Drive Controller Theory; General Description; Phases And Power Switches; Motor Torque - GE T2100 Service Manual

Drive Controller Theory

General Description

Phases and Power Switches

Motor Torque

Motor Speed

Revision A
Theory of Operation: Drive Controller Theory
A brushless DC motor consists of permanent magnets that create a static
magnetic field and electromagnets that, when energized, provide motion.
The magnets attach to a shaft to form a rotor with an even number of
magnetic poles. One or more electromagnets are wound on a laminated
steel stator to form the motor phases. Typically, brushless DC motors
have four, six, or eight magnetic poles with three winding phases.
With multiple motor phases, there is always one of the phases that can
be energized to provide rotational torque-regardless of the rotor position
with respect to the stator. Energizing the phases in the proper sequence
and polarity provides constant unidirectional torque. Transistors have
replaced brushes to accomplish phase switching. Sensors determine the
rotor position and turn on the correct motor phase.
The most common arrangement of phases and power switches is the
three-phase, Y-connected windings with six power switches. Each phase
consists of two windings in series, spaced 120 electrical degrees apart.
Each phase can be energized in either direction by turning on two of the
six power devices. This arrangement of switches and motor windings is
identical to an AC motor drive. The number of electrical cycles per
mechanical revolution is equal to the number of rotor poles divided by
two.
To control motor torque, it is necessary to control the current through the
motor windings since torque is directly proportional to motor current.
Rapidly switching the power devices on and off limits the effective
voltage applied to the motor winding. Using a feedback loop, a voltage
command controls the motor current. This is called the current loop or
inner loop of the drive.
Control of motor torque is only half of the motor control process, since the
ultimate goal of a motor drive is to provide an adjustable motor speed.
The simplest speed control loop consist of a summing junction to
determine the difference between the desired motor speed and the actual
motor speed, along with a gain block that feeds into the current loop. The
current loop may be considered a single block with voltage as an input
and motor current as an output. The overall feedback loop is called the
velocity loop or outer loop of the drive. While this system controls motor
speed, it cannot control the exact motor speed since some error signal is
GE T2100 Treadmill
2021403-031
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