VFD-Pyrometer interference

S

Surly

Joined
Apr 26, 2000
Messages
63
Guys, I thought I'd share my problem (and current resolution) here and get some feedback. My shop is currently 110V only. I recently rescued an old, small Cressi 100 electric kiln. The element was ok, but the thermocouple and pyrometer were shot. Sitting in 6' of water is apparently not that great for electronics.

I replaced the thermocouple and the pyrometer, but couldn't get a decent reading from the pyrometer. It would jump around and give odd negative numbers, etc. After several phone calls to the company in frustration, I took it home to box it up and ship it back. I decided to try it one more time, and surprise, it functioned just fine.

I eventually discovered that the VFD connected to the grinder is screwing up the pyrometer. As soon as I unplug the VFD, the pyrometer works.

Does anybody know how to "isolate" the VFD? Worst case scenario is, I can't heat treat and grind at the same time. Not a huge deal, but if there is a simple solution, I'd like to use it.

Here's a photo of said oven:
heattreatoven0017dn.jpg
 
can the pryometer be converted to run on battery power?

I have a Bartlett pyro that runs on 9VDC
 
I can't answer your question, but I do have a similar problem with my VFD. I can't listen to the radio while the VFD is on, all I hear is a faint humming. No problem playing CD's, however :).

I'd say unplug the VFD when you want to run your oven, or move the oven to another room. Sorry I can't be of more help.

Matt
 
Daniel, I don't know. I know they sell one that is battery powered, but I not sure if it's possible to convert this particular unit. The adapter is hard wired in there.
 
You can also buy a switchbox. Essentially a single-pole, double throw switch that will hook either one or the other at a time. I'm considering this option for a couple of my 220V tools myself.
 
One of the potential problems with VFD's is 'noise' generation back onto the the supply line. This 'noise' is called harmonics and can cause some pretty severe problems for other electronics devices using the same supply source. Some devices are more sensitive to this than others.

One thing to consider checking is to make sure that the 120 volt circuit you are feeding the VFD is NOT the same phase and/or circuit as the circuit that supplies your pyrometer. If they are the same phase, simply swapping your pyrometer to the other phase may very well resolve this problem.

If this is not your problem or doesn't help then........

The best fix may be to get an isolation transformer to supply your VFD. Basically what this is, is a transformer that is 120 vac input and 120 vac output. Since there is no direct connection between the incoming power and the output power on this transformer, the noise is generally filtered out.

The transformer has to sized to handle the full load current of the motor and I have no idea of how costly one might be. There should be a local motor shop somewhere near you that could help out with this. But make sure you ask about an isolation transformer.

Mike
 
VFD's, like any other piece of electronic/electrical equipment, can produce two kinds of interference. The first is conducted through the mains supply connection. The second is radiated through the air. There are regulations relating to both forms of interference and it takes a lot of R&D work to make a VFD meet these regulations. Without being able to connect some instruments on your setup, it is difficult to know which form of interference you do have but is most likely that your interference is conducted.

Check that the connection of ground wire for your house to the ground rod is good. A poor ground will reduce the effectiveness of any solutions suggested.

Your least expensive option is to do what Mike suggested and run the oven using a different circuit. This will give the VFD and pyrometer separate Ground return paths. You are looking for two circuits that are on different circuit breakers in your main panel.

The VFD Applications Engineer that I spoke with here at work suggested putting an noise filter on the mains input of your VFD. For a 1 - 2 hp VFD, the cost of such a filter would be in the range of $200 - $300.

If the pyrometer and the oven heating elements each have their own power cord, it would be simpler to put a noise filter or isolation transformer on the mains input for the oven controller. The pyrometer draws much less power so any filter or isolation transformer that you use would be much smaller and cheaper than one that supplies the VFD. E-mail me if you want to try a noise filter for your pyrometer. My employer makes VFD's and other electrical equipment and they scrap out a lot of stuff that is too expensive to repair. Many components, like the noise filter, survive the blow-ups.

Putting a ferrite core around the power cord for your VFD and or your pyrometer may also help if the noise is in the higher frequency ranges. Radio Shack sells one (273-105) for $5 - $6. Check that your power cord will fit first.

Good luck.

Phil
 
That is very interesting and can understand the possibilty of that occuring, although I would not have wondered about it before it happened to me. I have seen many and a many electronic mysteries. The obvious first choice of elemenation I might have is to be sure all connections are mechanically sound and both case and all other shieldings are well earth grounded. There is a possibilty, and I am no expert on the specific theory of VFD or electronic pyrometrics, of the VFD (variable frequency - remember) interfering with the pryrometer electronics (hence the possible necessity for adaquate - grounded - sheilding). Other than that it beats me.

RL
 
Phil's suggestion on filtering the power to the pyrometer is a good one (definitely cheaper!) and I can't imagine why I didn't think of it---senior moments are becoming FAR too common!:eek:

What I was thinking of when I suggested putting your oven and VFD on diferent phases/circuits was a little more involved than just putting them on seperate breakers (though that may work). Seperate breakers in a panel can actually be the same phase and you may still get the harmonics interference. The opposite phase can provide more isolation.
In your 120 volt circuit breaker panel you will almost certainly have two hot wires feeding it. These two wires have 220 volts between them but only 120 between each one and neutral/ground.

If you need more specifics, shoot me a PM.



Good Luck!

Mike
 
Thanks guys, that's a ton of info! I'm working on mapping the power/temp curve for the oven right now.

I'll post it for grins once I get done.
 
Hello Surly

What is the purpose of "mapping the power/temp curve for the oven"?

Phil
 
pso said:
Hello Surly

What is the purpose of "mapping the power/temp curve for the oven"?

Phil
Click to expand...

Perhaps generating a analog dial-setting % versus observed temp?? Just guessing, since I'm not "Surly", just surly... :)
 
Exactly. It's an analog dial, so I need to figure out what each setting corresponds to (at least within a few degree range) if I want to have any kind of accuracy.
 
You must log in or register to reply here.
Back
Top