Electronic Circuits and Projects Forum

let the turbine turn the motor while the resistance in the potentiometer is high then to slow down the motor, lower the resistance. Would this work or is it bad to do this?

I think you're on the right track there.
You said you can stop the turbine easily with a finger. Suppose you need a force of 1 lb at a radius of 6" to do this. That means the turbine torque is 1 x 6 / 12 = 0.5 ft.lb, so the turbine energy output is 0.5 x 2 x pi x 25rpm / 60 = 1.3 ft.lb/sec = 1.8 Watts. Hence if the stopping force were 5 lb then the energy would be 5 x 1.8 = 9 Watts.
The motor you linked to is rated at ~ 200 Watts peak. Let's allow a hefty safety margin and say 50 Watts. That's far more than the 9 Watts output from the turbine, so if you apply a variable current to it and use it as a motor (not generator) to attempt to drive the turbine backwards through 1:5 gearing you should get the braking effect you want. The motor will be operating under stall conditions because it's trying to drive one way but the turbine is forcing it the opposite way.

Edit: At 50 Watts using a 10V supply the motor the motor needs 5 Amps. So a potentiometer (unless a high current expensive one) won't cope. A PWM-controlled FET would be better.

@Duffy Yes I have a spare stepper motor and felt those jumps while turning the shaft. That might cause problems when viewing the wake behind the turbine. To measure the turbulence I am using PIV to look at the vortex shedding off the tip of the blade. The turbine does spin pretty flow so that's why I mentioned before that I wasn't sure if I should buy one of the outrunners. I only found them because I wanted a brushless motor. The brushed motors have that same type of jumping effect that the stepper does and is harder to turn.

@alec_t So if I can find a smaller motor then I should be able to use a standard potentiometer? Is it bad to run a motor under stall conditions?

edit- Looking into it a little more, PWM motor control would probably be better and its not hard to build the circuit

Sounds like it must be a pretty huge water tunnel to hold a 15" diameter prototype, several hundred gallons at least. Is it your own, or a university's or something?

The more I think about it, the more I think you might want to bite the bullet and build a custom powder brake. Not only will it give you smooth rotational motion, you could pulse it at various speeds to simulate the periodic angular acceleration inherent with some of the things a turbine blade might be coupled to. It appears to be nothing more than a steel shaft running through the center of a solenoidal electromagnet with some iron filings in the gap and a couple of seals on the ends.

It's only bad to run a motor under stall if the current is high, and something you can stop with your finger isn't going to be putting 50W of friction power into your finger (unless you are seriously masochistic).

So if I can find a smaller motor then I should be able to use a standard potentiometer?

Even a motor at say 1/5 the size is going to take 1 Amp. That's still far too much for a standard pot (which can only handle tens of mA) to be used directly; but a standard pot can control current through a power transistor (FET or BJT) driving the motor. PWM is the most efficient way (less heating of the FET/BJT).

Is it bad to run a motor under stall conditions?

See post #24. Provided the stall current is well below the maximum specified current for the motor it's perfectly ok.

Would a mag particle brake work out for this?

Ron