stalling a DC motor: what will happen?

[Hero999]

You need a series resistor to provide a holding current.

What do you mean by holding current?

I don't think you've understood how a polyswitch works.

It behaves like a slow blow fuse with a resistor in parallel with it.

There is no need to use a series resistor because the large starting current won't last for long enough to trip it.

When a motor is running at max efficiency then the current it draws is the maximum it can draw indefinitely. If we stall the motor then it will not get the cooling effect of the rotating armature and will therefore overheat at that current, hence my suggestion of 75% of the max efficiency current.

However if we limit the current, the voltage will decay to a very low value when the motor is stalled so the power dissipation will be much lower.

 

[3v0]

How about an update. Have you tried small motors yet to see if you can get enough speed and torque to work as fingers ?

 

[Hank Fletcher]

I'm definitely looking into them. I don't want to get too distracted right now, because I want to put the finishing touches on my first recorder prototype, RCD-1. It's at a bit of a sensitive stage right now, as I'm doing my best to get it done while working within inherent flaws in the design. For instance, the recorder will now be seated upside-down in the holder, which hopefully will allow gravity to return the majority of the solenoids.

I should definitely have that done in the next two weeks. After that, I'm really going to throw myself at the small motor actuator idea. I'm going to ask my Dad to help me with the design to see if I can get something that works well as a universal finger for various instruments, and that might be helpful for people who could use the finger for something else. Even forgetting the locking mechanism for a moment, I think it might be practical to do something that just stalls a single motor (so it has some "grip") as long as good enough allowances are made for power dissipation, motor specs, and the knowledge that it will put some wear on the motor.

Right now I'm thinking of a single, cheap (<=$1), small motor with a custom pulley about 1 to 1 1/2" diameter attached to the shaft, and a line attached to a Bowden cable of sorts which then attaches to the actuator, so that the actuator can be powered remotely from the motor. For the actuator, I'm thinking something slim but strong, so that it can transfer pressing power without flexing and several can be positioning close to each other.

The little tinkering I've done tells me that, for my interests, it will be easier in the long run since I have a variety of interests that could use it. If you figure conservatively that a loaded motor (meaning the key it's pressing, not the stall point) would still be able to rotate once every 10ms or 20ms, then that's fast enough for me.