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Single phase/ three phase questions

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Doug71

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After a a bit of advice from the knowledgeable people out there.

This started as what was going to be a simple question but has got a bit more complicated, I will start with the original question.

When I turn on my panel saw (240v 16amp) my workshop lights dim for a couple of seconds, it was the same with my old table saw. The question was will this this do any damage to my saw in the long term?

Things took a turn when I checked the identification plate on the saw to see what size the motor was, the plate says it's 3 phase 400v, I guess someone changed the motor at some point :dunno:

On the odd occasion the saw will stall and again the lights will dim so I guess my single phase supply isn't big enough, I'm in rented premises so can't do anything about this?

I do have three phase and most of my other machines are three phase which brings me to my next questions.

How easy/expensive is it to swap the motor back to a three phase? Do the switches etc need changing or basically just the motor (I wouldn't do it, it would be my electrician). The saw doesn't have an electric brake but now wonder if it did originally?

Another idea I had is the three phase supply seems okay power wise, could I somehow take power from that to run the saw with it's current motor?

The saw is a Minimax SC4 from 1994 if that makes any difference.

Thanks for any advice, Doug
 

topchippyles

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Worrying about nothing doug and a dip in lights is quite common on a big machine on start up. Is everything working as it should when running ?
 

Doug B

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I had a similar problem of the lights dimming when turning on an old Wadkin planer that had been converted to single phase, I was told that it could cause damage to the motor windings, I ended up putting a bigger supply to the workshop.

A 3 phase motor & DOL starter shouldn’t be too expensive to convert the saw I’ve no idea how much a brake would be but I believe they aren’t cheap.
Isn't you single phase just taken off one of the incoming 3 phases?
 

guineafowl21

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New three phase TEC motor would be about £150. Assuming your saw’s motor is a standard frame type (take a picture of the rating plate on the motor), it should be an easy swap. You’d need a new DOL starter for similar money, or a Crompton DC brake/starter combo which would be about £500. It shouldn’t take more than 3 hours to fit all that. Factor in a bit more for fitting an extra socket outlet for it.

You could take one phase and a neutral from the three phase supply - essentially just adding another circuit. This is assuming your three and single phase are separate supplies.

Or just put up with the dimming.

Not sure which I’d choose - if you have three phase available, I would use it, as it’s just better, so maybe option 1.
 

Doug71

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Is everything working as it should when running ?
Yes everything seems to work fine.

Isn't you single phase just taken off one of the incoming 3 phases?
It's not straightforward, as I say it's a rented workshop unit and for some reason my three phase has a meter in my workshop but my single phase has it's meter in my landlords workshop, that's why I wondered about taking another single phase off my three phase.
 

topchippyles

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Its working fine why alter things honestly some of the posts on here look for issues when there are none
 

Doug71

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New three phase TEC motor would be about £150. Assuming your saw’s motor is a standard frame type (take a picture of the rating plate on the motor), it should be an easy swap. You’d need a new DOL starter for similar money, or a Crompton DC brake/starter combo which would be about £500. It shouldn’t take more than 3 hours to fit all that. Factor in a bit more for fitting an extra socket outlet for it.

You could take one phase and a neutral from the three phase supply - essentially just adding another circuit. This is assuming your three and single phase are separate supplies.

Or just put up with the dimming.

Not sure which I’d choose - if you have three phase available, I would use it, as it’s just better, so maybe option 1.


The dimming does annoy me but my concern was (as Doug mentions above and I had read elsewhere) that it will be damaging the motor?

As you say three phase always seems better so that is probably the option I would go for.
 

Spectric

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It's not straightforward, as I say it's a rented workshop unit and for some reason my three phase has a meter in my workshop but my single phase has it's meter in my landlords workshop, that's why I wondered about taking another single phase off my three phase.
All that you are witnessing is a volt drop under initial load, it could be that the supply to these buildings is either old or on the limits. I have seen this before where people string supplies from one building to another and over time the potential loads just keep rising . It sounds like you have a strange setup, there is no reason why the single phase supply is not be taken of the three phase supply because that is where a single phase originates but it could be that when that single phase supply was installed the electrician was not versed in three phase or someone thought the three phase meter could only supply three phase.
 

Doug B

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The dimming does annoy me but my concern was (as Doug mentions above and I had read elsewhere) that it will be damaging the motor?
I believe it overheats the windings, I’m surprised you get the lights dimming on a saw as there is very little load on start up compared to a large cutter block as in my Wadkin planer.
The chap who sorted mine put an amp metre on the incoming live & the amp draw on start up was enormous so no wonder the lights were dimming but it wasn’t tripping anything.
For me changing the incoming main cable wasn’t that difficult or expensive but I imagine in your case the 3 phase route will be the cheapest/easiest option.
 

guineafowl21

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Undervolting a motor will cause increased current draw and heat, yes. Not so much from initial start-up, admittedly, but if it’s on and off a lot, and stalling as mentioned, it could build up to a problem in the future. Seems silly to run from an inadequate supply when there’s juicy three phase sitting right there.

I wouldn’t say the ‘if it ain’t broke...’ philosophy translates too well to electrical stuff. If there are concerns, it’s worth a check over if only for peace of mind, eg that there isn’t a loose connection slowly fizzing away somewhere.
 

Vann

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...You’d need a new DOL starter....
I don't know about modern DOL starters, but the older models of MEM starter I have on a few of my machines (Memota and Startex) can be wired for 3-phase or single phase. So chances are that the DOL starter on your machine may have been rewired from 3 to single phase - and could be wired back to 3-phase (although the amp settings may have required a new DOL starter at the time of conversion).

...if you have three phase available, I would use it, as it’s just better, so maybe option 1.
What he says.

Cheers, Vann.
 

Spectric

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So chances are that the DOL starter on your machine may have been rewired from 3 to single phase
Providing the coil is 230 volt and not 400 volt, the contactors are three way and you will need to size the correct overload in the DOL for the new three phase motor.
 
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