HELP: Variable Torque and therefore acceleration in an electric motor.

1. Nov 16, 2009

Sib08

I have recently been searching for a way to calculate the time take for a motor to turn to 1/4 rotation in an magnetic field. Instead of using a commutator with a DC power supply I was hoping to be able to use an AC supply with the correct time period to reverse the current and continue the acceleration.

I came across a few problems.

Variable acceleration: Although the acceleration is dependent on the torque I have found that the torque was also dependent on the angular displacement of the motor spindle which lead me into a mathematical loop.
such as:

a= (1/m)Fcos(s + ut + 1/2at "squared") although it may not be clear by this I ended up with page lengths of continuous cos(...)s and Fs

Any help would be great. x

2. Nov 16, 2009

Bob S

Some modern dc motors without commutators (or slip rings) use permanent magnet multipole field rotors, and a Hall Effect probe to sense rotor angle. The Hall Effect sensor is then used to commutate the current in the stator windings using solid state H-bridge switches. The electric vehicle motors, which run on dc, are similar.
Bob S

3. Nov 17, 2009

Sib08

Although i do not fully understand. I will look into that, sounds very complex but useful. Wikipedia here i come.

Thanks

4. Nov 17, 2009

Bob S

This might help:
http://74.125.95.132/search?q=cache...nent+magnet+dc+motor&cd=1&hl=en&ct=clnk&gl=us

(sorry about the highlights in above url). Usually transistor switches are used to commutate the polarity of currents in the stator. The switches are in the form of "H bridges" because the dc source, ground, the four transistors, and the winding form an "H".
http://en.wikipedia.org/wiki/H-bridge
For large currents, devices called IGBTs (isolated gate bipolar transistors) are used. usually, IGBTs are supplied as half-bridges, so two modules form a complete H-bridge. This units switch 100's of amps at voltages up to about 1000 volts.
Bob S

Last edited by a moderator: Apr 24, 2017