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Single phase milling machine recommendations

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not done it yet28/06/2022 08:01:41
6888 forum posts
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As per Andrew, really. But in addition, there is likely a 3 phase supply quite close - as two thirds of of the houses, nearby, will likely be fed by one of the other two phases.

Other advantages of a VFD include soft-starting, braking, safety trips and others. No need for the usual starters/overloads as the VFD replaces that item.

John Haine28/06/2022 08:15:31
4718 forum posts
273 photos

Why YT? This https://topcharger.co.uk/can-i-get-three-phase-power-at-home/ May be just as useful.

**LINK**

Suggests 4000 to 7000 sqids. A decent vfd looks a bargain. Why bother with horrible capacitor bodges?

Macolm28/06/2022 22:03:17
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I agree with the sentiment that converting the three phase main drive motor of a machine to single phase with a capacitor is a poor idea. However, for something like a coolant pump motor, it seems scarcely worth the expense of a VHD if it runs well enough. This is what I converted, and anyway it was before the availability of affordable VFDs.

I perhaps did not make it clear enough what the (mild) concern is. At resonance, the inductive and capacitive reactances are equal in magnitude but opposite in phase, so they cancel and thus there would be no phase shift of the start winding current. Thus if it were possible for resonance to happen, the motor would likely not start or would run slowly, but would draw excessive current in the start winding as well as stall current in the run winding, a recipe for rapid destruction.

I just wondered whether there is any reason to caution about experimenting with a wide range of capacitor values.

John Haine29/06/2022 06:56:38
4718 forum posts
273 photos
Posted by Macolm on 28/06/2022 22:03:17:

.......

I perhaps did not make it clear enough what the (mild) concern is. At resonance, the inductive and capacitive reactances are equal in magnitude but opposite in phase, so they cancel and thus there would be no phase shift of the start winding current. Thus if it were possible for resonance to happen, the motor would likely not start or would run slowly, but would draw excessive current in the start winding as well as stall current in the run winding, a recipe for rapid destruction.

 

I just wondered whether there is any reason to caution about experimenting with a wide range of capacitor values.

In the situation you describe, though there would be no phase shift of the current in the start winding, there would be in the RUN winding since if the motor is stopped it will look inductive. The start winding current though would be limited only by the winding resistance so it had better start quickly!  This thread might be interesting:

https://www.model-engineer.co.uk/forums/postings.asp?th=133409

https://www.model-engineer.co.uk/forums/postings.asp?th=133409

Edited By John Haine on 29/06/2022 07:18:41

SillyOldDuffer29/06/2022 11:41:32
Moderator
8903 forum posts
1999 photos

The cost of installing three-phase seems to depend mostly on how close and convenient the source is. My present home gets single phase tapped from three-phase wires on poles outside. 3-phase is close to the house, an easy fit, and I guess it would be fairly cheap.

When I lived in a city suburb, much more difficult. Getting three-phase involved digging a long trench, and trenches are expensive. Seriously expensive if they have to cross or run along a main road.

Single phase can be pricey too, if you can't share the infrastructure with other nearby properties. My mum's hairdresser and husband have been working for about 5 years on a part barn conversion/new build just outside a village. Quote for connecting electricity this year: £14,000.

VFDs are the best solution for most of us.

Dave

Macolm29/06/2022 21:04:31
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85 forum posts
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Thanks for providing this link -

https://www.model-engineer.co.uk/forums/postings.asp?th=133409

**LINK**

 

The voltages across L and C components must be in antiphase, so greater than applied supply voltage is to be expected. The current is the key measure, and in the link it is clear that while nowhere near resonance, it might well be possible to get pretty near by varying the value of the C. Assuming only the 35 ohms counts, the current in the mesh would be 7 amps at resonance, but in practice the magnetic circuit would contribute additional resistive load/losses.

 

It does confirm, at least to me, that caution is necessary in experimenting with capacitors to convert a three phase motor to single phase. It would very likely be start/run configuration, so has to be ensured to be safe over the whole operating envelope.

Edited By Macolm on 29/06/2022 21:05:54

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