Variable speed drive with integral bypass contactor

A method of operating a VSD powering a compressor/motor in a chiller system to eliminate losses associated with the VSD at full power, the method comprising:

• monitoring a voltage of an AC input source connected to the VSD, an input frequency of the AC input source, and a distortion level of an output voltage of the VSD;
• initiating a transfer process in response to the AC input source voltage and frequency being equal to an output voltage and frequency of the VSD, and the distortion level of the VSD output voltage being less than a predetermined distortion level;
• increasing the output frequency of the VSD to approximately equal the input frequency of the AC voltage;
• adjusting a DC link voltage of the VSD until the output voltage of the VSD is approximately equal to the voltage of the AC input source;
• enabling a current limit of the VSD;
• closing a bypass contactor, the bypass contactor being connected in parallel with the VSD, between the AC input source and the compressor/motor;
• and
• disabling the VSD while the VSD is operating at a frequency within a predetermined range of frequency of an AC input source.

A variable speed drive system (VSD) configured to receive an input AC voltage at a fixed AC input voltage and frequency, and provide an output AC power at a variable voltage and variable frequency, the variable speed drive comprising:

• a converter stage connected to an AC power source providing the input AC voltage, the converter stage being configured to convert the input AC voltage to a boosted DC voltage;
• a DC link connected to the converter stage, the DC link being configured to filter and store the boosted DC voltage from the converter stage;
• an inverter stage connected to the DC link, the inverter stage being configured to convert the boosted DC voltage from the DC link into the output AC power having the variable voltage and the variable frequency;
• and
• an integral bypass contactor connected in parallel with the VSD between the AC power source and the AC output power, the integral bypass contactor configured to bypass the VSD when the VSD output frequency and voltage are approximately equal with the AC input voltage and frequency;
• wherein the VSD is configured to transfer a load from a VSD operation to a bypass contactor operation in response to the control panel calling for a connected load to operate at or about the frequency of the AC input voltage, wherein a range of deviation of the AC input voltage frequency is determined by plotting an efficiency of a non-VSD equipped chiller system against the chiller system equipped with the VSD using an integrated part-load value.