Switching it up: IGBTs

Posted by Mackenzie Inman 15/08/2014 10 Comment(s) Variable Frequency Drives,

An Insulated Gate Bipolar Transistor (IGBT) is a key component in what makes up a VFD (Variable Frequency Drive). If you break down a VFD, one easy way to analyze it is to think of it in three main parts: the bridge converter, DC link, and what we will talk about today, the inverter. An IGBT is the inverter element in a VFD, pulsing voltage faster than we can even blink.

IGBTs have come a long way since they were first developed in the 1980’s. The IGBTs of today are much more advanced than their predecessors, which were slow at switching current on and off and often had problems overheating when passing a high current. With each new generation, IGBTs have continued to improve. No longer plagued by slow speeds, IGBTs have become highly reliable devices that can handle high voltage devices and are able to switch in less than a nanosecond (that’s a billionth of a second)!

IGBTs are the “Gatekeepers” of Current

To understand an IGBT’s role in a VFD, it is important to identify how an IGBT works on a smaller scale. As defined by being a transistor, an IGBT is a semiconductor with three terminals which work as a switch for moving electrical current. Just as the word “gate” suggests, when voltage is applied to the gate, it opens or “turns on” and creates a path for current to flow between the layers. If no voltage is applied to the gate, or if the voltage is not high enough, the gate remains closed and there will be no flow of electricity. In this way, an IGBT behaves like a switch; on when the gate is open and flowing current and off when it is closed. 

In this way, the IGBT acts as the switch used to create Pulse-Width Modulation (PWM). An IGBT will switch the current on and off so rapidly that less voltage will be channeled to the motor, helping to create the PWM wave. For example, although the input voltage may in reality be 650V, the motor perceives it as more like 480V by using PWM (shown in diagrams below). This PWM wave is key to a VFDs operation because it is the variable voltage and frequency created by the PWM wave that will allow a VFD to control the speed of the motor. Therefore, without the IGBT switching the current on and off so rapidly a PWM wave—and the speed control that comes with it— could not be created.

480V 60Hz

80V 10Hz

The number of pulses per second from the IGBTs is known as carrier frequency. Since carrier frequency is an adjustable parameter on most VFDs, you can essentially set it as high or as low as you want. Although, adjusting the carrier frequency comes with a few tradeoffs. Setting the carrier frequency too high will reduce the acoustic noise level produced from the VFD, but it will also shorten the expected VFD life due to heat. A higher carrier frequency will also contribute to an increase in motor heating and affect the overall efficiency of the motor. On the other hand, if you are in a sound sensitive environment – or if you just don’t want a headache – setting the carrier frequency too low can create a lot of motor noise or whining from the VFD. We have found that setting your carrier frequency at about 2 Kilohertz will achieve a nice balance between the audible acoustics while still keeping your VFD running efficiently. 

In a typical six pulse drive there are six IGBTs pulsing voltage up to 15,000 times per second. Since their introduction in the 1980’s, IGBTs have literally switched up the market and now play a large role in many modern day power electronics applications where speed and process control are needed. It is clear that IGBTs play a large role in many power electronic applications and will continue to as they become more and more advanced in their technology. Hopefully taking this in depth look at the small part an IGBT plays has helped you to understand the overall functionality of a VFD as well. Check out our other featured articles for everything you need to know about VFDs and motors at www.vfds.com/blog, or click on the banner below to find the VFD you are looking for out of our 2,000+ inventory!

Leave a Comment

10 Comment(s)

30/08/2017, 06:58:27 AM

Deeply described on it. Useful information who is looking for theory knowledge.

Mitchell Friedman:
27/07/2017, 08:50:46 AM

We require a small (1 amp @ 120v 60 Hz input) power supply to provide 120v 60 Hz adjustable to 75 Hz output for a production application.

rahul gupta:
06/01/2017, 06:18:52 AM

good block for learning

25/08/2016, 03:18:19 AM

what is the max temperature that IGBT can withstand in AC drives??
What is the max temp AC drives can withstand??

04/06/2016, 07:44:14 AM

Is it possible to run the motor of 275 kW with 250kw drive?

06/06/2016, 08:04:43 AM

Hello Ayoob,

If the VFD has a maximum current rating higher than the motor's full load amperage (FLA), and there is no reason the VFD would need to be de-rated (high elevation, high temperature, single-phase input), then the VFD will be able to drive the motor without a problem.

naresh kumar:
27/11/2015, 11:38:12 PM


15/08/2015, 07:58:17 AM

Hi, thanks for nice site.
Does the output of the inverter 200 V to 12 V, 12 V, 50 Hz with reduced and motor launches.
Thank you.

Jeff Vollin:
29/07/2015, 02:43:57 PM

Where do you get IGBT's that switch in under a nanosecond? I am not familiar with any device that fast, and I would be very interested if it existed.

10/09/2016, 11:05:48 PM

take any faulty VFD break it u will find a three terminal device in it which will be in rectangular in shape. if u want more knowledge about it then refer to its manual.

Marius Hauki :
17/05/2015, 05:50:55 AM

The rotor loss in a squirrel caged asynchronous AC motor is proportional to Pmech_axle * s/(1-s) where s = (ns - n) / ns.

ns = synchronous speed
n = axle speed
s = slip

If a voltage speed regulation method is used, the motor moves closer to Mmax and s increases. The rotor losses increase significantly since the artio s/(1-s) increases. Furthermore Mmax is proportional to Uin squared , so the M (mechanical moment) lost may be significant.

If a slip ring rotor resistance regulation method is used, s still increases and loss increases, but the Moment Mmax is constant but moved lower in rpm.

If a frequency based speed regulation method (VFD) is used, the ns is altered so s (the slip) can be held much lower. Therefore the rotor loss should be lower. The Mmax is also more or less constant. Therefore a VFD method gives less loss when we look at the fundamental frequency of the drive current.

Harmonics may give loss components, but the VFD should have proper filtering and design to prevent harmonic current loss in the motor.

al archambault:
30/12/2014, 11:32:43 AM

Your comment about higher xarrier frequencies increasing losses in the motor is not correct.

A higher carrier frequency actually reduces the motor losses but as you said higher carrier frequency increases the losses in the inverter section of the VFD.

09/01/2015, 04:23:00 PM


Thank you for your comment. We always try to give as accurate and reliable information as possible. We spoke with several of our engineers to learn their experiences with how a higher or lower carrier frequency affects a motor. They really felt that the majority of the time, losses either way (with a higher or lower carrier frequency) were negligible. They have, however, seen that with a high carrier frequency there were occasions where the motor would run hotter. We'll make sure to update our post to make that more clear, so thank you for bringing that to our attention.