I am helping a friend who has an AC motor that was originally wired to run on 220VAC but from the looks it is wired to use two capacitors to (somehow) convert a single phase to a 3 phase... the goal is to use a VFD to control its speed/ramp rather than just on/off. I looked online and found a few pages (like this one) that discuss how a capacitor could have been used (and perhaps one is a start and the other a running cap?), but I am somewhat stumped about having 8 wires rather than the 6 I was expecting for the VFD. I understand that there are Y and Delta configurations where the coils are either in series or parallel, but that would mean 9 wires..? Am I missing something?

Here's an image:Capacitors and Wires

Unfortunately I have no mfg# or any specs... wires are also, annoyingly, not labeled.

Any idea on how I would go about figuring out what wire is/does what? I can measure resistances and inductance, but that is about it. I didn't want to hack at this any more than I need to before I have some strategy... but I suppose I could open it up in the hope of seeing something obvious? What I'm really after, is to get feedback on a good approach to determining what each of those wires does and then how I would go about connecting them to a configuration that is suitable for a VFD.

I'd greatly appreciate any thoughts on this... Thx, J


closed as off-topic by uint128_t, Voltage Spike, Wesley Lee, Dmitry Grigoryev, Enric Blanco Apr 28 '17 at 1:21

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  • \$\begingroup\$ Maybe it's a capacitor start capaitor run (cscr) motor check this link \$\endgroup\$ – iMohaned Apr 12 '17 at 3:11
  • \$\begingroup\$ Is there a nameplate on the motor showing power, RPM, etc? \$\endgroup\$ – mkeith Apr 12 '17 at 5:30
  • \$\begingroup\$ That is a single phase motor. Use it as-is or buy a three-phase motor. Sometimes you can find good deals on small three-phase motors on craigslist, if they have that where you are. \$\endgroup\$ – mkeith Apr 12 '17 at 15:16

The motor you have is a single phase motor and it has nothing to do with the link you have pointed to. The link is talking about a way to make a 3 phase induction motor work on 1 signle phase supply.

Single phase motors can not be connected to 3 phase output VFDs. These type of motors have their own type of VFDs which are called single phase output VFDs as this one.

Other things to be taken into consideration:

  1. Capacitors on the output of variable frequency inverters are a bad thing unless otherwise stated in the VFD's user manual.

  2. Single phase motors aren't ideal for inverter applications, the motor may not be rated for inverter use, and if they have switches or whatever inside for starting that can cause problems.

Look here for more information.

Assuming you have taken these considerations into account, just two wires should go from the motor to the VFD. This link has a schematic that could help you make the motor section connections.

My recommendation is that 3 phase induction motors are cheaper, have higher torque, have more efficiency, and you can easily find the VFD that suits your application's requirements.

  • \$\begingroup\$ Not all single phase motors are compatible with VFD's. \$\endgroup\$ – mkeith Apr 12 '17 at 5:29
  • \$\begingroup\$ @mkeith Yes you are right. \$\endgroup\$ – Macit Apr 12 '17 at 10:16
  • \$\begingroup\$ I was thinking maybe you could update your answer to note that only certain AC motors can be driven by VFD, and generally capacitor start motors cannot. \$\endgroup\$ – mkeith Apr 12 '17 at 15:08
  • \$\begingroup\$ Thanks to all for the valuable inputs. I got the chance to take some quick measurements today and conclude that this is (likely) a capacitor start/capacitor run or two value capacitor type (with an extra winding to determine startup direction?). What I measured was Yel-Yel:5.3ohm/37mH, Red1-Red1:1.2ohm/13mH, Red2-Red2: 1.2ohm/13mH. Blk-Blk is presumably a centrifugal switch since it showed 0ohm. So based on this and previous comments, it makes sense to purchase a new motor, probably a 2Hp, that way I know what we have and can use the VFD... \$\endgroup\$ – JoeEg Apr 12 '17 at 21:01

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