PWM and motor control -- request clarification

Dec 26, 2011 23 Replies

No. What I said. *Assuming that the speed stays the same*, as you load the shaft, the back EMF stays the same. The *current* goes up, and with it the IR component of the voltage.

With clever enough IR compensation, the back EMF can be recovered intact, and used for speed control.

There are time-honored analog ways of doing this, which usually require fine tuning under varying load conditions.

Modern drives frequently have an "autotune" facility, which removes the field excitation, so the motor does not revolve (maybe you have to lock the shaft as well, if the poles and/or frame are badly retentive), then runs the armature current up and down,and calculates and stores the IR characteristic automatically.

With small motors, say quarter-horsepower down, you can usually get away with ignoring IR, unless you want close speed control.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

e
e
e

Copper has a positive temperature coefficient of resistance

formatting link

of about +0.4% per degree Kelvin so the IR compensation has to vary - a bit - with the load on the motor.Since the magnetic field generating the back EMF for a given rotation rate tends to decrease (rather faster) with increasing motor temperature, temperature compensation can get tricky if you want any kind of accuracy.

-- Bill Sloman, Nijmegen

voltage

account

the

the

intact,

OK, i see the difference now.

?-)

he

=20

=20

Jamie was discussing Allison, not Hobbs.

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required