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Welcome to Part 2 of Variable Frequency Drives Explained!
You can find the link to Part 1 in the video description.
As we have learned in the previous lesson,
there are many applications where
in order to meet varying demand,
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!
Going back to the VFD diagram we were previously studying in Part 1,
and quickly recapping:
Firstly, the VFD checks for a ground fault.
Next, the alternating current input power
is converted into direct current
by the six pulse rectifier or converter,
resulting in a DC waveform.
Following comes the DC bus;
but to prevent any inrush damage,
the pre-charge circuit slows
the change rate of the bus capacitors during power-up.
The DC bus then filters and buffers the signal,
which now follows the last step of the VFD’s drive output:
the IGBT module.
We know that this last step will then convert the direct current
into the alternating current
as a Pulse Width Modulated output.
But… What does that mean?
And how does that happen?
It is impossible to talk about VFDs
and Insulated Gate Bipolar Transistors,
also known as IGBTs
without understanding what Pulse Width Modulation,
or 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,
or 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 solid-state 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.
Let’s look closely at a typical IGBT:
Its terminals 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.
Here is an electrical drawing representation of it:
The IGBT behaves similarly to a switch:
As the word itself suggests,
when a positive voltage is applied,
the Gate will close, or turn on,
allowing current to flow between the collector and the emitter;
if not enough voltage is applied,
the Gate remains open,
or turned off,
not allowing for any current to flow between the collector and the emitter.
To prevent the device from closing,
this voltage is kept negative in this off state.
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 simpler understanding.
The top IGBTs,
which in our representation are switches
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.
The control processor of a VFD
contains a program that is not typically user-accessible,
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, or PID
Minimum and Maximum Speed,
and Current Limit, Among many others!
The number of parameters varies
based on the level of complexity of the VFD.
They can range from 50 to over 200 parameters!
Now let’s combine and review once again what we have learned.
Variable Frequency Drives are motor controller devices
used in numeral applications,
from small appliances to large compressors,
and much more in between!
The main function of the VFD
is to drive and control motor speed and torque
to meet application requirements
by varying supply voltage and frequency.
There are multiple stages inside a VFD controller,
which can be briefly summarized as:
Drive Input stage
is when the alternating current is being fed into the inverter.
For Housekeeping,
current transformers measure and compare
current entering and leaving VFD,
checking for ground faults;
Also, initial inrush damage
is prevented by the use of a pre-charge circuit.
During the Drive and Drive Output stage,
the alternating current then gets converted into direct current
by the six-pulse rectifier or converter.
This direct current gets buffered and filtered in the DC bus
where its resistor function is to divide voltage
and to guarantee that all capacitors have the same voltage.
All so that this now filtered direct current
can get converted once again into alternating current
by the IGBTs acting similarly
to switches allowing control of the phases and frequency of our signal
in form of pulse width modulated output.
Lastly, the Motor and drive train stage is
when this pulse width modulated output
then allows for the control of the motor's speed,
and consequently, its mechanical output.
VFDs have evolved tremendously.
They have developed into highly sophisticated high-frequency power devices
in the form of microprocessor controllers.
The use of VFDs allows for predictable,
smoother,
durable, and highly efficient operation processes!
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