Running 3 phase motor off single phase. Y or Delta?

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flanajb

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I have been reading some interesting articles about running 3 phase off single phase using start and run capacitors. I thought I has it sussed last night, until I read another article where the author said it's better to have the motor configured in star mode rather than delta.

The motor I have can be configured in either star or delta and I have the required start / run capacitors.

I posted a few weeks back about doing this, but felt another topic was suited here as it's more technical than the previous post.

Is anyone able to explain the merits of running in start or delta when using capacitors and a single phase supply?

Once again, here are the motor plate details

15684958632_f1bee86ce8_b.jpg
 
If you are running off 240 volt mains then you have no choice but to use Delta.

The windings on the motor you image are rated for 240 volt.

If you connect in Star you have 240 + 240 = 480 volt.
 
Myfordman":25lu8ajo said:
CHJ":25lu8ajo said:
.

The windings on the motor you image are rated for 240 volt.

If you connect in Star you have 240 + 240 = 480 volt.

No! connecting in star will give 240 x Sqrt 3 = around 415v

Yes I know, there are several aircraft flying around with my 60kva alternator installations in them.

The OP does not have a great understanding of 3ph supplies, I simply quoted figures from his motor label that would give him an understandable explanation of what the difference would be as a straight single phase load over two windings without taking into account any phase shift of supplies, which to my mind he does not need to be confused with.
 
Now I am confused. So if connecting in star will give me 415v, then that to me is the best option if I don't want to run the motor underpowered which is what will happen if I run in delta?

I am already confused by this, but just need to know whether star or delta is the best way. The capacitors I have are rated at 380/400V
 
Ok. So it's all running ok, but when I connect a volt meter across the wires I get readings from 240 to 265V. Now I am a little worried that the 30uf run capacitor might be slightly oversized, as I don't need to bring in the start capacitor to get the motor running.

Anyone have the formula to hand so that I can double check how many micro farads I need. Just worried that 265v is too high and will burn out the windings?

Thanks
 
Stop worrying about it, there are all sorts of supply voltage inaccuracies going on using a simple voltmeter that you don't need to know about, as long as the motor does not show excessive heat build up and provides adequate torque for your purpose just accept it.

If you want to know why I say that just look at the waveform timings in bobs diagrams, unless you have an oscilloscope or a suitable piece of equipment to measure RMS power for you correctly, related to waveform timings measuring with a simple voltmeter is only a guide to being somewhere in the ball park.

The actual power being consumed by the motor will change as soon as you load it under work conditions, as long as it settles on a figure that gives you a working machine on something like a pillar drill that has short working cycles without overheating it is not worth worrying about.
 
CHJ":ln5hvyg5 said:
Stop worrying about it, there are all sorts of supply voltage inaccuracies going on using a simple voltmeter that you don't need to know about, as long as the motor does not show excessive heat build up and provides adequate torque for your purpose just accept it.
Point noted. I wish 'over analysing' was an Olympic sport as I'd be a gold medallist!

I will monitor the motor casing to see if it gets too hot.

Thanks
 
flanajb":24i562de said:
.....
I will monitor the motor casing to see if it gets too hot.

Thanks

Out of curiosity, I wonder what 'too hot' is in this context? Hold a hand on the motor casing indefinitely = OK and so not too hot?
 
Just a two penny worth over something that may be causing confusion. The diagram to which the link drops you is correct on the top diagrams, but I believe inaccurate when it shows the motor windings. The motar windings when connected as shown in the diagrams with the wires for 1 phase will produce a delta connected motor rather than the star connection shown. Just use the wiring diagram and don't worry about the how the windings are shown.

Again it may help, there is a lot of mystery surrounding 3ph and its association with 415V. To simplify, down the street there is a 3ph supply, just like the one that's connected into a 3ph machine. In turn normally each house down the street is connected to one of the 3 phases, so the first house gets the blue wire, the second the yellow and the third the red, the pattern then repeats itself. All phases of a 3 phase supply are at a nominal 240v.

A 3ph motor normally (for any reasonable size of motor) has a two stage startup. The first stage is to get the motor spinning, this takes the most energy, and for this reason the motor is normally connected in Star configuration. Once spinning and after a short period of time, the control electronics (or simple timed relay) switches the motor configuration over to delta. To keep the motor running takes less energy than getting it started, and in delta it uses less power. There are exceptions to this where keeping a motor in star configuration is necessary, I.e. Where there is a need for high torque typically used for fans and pumps. Woodworking in general I.e, P/T, circular saws are also high torque applications. However, if you are not in production, using the machine continuously at the limits of what it can do, and prepared to accept the thermal cut out trip once every now and again, connected in delta is not significant (if you hear it starting to slow reduce the feed rate is a typical get around). Delta configuration does not affect the no load speed, but does affect the the ability of the motor to keep running at speed under loads. As the motor slows down, the coils heat up as they need to do more work to keep the motor running.

Putting into perspective, most don't if they have a circular saw capable of cutting 4" cut 4" all of the time, typically 1~3". You will probably never notice the difference of running in Delta. Cutting 4" seasoned oak may cause the motor to start to slow / the thermal trip to interrupt proceedings. However, slowing the feed rate down, and taking a little extra time and the motor will normally cope with what is being asked if it. If you cut 4" seasoned oak day in day out, you need a proper 3ph supply (rotary converter typically) to run the machine off a domestic supply.

This is a very simply explanation of what is a very complex subject, so please don't shoot be down for the glaring gloss over of the technicalities.
 
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