Variable Frequency Drives

How do customers control electrical motors? 

Customers have traditionally utilized the full voltage starter, which applies the full line voltage to the motor when it is turned on. When the full voltage start is too strong and the motor accelerates too quickly, we can use a soft starting to lessen the voltage given to the motor, but once we get to speed, the motor is trapped there until we switch it off. To modify the motor speed, we utilize a device known as a variable frequency drive (VFD). You may also hear it referred to as an adjustable frequency driver, inverter frequency drive, variable speed drive, or often drives, but they all refer to the same thing. 

When a customer uses drives, the power from the utility first passes through a circuit breaker and then continues to the drive's input. When the power exits the drive, it then continues to the motor. If we want to run the motor at full speed, the drive will supply the motor with 50 Hz or 60 Hz, depending on the rating of the drive and the supply frequency, but if we want to slow down the motor, the drive is able to reduce the frequency, which causes the motor to run much slower. The drive gives us complete control over the speed of the motor at any time.

Understanding how the frequency of the power impacts the three phase motor can help you grasp how this works. 

Back in the late 1800s, a man named Nikola Telsa invented the three-phase alternating current (AC) motor. In his design, there are two main parts, the stator and the rotor. The stator is the portion around the outside of the motor made of many windings of thin copper wires, and with the three-phase motor, there are windings specifically powered by phase 1, windings for phase 2, and windings for phase 3. The rotor is located in the heart of the motor and is the component that spins when the motor is turned on. It is also connected to the shaft, which connects the motor to whatever is running. 

Let's take a look inside to see how it works: 

When the first phase enters the motor winding, the electricity flow makes one set of windings positive and the opposite set of windings negative, creating a magnetic field between them. The sin wave pattern of the power flowing into the motor is shown below. When the positive side of the sine wave is the strongest then windings on that side become positively charged and the opposite becomes a negative charge. When the negative side of the sine wave is the strongest the polarity inside of the motor reverse making the opposite windings positive and negative. 

The rotor in the center is magnetically charged as well and it tries to align with the magnetic field generated by the stator but with only one phase going it makes for a pretty choppy spin of the motor. 

Let's now include the B phase, which strengthens as the A phase weakens and keeps the magnetic field in motion.

Let's add the C phase, which fills in the void left by the A and B phases losing strength, to make it beautiful and smooth.

Now, when power flows into the motor continuously, the AC sine wave switches back and forth, spinning the magnetic field. The faster the sine wave enters the motor, the faster the magnetic field spins; conversely, the slower the sine wave enters the motor, the slower the motor will spin.

How can the three-phase motor be slowed down?

The magnetic field will spin more slowly if we can spread out those sine waves so that there are fewer of them entering the motor each second and lower the frequency of the power.

Basic of VFD 

A drive is composed mostly of three parts. AC power from the utility enters the drive and is converted to DC. After that, the DC is carried over and converted back to AC. These three parts are known as the converter section, the DC bus, and the inverter section.

Converter section

We have clean AC power flowing from the utility on one side of the converter portion, as seen below:

Inside the converter, there are diodes that chop up that AC power and spit it out as DC.

                               Diodes

Capacitors are employed inside the drive to smooth out this DC, resulting in somewhat of the power seen below:

It is flat when it is on until it is turned off. The DC power is then transferred to the DC bus, which sends it to the inverter portion.

Inside the inverter, there are small circuit board components called insulated gate bipolar transistors, sometimes known as IGBTs

IGBTs serve as tiny triggers that accept DC power and release it in brief bursts. The DC is therefore released by the IGBTs in brief bursts, followed by additional ones. The DC was then released for a bit longer, then for a little longer, then for a little shorter, and so on. After that, they take a DC and reverse the polarity. On and off, on and off, the DC is turned. Then it flips again after flipping once more. What is beginning to resemble this below:

The usable power in there averages out to look much like this below to the motor due to the spacing of the DC bursts. 

Although it isn't perfect, motors are stupid. Additionally, it is close enough to AC that the motor hardly notices the difference. How does this cool? The frequency of our fictitious AC sine wave can be efficiently changed by spacing out or tightening up them to effectively change the frequency of our imitation AC sine wave. 

Pulse width modulation, or PWM, is the name given to the fake sine wave that emerges from the inverter. Since we already discussed it, we are aware that altering the frequency that the motor receives will alter its speed. We now have a method for managing that frequency. Let's look at how we can use this rather great technology.

VFD Applications

Why would you want the motor's speed to be altered? Well, take this industrial facility as an example, where the workers are kept cool by a huge ceiling fan. The fan is operating at its maximum speed when the motor is operating at our customary 50 or 60 hertz. The midst of winter may not be the best time for this, but the middle of summer might be. while it's cooler and you don't require the fan's maximum power. We can use a VFD to reduce the motor's frequency and increase the fan speed accordingly.

Our fan example is what's referred to as a variable torque load. As the fan speeds up, the amount of torque or force required to move the air increases.

Blowers, spinning pumps, and spinning compressors are more examples of variable torque loads in addition to fans.

According to the general rule of thumb, a variable torque load is more likely to be moving air or a liquid that is reasonably easy to move, such as pure water. In order for a motor to operate at its maximum speed, every drive must be able to produce the FLA, or full load amps, that the motor requires. However, we need the drive to be able to handle any temporary fluctuations in load or additional load from the motor without shutting down. You never truly met chunky air, and both are always smooth if you're moving clean water. Thus, you don't need much extra capacity beyond what the motors FLA can handle. Therefore, with a variable torque load, we size the drive to be able to temporarily deliver 110% of the motor's FLA.

Due to its capacity to supply 110% of the FLA, this drive is what we refer to as having low overload capability. If the load is not variable torque, it is probably a load with constant torque. It takes the same amount of effort to move a constant torque load at low speeds as it does at higher speeds. 

Extruders, elevators, shredders, and conveyors are a few examples of constant torque loads. 

Because constant torque loads might fluctuate regularly, such as when new boxes are put to a conveyor belt, the drive must be able to manage transient spikes in the motor's additional current consumption. We size the drive at 150% of the motor's FLA for constant torque loads in order to provide it enough additional capacity. Given that it can deliver 150% of the FLA, we refer to this drive as having a high overload capability. Rewind for a moment to varying torque loads. Because of the way fluids like air or water travel, variable torque loads are rather unique. 

The affinity rules, a physical principle, govern how they behave. According to the affinity laws, the flow of air or liquid increases linearly with speed when a variable torque load is applied.

Therefore, if the pump is operating at 50% speed, the water flow is also 50%. When the speed and pressure of a pipe or duct are compared, the pressure rises at a rate equal to the speed squared. 

The power is equal to the speed cubed when compared to the speed needed to transport the air or liquid. 

The highlight is this. because of this relationship. My power consumption will be significantly decreased if I can run my motor at a lower speed with a variable torque load. If I reduce my motor speed to 70%, I'll only need about 35% of the electricity that was needed at maximum speed. Your electrical bill will reflect the reduction in energy required.

The formula for calculating the power needed at various speeds is quite straightforward. The square root of the percentage speed is the power. I would therefore want to use 0.7 to represent 70% in the preceding example if I were to plug in 70% of full speed or if I were using a calculator. After that, I multiply 70% by 70% by 70%. And when I run at maximum speed, I only obtain around 35% of the power needed. 

Power = (%Speed)³

            = (70%)³

            = 70% x 70% x 70%

            = 35%

Therefore, the savings begin to accrue if a customer is able to turn their fan down from maximum speed. 

We now understand the fundamentals of VFDs and can see that they all operate according to the same fundamental principles, regardless of their size, shape, or color. A VFD receives power at a set 60 or 50 Hertz. However, the VFD enables us complete speed and torque control at any time by allowing us to change the frequency and voltage entering the motor. How we size the VFD depends in part on the type of load we apply to the motor. Furthermore, while operating below maximum speed, variable torque applications result in significant energy savings. Our discussion on variable frequency drives has come to an end. 

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