# Heat dissipated by inverters

I am part of a team designing an electric car and we decided to use a bldc and I need to decide to use square wave or a sine wave controller. Since we have a cooling issue I must know which type of inverters generally dissipates more heat square wave or pure sine wave (same power rating)? I think it's the pure sine wave because of higher mosfet switching frequency and passive filters used, but I am not sure if I am right.

• I'd say you're right, but, there's always a but, what is the actual question? I doubt you want a "you're right" answer. – Harry Svensson Nov 18 '17 at 1:10
• I am a part of a time designing an electric car and we decided to use a bldc and i need to decide to use square wave or a sine wave controller. – user28324 Nov 18 '17 at 1:12
• Since we have cooling issue i need to know which will dissipate less heat, hence the question :) – user28324 Nov 18 '17 at 1:13
• This is great information, I'd recommend you to edit your question and put everything there so anyone else who attempts to solve your question won't have to find crucial information in the comments. – Harry Svensson Nov 18 '17 at 1:16
• Is this "electric car" a RC car or an actual car that you can sit in? In other words, what are the power requirements? Either way I doubt that the main issue is with the shape of the wave form. I believe your problem is either with weak motors trying to do more than what they are made for, or using too high voltage, or the ESC (Electronic Speed Control) is not good enough for your motor. The more you share, the more we'll be able to pinpoint the actual culprit. I say we even when it's just me here, but more people will come. – Harry Svensson Nov 18 '17 at 1:28

In an inverter, each switching event loses some energy in the inverter. You therefore need to minimise the switching frequency in order to maximise inverter efficiency. This means (a) square wave output rather than sine and (b) lowest frequency waveform.

HOWEVER

You probably don't want to maximise inverter efficiency. You probably want to maximise system efficiency, which includes the motors.

The output torque of the motors is proportional to current. The motor winding $I^2R$ loss is proportional to current squared. This means that the motors will run more efficiently with a low ripple current (like you get from a sine output converter) than with a high ripple current.

A low ripple current requires either a faster PWM'ing inverter, or a filter between inverter and motor, or both.

In summary

Square wave -> efficient inverter, inefficient motors
Sine wave -> inefficient inverter, efficient motors

To get best overall system efficiency you probably need to use a custom inverter, and adjust it to your motor parameters, while measuring the performance of the system as a whole.

Square wave inverter will be somewhat more efficient than a sine wave inverted.

The square wave inverter switches at the same frequency as the AC which it generates. The pure sine wave inverter switches at a much higher frequency and modulates (PWM) the duty cycle such that the duty cycle in a sine function. Additional power is dissipated during each switching event.

(illustration from here, similar here)

The inverters which are intended for driving AC motors usually go under the name Variable Frequency Drive (VFD).

Don't know about square wave inverter output. Did you mean, six step pulse control for BLDC? If so, and this is the only control method for BLDC that outputs square wave, then it might be useless, since your car will be always accelerating with maximum acceleration until reaching the final speed. Therefore no speed control, which indeed has to be implemented in a car/bike drive.

If you want to add the speed control, then here it comes the PWM output. Still you might have six step pulse or three phase sine wave output. So you might exclude the square output from further calculations.