# Homework Help: Quick question about tourque.

1. Dec 11, 2007

### seang

I don't often deal with torque, but I'm dealing with a question that's a bit out of my league, so here goes.

Consider a system where a motor is connected to a generator via a shaft. The generator has been found to require a certain torque. Assuming the shaft is rigid, the motor must supply the exact amount of torque which exists in the generator, right? regardless of other properties of the motor/generator.

In other words, in terms of homework problems, is this a constraint I can use? If I find that the torque at the generator is x, can I say that the motor must supply x? (or maybe negative x)

2. Dec 11, 2007

### catkin

Yes, assuming losses are negligible. If the motor supplies more torque the output from the generator increases above intended and if less then the generator output falls below.

3. Dec 11, 2007

### Shooting Star

Suppose the generator has a big wheel, to the circumference of which is attached the shaft, the other end of the shaft being attached to a point on the circumference of the motor wheel, which is smaller in radius. (Have you guys seen an old steam locomotive?) I think this is basically the question asked?

Grossly speaking, the force in the shaft is same at both ends, and so the torque will be lesser in the smaller wheel, because the radius is smaller. The exact calculation will involve a lot of other things -- the shaft angle will change, and the shaft has to have a hinge to compensate for the changing distance between the points of attachments etc.

4. Dec 11, 2007

### catkin

Hi Shooting Star :-)

You are describing a crank and connecting rod arrangement but I don't think the question requires it -- the motor and generator could share a common axle shaft (leaving aside practical considerations of alignment etc. requiring a flexible coupling).

5. Dec 11, 2007

### Shooting Star

Oh, so you mean just like an axle being rotated at one end. If the cross-section is same, then the torque will be the same.

6. Dec 11, 2007

thanks guys