.

Would using a variac damage the motor? Not instantly, if at all, you would notice it getting very hot first.This is a case of suck it and see, if it works then fine, if it doesn’t well at least you tried.

Would an auto wiper motor have enough torque for the purpose? You could use the associated speed controller hooked up to a 12 volt DC supply. But I guess you would not want to have to reconstruct the mounting, sorry.

I am not an EE (I am an PE-mechanical), but I play one everyday in real life. An AC motor’s speed is dependant on two things: THe number of poles, and the frequency of the electrical energy supplied to it. An inverter, or variable frequency drive, changes the frequency of the electricity supplied to the motor. The number of poles is pretty much determined when the motor is built, although two speed motors are available. With 60 hz electric energy, the maximum speed of a synchronos motor is 3600 rpm, and these motors have two poles. As you add more poles, even numbers only, the speed drops, so with 4 poles, you get 1800 rpm, with 6, 1200 rpm, and so on. A motor’s diameter is indicative of it’s torque, which is also related to the amp draw.

The load (brake horsepower) attached to the motor is what determines the motors amp draw. An AC motor will try to bring itself to full speed regardless of the load attached to it by increasing it’s amp draw. At full load, the motor’s windings can handle the required amp draw. Most motors have a service factor too, of about 15%, so you can overload them a little. Higher amps than that will cause the windings to burn up, and the motor will be destroyed. Dropping the voltage into the motor will have the same effect and will not reduce the speed at all. Don’t use a rheostat. I don’t know what most electric drills use for speed control, but I am guessing it isn’t a freq-drive, so it probably won’t work on your motor.

If it is an AC motor, you can either get a new motor with a lower speed or a freq drive from Grainger, or do a number with gears or a belt drive. An old bicycle might provide you with a workable gear train, but a new motor with the same frame size (probably on the existing motor’s nameplate) and half the speed (and therefore horsepower) would be a quick and elegant solution. Probably fairly cheap too.

Just checked up on the hand drills. They use Wound Rotor motor control, which is basically using resistors to drop the voltage on the rotor circuit. This is a specific type of motor and you can’t just put the rheostat (trigger) on another motor.

I was assuming the motor you are using is just a single-phase induction motor.

Thanks to all for the information and ideas. I think I will leave it be for now. Halloween is to close to be trying a major overhaul. Spare parts and bubble gum only get you so far. I have an old variable speed drill, so I think I will use it for version II next year.

Keep fighting ignorance! Thanks again.

There are two main kinds of motors and the answer depends on which type you have. A synchronous motor (no brushes, heavier for the same power) cannot only be controlled by the frequency so you can forget about it.

An async motor (has brushes, gives more power) can be controlled within certain limits with a rheostat or electronic speed controller. it will slow down with load. these are the motors you would find in a hand drill and similar tools.