HomeDefinitionVFD "Motor Pre-heat function"

VFD "Motor Pre-heat function"

Some VFD Manufacturers have a function called "Motor Pre-heat function" that can be enabled via a couple of parameters. This function produces a low level DC Current (actually AC at 0 Hz) that heats up the motor windings to prevent condensation, while motor is de-energized/at rest, which is the same function of motor space heaters.

This feature actually came from a feature needed in controlling Container Cranes, called "level lufting". That function is to provide full load torque at zero speed to hold container in place while suspended in space while in lifting cycle but at zero speed. This function also proved valuable to use as "motor pre-heat function".

A good application of this "Motor Preheat function feature in HVAC is on Cooling Towers in conjunction with "Sleep/Wake Mode".
If your motor has heaters in it pretty much every VFD has relay outputs. If power is within limitation of relay it can be done right off the variable frequency drive. If not add a control relay. Program for not running and it will function like it would from a motor starter setup.

Many of the HVAC VFD manufacturers simply call that feature "DC Injection at Start".

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