The practice of allowing a VFD to “hold” or “lock” the shaft, commonly called “fluxing-up” is a practice that is becoming more frequently utilized in some applications to not only hold the shaft in a stationary position, but to pre-heat the motor to prevent condensation from forming inside the motor.
Different VFD manufacturers utilize different methods of achieving this. Some will switch between a forward and reverse direction with AC, at a high frequency to keep the shaft from moving. Others will allow an adjustable amount of DC to flow, which also effectively holds the shaft in a stationary position.
For example, the current model DanFoss VFDs are equipped with a “Motor Hold Preheat” function located within parameter 1-80. This would need to be set to “On” to allow the VFD to produce a DC voltage to the windings. This effectively holds the rotor stationary and in doing so, the current flow will produce heat as a by-product.
There is a second parameter that must be set in conjunction with the 1-80. This parameter, 2-00, sets the level of current supplied to the motor when 1-80 is on. This will be set as a percentage of the FLA. If indeed you were utilizing this to hold the shaft in a stationary position, this percentage would be in direct relation to the forces being applied to attempt to move it. However if you are only interested in keeping the motor heated, you would need to calculate the percentage of FLA to achieve the recommended wattage (XX watts for the frame size motor that you have in service per EASA recommendations) to keep the motor heated sufficiently.
All of the current EASA ( Electrical Apparatus Service Association) information gathered has indicated that this does not cause any issues with the motor, its lifetime or warranty. This practice will reduce your overall energy efficiency for the system as the VFD is powering the motor. This practice is really recommended as primarily a stop-gap measure until a proper cooling tower motor or TEFC motor with internal heaters is installed.