using a dynamo as an electric brake

[gareth78]

Hi,

ultimately i need to control the tension of tape coming off a reel, but as the diameter decreases, the tension increases, and i dont want to use a feedback system to control a brake.

has anyone heard about or had experience of using a dynamo as a brake. if a constant power is somehow dumped from the dynamo, i am thinking the tension will be kept constant...?

thanks in advance

 

[cbarn24050]

Hi, no the tension will not be constant, you have exactly the same problem. However if you feed the tape through 2 rollers 1 of which is coupled to the dynamo then you can control the tension, but you still need somekind of current control system.

 

[RichLeimbach]

You could probably keep your torque constant, but with a varying position of the tape, this won't help you much (since your tension will still vary as the radius of the tape changes).
Without a feedback loop or a second set of rollers with constant radius (as Cbarn has suggested) you might be stuck. However, I don't think it would be very difficult to have some sort of simple mechanical or electro-mechanical system to modulate the torque on your reel (could be based off of length of tape played out, position of a tensioning arm, tape speed vs. reel RPM, etc.). It would not have to directly sense tension, just needs to have something that defines what state the system is at (the effective radius of the tape) and changes the reel torque accordingly.

 

[peebee]

This sounds very similar to two other devices, maybe these will give you some ideas:

1. An electrical contractor pulling a wire into a conduit off a spool of wire. Usually the tension is measured at the pulling end. I have no idea if they might sell a device that could be easily adapted, but who knows, maybe they offer something off the shelf that would work for you.

2. The capstan and pinch roller in a cassette tape player. This is almost identical to what you're looking for, but on a much smaller scale.

Hope this helps.

 

[jbartos]

Suggestion: It appears that a feedback is the proper and accurate way to go. The other approaches will not be as accurate for torque control to be at a constant value.