VFD to 2 motors

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VelocityDuck
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VFD to 2 motors

Post by VelocityDuck »

So I have this idea...

Moving the belts on my ShopTask BridgeMill to change speeds is a pain. I've sourced the motors for the lathe and mill along with the VFD controller. I got lucky and Clough42 did all the work for me there. But I was thinking. The lathe and mill motor will never be used at the same time. So if I put a switch between the controller and the motors, I can switch between the two motors and the controller I'll be able to have less cluttered install and save some money at the same time.

My problem is the switch. I've found some switches that are rated for 250V at 20a. But I haven't found any 3p. I suppose that I could tie the neutrals together and only switch the live legs. Not sure if that's a good idea though.

Thoughts?

Suggestions?
John Hasler
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Re: VFD to 2 motors

Post by John Hasler »

All three legs are live. Opening only two would effectively disconnect the motor so that it would not draw any current but that third leg would still be hot as would all the wiring in the "unpowered" motor. This could be a safety hazard. Don't do it.
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Re: VFD to 2 motors

Post by VelocityDuck »

Thanks.

Any suggestions on a 3P2T switch?
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Steggy
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Re: VFD to 2 motors

Post by Steggy »

VelocityDuck wrote: Thu Dec 31, 2020 8:37 amSo I have this idea...I suppose that I could tie the neutrals together and only switch the live legs.
There is no neutral in three-phase power. As John said, all three legs are "hot" with respect to each other, as well as to ground.

The ideal arrangement is to have a three-pole, double-throw (3 form C) switch of the rated capacity. Note that the switch's rating should be for the larger motor's full-load amperes. Such switches do exist, but you'll likely pay as much for one as for a second VFD.

The alternative is to switch two of the phases and let both motors share one phase. You'd need a double-pole, double-throw (2 form C) switch, which won't be as costly as the 3 form C switch. Again, as John said, doing this creates a potential safety hazard, since the unused motor will be energized with respect to ground. Think carefully about this before you proceed.
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John Hasler
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Re: VFD to 2 motors

Post by John Hasler »

>There is no neutral in three-phase power.

There is, but it isn't one of those legs. Three phase motors don't generally need a neutral.
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Re: VFD to 2 motors

Post by toddalin »

VelocityDuck wrote: Thu Dec 31, 2020 11:33 am Thanks.

Any suggestions on a 3P2T switch?
Don't use a switch. Use a 12 volt contactor and use a wall wart to switch that. That's how I did my table limit switches.

In fact, I never switch the forward/reverse "on switch" for either the lathe or mill when live. I always leave the contactor off until this connection is made. Therefore, the cheap Chinese switches never have to switch the load and the points don't arc/burn.
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Steggy
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Re: VFD to 2 motors

Post by Steggy »

John Hasler wrote: Thu Dec 31, 2020 2:09 pm >There is no neutral in three-phase power.

There is, but it isn't one of those legs. Three phase motors don't generally need a neutral.
There is no neutral in three-phase power and, in fact, neutral usually doesn't exist in power transmission and distribution until the final distribution transformer (e.g., the one on the pole in the alley behind your house) is reached. At that point, neutral is provisioned for use by single-phase loads by connecting to the center (common) connection of a wye-connected transformer secondary, or by connecting to a center tap on one secondary in a delta-connected transformer.

Using a wye connection, the voltage from any phase to neutral will be E × 0.577, where E is the voltage across any two phases. In a delta arrangement, the voltage from phases A or C to neutral will be E × 0.5. The voltage between phase B (the "wild" leg) and neutral will be E × 0.866.

In either case, the neutral is used by single-phase loads. In a machine tool, for example, control and DRO circuits, and possibly a work light, would be single phase. The motor running the machine would be three-phase and would have no need to connect to the neutral.
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Re: VFD to 2 motors

Post by toddalin »

After a bit of thought..., cheapest and easiest way to go with no concerns for power handling, because you can get big-azz contactors fairly cheap.

Use two three pole contactors. They can be low voltage with a wall wart, or full 120 or 240 volt.

Use a dpdt center off toggle switch to send power to whichever of the two contactors you want to activate.

It's that easy and <$20 from the 'bay.
Last edited by toddalin on Thu Dec 31, 2020 4:11 pm, edited 2 times in total.
John Hasler
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Re: VFD to 2 motors

Post by John Hasler »

The center of the wye is neutral. It's virtual in a three wire system but can always be regenerated. "Neutral" and "grounded conductor" are not the same thing. The grounded conductor in a three phase system can be the neutral, one of the phases, or a center tap off a delta connected transformer.

Calling the grounded conductor in a single phase system "neutral" is, strictly speaking, a misnomer.
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Re: VFD to 2 motors

Post by Steggy »

John Hasler wrote: Thu Dec 31, 2020 3:54 pmCalling the grounded conductor in a single phase system "neutral" is, strictly speaking, a misnomer.
True that, although neutral will be bonded to ground in the breaker/fuse panel.

Back in the days when my company used to regularly install serial-interface terminals as part of computer systems, we'd occasionally run into ground plane potential imbalance (GPPI). This would manifest itself as a cyclic current flow through the data cables' shield, which is grounded as a matter of course. At best, it introduced noise into the circuit and caused occasional errors. At worst, it would damage equipment.

In the majority of these cases, it was found that some bozo had assumed that ground and neutral were one and the same, and had tied what should have been the neutral of a load to the local ground (the box and conduit) instead of an actual neutral wire. Since the interconnected network of EMT and boxes has a small amount of resistance, voltage drop would develop, leading to GPPI.

GPPI was particularly a problem when the server was in one building and peripherals (terminals, printers, etc.) were in another. In such cases, we'd resort to a fiber optic link.
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John Hasler
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Re: VFD to 2 motors

Post by John Hasler »

Shields should not be grounded at both ends. Between buildings use RS422.
VelocityDuck
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Re: VFD to 2 motors

Post by VelocityDuck »

toddalin wrote: Thu Dec 31, 2020 2:38 pm
VelocityDuck wrote: Thu Dec 31, 2020 11:33 am Thanks.

Any suggestions on a 3P2T switch?
Don't use a switch. Use a 12 volt contactor and use a wall wart to switch that. That's how I did my table limit switches.

In fact, I never switch the forward/reverse "on switch" for either the lathe or mill when live. I always leave the contactor off until this connection is made. Therefore, the cheap Chinese switches never have to switch the load and the points don't arc/burn.
Can you suggest a contactor? I'm guessing that I would need three?
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