VFD 101 Basics

welcome to Eton's variable frequency drives 101 how do customers control industrial motors well traditionally customers use our trusty friend the full voltage starter which applies the full line voltage to the motor when it turns on when a full voltage start is too harsh where accelerates too fast we could use a soft starter to reduce the voltage sent to the motor and slowly ramp it up to full voltage but with both of these controllers once we get to full speed the motors stuck there until we turn it off in order to change the motor speed we can use a magical box called a variable frequency drive or a VFD you may have heard it called a variable frequency drive an adjustable frequency drive a variable speed drive an inverter drive or often just the term drive but they're all referring to the same thing when a customer uses a drive the power comes from upstream which we can call the utility power first it passes through a circuit protective device like a circuit breaker and then it continues into the input of the drive the frequency of this power is at 60 Hertz when the power exits the drive it then continues to the motor if we wanted to run the motor at full speed the drive would supply the motor with 60 Hertz but if we want to slow down the motor the drive is able to cut the frequency down to say 20 Hertz which causes the motor to run much slower or we can bump it back up to a faster speed at say 40 Hertz the drive allows us to have full control over the speed of the motor anytime we want in order to better understand how this works it will help to understand how the frequency of the power affects a three-phase motor back in the late 1800s a sharply dressed Serbian named Nikola Tesla invented the three-phase AC motor not bed 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 lots of windings of thin copper wire with a three-phase motor there are windings that are specifically powered by the a phase windings for the B phase and windings for the C phase the rotor is in the center of the motor and is the portion that spins when the motor is turned on the rotor is also attached to the motor shaft which connects the motor to whatever it's running let's look at the inside of the motor to understand how it operates when the a phase enters the windings in the motor the electricity flowing causes one set of windings to be positive and the opposite set of windings to become negative thus creating a magnetic field between them on the right hand side you can see the sine wave pattern of the power coming into the motor now let me slow it down for a minute when the positive side of the sine wave is strongest then the windings on that side become positively charged and the opposite side is negatively charged when the negative side of the sine wave is strongest the polarity inside the motor reverses making the opposite windings positive and negative the rotor in the center is magnetically charged as well and it tries to align itself with the magnetic field being generated by the stator but with only one phase going it makes for a pretty choppy spin on the motor let's add in the B phase I'll slow it down so you can see that as the a phase loses strength the B phase gain strength and keeps the magnetic field moving to make it nice and smooth let's add in the C phase which fills in the gaps when the a and B phases lose strength now as the power continually comes into the motor the switching back and forth of the AC sine waves causes a magnetic field to appear to spin the faster the sine waves enter the motor the faster the magnetic field spins the slower the sine waves enter the motor the slower the motor will spin so let's crank this up to full speed and feed the motor with the full 60 Hertz power being generated by the utility [Music] our buddy Nick Tesla is starting to get dizzy so how do we slow down the motor if we can stretch out those sine waves so that less of them are entering the motor per second thus lowering the frequency of the power then the magnetic field will spin slower [Music] since the utility is always pumping out 60 Hertz let's see if we can change that frequency inside of a drive it can be broken up into three main parts the AC power from the utility enters the drive and is converted from AC into DC then the DC is carried over and converted from DC back into AC these three parts are called the converter section the DC bus and the inverter section that seems like a lot of work to take AC power mess around with it and then spit it back out so let's figure out why in the converter section we have nice clean AC power coming from the utility on one side which looks like this inside the converter there are diodes that chop up that AC power and spit it out as DC we use capacitors inside the drive to smooth out this DC so that the power looks kind of like this when it's on its flat until it turns off then the DC power gets onto the DC bus which carries it over to the inverter section inside the inverter there are small circuit board components called insulated gate bipolar transistors which as you can imagine everyone calls AG BTS the IGBTs act as little triggers that take the DC power and fire it out in short bursts so the igbts spit out the DC in a short burst then another then another then they let out the DC a little longer and then a little longer and then a little longer and then a little shorter then they take the DC and flip the polarity the DC is turned on and off on and off pow-pow-pow paddle then it flips back then it flips again what is this starting to look like yeah a really lousy looking sine wave well with the spacing of the bursts of the DC the usable power in there averages out to look something like this to the motor it's not perfect but motors are dumb and it's close enough to AC that the motor doesn't really know the difference why is this cool well if the igbts can control how often and how long you spit out the DC then you can space them out or tighten them up to effectively change the frequency of our imitation AC sine wave this imitation sine wave coming out of the inverter is called pulse width modulation or PWM since we know from our earlier conversation that changing the frequency going to the motor will change the speed of the motor now we have a way to control that frequency pretty cool technology but let's see how we can apply it why would you want to change the speed of the motor well consider this example of a large ceiling fan in this manufacturing facility used to keep the employees cool with the motor running at our standard 60 Hertz the fan is spinning at full speed this may be great for the middle of summer but not so great for the middle of winter when the temperature is cooler and the full capacity of the fan isn't needed we can use a VFD to lower the frequency of the motor to a more appropriate fan speed our fan example is what's called a variable torque load as the fan speeds up the amount of torque or force required to move the air increases other types of variable torque loads besides fans our blowers spinning pumps and spinning compressors the general rule of thumb is that if the load being moved is air or a relatively easy to move liquid like clean water then it's likely a variable torque load every drive needs to be able to supply the motor with its full load amps or FLA in order for the motor to run at full speed but if the load changes or encounters extra load momentarily we need the drive to handle the extra current draw from the motor without shutting down since you never really encounter chunky air or if you're moving clean water both are consistently smooth so you don't need to have much extra capacity over the motors Fla so with a variable torque load we sized the drive so it has the ability to pump out a hundred and ten percent of the motors FLA for a brief period of time we call this a drive with low overload capability as it's able to provide a hundred and ten percent of the fla if the load is not variable torque it's likely what's called a constant torque load constant torque loads require the same amount of force to move the load at slow speeds as it does at higher speeds examples of constant torque loads are conveyors elevators shredders or extruders that extrude the pasta that you buy at the grocery store basically any load that isn't a fan pump blower or compressor since constant torque loads can change frequently like more boxes being added to a conveyor belt the drive must be able to handle brief periods of extra current drawn by the motor so to give the drive enough extra capacity we size it 150 percent of the motors Fla for constant torque loads we call this a drive with a high overload capability as it can provide 150 percent of the Fla let's come back to variable torque loads for a minute variable torque loads are kind of special because of how fluids like air or water move they behave according to a law of physics called the affinity laws the affinity laws state that as the speed increases with a variable torque load so does the flow of air or liquid at a linear rate so if the pump is running at 50% speed then the flow of water is 50% if you compare the speed against the pressure in a pipe or duct the pressure increases at a rate of the speed squared if you compare the speed against the power required to move the air or liquid the power is equal to the speed cubed this is the best part because of this relationship if I'm able to run my motor at a reduced speed with a variable torque load I consume substantially less power let's say I drop down to 70% speed well then I only require roughly 35% of the power that was required at full speed that difference in power required is energy savings on your electrical bill the equation for figuring out the power required at different speeds is pretty simple the power is equal to the percent speed cubed so with my previous example if i plug in 70 percent of full speed or if I were using a calculator I'd want to use 0.7 to represent 70 percent then I multiply 70 percent times 70 percent times 70 percent and I end up getting about 35 percent of the power required at full speed so if a customer is able to run their fan below full speed then the savings start to roll in now that we know the basics of VFDs regardless of size shape or color they all work behind the same general principles the power going into a VFD is fixed at 60 Hertz but the VFD allows us to adjust the frequency and the voltage entering the motor which gives us full speed and torque control anytime we want the type of load we put on the motor can affect how we size the VFD and with variable torque applications we see significant energy savings while running below full speed that concludes our lesson on variable frequency drives for more training opportunities and resources please visit the electrical training group's website at WWE 10.com ford slash training you

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Length: 15min 51sec (951 seconds)

Published: Fri Jan 15 2016