Cheapest easy DC motor to survive locked rotor

Put a resistor in series with the motor windings. The unloaded speed won't be affected much, but the stall current will be. You could get fancy and order a polyswitch from Digi-Key -- they are little resistors made of a polymer material with a strong positive temperature coefficient; they work like little resettable circuit breakers. The nice thing in this application is that they're slow to turn off and they hold their value automatically. Their mechanism is entirely thermal so if you tape the thing to the motor case it'll tend to go off sooner when things are toasty.

This is normaly done by two end switches and two diodes. You'll find them in your x-t recorder, I guess.

Nick

What about torque, it needs some torque to turn the gate (something that I can help with by use of a little rubber band).

I will definitely play with this, as this seems to be the easiest solution. I will try to measure DC resistance of this motor and add perhaps 3x as much resistance.

It seems that I found something that my 4.5 year old takes long term interest in (control of his electric railroad) and I want to keep it going.

Very nice. I will try to use resistors as you suggested.

Your DC motor isn't overheating simply because it's stalled. Limit the current to the motor and it'll be happy applying a torque at zero speed indefinitely. For what you're trying to do, a resistor would probably do the trick.

Ned Simmons

Ned, thanks, great idea, will try tonight.

Motor torque goes with current, so it will definitely go down with the resistor. The polyswitch should be better in this regard. Are you driving the gate directly from the motor? If so you're really not matching the motor's capabilities to the problem. If you have an old RC servo lying around you could strip out the electronics and just use the geartrain and arm from that, with an appropriate resistor to limit the current, of course.

Or... consider holding a spring-loaded solenoid on for one position, and off for the other.

LLoyd

Or... consider holding a spring-loaded solenoid on for one position, and off for the other.

LLoyd

Makes sense.

Yes, it is just a little stick on the shaft.

You know, I may be able to just use a little counterweight and reduce torque requirements to next to nothing. Then this motor and a resistor would just be perfect. Torque is a non issue, as I am realizing. Sorry to have brought it up. As long as I limit current and overheating, I will be perfectly fine.

Yes, it is a good one, but the solenoid would need dual coils, which costs $$$. I will check them out though.

momentary

locked

little

I did a very similar thing on a dc motor that advances the tool turret of my small cnc lathe. It positions the toolpost, then falls back to a lowish current to hold the motor against a dentent, only difference was that this motor had to run forwards to position, over-run a ratchet then reverse onto the ratchet stop then reduce the current.

AWEM

No... if it's spring loaded to one position, it will hold there until energized. When de-energized, it will return automatically.

LLoyd

What about the motor & worm-gear from a salavged CDRom drive?

Run the motor with reduced voltage. It will run more slowly ( which may make the RR gate more realistic) and it won't overheat if run stalled. Determine at what voltage (current, really) the motor has just enough torque to close the gate reliably. It might be just a couple of volts.

A resistor would also limit current but it won't reduce speed.

If the motor is a slow gearmotor, a microswitch at the travel limit could switch in a resistor to just hold the gate but limit current in that position.

You could operate the gate with a directly-driven pulley with an O-ring tire on it that rides on the motor shaft and just slips when the gate is fully open.

This could be a good app for "memory metal" aka nitenol, which is easily deformed when cool but exerts force to resume its original shape when heated. A piece of it, working against a spring or counterweight, could close the gate when heated (by electric current), would "relax" when it cools allowing the gate to re-open.

Bimetal, as found in thermostats, bends when heated but it doesn't exert much force.

A solenoid would be too fast and slam shut unless you include some viscous damping.

With a 555 and 2 LED's you could make a flashing grade-crossing light.

I'm not sure that a solenoid would need dual coils. Is this to avoid overheating? If so, you can get some solenoids which can remain energised with no problem.

I think I have a few solenoids in the shed. Some were used for remotely unlocking doors, and others were from a printing press. If you're interested I can have a look. You can have one for the cost of shipping to the US if they're what you want.

Chris

Hey Iggy,

Simply, put a light bulb in series with the motor. When the current to drive the motor is low, the lamp remains cold and dark. When the current rises enough, the bulb filament will heat and glow, increasing it's resistance and therefore reducing the voltage across the motor. Heat is always generated at the point of highest resistance. It will take some experimentation to get the best/proper bulb.

Take care.

Brian Laws>We were playing with my son last night and made a toy railroad

Heh. Excellent idea there. Nonlinear resistance so you get more torque for a given stall current and only 1 part. Maybe you could use an xmas bulb.

Best regards, Spehro Pefhany

Yes.

That's nice, I did not realize that. The pull needs to be very minimal.

Well, sure, see if you have 12 VDC ones, which is what is the power supply that we use on his toy railroad.

That would be a one shot way of using it, right?

Very nice to know. All this screwing around is highly educational for my son.