3 phase

[les]

The rating on the 3 phase motor is obscured, I'm thinking 3/4, I should have a handbook soon and maybe get a clearer picture of the hp. The motor I may fit is definately 3/4hp. All this has been very helpful knowledge to me, thanks.

[bmcecosse]

I would say speed more important than HP!

[Declan_Burns]

Bear in mind that a three phase motor started direct on line (DOL) has a starting current six to seven times it's rated current. That's the advantage of the three phase motor on the single phase frequency inverter. You can ramp the motor current on starting so you have a soft start effect and you won't blow the domestic fuses. Just make sure the output current on the inverter is slightly higher than the motor rated current. One size up is the rule of thumb in industrial applications. Take a photo of the motor nameplate and show it to your potential supplier if possible.
Regards

[les]

I've since discovered the 3 phase motor is 1/3rd hp. Also someone has reminded me of a s/h machinery company that sells lathes etc near where I work, I think it is worth enquiring after a single phase motor, hopefully one from a similar lathe to mine giving me an easy swop.

[Alec]

Hello Les,

that doesn't add up given the figures you quote at the start of this thread.

The formula is V x I x 1.732 x power factor (nominally 0.8 ) which comes out near enough to 700 watts. 746 watts = 1H.P.

Alec

[les]

Hi Alec, I have in fact borrowed a manual from a friend with the same lathe and the hp quoted is 1/3, hence the assumption. So whilst not doubting your calculations, I'm never the less puzzled. Although not necessarily significant the 3/4hp single phase motor I mentioned is a fair bit larger physically than the one fitted to the lathe. Unfortunately the motor plate is unclear re the hp.

[bmcecosse]

I'm surprised to see a 3 ph motor - at only 1/3 Hp ??? Are you sure ?

[les]

Well it appears in the manual as 1/3hp and the motor fitted is original. I'll take another look at the plate.

[bmcecosse]

It may be 1 HP in Star - and 3 HP in Delta!

[Declan_Burns]

Roy,
A lot lower than 1/3HP. Nowadays standard 3 phase IEC motors start at 0.06kW for 4 pole (1500 rpm)-used to be 0.09kW. Motors and inverters-It's my daily bread and butter.
https://www.automation.siemens.com/mcms ... 008_en.pdf
Regards

[Alec]

Hello Les,

the physical size is not in itself a guide to power.

Declan, has motor terminology changed, when I did my time it was pole pairs so a 1500 would be two pole (pairs) not four pole?

Alec

[Declan_Burns]

Alec,
Yes it has changed. Nowadays they refer to the actual no. of poles and don't refer to pole pairs any longer. So 1500rpm= 4 pole, 3000rpm=2 pole on 50Hz etc.

Regards

[bmcecosse]

It was always so in my day! Do you have 50 c/s in Deutschland ?

[Declan_Burns]

Yes it's 50Hz here 400/230V. Pretty much the same all across Europe. I've only come across one country in Asia where they have both 50 and 60 Hz -Japan!
Here'a list for reference-there are some strange ones! There are a few listed as "unavailable"-that could very well imply no power at all!
http://users.telenet.be/worldstandards/ ... m#3voltage
Regards

[Alec]

Hello BMCE,

"It was always so in my day! Do you have 50 c/s in Deutschland "

rather a contradictory statement as if you were brought up in cycles per second then pole pairs would be the convention. Of course people never said pole pairs just poles.

I came out of my time just over forty years ago and Hertz was being used then, I seem to remember?

Alec

[bmcecosse]

Ahh - but i'm an old 'stick in the mud' - and I like cycles per second ! It tells exactly what's happening - where as 'hertz' to me is something to do with car hiring ????

[Alec]

Hello BMCE,

yes, it makes more sense. One of our tutors commenting on this new fangled Hertz idea that we would be doing 'Fords' not miles per hour.

Alec