# How best to control 6 IGBTs for 3-phase controlled rectification?

I am using a BLDC as a generator. I am hoping to get a steady 50 V at 100 A. The frequency and voltage of the 3-phase output are variable with motor speed and linearly related, and I measure a frequency of about 1800 Hz at 50 V. Max. AC input will be limited to 60 V based on engine RPM.

I would like to drive a "6 pack" IGBT rectifier module to provide a steady 50 VDC output, even imwhen the generator is providing more.

I am thinking of using an Arduino to drive the 6 IGBTs, but struggling with the calculation for PWM outputs given the variable frequencies input.

1. Do I need a zero crossing detector and a fixed firing angle, or can I just use a random PWM signal at a much higher frequency than the input? Does PWM really need to be synchronous with zero crossing?
2. Is there a chip available that would do this without an Arduino?
3. Most chips I have seen are for motor drives and not rectification, and use Hall-effect sensors on the motor. Could I modify these to use as a rectifier driver?
• What is the maximum voltage that the BLDC device might generate? Edit your question with this information. The usual decision point between using MOSFET vs. IGBT is 400 to 600 volts, and even that depends heavily on the circuit designer's opinion. And -- how willing are you to learn? In electronics engineering terms, an Arduino is the kiddie pool -- you're proposing a dive off the deep end after the lifeguards have gone home for the day. Nov 23, 2022 at 15:42
• Note that the easy way to do this is just use six Schottkey diodes to rectify, then use a plain old buck converter to generate your "no more than 50V". You get a little bit more loss in the Shottkeys than you would with synchronous rectification with MOSFETs -- but less than you would with your IGBTs. Nov 23, 2022 at 15:45
• I am thinking regulating the AC will be easier, spreading the task over 6 MOSFETs rather than 1 or 2. I did simulate regulating on the DC side. Because I will have currents in the order of 100 amps at 50 volts, my smoothing circuit would require huge inductors and capacitors, both right after the bridge rectifier and after the PWM mosfet. Seems more efficient to regulate 3 ac inputs then rectify and smooth those. This is for a drone so weight is very important Nov 23, 2022 at 16:23
• Not sure what you're doing, but do you need a regulated 50V rail? What happens if you control the engine speed and let the 50V rail be whatever it is? Nov 23, 2022 at 23:29
• The pack will stabilize the voltage. You should connect a BLDC motor controller to the generator. One that supports FOC and regenerative braking. Operate it in torque control mode and set the torque based on various conditions. This will provide a near constant load to the motor so it will be easy for the speed to stay nearly constant also. If you need more or less power you can ramp the torque slowly (over a second or so) so the 7 hp ICE can adjust slowly. The batteries can buffer the voltage in the short term. Nov 24, 2022 at 2:20

It sounds like you are running an internal combustion engine as a "prime mover" for a generator to keep batteries charged. The batteries power some type of drone (multi-rotor, I presume... not fixed wing).

How to control the generator? If you are determined to use a BLDC for generation, I strongly recommend you use a motor controller capable of field oriented control (FOC) and regenerative braking.

Why FOC? FOC is capable of controlling torque very dynamically which will be very important for you to otpimize the loading of the motor. FOC can control torque over a much wider range than other control methods (such as passive rectification with variable duty cycle). In some cases, passive rectification will be unable to generate sufficient torque to meet your energy demands. Whether this applies to you or not is difficult to say and depends on the properties of the motor and the speed it will be operated at. But field oriented control will not suffer from this problem. It can control torque when the motor is spinning fast or slow. Note that, since torque * speed = power, the ability to control torque is the ability to control power.

You have some basic system engineering to do. There has to be kind of a voltage match between the motor and the battery. In this case, since you are doing regen exclusively, you need to make sure that the back EMF of the motor does not exceed the battery voltage because if it does, you can no longer control the torque effectively. The MOSFET or IGBT body diodes will act as rectifiers even when they are off. By the way, I suggest using MOSFETs not IGBTs.

It may be instructive to look at how automotive alternators work and how much they weigh. A Balmar 97EHD-190-24 alternator when cool, can put out 190 Amps at 24 V and it weighs 36 lbs (about 16.4 kg).

Alternators use a field winding rather than permanent magnets to generate back EMF. Increasing the current in the field winding creates a stronger magnetic field and therefore more back EMF. Then they rectify the current to ripply DC. By varying the field current, they can control the output current.

A BLDC generator will probably be lighter than an alternator. But a BLDC generator capable of running at 5 kW and putting out 100 Amps will not be a tiny thing.

The other thing you seem to be assuming is that a 7 HP motor can drive a 5 kW generator. This would require very high efficiency in the generation process (over 95 percent). It is possible to design a 95 percent efficient generator. But making it very efficient will tend to make it heavy also. And weight is your enemy. So I am concerned that the specifications are not realistic yet. Planning for 70 percent efficiency would be more realistic. The generator will require cooling. 30 percent of 5 kW = 1.5 kW of heat you need to transfer to the environment somehow.