PowerTec 2000C Instruction Manual page 10

© copyright 1995 by
Powertec Industrial Motors
operation. The motor is converting the electrical energy from the
supply into mechanical work at the motor shaft.
When any electric motor rotates, it produces a voltage at its
terminals due to the movement of the windings through a mag-
netic field, as in the case of the brush-type DC motor or the
induction AC motor. This potential is called Counter-Electro-
Motive-Force (CEMF,
for short). CEMF is
produced by the motor
even when it is
drawing power from
the supply, and the
CEMF tends to oppose
the flow of current
from the supply to the
motor. In the case of
the Brushless DC
motor, a field pro-
duced by the magnets
on the rotor is moving
around the stationary
windings of the stator.
If the voltage pro-
duced by the rotation of the motor shaft (due to the CEMF)
exceeds the supply voltage, the motor cannot draw current from
the supply. Usually this condition is produced when motor speed
is greater than the speed commanded by the reference, perhaps
due to the amount of inertia of the load being greater than the
amount of inertia which can be slowed by the motor in the time
allotted, or when the load is being moved by another force faster
than the motor wants to turn.
A load in motion will come to a free-wheeling stop by
"coasting" in an amount of time determined by the speed, inertia,
and friction of the load. The faster a load is moving, the longer it
requires to stop. Larger inertias (generally speaking, more mass)
take longer to stop, but a higher friction load slows it down faster.
A moving load stops in a coasting situation by dissipating the
energy of motion as frictional heat, which acts as a brake. If
inertia is high and friction is low, the load will take a longer time
to stop. Mechanical brakes may be used to increase the amount of
friction.
Non-regenerative motor controls do not have the ability to
slow down a load in a time which is less than the motor would
normally slow down by itself, or come to a stop, by coasting. It
cannot act as a brake, so it shuts off and waits for speed to fall
below that commanded by the reference at the time (if the control
Figure 3: Stopping methods for motors compared.
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