rpm

[Uzzy]

Does anyone know if there is a formula to work out the RPM of an electric motor. The details I have from the makers plate are as follows:- FBV Motor Its 3 phase, 0.6HP, 50Hz, 860GIRI (I don't know what this is ?) and No 171 which I assume is the motor number. I am thinking of putting a single phase motor in but they come at two basic speed 3000rpm and 1500 rpm

[jackrae]

Without knowing the numbers of poles within the stator assembly there really isn't any way of determining its speed.

Assuming 50Hz, the synchronous speed of motors are 3000, 1500, 1000 and 750 for 2pole, 4 pole, 6 pole and 8 pole respectively. Actual speed is usually about 3% less.

Because the motor is relatively small it's unlikely to be either 6 or 8 pole.

If you can get hold of a stroboscope or a tachometer you could measure its running speed.

Perhaps you could work backwards and estimate the motor design speed. Ensuring the motor is fully isolated, set the machine onto a known speed, engage the clutch, then turn the motor by hand and count how many revs are required to turn the output shaft by one rev.

For example, if the output is normally 100RPM and the motor is a 1500RPM unit, then one rev at the output would require you to turn the motor 15 revs

regards

Jack

[the_viffer]

The easy practical way of measuring rpm is to couple a bit of metric screw thread with 1mm pitch to it. Run for n seconds with a nut on the thread and measure how far the nut runs up the thread. Simple calculation to give rpm

I learnt a lesson with my lathe doing it...

[ChrisCrosskey]

Giri translates from Italian into English as "Turns" .....well it does at Babelfish anyway.... so I'm guessing that's the rpm......

chrisc

[chris vine]

Yes, Giri is RPM in Italian!!

Chris v

[jackrae]

If it's rated at 860 RPM and the motor is designed for 50HZ ( 6 pole = 1000RPM) then either it has a gearbox fitted or its a rather strange (poor ?) design because 860 suggests 14% slip.

jack

[jackrae]

Just had a thought -

860 times the 171 (or rather 1.71) gives a motor shaft speed of 1470 which starts to make sense (2% slip)

So perhaps you actually have a 1500 RPM (actually 1470) motor with a 1.71 gearbox attached giving an output speed of 860

jack

[Uzzy]

There is no gearbox on the machine speed control is via pulley and belt. Its an Astra Horizontal Milling Machine seems to be very basic.