BENSHAW RediStart MX2 SEP, Руководство пользователя

Страница: 91 / 130

7 - Theory of Operation
91
7.1.4 Closed Loop Power Factor Control
The MX
2
SEP controller optionally provides closed loop motor power factor (PF) control. In this mode the motor
will be started as described in Sections 7.1.1 and 7.1.2 for the brush type or brushless type motor. After motor
synchronization has been completed, active PF control will be enabled. As the load and stator voltage of a
synchronous motor changes, the motor’s operating power factor will change. Typically a motor manufacturer
will provide what is called the “V” curves of a motor (see example below) that describe the motor power factor
and stator current, relative to the motor load and the field excitation current level.
In this example, the load curves are drawn at 0% load, 50% load, and 100% load. The dotted unity PF line is
drawn through the curves. When the motor is operating to the left of the unity PF line the motor’s PF will be
lagging (reactive) and when operating right of the unity PF line the motor’s PF will be leading (capacitive).
What can be seen from these curves is that the motor PF varies depending on motor load and DC field current
level. For example, if a motor is operating at unity PF at 50% load, and the load is increased to 100%, the motor
PF will become lagging. When MX
2
SEP PF control is used, the field current will be automatically increased so
that the motor continues to operate at its defined PF level.
Closed loop PF control has multiple benefits:
1. As the mechanical load changes on the motor, the motor PF will change. PF control will actively raise or
lower the field excitation current so that the motor operates at the desired PF set point. Also by operating
at the optimal PF, the stator current magnitude can be minimized, which minimizes losses. By raising the
field current as load increases, in order to keep a constant PF, the pull out torque capability of the motor
will also be increased as the field current is raised.
2. As input voltage levels vary, the PF control can stabilize the PF of the motor to prevent swings in motor
power factor that can occur.
3. A synchronous motor can be used to drive a mechanical load, as well as provide leading (capacitive)
Vars to a plant power system. These leading Vars can be used to improve the overall PF of the facility.
Closed loop PF control allows the user to set a given PF set point; then the PF control will adjust the
motor field excitation current to operate the motor at the desired set point.
Unity
Power Factor
Lagging PF
Leading PF
Field Excitation - DC Amps
St
a
to
r -
A
C
Am
p
s
Rated P
P=50%
P=0
I
