Section 9 - Brushless Motor Control; Description Of Brushless Controller; Brushless Motor Review; Starting The Brushless Motor - GE GEH-5201 Instructions Manual

SECTION 9 - Brushless Motor Control
9.1 Description of brushless
controller
A brushless controller must provide the functions peculiar to
starting and protecting a brush less synchronous motor. To
understand the functional requirements, it is necessary to
review the construction of a brushless motor.
9.1.1 Brushless motor review
A brushless motor is like a conventional slip-ring motor in
that it has rotor mounted field poles which must have dc
supplied to their windings so that the rotor poles can "lock" onto
the rotating stator field and run in synchronism. Also, like the slip
ring motor, amortisseur windings are built into the tips of the
rotor poles to provide acceleration and damping torques during
starting and normal operation. During start, the motor acceler-
ates to near synchronous speed. When the rotor is close enough
to synchronous speed for the field poles to pull the rotor into
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Figure 22. Typical rotating rectifier exciter schematic diagram
with synchronous motor
synchronism, dc is applied to the main field and the rotor then
pulls into step, and normally operates at some power factor
equal to or more leading than unity.
The brush less motor has, as its name implies, no brushes or
slip rings. Instead, it contains a rotating exciter with stator
mounted dc windings and the armature winding on the rotor. A
rotor-mounted, solid-state rectifier converts the ac from the
exciter to dc for the main-field poles. The Silicon Controlled
Rectifiers (SCR) and control circuitry are rotor mounted, along
with the field discharge resistor, to control the application of dc
to the main field at proper rotor speed and angle. From Figure
22, it is seen that the actual field control is provided with the
motor and is not part of the motor controller.
9.2 Starting the brush less
motor
The brushless motor is started by first applying power to the
stator windings followed by application of dc to the exciter field.
See Figure 23.
There are two basic timing functions a brush less controller
must provide during start.
1. Apply dc to exciter (not main) field a given pre-set time
after stator windings are energized.
2. Enable the power-factor, pull-out protection and provide
a contact for signalling external automatic motor loading devices
for motor loading. This is determined by a second given pre-set
time delay that allows sufficient time after the application of the
exciter field for the motor to synchronize and stabilize.
Both of these times can be entered as set-point parameters
in the CR192ILSPM. (See brushless programming, Section 9.6.)
9.3 Protecting the Brushless
Motor
The brush less motor is like the conventional synchronous
motor in that it requires special start protection and power factor
pullout protection. These two functions are provided in the
CR192ILSPM.
9.3.1 Stall protection
Stall protection is derived from stator current inputs from
two lines feeding the motor. The protection characteristic
IS
shown in Figure 24. After the motor synchronizes. the CR 192
35