Question

please explain how speed and torque in a dc motor are inversely proportinal?

Answer 1

i believe that a motor is able to operate as a result of torque. Since a motor is just an automated crank; the larger the radius is compared to the force applied the larger the torque that is generated.

where the outer part of this crank meets the contacts has roughly the same linear speed. distance covered is equated as the product of speed and time; d = st. if we consider the speed to be a constant, this indicates that time and distance are directly proportional.

The larger the radius is for our crank, the more distance you add to the circumference and the slower the central point of rotation becomes.

im sure that makes more sense in my head tho :)

Answer 2

The engine is designed to operate on some power rating and the power output from the axle is the power input minus the heat losses.

Now, the mechanical power output is torque*(angular velocity) and therefore if the power is kept constant, the product torque*(angular velocity) will keep constant and therefore, if you have small rotating speed, you have large torque and vice versa.

But the question *why* the power can be constant is much more complex question. Not all engines can give the same power output on different speeds.

If you compare a dc motor and a gasoline engine, there are some differences. For example, you can not get the same power from a gasoline engine if you run it on 700 rpm or 7000 rpm. That is because a gasoline engine can not give a large torque when running slow.

An electric motor can give large torque with a wide range of speed, and therefore you can get large power output on small and large rpm:s. That's why they build hybrid vehichles :-).

Answer 3

Speed or 'angular velocity' in motors is often expressed in revolutions per minute (RPM). The correct SI units for angular velocity are radians per second (r/s). Torque is rotational force. the SI units for torque are N.m.

Suppose you have a wheel turning at 3000 RPM that you can slow down and stop with your hand. Here the speed of the wheel/motor is high, but the torque or rotational force is low (because you can stop it with your hand).

The higher your RPM, the lower your torque. If your motor was attached to a large iron wheel but was not powerful enough to rotate it, the motor would just tremble and there would be a smell of burning components. So your RPM would be zero, but your torque would be at a maximum. In other words, rotational force is always at a maximum just before the motor begins to rotate. As the motor speeds up, torque will keep falling. The best example I can think of is a children's windmill. The wind will turn it at maybe 200RPM, but you will be able to stop it turning (because it has low torque) by touching it lightly with your hand. If instead you place this little windmill in a strong wind but jam your finger against it, it will not move (it will have zero RPM) but you will be able to feel the high torque against your hand (as the small plastic blade of the windmill cuts into your hand).