Motor Speed and Performance Curves

We’ve had a couple of cases recently that made us think about the importance of getting the gearing of an electric motor correct. Whilst it is true that electric motors can give good torque at low speeds, it is not always the case that the efficiency of a motor is good at that low speed. Here we take a quick look at the importance of considering the motor speed and performance curves that the motor manufacturer should be able to supply.

For background we have recently had a Lynch LEM 200 being used on a river boat, and a Motoenergy ME0708 being used on a [large] model loco. In both cases although the motors would run and put out some impressive power, they were not geared so as to be able reach their peak efficiency RPM. The consequence of this was that both the motors and the controllers [Pro-360s] started to get hot, up to the point of the thermal protection kicking in.

If we have a look at the curve for the ME0708 we can see that the efficiency is only above 80% when the motor is turning above 1600 RPM. In the case of the loco the motor was geared to allow 950 RPM at top speed [10 mph]. The consequence of this was that the motor was operating a long way from it’s  peak efficiency.

me0708 performance curve

After the gearing was changed, the motors were then able to get closer to the peak efficiency RPM when at full throttle, this allowed the motors and controllers to run much cooler and draw less current for the same power output. This will extend the range of the vehicles and the life of the motors and controllers.

We’ve put a speed to RPM calculator here. This lets you enter the number of teeth on the sprokets, the top speed required, the rolling wheel diameter, and will calculate the motor speed.

An explanation for the low efficiency is that at low RPM the back EMF of the motor has not yet built up sufficiently to reduce the current through the armature. Resistive heating losses are proportional to the square of the current, so high currents will lead to higher I2R losses thus reducing the energy available for driving. In the case of the ME0708 the armature resistance is 0.01 ohms, with a supply voltage of 24V this motor could pull a static stall current of 2400A!



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