Motor size help.

[dawesy]

Not sure if this is the right place for this so please feel free to move if needed.

As you may know I'm refurbishing a churchill redman cm lathe. It came with a. 3 phase 415 volt motor that's wired in star. It looks like the original motor and the lathe is 30/40's vintage.
The problem is the data plate is missing so I'm not sure what size this motor is. The overload in the switch is set at 2amps can I find the output of it from that info as I want to run it from an inverter. I've found the star connection so I can rewire to delta but I need to know the motor output to get the right size inverter for it. Some one told me that for a 415v motor a rule of thumb is 1.25 amps per hp?
Any help much appreciated.

[Lew_Merrick_PE]

Dawsey -- 1 HP = 746 W.  1 W = 1 V @ 1 A (true for DC, generally minor errors for AC).  Therefore (with some conversion factors missing that usually run in the 5%-10% range as I recall), 746 W/415 V = 1.80 A.  The thing to remember is that it is not uncommon for electric motors to run as much as 3X their nominal HP during start-up.  Your mileage may vary.

I most normally am figuring torque/load values used to specify the motor.  Everything I know about electricity is: Ohm's Law, Kirchhoff's Law, Don Lancaster, Forrest Mimms III, and, if you let the smoke out, the magic is gone...

[lordedmond]

As a rule the easy way for TP power on a balanced load is to take the phase to neutral volts eg in this case is 230 .

Then just do 230 X current 2amp and then X 3

This gives 230 X 2 X 3 = 1380 watts

As as been said 746 watts per hp. Gives 1.8 Hp

Yes it's not accurate to the nth dec place root 3 would help

Stuart

[awemawson]

Don't fall into the trap of 746 electrical watts = 1 mechanical horsepower as it doesn't - there is a loss in the conversion causing heating (and noise to a lesser extent).

So if the figures give 1.8 HP then it's probably rated at 1.5 or so actual horsepower (or 5 HP Chinese rating !)

Andrew

[dawesy]

Cheers for clearing that up guys. So if I went for a 2.2kw inverter I should have a bit of a buffer then?

[rotorhead]

Hi Dawesy,

I may be totally out of my tree here, but doesn't the motor need to be dual voltage ie: 220/430v to use an inverter.

Otherwise you would need to use a 3 phase 'converter' to get your 415v.

I ain't no authority on such matters but, I've been told to use inverters purely for dual voltage motors.

[dawesy]

Your kinda right. At the mo it's wired in star which makes it 415 only. But by breaking out the star connection in the windings and separating the 3 wires the were joined from the field coils and reconnecting the motor in delta it then becomes 240/415. At least that's my understanding.

[awemawson]

Your kinda right. At the mo it's wired in star which makes it 415 only. But by breaking out the star connection in the windings and separating the 3 wires the were joined from the field coils and reconnecting the motor in delta it then becomes 240/415. At least that's my understanding.

Correct - been there, done that 

Andrew

[Noitoen]

And if you connect a 230V motor to a 400V inverter and configure it correctly, you get a 43% increase of power/RPM 

[lordedmond]

And if you connect a 230V motor to a 400V inverter and configure it correctly, you get a 43% increase of power/RPM 

Please explain as it's a new one on me I spent a few years stator winding when I served my time and the only way to increase the rpm is to up the HZ

Yes the torque will up when you use the motor in a star delta setup as in a S D starter eg low cuernt and torque on startup full power in delta

If a motor is a 1 hp when manufactured as a star only unit then you may see more hp but not any more rpm

Stuart

[Jonny]

I may be totally out of my tree here, but doesn't the motor need to be dual voltage ie: 220/430v to use an inverter.

Simple answer no, just get a single PH 240V input to 400V 3PH output inverter, had two and use one daily in 'plug and play'. That means when the inverter goes down I can plug my lathe direct in to a 3ph Transwave rotary convertor in the way it was intended. ie not wired direct to motor mines just as it would be powered in a factory.

[Noitoen]

And if you connect a 230V motor to a 400V inverter and configure it correctly, you get a 43% increase of power/RPM 

Please explain as it's a new one on me I spent a few years stator winding when I served my time and the only way to increase the rpm is to up the HZ

Yes the torque will up when you use the motor in a star delta setup as in a S D starter eg low cuernt and torque on startup full power in delta

If a motor is a 1 hp when manufactured as a star only unit then you may see more hp but not any more rpm

Stuart

I'm a motor "rewinder" too. A normal motor, when designed, the core/winding/power relation is calculated around a target frequency, normally 50Hz in Europe. Motors core have a linear voltage frequency relation so, if you apply 25Hz to a 400V/50Hz motor with a standard VFD and measure the voltage at the motor's terminals using a RMS multimeter, you will find 200V, so at 50Hz you will get 400V, the maximum voltage a VFD can produce.
Back to a normal motor's name plate, you will see that a 1,5Kw motor 50/60Hz, it will be 1,5/1,8Kw.
To increase the power/torque over 50Hz, you have to increase the voltage in the same proportion. With a modern VFD, this can be done by connecting a 230/400 D/Y motor to 230, programming this in the motor menu and the 400v VFD will "give" the motor 230V @ 50Hz and raise linearly the voltage to 400V @ around 86,5Hz with the proportional increase of power/torque and speed without straining the motor. After 86,5Hz the speed will still increase but the power/torque will stay stationary.

If you take a normal 0,25Kw 400V 1500RPM motor and rewind it to 100V by reducing the turns and increasing the wire's cross section by a factor of 4, you can run it on a 1Kw 400V VFD to 6000 RPM and it will have 1 Kw power. It's not recommended to go much higher in frequency because of core losses.