VVVF Drives Questions

1. CDF Means "Cyclic Duration Factor"; it is typically (Total loading time /total operation time); but the definition varies for various duty classes of motors.

2. Not necessarily. If you do so - selecting 205 higher rated motor than what is required - you are loosing out on the efficiency of the motor. But, this not-so-good practice of "cushioning" is prevalent still in the industry.

3. Again, not necessarily. Only thing important in Contactor Selection is to take care about the Utilisation Category. You are not economical, if you so oversize your contactor.

If I may elaborate a bit on 2 and 3.

2. Sometimes people think that the problem with running motors on VFDs is one of heat, because when you run a motor at slow speed and the motor depends on an internal cooling fan, then the fan moves less air and the motor over heats. You can compensate by using a larger motor so you have more mass to absorb the thermal energy, but that is actually a skin-flint's folly. It would be less expensive to use a proper motor designed to be operated by a VFD ("Inverter Duty") and/or an externally powered cooling fan.

3. I have seen this contactor sizing issue misapplied as well. People put contactors on the output of a VFD and then wonder why the contacts weld in us, so they think they are solving the problem by increasing the contactor rating. The reality is, they should NOT be putting contactors on the output of the VFD! It not only is causing the contacts to over heat (PWM is high frequency DC pulses that have higher arc temperatures as they open), but the act of opening a contactor on the output side will damage the transistors in the VFD. Maybe not immediately, but the damage is incremental and will cause premature failure. Most VFD manufacturers will void any warranties if they find that you have put a contactor, other than an emergency safety contactor, on the output side. So don't bother with over sizing them, just don't use them at all!

what is the meaning of 'Duty Class', or 'Inverter Duty'?

Regards,

Hi.

1. What is dead load, live load, vertical impact load, braking load, lateral load etc?

2. VVVF drives can use squirrel cage induction motor?

3. Gradual acceleration will be provided by reversing type vvvf. What is reversing type vvvf?

regards,