# Should A Vfd (speed Controller) Be Powered Continuously?



## modela (Sep 10, 2016)

I have a question regarding VFD drives.  I understand that if they set a long time the capacitors may discharge.  I have an installation of an exhaust fan that I just installed.  I installed an exhaust fan on my shop with a VFD unit that I switched on the input side.  I have used VFD units where they are continuously powered and switched on and off at the unit.

I am wondering if VFD units should be powered continuously or if the power to the units can be switched on and off.  The fan will be used occasionally.

Thanks,

Jim


----------



## mksj (Sep 10, 2016)

The issue with the capacitors which are electrolytic, is they have a finite lifespan, and heat/time can accelerate the degradation.  Historically older style electrolytic's had a shelf life and if not used for many years it was recommend that the voltage be applied slowly in steps (reforming the capacitor) when first powering up. Newer electrolytic capacitors, this is less of an issue. Power electrolytic capacitors do not like to be switched on and off frequently, as they can heat up and sometimes explode. Motor start/run capacitors, you can often see this happen. On newer  VFDs, I would not leave it on all the time to just use it occasionally (days or months), just turn it on when you need it. It is also not good to turn VFDs on and off repetitively over short intervals (minutes). So if I am working on a machine with a VFD, I leave it on during the day if I am going to be using the machine, switch it off when I am done that day. VFDs usually have a momentary timer which inserts a resistor in the DC bus line when you turn on the VFD, this softens the current inrush as the capacitors charge (seconds), also even after you shut off the VFD there is enough charge in the capacitors that the VFD will stay operational for ~ 20-30 seconds.


----------



## modela (Sep 10, 2016)

Thank you.  That is exactly the answer I was looking for.  I have others that I keep powered all of the time and only use occasionally.  I will power them down when I don't need them.


----------



## JimDawson (Sep 10, 2016)

I guess I have been doing it wrong all these years, all of mine are powered up continuously and have been for years.  The thought is that it keeps them warm and dry.  And am not thermal cycling the circuit boards.


----------



## mksj (Sep 11, 2016)

I mostly am basing this information on general electronic information and practices with tube/electronic gear that use electrolytic capacitors, there is no specific VFD guidelines that I am aware of. A bit like should you leave the computer on all the time or shut it off at the end of the day. Since heat is a primary factor in electrolytic capacitors, not staying on all the time would generate less heat. On the flip side if your shop is not heated and exposed to freezing conditions, you would be better off leaving it on 24/7.  There are also other components that can fail with their operational time. Since VFDs have a internal clock that counts down the hours on the capacitor life, you will use up this time if left on 24/7. Still, most VFDs are designed to operate for ~10 years, if I recall the Hitachi WJ200 states something like 7000 hours capacitor life before an error code pops up. There are newer capacitors that have a longer life span, the rating is usually at the rated maximum voltage of the capacitor, rated maximum temperature and ripple current. If these factors are reduced, then service life can increase significantly. This is one reason why 3 phase VFD need to be derated for single phase use. 

Now if a VFD is not used for more than 9-12 months, many manufactures state to power up the VFD at a reduced voltage for at least one  hour with no motor connected. After this period, shut down the VFD for 24 hours and then it may be used. This process reforms the capacitors. This information is from ABB and WEG.
https://library.e.abb.com/public/cf...bd7be/Guide_for_capacitor_reforming_Rev_G.pdf

So it all come down to use, but if not using a VFD on a daily basis or in a continuous mode, I would turn it off if it is not going to be used daily or using it for just a couple of hours. When I am in the shop "garage" and using a VFD machine, I leave it on for the time I am in the shop. In either case, they are designed to last for many years.


----------



## Thread (Sep 20, 2016)

The biggest reason not to cycle power on VFD's is to prevent excess heat in the resistors that dissipate the DC buss voltage in the capacitors. cycling a drive once an hour will increase the operating temp quite a bit. Once a day, no problem. The additional heat from cycling added to the operating heat will cause premature failure.


----------

