Precor TRM 885.v2 Service Manual page 29

The frequency is then sent to the motor over 3 lines; with each of those lines being phase shifted
120 degrees from each other. This type of AC motor is referred to as a 3 phase AC motor and is
used to allow enough torque to be applied over a continuous duty run time. The amplitude
(voltage/current) of all three sine waves is the same at any given time and allows the torque to be
applied smoothly throughout the rotation of the motor. Increasing the amplitude increases the
torque and decreasing the amplitude decreases the torque.
The Motor Controller (MC) is responsible for sending the proper frequency to control speed and the
proper amplitude (same on all 3 lines) to control torque. The motor must then be balanced both
mechanically and electrically in order to translate the frequency and amplitude into fluid motion.
Mechanical balance is achieved by balancing the weight of the flywheel, and electrical balance is
achieved by all 3 windings being equal (same number of ohms).
Other features of this AC drive motor controller include "Dynamic Braking" and "Power Factor
Correction". Dynamic Braking addresses an issue where an over-speed condition could occur. If a
heavy user runs at a high incline, the weight of the user has the potential to push the running belt to
go faster than the motor control was trying to maintain. The dynamic brake circuit senses the load
variations and applies a braking force within the motor. The system utilizes an external power
resistor to determine the point at which the braking force is applied, and to absorb the extra power
being created by the motor. Power Factor Correction is a feature that attempts to reduce power
consumption. The system monitors that this power factor is held within certain parameters. If it falls
outside those parameters (i.e., the system is suing more current than expected), the system will
flag an ERROR 27. Error codes are addressed in detail in the troubleshooting section of this
manual.
Figure 10: AC Motor Phase Chart
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