Electronic – Why does Ohm’s law not work for vacuum cleaners


I have been learning about Ohm's law and testing the resistance across the plug of my household appliances and calculating the current.

For example, my kettle was 22 ohms (10.45 amperes) and is protected by a 13 A fuse.

This makes sense, and I'm okay with it, but then I tested the vacuum cleaner which had a resistance of 7.7 ohms which equates to 29.8 amperes which surely should blow the 13 A fuse, but it doesn't. I have now tested two different vacuum cleaners which have the same small resistance reading across the live and neutral.

Surely this would be a direct short, but it works fine so does the resistance change or what?

Best Answer

The 7.7 ohms you measured is the winding resistance of the motor. But that is not the only factor that determines its operating current.

Your vacuum cleaner might draw close to the calculated 30A the instant power is applied, but as soon as the motor starts to rotate, it generates a voltage that is proportional to speed (called back emf) that opposes the applied voltage, decreasing the net voltage available to drive current through the windings. As the motor speed increases, the current (and therefore the torque produced by the motor) decreases, and the speed settles at the point where the torque produced by the motor matches the torque required to drive the load at that speed.

Fuses don't blow instantly. But if you were to lock the motor so it couldn't rotate, that fuse wouldn't last long.