Help spec'ing and purchasing MOV's

I wan't to use MOV's to supress any voltage spikes in my 3 phase system. I will be running around 10 Hp of equipment. The biggest motor will be a 7.5 switched while idling.
I would like some help spec'ing the VOM's I will put between the legs of my 3 phase. I need a good source to buy them as well.
Thank you,
Treven.

"I wan't to use VOM's to supress any voltage spikes in my 3 phase system."

I'm sure you meant MOVs ... metal oxide varistors.

Most of the "usual suspect" distributors carry MOVs. Even Chicken Shack does. Perhaps also HSC.

The trick will be specifing these for large loads, as the easily available ones are intended for protecting individual loads such as consumer small appliances and PCs.

Here is a page with some specs for MOV's:
http://www.nteinc.com/Web_pgs/MOV.html

I'm thinking the 1v250 - left colum 3/4 of the way down the page?

Whoops! yes I meant MOV's - Thanks Peter.
Santa Cruz electronics can get all the ones from that web page if I can just pick the right one. Unfortunatley I don/t quite understand the specifications they have written. I think I just need to know the inital breakover voltage and the the amp rating and the flow voltage. I would want to make sure that it shuts off above the VFD voltage.

Im no EE but Im very suspect about these doing a darn thing for you. You can shock load a VFD and never exceed the design voltage of the MOV.

I spoke to an engineer at GE Drive that told me voltage spike on the output would back feed into the VFD and kick in the Over Load protection so there shouldn't be a problem. So much vague info on the subject........

Maybe I should just go low tech and put some light bulbs between the 3 legs. Two 100 wat light bulbs between each leg in series should help.....

"Maybe I should just go low tech and put some light bulbs between the 3 legs."

I would install line reactors instead.

You need something which is an energy storage device, not a pure resistance (an energy dissipative device) such as an incandescent lamp.

Forrest Addy described how to make a simple but effective line reactor Here.

Both the inductors and the capacitors are energy storage devices, and these work in concert to eliminate hash (transients).

"Even Chicken Shack does. " Wanna bet? I recently went in there to buy a simple surge supressor for a coil that was causing noise problems on a circuit. A) they had no "surge supressors" already packaged and ready to use, which I thought was interesting given the name of their company, and B) when I told the guy I would make them with a resistor, cap and MOV, he looked at me like I had 3 eyes, and said "what's an MOV?" They should really change their name to Cell Phone Shack because that's all they really do there any more.

Forget the MOVs to try to protect the VFD, they are "one shot" devices anyway and you won't know when they are already dead unless they explode, which is a likely scenario as well. Peter's advice on the Line Reactor is the best as far as protecting the VFD from problems in the incoming service, but if you want to protect the drive and motor from transients on the load side, put in a load reactor as well. If you can only afford one, put it on the load side.

"Peter's advice on the Line Reactor is the best as far as protecting the VFD from problems in the incoming service, but if you want to protect the drive and motor from transients on the load side, put in a load reactor as well. If you can only afford one, put it on the load side."

Same device, just on the load side of the VFD.

Review Forrest's design.

While small caps on the load side of a VFD usually aren't a problem, do avoid large caps on the load side.

Jraef -

Have I been living a charmed life? I have always made the decision to put the reactor on the line side when only one would be purchased.

VFDs reflect all kinds of harmonics back on the line - line reactors help suppress this.

Line reactors also afford a bit of protection for the drive during line transients.

Load reactors are typically applied to protect the motor from switching current induced voltage spikes - most motors with inverter spike resistant wiring suffer no ill effects from standard PWM synthesized sinusoidal currents.

I just posted on another thread our experience with quite a number of drives - with long motor leads - operating at 480V - with no load filters - with nary a winding failure.

Now if the purported justification of the load reactor is to protect the drive from load transients . . . the transients that a load reactor would be effective against would be disconnecting / reconnecting of the motor from the drive - which in my opinion is not unlike using your manual transmission without bothering with the clutch, it can be done - but why would anyone want to abuse the drive in that manner?

I respect your opinion on these matters, but would have to say on this topic that I have not experienced any ill effects from not having load reactors . . .

If I were spending the money, I would put in both, they both have merits. But my comment on having only one and putting on the load has to do with my experience in the field. Far more drives die as a result of motor circuit ground faults and shorted windings than anything coming in from the line. Most VFDs have GF and short circuit protection built in, but the load reactor gives it time to react and shut down the transistors before damage is done. Most common source of load faults? Water in the conduit from drive to motor! Condensation forming on the warm conductors going in a cool place, like under ground or inside a cabinet, gets trapped in the conduit and rarely escapes. It builds up until it finds a weak spot such as a pin hole in the insulation, and it's all over for the VFD. Motor windings go bad all the time, hence the need for rewind shops, but without a load reactor you typically lose the VFD too.

Line reactors protect the line from harmonics, not the drive for the most part. I'm not saying they aren't valuable, I'm just expressing my "Sophie's choice" preference so to speak.

As Jraef says MOV's are one shot protection. If it is ever used (i.e, shunting a lightning strike to ground) it is most probabily blown. MOV's are sacrificial elements.

For more information on the shortcomings of MOV's check out 'www.brickwall.com'. I am not connected with them in any way (usual disclaimer) but their Brickwall protection devices have a pretty good reputation in the television and audio recording industry.

John

The MOV's that I saw on the web were tested for thousands of cycles before failing. I belive that was momentary or short voltage spikes not long time over voltage.

[ 07-08-2005, 03:44 PM: Message edited by: chevy43 ]

I have watched drives puff smoke from the enclosures without blowing from the MOV's - they do this a few times before they finally blow up and take the drive out catastrophically -

One plant would have a cabinet full of a dozen drives all mysteriously explode at 5PM on Saturday - 3 weeks in a row!

Turns out they had a 1000HP compressor that they would turn on to run the plant saturday evening so that they could do PMs on the regular compressor. The standby compressor had a phase to ground fault that was powered from a floating transformer. This would cause a huge voltage spike on one of the lines relative to ground on adjacent power distribution systems and blow the phase to ground MOVs out of the drives.

MOVs are rated in Joules as far as the energy they will absorb

As far as VFDs protecting themselves against load side ground faults and phase-to-phase shorts - I have not experienced a failure of this type on any installation to date. The drives we use were subject to a "crowbar" test where the outputs are shorted out while under load and the drive faults with an IOC fault (Intantanious Over Current). Same fault when a loaded conductor is brought to ground.

I haven't had the moisture problems you have experienced, more line power / plant power problems.