View Full Version : single and 3 phase motor sizing and mounting ?

27-10-2013, 12:10 PM
Mate across way from me has an electric hydraulic press for cutting leather.

It was powered by a 3 phase 1HP motor run from a Machine Mart convertor.

Going by the smell of cooking windings, the motor has bought it, it still rotates but its down on power and sure don`t sound or smell good :-/

As the motor is only 1hp (740 W) 1400 RPM is there any need to replace it with another 3 phase motor, single phase 1HP motors aren`t rare, or are the 3 phase windings needed for start up torque or something?

Motor mounts with a flange on its face around the poutput shaft rather than on a foot, is there a common name for the mounting?


Clive S
27-10-2013, 01:03 PM
This might be helpful for a start Marelli - 230V Single Phase Motor 0.75kW (1HP) Cap Run 4P 80F Flange - AC Motors (Single Phase) (http://www.inverterdrive.com/group/Single-Phase-Motors/Marelli-Single-Phase-Motor-750W-1400RPM-80-B5-LST/) but I believe there are different sizes of flanges etc. also the starter would have to be replaced. But best to check with others first.

I think this one is a higher starting torque http://www.inverterdrive.com/group/Single-Phase-Motors/Marelli-Single-Phase-Motor-750W-1400RPM-80-B5/ .. Clive

27-10-2013, 02:47 PM
I'm going to guess it was one of the standard/non-rotary phase converters he used, and the motor has burnt out because you have to derate them when running on a static converter.

What you can replace it with depends on how much starting torque is needed. You can get high starting torque single phase motors, but they are quite a bit more expensive (search for capacitor start motors).

27-10-2013, 03:16 PM
If it's 1400RPM, then it's most likely a 4-pole motor - the question is: have the manufacturers used a 4-pole for the extra torque or was it purely a speed related thing? Most single phase motors will have a capacitor start. Basically, the bigger the capacitor, the greater the starting torque.

You'll probably need to match the motor frame size and mounting type, but I see no reason why a single phase motor can't be used. The other option is to go for a 3-phase motor but use a VFD (Variable Frequency Drive) inverter to drive it. You can get some decent single phase in/ three phase out drives nowadays for around 100. TEC motors are decent and usually a bit cheaper than the Italian ones. Another option is to get a used 3-phase motor from a scrapyard. They're usually good motors which have been removed from industrial machinery. More importantly, they're about 5 each and can be driven by a VFD...

27-10-2013, 03:35 PM
Capacitor start motors (I think officialy they should be called capacitor start/capacitor run) use a centrifugal switch to disconnect the start capacitor once upto speed, otherwise the motor will cook itself pretty quickly.

Having said that, a 3phase and VFD is probably a more practical option. If a high startup load is involved, you might need to oversize the VFD a bit to give a bit more current for startup purposes.

27-10-2013, 08:58 PM
According to the data sheets the 0.75kw 3 phase 4 pole motor produces about 5.1Nm torque but you have to go to a 1.5kw single phase 4 pole motor for that same torque.