How to drive a BLDC

A BLDC, also known as a brushless DC motor, is a three-pole component that provides torque to its rotor, when a sinusoidal input is provided to its terminals. It’s funny to call it a DC motor, since the input to the motor is anything but DC. Let’s break it down.

A brushless DC motor works by pulsing coils of copper wire with current to generate a magnetic field, which “tugs” on the rotor, and this creates torque. The motor also includes a hall effect output, so that you can tell which coils to give current to. As the rotor spins up, the hall effect outputs continue to change and tell you which coils to energize, so with powerful transistors with fast switching, you simply give these signals to the gate of the transistor, and let the motor rev to as high of a speed as you would enjoy.

Now, what makes a controller sinusoidal? Instead of using a single transistor for each of the three poles, you can use two transistors for each pole. This suddenly gives you the ability to provide a negative current to motor coils, instead of only positive currents, as your inverter has now gone from a half-bridge to a full-bridge. This is what makes the fancy, powerful motors sinusoidal, as the input waveform approximates a sinusoid better, sweeping from a negative, to positive voltage, with a small down time at zero volts.

If you’re building electric cars, you can get even fancier by switching the voltage at a very high frequency, and encode a sinusoid just as you would in a class D amplifier, and have the inductance of the motor windings filter the current into a clean sinusoid to drive the rotor. We’re going too far into the land of ultra-efficient motor drivers that cost thousands of dollars.

Anyways, let’s inspect a giant motor use this technology to rev up in preparation for driving a monster go-kart I am designing for senior design project. The motor is a Motenergy ME1306, and the controller is a Kelly KLS48100NC. The motor is to be driven by a Porsche Taycan 24V 2.7 kWh battery module. The motor controller and motor can soak peak powers of 37 kW! If you drew that much power from your house’s fuse panel, you’d pop the main 200A breaker!

Obviously it sounds great when it’s actually spinning, but I’ll still have to play with the controller parameters to fully calibrate the motor to this controller.