# dc motor/generator

#### suzuki

Joined Aug 10, 2011
119
Hi,

I'd to get some more information on the torque/speed characteristic of a dc motor that is being used as a generator.

i know that torque is >0 and decreases linearly as speed is increased (positively) in a motor. in the generator mode, is it the torque or the voltage that will be negative? is there such a plot of torque speed that has all 4 Cartesian quadrants to show this?

I'm asking this because in a lab experiment, i found that when i supply a rectified ac voltage to the dc machine, it is motoring and spins clockwise. however, when in apply some positive dc source (to the load) the direction of the spinning is now counterclockwise. So I suppose where i am confused is, since the speed is now negative, does this indicate that we are in the generating mode? I'm not certain since I thought it would be possible to get this rotation by just switching the leads (reversing the polarity) of the input supply.

tia

#### t_n_k

Joined Mar 6, 2009
5,455
Perhaps if you post a diagram of your experimental setup it might be easier to understand the questions a little better.

#### BMorse

Joined Sep 26, 2009
2,675
If you are using the motor as a generator, why would you apply any voltages to it's leads?? If a motor is used as a generator, you are supposed to spin the shaft, and get power out of it.... the output would be alternating so you would have to rectify it.

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#### strantor

Joined Oct 3, 2010
6,778
What BMorse said, + it will only work if it is a permanent magnet motor. If it is a wound field then you have to supply current to the field.

When you were supplying rectified AC, if you had reversed the leads, it probably would have reversed direction just like reversing the leads with DC. That is, unless it is a series would motor, in which case it should always spin the same direction (AC or DC) until you reverse the field winding.