Mixed frequencies in same conduit

[mshreff]

Can a 480 volt 3 phase 60 hz circuit supplying a VFD be in the same emt conduit as the 480 volt 3 phase variable frequency load suppling the motor? Are there any issues with this?

 

[S'mise]

Can a 480 volt 3 phase 60 hz circuit supplying a VFD be in the same emt conduit as the 480 volt 3 phase variable frequency load suppling the motor? Are there any issues with this?

I never heard of any.

 

[mike_kilroy]

only if you are a glutton for punishment...... do it and you will likely be fighting such horrendous EMI noise that after hours of not being able to get other nearby stuff and machines to work properly you will eventually remove the motor leads and put them in their own conduit or at least run them separate from any other power or signal lines, preferably in 4 conductor shielded cable. No, you do not want to go there. Keep motor leads separate from everything.

 

[petersonra]

It may have some practical issues, but it is not forbidden by code.

If you do this, you may well get some EMI fed back on the line as another poster suggested.

I suggest not doing this, but it will probably work OK. Note use of the word "probably".

If you are forced by circumstance to do so, maybe some line reactors at the feed point are in order.

 

[stevephillips]

I did an install for a MG set to convert from 60hz to 400hz last month and at first, ran the supply and load cables in the same conduit. I wanted to run it in separate conduits but because the MG set was outside, the customer wanted the wire run in one conduit (100 feet rigid, sized properly per code). This company did avionics testing and certs.

Well, it did not work so well. The 60hz side was full of noise, so much so that the VFDs on the HVAC were having issues (random tripping, etc). The 400hz side was equally bad. The testing equipment that they were using simply would not work and the instruments could not be calibrated.

I came back out and installed new conduit just for the 400 hz (labeled it as well) and this solved the problem.

Did a similar install a few years back for another company but had a 60hz to 50hz MG set and had the same problem with the same solution.

Take away message, use separate conduits for different frequencies. May not be required as per code, but IMO its the way to do it.

 

[iwire]

I think you will find most VFD instructions say not to do so which could make it an NEC violation per 110.3(B).

 

[Besoeker]

Can a 480 volt 3 phase 60 hz circuit supplying a VFD be in the same emt conduit as the 480 volt 3 phase variable frequency load suppling the motor? Are there any issues with this?

I'm with the others. Don't do it.

I can't say it's a problem I've come across, nor one that I would want to.
We (here in UK) hardly ever use conduit and not at all for power that I can recall in decades. We generally use steel wire armoured cable for power so the issue doesn't arise.

And iWire is right. Check what the VSD user manual says. This is the motor cable requirement from the drive manual of one we often use:

Symmetrical power cable equipped with concentric protection wire and intended for the specific mains voltage.

 

[hillbilly1]

I have heard also to keep them seperate, and by the way Steve, they make electronic freq converters now to replace motor/generator sets. I installed one at an air force base testing facility, much lighter, and extremly efficient.

 

[mike_kilroy]

the simple MG set 60/400hz was just nice clean AC non-harmonic filled current and voltages..... the vfd is totally different; not just different frequencies to mix but at 480v you have 1200v spikes at the pwm switching frequency on those motor output leads. THAT noise over 100ft WILL guarantee it will not work. NO amount of inductors or filters would allow that system to not cause major problems.

 

[Jraef]

When you have multiple conductors in the same conduit at the same frequencies, any mutual inductance onto each other is canceled out. But when you have conductors in the conduit that are at DIFFERENT frequencies, they no longer do, so you end up inducing voltages and current onto each other. I have seen situations with multiple drives all feeding their outputs into the same large conduit to separate motors where the voltage spikes coming back onto the drives were destroying the transistors. In the case of the input and output cables being in the same conduit., it generally results in turning the input cables into radio transmitters and harming other nearby equipment.

But it also depends on the length of cables. A few feet probably won't make much of a difference, you can get that much inside of a control panel or MCC. But at what distance it becomes a problem is indeterminate, you won't know until it happens, at which point you have to start over, which costs more than just doing it right the first time.

 

[mike_kilroy]

I thought I read the conduit length was 100 feet: I think I mixed posts. @ 100 ft I stand by my 'guaranteed will not work.' If much less (prob less than 15'), then Jraef is of course right and it slightly MAY work.

I hope the OP will list the actual length and results.

 

[stevephillips]

The OPs question was 15 feet. I posted an experience that I had where the run was ~100ft.

 

[Jraef]

The OPs question was 15 feet. I posted an experience that I had where the run was ~100ft.

Actually I don't see that in the OP. Did he PM you or something?

 

[stevephillips]

I guess I assumed that and did not realize it.