(ET) AC motor braking

["Rhett T. George" <rtg ee duke edu>]

 - Greetings -

Glad to see the discussion of baking in this venue.

The induction motor has lots of places to put AC voltage drops in its
equivalent circuit.  Simple resistance is a drop in the bucket.  [No,
I shouldn't make a pun that terrible.}  Take a look at the rated current
of the induction motor.  This is continuous without getting too hot and,
of course, with its cooling fan running.  Now, measure the DC resistance
of the motor winding.  Multiply that number by the rated current and you
have a figure for voltage.  This voltage applied for no more than, say, 
two minutes will do a heck of a braking job.  If you remind yourself to
use the braking for no more than 30 seconds, you can use twice the 
voltage with a resulting twice the current.  Some motor windings may be
too tender for twice the rated current for more than 10 seconds.

What happens inside is that you have established a big, constant magnetic
field across a spinning set of conductors which are shorted at their ends.
Lots of current flows in these conductors in the rotor and oppose the 
rotation.  From this comes dynamic braking.

A couple of caveats.  The DC voltage is likely too be much lower than 
your full battery voltage.  Don't make a dumb assumption about the bat-
tery voltage being `just right.'  [The assumption of `just right' got
Goldilocks into trouble.]  Also, now that you have this magnetic field
built up to a high energy level, what are you going to do with the 
energy when you are done with the braking?  Simply opening the braking
switch is going to draw a fat, welding-quality, arc.  One method of
handling this energy is to put a resistor in parallel with the motor
winding when braking commences.  Select a resistor (big in wattage 
capability) which yields approx the rated motor voltage when the 
resistance is multiplied by the braking current.  This keeps the energy
from developing too big a voltage in the winding, a voltage perhaps so
big that it causes a breakdown of motor insulation.  Remove the 
resistor from the circuit a few seconds after turning off the braking
current.

Hope this helps.

                                        Rhett George