three phase motors

[Chee]

hi all,
i have a machine which requires 3 phase power. Its a cylinder grinder which sharpens cutting cylinders on grass machinery. The grinder has three motors in total, on the identification plates two of them read 0.37 kw. One of them reads 1.1kw.
I thought this was fine and i could pick up a generator to run it at a reasonable price. An electrician has now told me that those figures could be per phase.
This meaning 0.37 x3 & 1.1 x3. ive never heard of this.
Can anyone help me with this??
thanks

 

[brightspark]

hi all,
i have a machine which requires 3 phase power. Its a cylinder grinder which sharpens cutting cylinders on grass machinery. The grinder has three motors in total, on the identification plates two of them read 0.37 kw. One of them reads 1.1kw.
I thought this was fine and i could pick up a generator to run it at a reasonable price. An electrician has now told me that those figures could be per phase.
This meaning 0.37 x3 & 1.1 x3. ive never heard of this.
Can anyone help me with this??
thanks

most 3 phase motors give the full load amps. whatever is on the plate is each phase so if it said 5,0 full load amps that would be each phase .15 amp over the 3 phases

 

[Chee]

ok so that would mean the machines total for all 3 motors is 5.5kw? which would be higher on initial start up?

 

[jasonmcc]

The total Kw of the grinder motors is 0.37Kw+0.37Kw+1.1Kw=1.83Kw. Your generator would need to be 2-3 Kva Minimum, something bigger would be better as running any thing flat out is going to reduce its life. If you are thinking about buying a "cheapanese" generator from somewhere like a well known auction site, make sure it has a waranty or refund policy. I recently had a customer bring in a 5.5Kva geny to be tested it was only two weeks old and had died without warning. It cost them nearly £700 to buy and no waranty given !!!!

 

[shenion]

If the motors are rated in kw, that would be the power, not current. There is no such thing as power per phase as power is between phases. Most likely it is power at the output shaft. Input current/power would be higher.

0.37 is 1/2HP. 1.1kw is about 1.5HP. Sounds like standard size motord.

Post a pic of the data plates. Maybe there is more info.

 

[Luke]

Power ratings are per motor, not per phase.

However, If you plan to run them direct from a generator, you need to consider starting current, which may be up to 3x rated current.

 

[Chee]

I'm looking at a pramac s8000. 8kva. I'll just put it on my hire fleet to cover some of the cost. I'll take it a pic of plates on motors tomorrow and post them up. Thanks

 

[piman]

Hello Chee,

I presume you start each motor one by one so the worst start current will be the 1.1Kw, which should easily be run off an 8KVA generator.

Generators rated in KVA and motors in Kw, confusing isn't it? Multiply the generator output by 0.8 will giver an approximate Kw output for the generator for an average inductive load.

Alec

 

[shenion]

start currents are typically 3-8 times the running load. So that 1.1kW motor would be 3.3-8.8kVA to start and probably more due to inefficiency..

 

[normspanners]

When starting motors on a small generator it is sometimes best to start the smaller motors first, as long as they are running free, once started. Because with them in the system running light they have a combined 'flywheel effect' and can actually help the larger motor start.

 

[piman]

Hello Scott,

I was taught in college that induction motor start currents were 7 to 10 times full load but in practice it seems that half that figure is more realistic? The initial maximum surge is of very short duration on small motors and it soon drops off as the motor picks up speed. I would say that the momentum of the generator quoted will cope quite easily with such a load?

Alec

 

[Big Pete]

for those motors I would be tempted to buy an inverter drive setup, it would likely be cheaper, and less hassle .

 

[Chee]

What do you mean by inverter drive? A generator seems easy as I rarely use the machine, mainly in winter. Also the workshop I'm in is temporary and hope to be in my new workshop by the summer.

 

[Big Pete]

an inverter drive is a box of electronics which will convert single phase to 3 phase. They can do other things as well, like soft start, variable speed, DC injection braking etc, depending on how sophisticated they are. They are also known as VFD/VSDs (variable frequency/speed drives). Some (usually the cheaper ones) take single phase 240v in and give 3 phase 240 out, so you need to rewire the motors (just in the terminal box) to run on delta, others (usually pricier) will take single phase 240v in, and give 415 3 phase out so there is no rewiring required. Drives direct and Newton Tesla are pretty helpfull in sorting out suitable systems, but if you red up on them they aren't all that complex to install. I've got a few driving kit in my garage.