Notes on AC Drives (VFDs)
What is an AC Drive or Variable Frequency Drive?
AC Drive & VFD Theory


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By James M. Shumberg

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On with the AC drives article:

About AC Drive Technicians
"A good AC drive technician understands the operation of the variable speed drive and the functions of its components.

An outstanding AC drive technician also understands the effects of the load on the drive and the effects of the drive on the load."
...James Shumberg


Understanding AC Motor-Drive Systems
(Sometimes called Variable Frequency Drives or VFDs)

Maybe your objective is to learn how to troubleshoot variable frequency drives or maybe you want to know how to properly set-up a drive through programming. Maybe you just want the question answered: how does an AC variable frequency drive work?

During this article I will definitely cover the theory of operation of the variable frequency drive (VFD) and several typical AC drive applications including multi-motor AC drive applications.

In any case, to gain a useful working knowledge of the drive it is mandatory that you understand how AC induction motors produce mechanical force, called torque, and the limitations of AC motors. Therefore, what follows includes quite a lot of AC motor theory followed by heavy AC drive theory.

The very first thing you need to know is that the AC drive does not produce any torque - in fact, it does not produce even one ounce of load moving torque. All the mechanical force that moves your machine and al of the mechanical force that is produced by the motor & drive system is produced by the motor. The drive itself just has to provide the motor with the proper voltage and current necessary in a form that is usable by the motor. The drive itself, is only an electrical power supply.

The motor is the all-important prime mover. If a particular motor does not have the capability to produce the required torque and speed there is absolutely nothing you can do to the drive, programmatically or otherwise, to enable the motor to power your machine!

Therefore, when designing a motor-drive system proper motor selection is crucial.

Above all never, never, never match a "High Efficient" design motor to an AC drive! I am emphatic about this because I see it happen all the time and it is the worst thing you can do. (Well, not really the worst thing you can do, but I do want to get my point across.)

Do not buy a high-effecient motor to operate on an AC drive!

Interested?
If you have an interest in this topic you should consider registering in my AC drive seminar and bring a co-worker (because no one else back on the job is going to believe what I told you during the seminar.) Also, keep reading, I will tell you things about AC drives you never before heard and never will hear anywhere else.


Exactly what is an AC drive?
The word "drive" is used loosely in the industry. It seems that people involved primarily in the world of gear boxes and pulleys refer to any collection of mechanical and electro-mechanical components, which when connected together will move a load, as a "drive". When speaking to these people, an AC drive may be considered by them as the variable frequency inverter and motor combination. It may even include the motor's pulley - I am not sure.

People in the electrical field and electrical suppliers usually refer to a variable frequency inverter unit alone, or an SCR power module alone (when discussing DC drives) as the "drive" and the motor as the "motor".

Manufacturers of variable frequency drives (VFD) used to refer to the drive as just that, a "variable frequency drive". More manufacturers are referring to their drive as an "adjustable speed AC drive". To make matters worse when a motor is included in the package it may be referred to as an "adjustable speed AC drive system".

A variable frequency drive is an adjustable speed drive. Adjustable speed drives include all types; mechanical and electrical. Now is it clear? Don't worry about it. It's not clear to anyone. As you read on, when I refer to the "drive" I  am referring to the variable frequency inverter alone.

 

A Little About AC Drives
The main power components of an AC drive, have to be able to supply the required level of current and voltage in a form the motor can use. The controls have to be able to provide the user with necessary adjustments such as minimum and maximum speed settings, so that the drive can be adapted to the user's process. Spare parts have to be available and the repair manual has to be readable. It's nice if the drive can shut itself down when detecting either an internal or an external problem. It's also nice if  the drive components are all packaged in a single enclosure to aid in installation but that's about it.

The Dumb Trap
The paradox facing drive manufacturers today is that as they make their drives easier to use,  the amount of training with which they must provide their users increases. This is because as drives become easier to use they are purchased more and more by people of less and less technical capability. As less technical people get involved in drive purchases the number of misapplications goes way up. I call this phenomenon the "dumb trap". (When manufactures discover this phenomenon they simultaneously discover how dumb they've been. Some have not yet discovered it.)

 

Ambiguous Motor Theory
The real action in an AC variable frequency drive system is in the motor. This is really where it all happens.
To be an AC drive application Wizard (which is several levels higher then Guru) one must understand how motors use electric power. It is essential. I cannot emphasize the importance of this.

All loads moved by electric motors are really moved by magnetism. The purpose of every component in a motor is to help harness, control, and use magnetic force. When applying an AC drive system it helps to remember you are actually applying magnets to move a load. To move a load fast does not require more magnets, you just move the magnets fast. To move a heavier load or to decrease acceleration time (accelerate faster) more magnets (more torque) are needed. This is the basis for all motor applications.
 
Where does the real action happen in a AC drive system?

Above is a cross-sectional view a motor rotor and field magnetic core. Looking from the side would look something like a looking at a can:
We can add magnets (and torque) to our drive system by using a motor with a core that is either longer, larger in cross-sectional diameter, or some combination of both.

A Side Note About Fishing, Electro-magnets, Current, and Magnetic Conductivity
When we go fishing we put bait on a hook and throw it in water knowing that according to generally accepted theory, a hungry fish will sooner or later, bite. Well the truth is we don't know why the fish bite. No one to date, has talked to a fish (well maybe a few people talk to fish). The fact the we get hungry and therefore fish must too, seems like a safe assumption. But it doesn't really matter because we do know that putting bait on a hook will get fish into the boat.

Magnetism and electricity are the same way. We have some well accepted theories that we can use to explain how magnets can move our load but no one really knows what magnetism and electricity are (regardless of what they say). When it comes to using magnetic force to move our load, how it works just doesn't matter. We do know that it works. We have even noticed a few peculiar things.

We have noticed that when you wrap a coil of wire around a piece of iron and apply electric current the piece of iron becomes magnetic. We call this an electro-magnet.

Schematic of Electro-Magnet
Electro-Magnet

About Electro-Magnets (The Torque Producers Inside Every Motor)
We have noticed a lot of things about electro-magnets that are very important to the drive application wizard (you'll see why later):

  • After we apply the electric current, the magnet field grows at a finite rate to a finite size .
  • After voltage is applied and full current is reached, which always takes a little time, the field quits growing and becomes a constant size. If we increase the applied voltage the field grows and becomes stronger, decrease the voltage and the field weakens and shrinks.
  • When we remove electric power to the coil the field does not just disappear. It just decreases in size until it does disappear. It collapses over time so to speak.
  • The more current our coil draws (which we can force by increasing the applied voltage level ) the stronger and larger our magnetic field becomes. I know I said it twice. It's that important.
  • When we increase voltage to our electro-magnet, current will increase directly proportional up to a point. After that point current increases exponentially. THIS IS IMPORTANT! Generally accepted theory says that the iron core or any material, can only conduct a limited amount of magnetic flux. Once that point is reached current can become very high with a very small increase in voltage. This is called magnetic saturation and is sometimes seen in motor applications. Motor life becomes very short when the core reaches saturation - about 15 seconds in some cases. We will look at this and some of the causes later.
  • Some energy is consumed by simply magnetizing the iron core. Different materials consume different amounts of energy. This is usually considered an energy loss.
  • Some energy is converted into heat within the iron core. Different materials convert different amounts of energy. This is also usually considered an energy loss.
  • Once a core is magnetized, demagnetization and reverse polarity re-magnetization consumes more energy and takes quite a long time, relatively speaking. (Remember, an existing field has to collapse over time.) The amount of this loss is proportional to the frequency of polarization reversals. This happens 120 times per second when operating an AC motor at 60 hertz. We will touch on the importance of this later. (Are you beginning to see where all this is going?)
 

The Magnets Within the Motor and Torque

The motor stator shown below is a two-pole motor meaning it is wound with with two field coils for each phase. In the industry this would be called a "2-pole motor".

For simplicity, only one phase is shown. In reality, a 3-phase,  two pole motor requires six coils, evenly spaced around the core - a minimum of two coils is required, to generate two electro-magnetic poles, for each of the three phases.

(A 4-pole motor will have four coils per phase or 12 total coils for a 3-phase motor.)


Motors are designed so that the  electro-magnets are made as strong as possible with acceptable risk of core saturation. This will maximize the torque capability of the motor but also means that during normal operation every motor may at some point, operate close to saturation. How close a motor runs to saturation depends upon the amount and type of core material used. So naturally, this point varies from manufacturer to manufacturer. There really is a difference in motors and you get what you pay for.


When the voltage applied to a motor is increased current to the electro-magnets increases resulting in higher field strength and increased motor torque output. This is a commonly used technique, especially in AC drive applications. It is a very good way to gain torque capability when needed.

This technique can increase motor torque it will also cause higher than normal motor heating resulting in reduced motor life. Close monitoring of the motor is required. Avoid saturating the core!
   

All Three Phases of a 2-Pole Motor
The image to the right shows all three phases wound into a 2-pole motor.

Note how the end connections of each phase are connected together at the "Y" point. This allows for three lead wires to be brought out of the terminal box to be connected to a 3-phase power system.



 

A Coil-Ectomy
If you could remove the coils from the above motor without breaking a connection, and lay them side-by-side, this is what you would have. What is shown are three phases: A, B, and C phase connected together (see the arrow) at a "star" or "Y" point. There are other motor connection schemes but this is the most typical:


AC Generator
If a magnet is passed along the coils, an electric current is generated in each of the three phases. In fact, there is little difference between AC generator and motor field windings.

The faster you move the magnet the higher the AC output frequency. Variable frequency drives control the frequency electronically. We'll get to more on that later.

When an iron core is placed so a moving magnetic field passes through it, a magnet field is generated within the iron core. It takes time to generate a field therefore, the new field reaches peak strength after the peak of the generating field has passed. The bar (rotor) is "pulled" by the magnetic field thus producing torque.

The magnetic field has to pass through the rotor to generate a rotor field and pull. If the rotor travels at the same speed as the magnetic field, induction into the rotor will cease, the magnetic field will disappear and the rotor will loose its pull and slow down. Pull (torque) is obtained when fields are passed through the rotor in quick succession. Remember though, it takes a long time to generate a field. If the frequency of fields passing through the rotor is too low, effectiveness is lost.  If the frequency of the generating field is held constant, and the torque is great enough to move the rotor, the rotor will reach an equilibrium speed, where at any higher speed induction and torque are reduced and the rotor slows back down to equilibrium.

More About AC Induction Motors

Typical Rotor (Rotating Part) Typical Stator (Stationary Part)
Important Motor Formula - Calculating the Synchronous Speed:
("120" is a constant in the formula - don't worry about it.)
 

"Synchronous RPM" is the RPM the motor would run if the rotor did not slip. All AC induction motors slip. ("Synchronous motors", a special kind of induction motor, do not slip- at least least they are not supposed to. More about synchronous motors will have to be covered in another article.)

A note about nominal RPM ratings:
An AC motor referred to, in the industry, as an 1800 RPM motor will be name-plated with a speed of something less, usually around 1735 RPM. 1735 RPM is a typical RPM rating but can be higher or lower but is always less than the synchronous speed (1800 RPM). The difference between the synchronous and the actual RPM is called "slip". Adjusting slip is an important technique in AC drive applications. A lot more about slip will come later.

 
The following is surprisingly simple but important! Don't let it scare you.
You do not have to memorize it - just understand it.
Calculation of Synchronous Speed
(The "Poles" are the number of electro-magnetic poles wound into the motor. Motors can have any even number of poles wound into them but a minimum of 2 poles for each of the 3 phases are required. The most common AC motors are wound with either 2, 4, 6, or 8 poles.)
Looking at the calculation above you can see that a motor name-plated approximately 3450 RPM and 60 HZ is obviously a 2-pole motor with a synchronous speed of 3600 RPM.
Calculation of % Slip
(Typical induction motors slip anywhere from 3% to 5% when they are fully mechanically loaded.)
Example of % Slip Calculation
 
 

Why are some motors called "induction motors" and what is induction?
When an electric current is applied to a conductor a magnetic field builds around that conductor. If another conductor is in close proximity so that the building magnet field "cuts" through that conductor, a current of equal potential is produced with flow in the opposite direction of the original current. This conductor is called the secondary circuit and the principal is called induction.

When an electric current is applied to a conductor a magnetic field builds around that conductor. If another conductor is in close proximity so that the building magnet field "cuts" through that conductor, a current of equal potential is produced with flow in the opposite direction of the original current. This conductor is called the secondary circuit and the principal is called induction.

If the number conductors in the secondary is increased the output potential is increased in direct proportion. The inverse is also true.

This is called transformer action. It is because of transformer action that a current is created in the rotor (secondary circuit) of an AC induction motor and a resulting magnetic force, within and around the rotor, is also created. 

Note:
If the magnetic field reaches maximum strength and quits growing, the current flow in the secondary returns to zero regardless of the level of current flow in the primary. In other words, there is a secondary current induced only when the magnetic field around the primary is either increasing or decreasing in size.


This article will be continued on our sister web site: www.drivesys.com
There is not much sense in having two long articles on two web sites and causing you to wait so long for this web page regarding AC drives, to take a long time to load into your browser. (And you may not even be interested in it.)

The continuation of this article will therefore be found, sooner or later, based on when I have time to write, at www.drivesys.com

If you would like to drop us a note just click here. I read everyone of our comments from readers.

Thanks for reading so far. .... James Shumberg

 

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