Precor TRM10/GEN06 Service Manual page 28

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 lower electronics module (IFT drive) 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.
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 (IE, the system
is suing more current than expected), the system will flag an ERROR 29.
Error codes are addressed in detail in the troubleshooting section of
this manual.
Figure 8: AC Motor Phase Chart
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