Page 23 of 108
M-AP-001-EN Rev. N
Functional Description
3.4.1 Motor Drive System Normally, AC power to the compressor remains on even when the compressor is in the idle
state. The compressor motor requires a variable-frequency three-phase source for variable-
speed operation. The AC line voltage is converted into a DC voltage by Silicon-Controlled
Rectiers (SCRs). DC capacitors at the SCR output serve as energy storage and lter out the
voltage ripple to provide a smooth DC voltage. The Insulated Gate Bipolar Transistor (IGBT)
is an inverter that converts the DC voltage into an adjustable three-phase AC voltage. Pulse
Width Modulation (PWM) signals from the Bearing Motor compressor Controller (BMCC)
control the inverter output frequency and voltage. By modulating the on and o times of
the inverter power switches, three-phase variable sinusoidal waveforms are obtained.
If the power should fail while the compressor is running, the motor switches into generator
mode, thereby sustaining the capacitor charge. The rotor can then spin down safely in a
controlled sequence preventing damage to components.
3.4.2 Soft-Start Board The Soft-Start Board limits inrush current by progressively increasing the conduction angle
of the SCRs. This technique is used at compressor start-up while the DC capacitors are
charging up.
The soft-start function and the variable-speed drive combined limit the inrush current at
startup.
3.4.3 Bearing Motor
Compressor Controller
The hardware and software for the compressor controller and the bearing/motor controller
physically reside in the BMCC. The BMCC is the central processor of the compressor.
3.4.4 Compressor Control The Compressor Controller is continuously updated with critical data from external sensors
that indicate the compressor’s operating status. Under program control, the compressor
controller can respond to changing conditions and requirements to ensure optimum system
performance.
Figures 3-5 to Figure 3-7 shows how the controller responds to chiller demands.
3.4.5 Capacity Control One of the Compressor Controller’s primary functions is to control the compressor’s motor
speed and IGV position in order to satisfy load requirements and to avoid surge and choke
conditions. However, the majority of capacity control can be achieved via motor speed.
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