Service manual

Revision D4-16
Troubleshooting: Brushless DC Drive Theory
Series 2000 Treadmill
409110-004
Brushless DC Drive Theory
General Description
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. These position sensors are typically Hall-effect devices because
of their low cost and immunity from environmental conditions.
Phases and Power
Switches
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.
Motor Torque
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.
Motor Speed
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 required to drive the current loop. The actual motor
speed is dependent on the speed command, the load, the error gain, and
the characteristics of the motor itself.
Adding an integrator to the error amplifier allows exact speed control.
Any speed error results in an output from the integrator that builds with
time and causes an output to the current loop. This loop, therefore, will
seek zero speed error. This is the type of velocity loop used in the
Marquette Series 2000 Treadmill drive.