Danfoss Turbocor TTS Series, Applications And Installation Manual

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voltage ripple to provide a smooth DC voltage. The inverter that converts the DC voltage into an adjustable three-
phase AC voltage. PWM signals from the BMCC control the inverter output frequency and voltage. By modulating
the on and off 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.
NOTE
The variable frequency drive (VFD) or inverter, supplied as standard with all Turbocor compressors, is mechanically, electrically, and
logically integrated with the operation of the compressor and its supporting magnetic bearing system. One of the most critical functions
handled by this tight integration is the regenerative power feature which extracts power from the spinning rotor to ensure that the
magnetic bearing system is fully functional during a power loss event. Because of the close integration between the compressor and the
VFD, Danfoss LLC cannot support the use of non-integrated VFDs due to the extensive development required to ensure the same
functionality and reliability as the standard integrated VFD.
3.4.2 Soft Start
The Soft Start 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.
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.
3.4.6 Expansion Valve Control
The onboard Electronic Expansion Valve (EXV) driver uses manual control only. Depending on the application, a load
balancing (hot gas bypass) valve can be manually driven by the auxiliary EXV output. Load balancing allows the
compressor to obtain lower capacities at higher pressure ratios. The valve opens to lower the overall pressure ratio
and thereby reduces the lift, enabling the compressor to reduce speed/unload.
3.4.7 Motor/Bearing Control
The magnetic bearing system physically supports a rotating shaft while enabling non-contact between the shaft and
surrounding stationary surfaces. A digital bearing controller and motor controller provide the PWM command
signals to the Bearing PWM Amplifier and Inverter, respectively. The bearing controller also collects shaft position
inputs from sensors and uses the feedback to calculate and maintain the desired shaft position.
M-AP-001-EN Rev. S-9/8/2021 Page 31 of 136