Why does this DC motor increase in current and then decrease as the speed increases?

Question

Well, the general answer to this is that as the motor needs to overcome the inertia from its state of rest, it requires high starting torque and aka high current is consumed to do that work.

On the other hand, Speed is also inversely proportional to Torque. Torque is directly proportional to the current and Voltage is directly proportional to the Speed.

As a result, it would seem only natural for a dc motor to consume high current at the start and then gradually consume less as the speed increases and torque requirement goes down.

However in this particular case, where I'm trying to understand the Speed Current relationship of a Geared DC motor. Which operates at 12V with max RPM 900 as read on the label. (These were the motors salvaged from a drill machine), I see this

The above is a current vs time plot, where I am independently increasing the speed from 0 to its max rpm via PWM.

The Input is Given via PWM with a range of 0-255 8Bit resolution.

The Peak current is achieved around half the Dutycycle aka Half the max RPM. The downward slope makes sense from the above assertion since speed is increasing current should decrease.

What confuses me is the increasing slope observed in the beginning. Speed is increasing, current is also increasing. But until it's half the RPM. Whereas it should have been a decreasing plot since the start of its operation.

What I am expecting

My opinion about this is that, since the motor is self-locking it starts from a low start current as it has to overcome its own torque offered by the gears of the motor. And only after that point ie. Half the rated RPM is able to overcome the torque offered by the gears.

PS: The current sensor used is a shunt-based sensor if that helps.

Answer 1

The overall efficiency of a DC motor can be found with the following equation

Efficiency = Output Power / Input Power = Output Power / (Power Output + Losses) = (Input Power - Losses) / Input Power

where the input power is an electrical one and the output is a mechanical one.

Since efficiency depends on power output, during the starting phase, it is low due to low power output and constant losses (iron losses, field copper loss and mechanical ones).

An increase in the output power is traduced in an efficiency increase till a certain maximum value ( where variable losse are similar to the constant ones). If output power is still further increased, the efficiency start decreasing due to a rapid rise in variable losses.

Motors are normally designed to have maximum efficiency during an actuation with full load.

A typical efficiency curve is the follwoing one:

TO SUM UP:

When the motor is working with maximum efficiency, the current it absorbs is lesser.

I kept this answer generic because the topic is really big. I suggest you to read more in bibliography.