VFD vs Soft Starter for Power Factor Correction

I find that due to the fact that the variable frequency drive (VFD) rectifies the AC to a DC bus, the power factor drawn from the supply is near unity but has harmonics. Harmonic kill the power factor correction capacitors. You need to install harmonic filters to take out these unwanted inputs. GTOs used to power the electronics generally fail open circuit. If your VFD has a ramp down to stop you need to program in a fast stop. All safety devices like E-stop, pull ropes, and choke control are wired to the fast stop. This takes the rotating energy in the motor and dumps it to the bus of the drive. It may trip the VFD on high bus voltage but it will be faster than the ramp down. If it is critical fit a braking resistor and the energy is dumped to the resistor and it should not trip.

Soft starters on the other hand (new ones) only use thyristors to conduct until it reaches it set point and then the bypass contactor comes in. Now you run with all the challenges of a normal motor. When it comes to soft stopping you have the same challenge.

Most people do not put in upstream contactors with their Soft starters. Read the manual and see what the common methods of starting are. You will find that the recommended method it states is that the soft starter controls its own contactor. Thyristors tend to fail short circuit. This means that if one fails or generally two, the motor could continue to turn even though you have supplied the stop command as there is a short circuit. If the soft starter controls the up stream contactor, is sees that the stop command has been issued and there is still current flowing and it opens the up stream contactor and it guilds to a stop. If you have a soft stop programmed, and you press stop the motor will ramp down. Not very helpful if you have your arm being circulated around the coupling. The safety circuits are wired into the contactor circuit and thus you remove the power and it glides to a stop. Much better than a 60 sec down ramp.

Setting up power factor correction controller should delay to come in and fast to come out. Should you fail to come out, your installation can go leading power factor and conditions like Q factor come into play with circulating harmonics. Set the dwell time to at least 3 minutes so the caps can get a chance to discharge.

VFDs , Soft Start

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