# Battery and Motor

1. Feb 3, 2013

### Curious007

I saw 15 Amp motor which runs on 120 VDC with 5500 rpm [This motor was attached to Miter Saw tool]. I'm not sure what's HP of that motor, but I read 5.5 HP, however the size of motor is around 6 inch long and 4 inch diameter (Is it possible this small motor have 5.5 Hp? And can we calculate torque of the motor based on above known facts?).

Now my other question is : Can I run this 120 VDC motor using this 20 Volt DC with 3 Ah rating battery? If so how long I can run this motor using it?

Thank you.

2. Feb 3, 2013

### Staff: Mentor

Well, 15A at 120V and 95% efficiency would be 2.3 horsepower.

I seem to remember that vacuum cleaner manufacturers play games with horsepower ratings, so it wouldn't surprise me if circular saw manufacturers did too.

3. Feb 3, 2013

### Curious007

May be that was mistake of Shop guy who put 5.5 HP sticker on it.

But other thing is can we run that motor using 20 V DC battery with 3 Ah rating? for 1 or 1 minutes?

4. Feb 4, 2013

### sophiecentaur

A 120V motor will not work on 20V. I doubt whether, on only 17% of its rated voltage, it will even rotate at all. Basically, with 17% Volts, you could expect to get just 2.8% of its power (i.e. 17% of 17%) - even if everything else was perfectly pro-rata - and there is a threshold below which nothing will happen. You might expect about 2.5A to pass.

BTW, the current that the motor will pass is determined basically by the supply volts - too few supply volts will just not make enough current flow. A low voltage motor is designed with much fewer coils of fatter wire (as in a car starter motor).