Any quality VFD will have a precharge circuit..... and that drastically
reduces the problems with turn-on. Turn-on is otherwise a stressor for the
diodes and for the bus capacitors. Most small VFDs have precharge, and I do
not know of even one large VFD that does not have a precharge circuit.
Small ones use a resistor and relay, large ones typically use an SCR
pre-charge. SCR types have no particular operation limit, since teh SCR is
rated for full input, but the relay and resistor type have limits based on
resistor heating for multiple turn-ons.
I definitely agree, it makes NO sense to remove power and apply it again
many times per hour. Far better to use the provided "enable" input. That
may entail some re-wiring, but it is worth the trouble.
Only if there is a stringent energy saving requirement might the power
on/off make *some* sense, and in such a case the VFD will need to be
designed to handle that. Even in that extreme case, I would expect that
some reasonable design work would let the unit satisfy the requirement while
maintaining a power connection.
In any event, the subject Huanyang unit didn't fail in a diode or capacitor.
And, it probably has a precharge, based on the "T-90 style" relay I saw
(there would need to be more than one, since that type isn't rated for high
voltage DC as found after the rectifier.)
The Huanyang are known for a fairly high incidence of rapid failures, often
with a "lightshow", although some work for long periods trouble free.
Other brands have failures also, but they tand to be varied, dead analog
inputs, bad outputs, display problems, etc, not concentrated in the power
circuits. Just a word to the wise.... which is hopefully sufficient.
I expect we have beaten this horse thoroughly, and should bury him.
----- Original Message -----
From: "Curt Wuollet" <wideopen1@...>
Sent: Thursday, July 04, 2013 3:12 PM
Subject: Re: [atlas_craftsman] FYI VFD
> On a related note: Most VFDs are rated for limited charging cycles
> per hour. Switching the input on and off frequently stresses the diodes
> and can cause similar damage. Using the drive start and stop is much
> less stressful on everything. The AB Powerflex 40 says 80 cycles per hour
> some other drives are as low as 20. I've seen this cause problems where
> a VFD was added to a motor that was cycled like a compressor or some
> pumps. Machine spindles are often used this way.