120 Watts and 7.2 Volts, how many Amps?

For a project I need to switch on a simple handheld vacuum cleaner with an Arduino. I am thinking of using a relay or mosfet. I opened the case and a 7.4 V 2200 mAh Li-Ion battery powers the motor, through a regular switch.

On the manufacturer's website, the power of this vacuum cleaner is 120 W, so according to my understanding:

I = P/V

I = 120 W / 7.4 V = 16.2 A

So according Ohm's law, the resistance of the motor has to be 0.46 Ohm right? Isn't this too low?

Thanks.

• Isn't this too low? Too low for what? Commented Apr 30, 2019 at 12:12
• We need to take into consideration the inductance of the motor also. Commented Apr 30, 2019 at 12:13
• A brushed DC motor will have a much lower resistance at rest than you would expect from the current it draws when running. This is due to the back-EMF. That's why they draw a big surge when you first apply power. Commented Apr 30, 2019 at 12:17
• @SpehroPefhany What do you mean by "drawing"? Does the motor lower it's resistance under load, so that a higher current can flow? Commented Apr 30, 2019 at 12:22
• @Verjan Yes, higher load (higher torque) requires more current (power), so the apparent resistance of a motor (due to back-EMF) changes with varying load. Commented Apr 30, 2019 at 12:26