I'm working on a project to control single phase ac induction motor in which I planned to use two IGBTs, one for switching the PWM to the motor (for AC chopping, PWM frequency is fixed @16khz) and one for freewheeling the motor. A symbolic circuit is included here:


I know the dead time of IGBTs is going to have some effect on the system. I want to know how worst the system is going to be effected because of this dead time and what are the remedies with respect to hardware and software.

What will happen if i completely ignore the dead time?

  • 2
    \$\begingroup\$ I'd be concerned about running this circuit from AC - they are not like triacs i.e. they are not bidirectional devices. Maybe this one is? \$\endgroup\$
    – Andy aka
    Apr 2, 2013 at 9:54

2 Answers 2


You wouldn't hook up the IGBTs like that; IGBTs are transistors and don't switch AC. You want a single phase h-bridge:


simulate this circuit – Schematic created using CircuitLab

The "Battery" is your source power; it usually comes from a rectifier bank with a bunch of capacitance. A freewheeling circuit could be implemented with an SCR across the motor leads but more usually in these types of designs as with a fifth IGBT which connects the battery to a big resistor. You turn on two of the h-bridge IGBTs and turn on the regen IGBT as necessary to keep the bus voltage from riding up too high.

Now you generate an AC waveform by PWMing Q1/Q4 for one half and Q3/Q2 for the other half.

Dead time is an enforced "all transistors off" period which is used to ensure that Q1 and Q2 are never on at the same time. If both halves of one side of the bridge (Q1/Q2 or Q3/Q4) are on, you get a huge current spike through the affected transistors. Ignoring dead time might work as long as your supply is not too stiff and your devices are over-specc'd, but it's certainly not recommended.


If you completely ignore dead time calculations, it is possible that Q1 and Q2 will both be turned on at the same time, placing a short circuit across the AC supply. You should consider whether or not this will cause problems in your application.


In addition to heeding the dead time, please seriously consider Andrew Kohlsmith's answer; it is clearly the right approach using IGBTs.

The other big benefit of his approach is that starting from DC you can control the AC frequency you are generating. As previously explained you cannot properly control the speed of an AC induction motor with a fixed AC frequency, so you will have to do it this way eventually.

The only point worth changing in his answer is that you will be working with 320V DC not 160V, since your AC supply is 230Vrms and you have a 230V motor.


Not the answer you're looking for? Browse other questions tagged or ask your own question.