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⌚Timestamps
00:00 Intro
01:00 VFD diagram
01:54 How does an IGBT work?
03:58 IGBT module in VFD

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Welcome to Part 2 of Variable Frequency Drives Explained!

As we have learned in the previous lesson, there are many applications where in order to meet varying demands, motors need to throttle down output.

Additionally, constantly running a motor at its full speed can be costly.

A variable frequency drive is a type of controller that has the function of driving an electric motor by varying the frequency and voltage that is supplied to control its speed and power.

VFDs are widely used in both industrial and commercial applications, such as control of fans, pumps, compressors, HVACs, and even roller coasters!

It is impossible to talk about VFDs and Insulated Gate Bipolar Transistors, also known as IGBTs without understanding what Pulse Width Modulation (PWM) is.

An analog input signal can be modulated by generating variable width pulses to represent its amplitude. In a very brief summary, PWM is a way to control analog signals with a digital output.

To understand the IGBT’s function in a VFD, it is important to understand how an IGBT works singularly.

At the simplest level, an insulated gate bipolar transistor (IGBT) is a switch used to allow power to flow when it is turned on and to stop when it is turned off.

It is important to note, however, that they have the capability of switching on and off several thousand times every second!

An IGBT is a solidstate device, which means it has no moving parts.

Rather than opening and closing a physical connection, it is operated by applying a voltage to a semiconductor component, called a gate, that changes its properties to create or block an electrical path.

IGBT terminals (pins) represent the Gate, the Collector, and the Emitter.
Current flows along the conductance path composed by the Collector and the Emitter, while the Gate controls the device.

Now that we have an understanding of how our IGBTs work, let’s go back to the application of IGBTs in VFDs, and let’s represent our IGBTs as contact switches for a simpler understanding.

The top IGBTs are in the positive DC bus, and the lowers are in the negative DC bus, so when one of the top switches is closed, that motor phase and voltage then become positive.

On the other hand, when one of the lower switches is closed, that motor phase and voltage then become negative.

Therefore, by controlling the speed and sequence that those switches open and close, we can control the phases and frequency of our signal: zero, negative, or positive.

It is important to note that VFDs output signal is a PWM signal, which turns out to be a rectangular waveform.

This wave is crucial in the operation of a VFD, as it is this variable voltage and frequency that will enable the VFD to control the motor’s speed.

VFDs output signal is a PWM signal, which turns out to be a rectangular waveform. This wave is crucial in the operation of a VFD.

The control processor of a VFD contains a program that is not typically useraccessible, however, there are many parameters and settings that can be adjusted and tuned for optimal VFD operation for each application where it’s been used to meet specific motor and driven equipment specifications and needs.

The following are common adjustable parameters and settings in a VFD:
Tunable: Proportional, Integral, Derivative (PID)
Minimum and Maximum Speed
Current Limit

The number of parameters varies based on the level of complexity of the VFD. They can range from 50 to over 200 parameters!
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