3 phase generator

bizidea · 18-10-2019 8:24pm #1

Hi I've just bought a 3 phase saw and need to run it with a generator as there is no 3 phase running nearby.would it be the bigger the genny the better or can the genny be too big for the size of the motor

19-10-2019 8:35am

It is not possible to have an issue because the connected generator is "too big". When you think about it the mains supply that you normally use is made up of many very large generators all connected together.

The concern would be that the selected generator is too small. Depending on a number of factors the starting current of the motor can be too high for the generator which can cause it to trip out. The starting point is knowing the size and nature of the load. Read the data sheet / manual that came with the saw carefully.

bizidea · 19-10-2019 2:31pm

2011 wrote: »

It is not possible to have an issue because the connected generator is "too big". When you think about it the mains supply that you normally use is made up of many very large generators all connected together.

The concern would be that the selected generator is too small. Depending on a number of factors the starting current of the motor can be too high for the generator which can cause it to trip out. The starting point is knowing the size and nature of the load. Read the data sheet / manual that came with the saw carefully.

Thanks 2011 its a seconhand saw so no manual it has a 7.5 hp motor and is 8.24 kw unfortunately I know nothing about motors or electricity I was in contact with a guy who sells gennies and he was saying if its directly wired you need 8.24x6 kw or if its not directly wired I will need 8.24 x3 kw to run it there is a 60 kw gennie I was looking at but was just worried it would be too big

MrMusician18 · 19-10-2019 2:38pm

bizidea wrote: »

Thanks 2011 its a seconhand saw so no manual it has a 7.5 hp motor and is 8.24 kw unfortunately I know nothing about motors or electricity I was in contact with a guy who sells gennies and he was saying if its directly wired you need 8.24x6 kw or if its not directly wired I will need 8.24 x3 kw to run it there is a 60 kw gennie I was looking at but was just worried it would be too big

60kVA I'd imagine is far too large. I'm no expert in startup current s for motors, but if be thinking 15kVA would be more than enough

19-10-2019 6:26pm

bizidea wrote: »

Thanks 2011 its a seconhand saw so no manual it has a 7.5 hp motor and is 8.24 kw unfortunately I know nothing about motors or electricity I was in contact with a guy who sells gennies and he was saying if its directly wired you need 8.24x6 kw or if its not directly wired I will need 8.24 x3 kw to run it there is a 60 kw gennie I was looking at but was just worried it would be too big

When I convert 7.5 hp ) using an online conversion tool I get 6.6 kW. I would expect the motor to have a power factor of around 0.88 (conservative value selected). Therefore the saw would be around 7.5 kVA

7.5 kVA = 7500 VA

This means that the full load current of the motor = VA / (root 3 x Line Voltage)

FLC = 7500 / (1.732 x 400) = 10.8 amps

Assuming that your saw is direct on line the starting current would be 5 to 8 times the full load current. Worst case that is 86 amps for a brief moment. As the saw rotates the current rapidly drops off.

So a 7.5 kVA generator (which would not be that expensive) would be able to handle the full load of the saw but would trip out under starting conditions. A generator that could supply 86 amps would be around 60 kVA which would cost a fortune and would be inefficient to run with such a small load. At a guess I would say that a 20 kVA generator would do the trick if the MCB selected was of the right characteristic. The voltage would briefly drop, but that would be ok (once the generator does not trip out on under voltage). You could rent a generator for a day and try it out. Another approach would to install a soft starter on the saw. This would reduce the starting current dramatically to a manageable level. I would recommend Danfoss, ABB, Siemens or Schneider Electric. Another option would be to buy a VSD (variable speed drive) and connect it to your single phase supply. This could give you a 3 phase output and a low starting current suitable for your saw. No more messing around with generators. I would recommend the same makes for the VSD.

Plenty of food for though!

Bruthal · 19-10-2019 6:42pm

A generator around double the saw full loading would be ok id say. With no load it will be at starting current for a tiny amount of time.

Even though it may take circa six times full load at starting instant, it doesnt need that to start running. It will just be slightly slower getting up to speed, maybe not even noticable.

Having said that, i remember setting up a 3 phase convertor in a workshop, and a temp 2.5 cable from the owners house, where as a 10 square was needed. Some machines with heavy rotating blades took ages to get up to speed, but when the proper size cable was put in, they were flying.

Incandescent bulbs (translates to lamps if never heard of them), take probably 10 times their running current at cold switch on as a matter of interest, although they are a small individual load.

Bruthal · 19-10-2019 6:47pm

2011 wrote: »

Another option would be to buy a VSD (variable speed drive) and connect it to your single phase supply. This could give you a 3 phase output and a low starting current suitable for your saw. No more messing around with generators. I would recommend the same makes for the VSD.

Presumably they output 230v 3 phase.

bizidea · 19-10-2019 8:01pm

Thanks for your replies lads still confused lol like I said motors and electrics are one thing I haven't a clue about. The saw isnt something id be using the whole time it would be only for an hour or two a week and then the odd day maybe for a couple of hours so a gennie mightnt be that expensive to run

Tuco88 · 19-10-2019 11:19pm

The VSD is the way to go. Surely you can get one with a 230v single phase to 400v 3phase output?

Bruthal · 19-10-2019 11:28pm

Tuco88 wrote: »

The VSD is the way to go. Surely you can get one with a 230v single phase to 400v 3phase output?

Would need to see what the saw setup is. Is it a trigger to operate for short on/off runs. Is it 400v star connected with 230v windings?

Does it have soft starting already?

Lots of unknowns.

Del2005 · 19-10-2019 11:28pm

bizidea wrote: »

Thanks for your replies lads still confused lol like I said motors and electrics are one thing I haven't a clue about. The saw isnt something id be using the whole time it would be only for an hour or two a week and then the odd day maybe for a couple of hours so a gennie mightnt be that expensive to run

While the genie won't be expensive to run it'll be expensive to buy. Have you looked at the cost of a single phase saw v's buying, and maintaining, a genie.

19-10-2019 11:42pm

Bruthal wrote: »

Would need to see what the saw setup is. Is it a trigger to operate for short on/off runs. Is it 400v star connected with 230v windings?

That would be one solution the other would be to use a set up transformer.

MrMusician18 · 20-10-2019 1:52am

2011 wrote: »

That would be one solution the other would be to use a set up transformer.

?
How would a step up transformer work in this case?

Bruthal · 20-10-2019 7:35am

MrMusician18 wrote: »

?
How would a step up transformer work in this case?

A step up as well as VSD if 400v was required from 230v supply.

bizidea · 20-10-2019 9:52am

Del2005 wrote: »

While the genie won't be expensive to run it'll be expensive to buy. Have you looked at the cost of a single phase saw v's buying, and maintaining, a genie.

Single phase saws are available but much smaller and won't cut the same depth as a 3 phase

20-10-2019 10:04am

Can you post a photo of the nameplate on the saw? This would show the power, voltage requirements, if it is 400V delta connected or 400V star connected, power factor etc...

Bruthal · 20-10-2019 10:17am

bizidea wrote: »

Single phase saws are available but much smaller and won't cut the same depth as a 3 phase

Yea you wont realistically get any motor of that size in single phase.

Any photo of the plate on it?

That will give winding voltages etc.

What will it be cutting?

MrMusician18 · 21-10-2019 1:47am

Bruthal wrote: »

A step up as well as VSD if 400v was required from 230v supply.

I thought the vsd created three phases from the one input phase and the 400v is created through the difference in the voltage of the phases at any given time

Just like real three phase, no transformer required.

21-10-2019 6:58am

MrMusician18 wrote: »

I thought the bad created three phases from the one input phase and the 400v is created through the difference in the voltage of the phases at any given time

Just like real three phase, no transformer required.

I'm not really sure what your post means. However a VSD with a single phase 230V input can generate a 3 phase 230V output. If the motor is 400V star connected this would be ok as we can connect it in delta and get the same output. If a 400V 3 phase supply is required then a step up transformer would be one solution.

Bruthal · 21-10-2019 11:02am

MrMusician18 wrote: »

I thought the vsd created three phases from the one input phase and the 400v is created through the difference in the voltage of the phases at any given time

Just like real three phase, no transformer required.

No, the single to 3 VSD just outputs the 230v in timed sequences over the 3 output poles, its not creating 3 actual simultaneous analogue phases that have reference to each other at any instant in the same way real 3 phase does.It is in actual fact rapidly switching on DC pulses in sequence, and in one direction, then the other, to simulate 3 phase AC output.

These will be PWM switched to approximate sine wave shapes.

So in simple terms, if you imagine a 230v AC supply rectified to similar DC,,,

3 switches, 1 each from + to L1 and to L2 and L3, and 3 more switches from - to L1 and L2 and L3. If you switch them on and off in correct sequences, (never both to same terminal at same time as that`s a short), you can power any 2 of the 3 poles and therefore 1 winding at any time and in either polarity. Then next winding and the 3rd, in sequence. So a 3 phase AC supply can be simulated. But as can be seen, it does not have any reference from one true sine wave to another at anytime.

So from that, the 230v is maintained, not changed in any big way, in terms of magnitude.

21-10-2019 11:21am

This shows how a VSD uses “Pulse Width Modulation” of a DC output to generate a quasi sine wave:

Obviously the above is a simplified version of how it works. Active and / or passive filters are also employed to “smooth” the output.

hesker · 21-10-2019 1:46pm

I was gifted a nice table saw with a 3 phase dual voltage motor.

Following the guide below I was able to get it up and running with a VSD. Total cost about €100.

https://www.ukworkshop.co.uk/forums/3-phase-to-single-phase-startrite-275-conversion-from-3-ph-t91106.html

Bruthal · 21-10-2019 1:50pm

No problem getting it up and running in the OP case once the motor windings are 230v.

230v windings can be connected to 400v 3 phase in star, or 230v 3 phase in delta, for full output.

Might be expensive for the size required by the op though, and a high single phase load.

MrMusician18 · 21-10-2019 7:21pm

Bruthal wrote: »

No, the single to 3 VSD just outputs the 230v in timed sequences over the 3 output poles, its not creating 3 actual simultaneous analogue phases that have reference to each other at any instant in the same way real 3 phase does.It is in actual fact rapidly switching on DC pulses in sequence, and in one direction, then the other, to simulate 3 phase AC output.

These will be PWM switched to approximate sine wave shapes.

So in simple terms, if you imagine a 230v AC supply rectified to similar DC,,,

3 switches, 1 each from + to L1 and to L2 and L3, and 3 more switches from - to L1 and L2 and L3. If you switch them on and off in correct sequences, (never both to same terminal at same time as that`s a short), you can power any 2 of the 3 poles and therefore 1 winding at any time and in either polarity. Then next winding and the 3rd, in sequence. So a 3 phase AC supply can be simulated. But as can be seen, it does not have any reference from one true sine wave to another at anytime.

So from that, the 230v is maintained, not changed in any big way, in terms of magnitude.

2011 wrote: »

This shows how a VSD uses “Pulse Width Modulation” of a DC output to generate a quasi sine wave:

Obviously the above is a simplified version of how it works. Active and / or passive filters are also employed to “smooth” the output.

Cheers for explaining, I'm not an electrical engineer (as one might have guessed. Every day is a school day

3 phase generator

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