Why don't motors coupled to gearboxes spike up significantly?

Question

A 5,5KW motor is rated 11,40A for the delta connection at 400V. This motor is coupled to a gearbox that reduces it's speed from 1440rpm to around 121rpm. On the gearbox shaft is a sprocket and chain which reduces the speed even further.

According to what I know, during starting a motor will rise to about 4-7 times its rated current for certain seconds. So I was expecting this value to be above 44A, which turned out not to be the case. In fact, the current wasn't even double it's rated value.

Now here's the question: why didn't this value rise up as much as I expected? Since there's no back emf when the rotor is stationary, the current drawn by the motor should have been significantly higher. I'm struggling to understand why it wasn't.

NB: the motor is running direct on line, no soft starters are involved.

Answer 1

Your expectations are wrong. The "4-7 times" statement is what is known as a "rule of thumb", used for rough estimation. It doesn't mean that every motor ever built draws a minimum of 4× its rated current at start-up.

In your situation, it would seem that the motor is able to spin up fairly quickly, limiting the time duration of the current "spike". Plus, the inductance of the windings limits the rise time of any spike. Your meter may also have a limited response time. Together, these effects limit the height of the spike to what you see.

Answer 2

I agree, it likely has to do with the response time of your meter and the fact that when the speed is reduced by almost a factor of 12 with that gearbox, the shaft torque to the load is INCREASED by the inverse ratio; i.e. 12x the motor's rated torque. So the motor is likely able to get to it's full speed under load in a very very short time, too short for your meter to fully react to it. If you had a high speed meter or a scope, you would see it.