Calculating motor power

[ahmedragia21]

Hi, i got a motor in which written Power = 40W , and supply Voltage 24V, 110RPM , i want to know what current will the motor draw under no load and with full load ? can ohm's law apply here I=P/V=40/24 = 1.6A ?
and another motor Vs=24V , 0.5 A , is its total power =12 watt or another number ?

thanks

 

[dknguyen]

YOu cannot apply ohm's law. THe only real way to figure it out is to measure it or have some other information about the motor's performance curves (speed, voltage, current, torque).

If you know the motor's resistance then you can use V=IR to find it's stall current. It's easy enough and safe enough to measure the no-load current (unlike measuring stall current where you have to clamp the motor's shaft).

Also, the wattages given are the motor's output power. YOu have no idea what it's efficiency is at that operating point so you can't even use P=IV to figure out the current it will draw since you don't know the efficiency.

 

[ahmedragia21]

Cant with just the motor power sepecified measure the drawen current ?
P=I*V ,P=40,V=24, then I=1.5A , then V=I*R , then R=16 ohm ... is this correct ?

 

[dknguyen]

THe P in P=IV has to be the input power (since the V and I you are using are input voltage and current). THe P that you have is the output power, not the input power. You also don't know the motor's efficiency at the operating point that those numbers are given at (the efficiency changes under different operating conditions). THerefore, you can't figure out the motor's input power.

Oh, yeah, and it might be hard to measure the motor's resistance to figure out stall current since the resistance is so low and a regular multimeter is not sensitive enough. In that case you just have to clamp the motor shaft pulse it on and off quickly once (so it doesn't overheat) while measuring the current.

 

[mneary]

I would start by measuring no-load current. This should be easy; no need to guess.

You can estimate the stall current by measuring the stalled current at reduced voltage (1V?) and scaling it to your intended supply. It's not linear, but it's an estimate.

I'll hazard a guess that the output power is the maximum (why advertise anything else?), which is near 50% load and 50% speed. So this means they probably got 40W at 55 rpm. But your motor will last a lot longer when you operate at less than 25% load (75% speed).

These are all techniques to get an approximation. These approximations may be off; so good engineering practice should govern your use of the data that you get.

 

[Hero999]

The best way is to look at the datasheet for the motor, there should be a power/load curve.