SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 5.19 – Buried permanent magnet rotor.
Field Weakening
Field flux weakening (usually called field weakening or flux weakening) was originally a brushed
DC motor concept and its purpose is simple to understand. Assume that a separately excited brush
DC motor is loaded to maximum continuous rated torque at rated speed (known as base speed
[25, p.405]); the armature and field currents will be at their maximum rated values. The
magnitude of the bEMF is proportional to the rotational speed of the shaft, so at base speed it will
be at a maximum value as well. In steady-state the terminal voltage is the sum of the bEMF and
the resistive voltage drop (which is at maximum as well since the current is at maximum). The
supply voltage under these conditions is the rated supply voltage which should not be exceeded;
thus it can be though of as the maximum voltage available from the armature supply. Although
the armature and field currents cannot be pushed beyond their limits, the motor can safely operate
at speeds greater than base speed. With the armature voltage and current fixed, the only thing that
can be changed is the field current. The bEMF constant K E is proportional to field flux. If the
field current is lowered, less flux will be produced, which raises the value of the bEMF
“constant.” Since the voltage “across the bEMF” (that is, the terminal voltage minus the fixed IR
drop) is fixed, but K E has been increased, a greater shaft speed results. However, torque is
proportional to field flux 39 so this increase speed comes at the cost of a reduction in torque.
Therefore, the region of operation below base speed is characterized by constant maximum torque
at any speed; this is accomplished via armature current control. Operation above base speed is
39
Recall that ke( r) kt(
r)
and for a brush DC motor this simplifies to KE KT
, thus when K E is
lowered K T will be lowered as well.
225