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Variable Frequency Drives Explained | VFD Basics

Learn about VFD, its applications, and its main components.

In this article, we will learn about VFD and its applications. We will also cover in detail the converter and the DC link. Later on, we will cover the IGBT module, and how PWM allows for AC output to the motor for precise motor speed control.

AC motor rotational speed

Induction or alternating current electric motors rotate at a rate that is set by the number of poles inside the motor itself, and the power supplied.

Alternating current electric motors

The frequency (measured in Hertz) is directly related to the Revolutions Per Minute (RPM) of a motor. The higher the frequency, the faster the RPM or the higher the engine rotation speed.

In the United States, electric power utilities provide alternating energy with a frequency of 60Hz. A standard two-pole AC motor operating at this frequency provides a nominal rotation of 3600 RPM.

AC motor rotational speed

Speed reduction

If an application does not require an electric motor running at full speed of 3600 RPM, which is very common, a few solutions exist:

1: Using a mechanical speed reducer

It mechanically decreases the output speed by increasing torque – the output gear has more teeth than the input gear.

Mechanical speed reducer

They require lubrication, provide no flexibility, are subtle to vibration and noise, and are not suitable when shafts are distant.

2: Adding more sets of poles

It reduces the speed without altering it electrically. Currently, there are transistor systems that allow for poles inside motors to be turned on and off. However, those systems can be complex and don’t provide fine control.

Number of poles

3: Using a Variable Frequency Drive (VFD)

It can be configured and fine-tuned to generate a ramp, frequency, and voltage so that the motor operates according to the load requirements (desired speed and voltage).

Variable frequency drive

An important feature of the variable frequency drive is that as the motor speed requirements in a given application change, the drive can simply raise or lower the motor’s speed in order to meet new operating requirements, which would not otherwise be possible by using only the mechanical speed reducer, or the transistor system to add more poles.

VFD applications

The use of VFDs is widespread in numerous industrial and commercial applications.

– In industrial applications, VFDs are used to control from extruders, and electric cranes, to roller coasters, and mechanical bulls, with so much in between!

VFD industrial applications

– In commercial applications, VFDs are widely used in pumps to control flow and even volume in a tank, as well as in the HVAC industry, being considered green technology.

VFD commercial applications

Ultimately, a VFD varies the supplied frequency to an AC motor in order to control its speed; allowing a smooth startup, and adjusting motor speed as the application requires.

Now that we know why and where variable frequency drives are used, let’s dive into the How does a variable frequency drive work?

VFD electrical circuit

Let’s look at this VFD diagram:

VFD current transformers

The first thing we find (T3) is a current transformer, which has the function of measuring the current entering our VFD so that the VFD controls can compare the current entering and the current leaving the VFD – measured by the two current transformers indicated by T1 and T2.

If the current measured is different, the VFD will disarm due to what we call a ground fault.

Six-pulse rectifier or converter

Following, we have the converter, also called a rectifier. If we were to expand and look at the rectifier diagram, this is what it would look like:

Six-pulse rectifier or converter

This is called a six-pulse rectifier or converter, and it’s where the three-phase alternating current gets converted into direct current by the use of diodes.

Using an analogy of a hydraulic system, those (six) diodes can be equated to check valves. The form in which the diodes are connected with each other is very strategic.

Hydraulic equivalent of rectifier

Check valves only allow flow in one direction, such as our diodes with the current flow (the direction is shown by the arrow on the diode symbol).

The electric current passes through the diode in the direction of the arrow on the diode drawing, that is, the current goes from the anode to the cathode, as shown in this image:

Diode current direction

There will only be electrical current if the voltage at the anode is greater than at the cathode.

Therefore, when we connect a three-phase alternating current to the converter:

– When phase A is greater than phase B or C voltages, this diode opens, allowing current to flow,

– When phase B becomes greater than phase A, then it is phase B diode that opens while phase A diode is closed,

Diodes on the positive side of the bus

– The same is true for C, as well as for the three diodes on the negative side of the bus.

Diodes on the negative side of the bus

That results in six pulses of currents as each diode opens and closes. The resultant waveform will look like this:

Six pulses of currents

DC bus or DC filter and buffer

Next, we have the DC filter and buffer, also known as the DC bus. The DC bus is represented by only one capacitor and resistor on the diagram, but in reality, there are various capacitors and resistors associated in series and in parallel.

DC bus or DC filter and buffer

Since the capacitors are not charged, their impedance is very low. If we were to charge them, the initial inrush could damage the input power devices, or the rectifier/converter, in case the entry fuses didn’t interrupt the circuit.

Bus capacitors charge rate

So instead, we have a pre-charge circuit. Pre-charge is a current limiting circuit that slows the charge rate of the bus capacitors during power-up.

The pre-charge circuit shown here is composed of:

– a contact,

– a resistor,

– a thermostat.

DC bus pre-charge circuit

When we energize our VFD, and the bank of capacitors is not yet charged, the capacitors start to charge by the resistors – once the VFD controls identify that the DC link is fully charged, it will then close the contact becoming the path of least resistance for the electric current.

DC bus pre-charge

In the scenario of the contactor not closing, and the VFD still starting the motor, the current flowing through the resistor will increase, which will overheat the pre-charging resistor. The thermostat will then act, and it will disarm the VFD due to overheating.

DC bus pre-charge circuit thermostat

Now let’s go back to the DC bus – its resistor function is to divide voltage and it will guarantee that all capacitors have the same voltage.

DC bus resistor function

IGBT

Finally, we have the IGBTs, which is the last step of the drive output: the DC to AC converter and our PWM output. We will cover those in more detail soon. Stay tuned.

VFD IGBTs

Summary

– Variable frequency drives allow for precise motor speed control by varying the frequency and voltage of its power supply.

– There are three main stages to a VFD:

1) the converter, or rectifier – composed of diodes – converts AC to DC,

2) the DC link – composed of capacitors and resistors – filters and buffers the DC,

3) the IGBT module – composed of transistors – converts the DC back to AC.

We covered in detail the converter and the DC link. We will soon cover the IGBT module, and how PWM allows for AC output to the motor for precise motor speed control. Stay tuned.

Want to learn more?

If you have any questions about VFDs or about Drives and Drive Components in general, add them in the comments below and we will get back to you in less than 24 hours.

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Giovanna Tomiotto

Giovanna Tomiotto

Automation Engineer

Posted on Feb 21st, 2022

Giovanna Tomiotto.

Giovanna Tomiotto

Automation Engineer

Posted on Feb 21st, 2022

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