VFD Minimum Output Frequency 2

[FredRosse]

Can typical (60 Hz input, 100 + horsepower ) VFD drives allow output at say, 1 Hz, 10 Hz, 30 Hz ? I understand that driven equipment may have problems at very low speeds, but the question here is if VFD systems usually have a "built in" minimum output frequency, or can you set the VFD at , say 2 Hz to 66 Hz on a 60 cycle system?

 

[M0rt]

Most motor manufacturer's publish a de-rating curve for current relative to speed, from memory this begins at 15Hz - 20Hz. If it is a short-term state as described, then there probably won't be an issue.
Interestingly, this topic caused some conflict between the motor and VFD departments when developing a motor mounted drive. The motor guys work to a typical max. of 155C, but once a VFD is bolted to it, this is heavily restricted as the drive components will be generally rated to 85C.
Cheers,
Mort

 

[waross]

Motor zero speed or VFD zero speed.
To hold a motor rated at 1760 RPM at zero speed with full torque load may require the VFD to have an output equal to 40 RPM in reverse.
Don't forget the slip frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

Developing of nominal rotor current need 0.75-1Hz induced voltage; so at zero rotor speed, stator field move at minim this speed, to develop nominal torque.
At such low frequency, reactance are very small, and to keep flux constant, voltage must be low also, resulting an small average duty cycle; for nominal current, conduction loss are smaller but conduction time for one device may lead to a higer local temperature, depending of thermal impedance, cooling at zero speed, modulation methods.
Much worse in VFDs with buck front-end PFC, where average duty cycle may remain almost constant for entire speed range.