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I have a 15hp 3-phase motor on a 50A 120/208 wye circuit. I also have a small 1A dc motor with controller and some monitoring circuits - this secondary system needs 24vdc and 1.5A altogether. I have an ac-dc converter that's rated up to 240VAC and about 1A input current to provide the DC power.

  1. Is it safe to wire the converter with the incoming 3-phase for the motor? I figure since the starter coil is using L1/L2 I can connect the ac-dc unit to L2/L3 and have a balance on the 3 legs. The motor runs for a max of about 1hr continuously, it's fla is 39A. Most of it's operation is for short durations - about 10min at a time.

  2. If this is acceptable are there any extra precautions other than fusing the secondary unit? Also, in case the large starting current cuts the DC power, what would be a good solution to avoid the voltage dips besides a large capacitor?

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That seems ok. You may want to check to make sure there is not a code violation, but I believe that is permitted. It would be best for the capacitor to be designed into the DC supply rather than added on. The supply may already have sufficient capacitance. Also, if the input voltage range is wide, it may be ok as is.

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  • \$\begingroup\$ I actually haven't found any info on the regulations about this sort of thing. The whole system is mounted on a custom-built rig so I'm guessing since it's not a building installation it shouldn't be subject to NEC regulations beyond the breaker panel and disconnect - would this be a correct assumption? \$\endgroup\$ – user64054 Jan 29 '18 at 18:37
  • \$\begingroup\$ If it is powered by electric service to a building and not a motor vehicle or boat, it is probably covered by NEC. Local codes may not enforce equipment installations in factories if the place has a qualified maintenance person. A custom-built rig could be required to have a UL or equivalent label, but could be considered as an installation of several pieces of equipment. NEC does specifically allow one branch circuit for a motor plus other items and the requirements seem to be covered by your description. My NEC is quite old and I didn't read it carefully. \$\endgroup\$ – Charles Cowie Jan 29 '18 at 19:28
  • \$\begingroup\$ Ok I'll have to do some more digging to make sure it's compliant. Thanks Charles! \$\endgroup\$ – user64054 Jan 29 '18 at 19:30

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