3 ph motor starter used 1 ph to feed VFD and heaters

J_R_Thiele · Jul 18, 2008

I have some 3 phase square D manual starters, and wanted to use to turn on and protect the single phase input to a Teco FM 50. This is for a 1 hp bridgeport J head, The 3 phase heaters it has are sized for the same head- but for 480 volt service.

Any problems with replacing 2 of the heaters, removing the 3rd, and using this for the single phase. Is there any need for two heaters? I assume the heaters for single phase would be would be 3 times the rating for 3 phase heaters.

Any suggestions/comments appreciated.

JunkyardJ · Jul 19, 2008

You might not be able to use it.

Depending on the current rating, it might not work. It would have to be rated for the INPUT current for the VFD. Another thing is I dunno if having a heater type motor overload on the input side of a VFD will work. It might have some problems because it's not really designed for that. I'd say it should work, but they have better ways of doing that. If there's allready a breaker or fuse inline, I'd do away with the overloads completely, find a way to bypass them. Then you just have a power switch. Make SURE it can handle the current, the input side of the VFD is going to draw MORE than the motor did originally. If it's sized for the motor, it might not be able to handle the juice. :skep:

2Slow · Jul 19, 2008

Most VFDs have built in overload protection so I don't think you need to put a contactor ahead of it.

If you do do it, the single phase heaters should be sized for about 1.73 times the 3 phase FLA.

-Joe

toolnut · Jul 21, 2008

All you need in front of the VFD is a fused disconnect. Select fuses per the instructions in the VFD manual and just use the disconnect to power up and power down the VFD. A 30 amp fused disconnect cost about $20 at Home Centers and the fuses will be about $3 to $4.
The starter heaters will not provide short circuit protection as fuses or breakers will. If you do not use a fused disconnect then the VFD will need to be on dedicated circuit and the breaker sized to provide the short circuit protection. If the breaker is providing the short circuit protection, then you can use the manual starter as a switch just bypass the overloads. In this case, you do not need the fused disconnect.

I have been using fused disconnects on VFDs for many years and they are fine for home shops.

Jraef · Jul 21, 2008

JunkyardJ said:
You might not be able to use it.
Depending on the current rating, it might not work. It would have to be rated for the INPUT current for the VFD. Another thing is I dunno if having a heater type motor overload on the input side of a VFD will work. It might have some problems because it's not really designed for that. I'd say it should work, but they have better ways of doing that. If there's allready a breaker or fuse inline, I'd do away with the overloads completely, find a way to bypass them. Then you just have a power switch. Make SURE it can handle the current, the input side of the VFD is going to draw MORE than the motor did originally. If it's sized for the motor, it might not be able to handle the juice.

toolnut said:
All you need in front of the VFD is a fused disconnect. Select fuses per the instructions in the VFD manual and just use the disconnect to power up and power down the VFD. A 30 amp fused disconnect cost about $20 at Home Centers and the fuses will be about $3 to $4.
The starter heaters will not provide short circuit protection as fuses or breakers will. If you do not use a fused disconnect then the VFD will need to be on dedicated circuit and the breaker sized to provide the short circuit protection. If the breaker is providing the short circuit protection, then you can use the manual starter as a switch just bypass the overloads. In this case, you do not need the fused disconnect.

I have been using fused disconnects on VFDs for many years and they are fine for home shops.

What they said...
You are thinking too much. No need for the manual starter at all, in fact it is not a good idea. You need a means to disconnect power, and a short circuit protective device, i.e. fuses or a CB (dedicated). The FM-50 has the motor overload protection built it.

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3 ph motor starter used 1 ph to feed VFD and heaters

J_R_Thiele

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2Slow

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toolnut

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Jraef

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