I've been tasked with designing a motor disconnect & plug for a 30 HP 480V motor on a VFD. I thought it was going to be easy, but I have the complication of running a shielded cable with he VFD. A normal 480 Volt 3 phase.. I would use a 3 pole 4 wire disconnect & plug combo... but how to deal with the shield? Can I use 4 pole 5 wire combo to use the extra pole for the VFD cable shield?
VFD motor disconect & plug
- Thread starter cornbread
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You also have the problem of VFD's don't like to have load switched beyond the controller terminals, in particular closing circuit to a load while output is already energized. You may need disconnect with aux contacts that break the control signal if the disconnect is open, as well as some kind of interlock to break control signal if the cord is unplugged from the receptacle, which I think can be done, but isn't your standard straight blade or locking receptacles you see all the time either.
A receptacle and cord cap is/can be a disconnecting means, so what is reason you want both a disconnect and a receptacle?
A receptacle and cord cap is/can be a disconnecting means, so what is reason you want both a disconnect and a receptacle?
I'm spec-ing out a early break contact on the motor disconnect that will disable the drive in the event the operator forgets to hit the stop. The disconnect is interlock so the plug can not be removed until it open. The reason we are doing the plug is our lock out policy. We don't trust a disconnect has opened with out verifying it, open door and check for voltage. The plug will prevent the need to verify the disconnect as the plug provide proof positive the circuit is dis-connected.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You answered some additional concerns I brought up.
I don't think you necessarily need a 5 wire plug though. You can use steel raceway and standard conductors and the raceway is the shield, so I think as long as you find a way to bond the shield beyond the receptacle you are fine with a 4 wire receptacle, bonding it in motor terminal housing I would think would work. The disconnect enclosure and outlet box are the shield around those portions of the install.
I don't think you necessarily need a 5 wire plug though. You can use steel raceway and standard conductors and the raceway is the shield, so I think as long as you find a way to bond the shield beyond the receptacle you are fine with a 4 wire receptacle, bonding it in motor terminal housing I would think would work. The disconnect enclosure and outlet box are the shield around those portions of the install.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Lapp, one of the larger suppliers of VFD cables, has a line of pin and sleeve connectors that have EMI shielding rings for connecting to the cable shields, unfortunately I don't see a version that would interlock with a disconnect.
https://lappdigitallibrary.cld.bz/2016-LAPP-Catalog/431#zoom=z
So kwired is right, up TO AND INCLUDING the box with the disconnect, using steel conduit and practicing rigorous bonding is fine, but obviously you are going to need that last connection point to be shielded flexible cable, and you are going to need that shield continuity. I don't see how you would connect the shield to a pin.
https://lappdigitallibrary.cld.bz/2016-LAPP-Catalog/431#zoom=z
So kwired is right, up TO AND INCLUDING the box with the disconnect, using steel conduit and practicing rigorous bonding is fine, but obviously you are going to need that last connection point to be shielded flexible cable, and you are going to need that shield continuity. I don't see how you would connect the shield to a pin.
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