60 Hz Motors fed from Hi-Freq

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DHkorn

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I hope someone here can help me with a question.
Just recently I was called to do some service work on two different machines, each using some standard american 60 Hz motors fed with high frequency circuits.
One was a very old machine in a saw mill using a motor/generator set with a 120Hz output feeding four of the motors, thru a 3P 120Hz/60Hz double throw bypass switch.
The other was for a different customer but also a saw. It had a more modern static VFD feeding some of the motors.
My service work on each was on some of the 60 Hz circuits.
My question is does the NEC require any special calc's or treatments for the high frequency branches or feeders? And same question if the branch conductors are mized in a raceway with both frequencies? I checked the NEC and all I found was the 430.120 Adjustable drives info.
On these installations the conductors and sizes looked to be no different than we would use for standard 60Hz motors.
And besides the NEC are there any non required but good standard practices things to do differently, besides special color codes or labeling?
You guys are a great resource. Thanks in advance.
 
Jraef

Jraef

Moderator, OTD
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Location
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Occupation
Electrical Engineer
DHkorn said:
I hope someone here can help me with a question.
Just recently I was called to do some service work on two different machines, each using some standard american 60 Hz motors fed with high frequency circuits.
One was a very old machine in a saw mill using a motor/generator set with a 120Hz output feeding four of the motors, thru a 3P 120Hz/60Hz double throw bypass switch.
The other was for a different customer but also a saw. It had a more modern static VFD feeding some of the motors.
My service work on each was on some of the 60 Hz circuits.
My question is does the NEC require any special calc's or treatments for the high frequency branches or feeders? And same question if the branch conductors are mized in a raceway with both frequencies? I checked the NEC and all I found was the 430.120 Adjustable drives info.
On these installations the conductors and sizes looked to be no different than we would use for standard 60Hz motors.
And besides the NEC are there any non required but good standard practices things to do differently, besides special color codes or labeling?
You guys are a great resource. Thanks in advance.
Click to expand...
No differences in wiring are required based solely on frequency. In fact the higher the frequency the lower the losses in the wires themselves (leaving harmonics out of it for the moment) so if anything you might even be able to get away with slightly smaller wire. However the NEC doesn't allow that either, so don't bother worrying about it.

Mixing frequencies in raceways can be problematic if the variable frequency conductors are not shielded. The other cables can pick up induced voltages and RFI from the variable frequency conductors. The best practice is to use the newer specialized "Inverter Cable" that has shielded triplexed conductors with a ground.

Other considerations are more related to the motors themselves, such as using output reactors or filters on the circuits feeding older motors, or using Inverter Rated motors for new applications. But none of these issues would pertain to the one using the MG set to double the frequency.

This is also assuming someone knew what they were doing in applying higher frequencies to the application in the first place. It's not something that should be done without knowing all the ins and outs.
 
Besoeker

Besoeker

Senior Member
Location
UK
Jraef said:
No differences in wiring are required based solely on frequency. In fact the higher the frequency the lower the losses in the wires themselves.
Click to expand...
A conductor has distributed inductive reactance, capacitive reactance, and resistance. All increase with frequency.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
bob said:
J
I was about to make the same comment as Besoeker. Do you want to clear your statement up?
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OK, under 500kHz and in conductors under 1" thick where the skin effect will have a negligible increase in conductor resistance, you don't need to worry about it. But I seriously doubt many of this forum's users are designing power systems for radio or microwave transmitters.

As to my saying the losses may be lower, that was just wrong. I was thinking in terms of devices such as motors being able to be smaller etc. at higher frequencies, but that's not the conductors.

Still, with regards to the OP's question I don't believe there is any reason to take 120Hz into consideration when sizing the wire. There certainly is nothing in the NEC that says you must.
 
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