Electric Motor Power Analysis

A challenge with developing and testing the fuel pump and LOX pump prototypes for Bandit was troubleshooting and interpreting the motor data. This was particularly important with the fuel pump where I was struggling to reach full power and trying to locate the source of inefficiency. This process taught me a lot about motor constants and calculations. I’m definitely not an expert on the subject but I want to share and document what information was helpful to me.

Motor Constants

Equations

Trends

Current is directly proportional to torque due to the torque constant Kt. Efficiency decreases with torque and current since higher current corresponds to more heat loss. Motors want to spin faster with less torque to get high efficiency. Power has a quadratic relationship with torque since efficiency is inversely proportional to torque but mechanical output is proportional to torque.

For a hypothetical motor of:

Kv=1500 RPM/V

Phase Resistance = 0.07 Ohm

Input Voltage = 24V

we have the tools to generate the motor characteristic curves at 100% duty cycle.

MATLAB Code:

Method Validation by Comparing to Datasheet

The fuel pump motor has properties

Kv=200 RPM/V

Phase Resistance=0.115 Ohm

Nominal Input Voltage = 36V

The results are slightly different due to real-world effects like friction but the generated plot that only accounts for heat loss is actually a really good approximation.