3 phase Q for helpful sparks, please?

[ivan]

I'm replacing a 3 phase motor with single phase, and the origninal has lost it's (info) plate. No 3 Ph here, so I can't actually measure. I have some idea of the max current in each phase from the 3Ph motor's starter overload setting. If I estimate power from P=root3*Vline*Iline*cos(theta), what's a typical power factor for a small (say 1.5 to 3Hp) 3ph motor? Is Vline now taken to be 400V (not 415)?

Thanks in anticipation, etc!

 

[OLD]

My SIP 10'' table saw has a 2200w (3hp) motor that has to run of a 16a dedicated supply as it takes out 13a fuses on start up so i do not think you want to go higher than 3hp.

 

[Anonymous]

I have some idea of the max current in each phase from the 3Ph motor's starter overload setting

Not neceesarily true as I have seen them set to all manner of settings over the years.

Power factor for the motor will be around 0.7 to 0.8

 

[Digit]

Assuming that the overload protection device is adjustable, as Tony says it's pretty well useless as a source of info I'm afraid Ivan.
Unfortunately replacing, say a half HP 3 phase motor with a similar size single phase, is likely to result in disappointment as the 3 phase motor has more torque than the same size single, so knowing the original motor size may not be a great help anyway.
What is the machine and what is its size? It might be better to work from there, also without the commisioning plate the RPM is not known and that might be important in a direct drive machine.

Roy.

 

[ProShop]

Ivan,
I've been down this road before, and as the above posts, 3phase 3hp is more powerful than a single phase 3hp. 4hp is about as high as you'll find in single phase motors imho.

If your motor is 2hp, I'd go for a 4hp, but again as mentioned in above posts the motor speed is the key factor here.

Hope this helps

 

[ivan]

Starting torque not too much of a problem, motor is 4 pole externally mounted driving a ducted fan via V belt. My guess is around 1.5 to 2 kW.

 

[Digit]

But at what revs Ivan, and don't make the mistake of ignoring the diameter of the fan? For equal mass a large diameter fan needs more torque to accelerate it than a small diameter one. Work = force x distance.

 

[9fingers]

Roy, Ivan has told us the speed - it is a 4 pole motor so that means 1425-1450 rpm

Theoretical speed = 2 x frequency x 60/number of poles.
Practical speed allowing for slip is 5% or so below theoretical.

Bob

 

[Digit]

I missed that Bob. I had to read it twice more before I found that ref. Yep 1450.

Roy.