VFD controlers.

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It's in the name more or less. :)

VFD stands for Variable Frequency Drive.

As AC motors for the US are generally designed around a 60 cycle supply raising or lowering the frequency of the supply will raise or lower the RPM of the motor.

It is my understanding that the changing of the frequency is the major factor at work here but the voltage is also manipulated as well to keep the motor from overheating.

By controlling the voltage the VFD can control the current. The current will have a major impact on start up torque.

Most (if not all) VFDs can also serve as motor overload protection.

Many can also be rigged with optional 'dynamic braking' which means they can make the motor work like a generator by applying a load to the circuit. This can slow motors with high inertia quite quickly without burning up mechanical braking devices. Sort of a 'Jake Brake' for electric motors if you familiar with diesel engines.

I have also read that some VFDs can be supplied with single phase but produce a 3 phase output, I have not seen this done.

Something to keep in mind is that the circuit supplying a VFD must be sized to the VFD not the motor it is controlling. NEC 430.2

Use a 25 HP rated VFD for a 5 HP motor and the supply will have to be based on 25 HP.

I have worked with VFDs from 5 to 500 HP and am always impressed with what they can do.

The 500 HP VFDs I worked with where driving 500 HP fan motors, they where programed to ramp the fans up from 0 RPM to 1740 RPM in about 7 minutes. Done this way there is absolutely no start up spike of current. If you watch the ammeter when you hit start it would go to about 50 amps immediately and than slowly ramp up to about 450 amps.

If you tried to do an full line voltage start with these motors the amperage would probably be in the 2000 to 3000 amp range. :shock:

I can tell you with certainty that the local power company was not impressed with an across the line start during commissioning. :wink:

A very common use of VFDs that I see is for the control of building HVAC fans and pumps the fans and pumps will only be spun as fast as needed to produce the needed flow at any given time to reduce energy waste.
 
Bob,
I can tell you with certainty that the local power company was not impressed with an across the line start during commissioning.
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Was that because the load is served by a weak grid or because they were not aware of the large motor loads when they designed the supply to the service?
I have worked on a installation that had a 4500 and 1500 hp motor with across the line start. It was served by a large utility that had a138kV feeder to the substation that supplied the customer. Under deregulation, the customer switched suppliers and the substation was only supplied by a 33kV feeder. When they tried to start the 4500 hp, most of the motor starters in the plant would drop out because the voltage dropped below the hold-in level. They went back to the original supplier for their power.
Don
 
Don

don_resqcapt19 said:
Bob,
I can tell you with certainty that the local power company was not impressed with an across the line start during commissioning.
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Was that because the load is served by a weak grid or because they were not aware of the large motor loads when they designed the supply to the service?
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That would be yes to both of your thoughts.

This facility is literally in the middle of nowhere, it is miles from any other structures other than one family homes and from a main road.

I am sure the power company designed the 13.8 KV supply based on VFD start ups as the plant design did not have a means to bypass the VFDs.

At commissioning the engineers wanted for reasons unknown to me to go for an across the line start.

The VFDs where bypassed manually (load and line pulled off the VFD and jumpered) and the 1200 amp breaker that would normally supply the VFD was closed.

I was not there but the info I was told was it was not something that the workers involved wanted to repeat. The power company showed up shortly after and did some screaming.
 
Bob,
The VFDs where bypassed manually (load and line pulled off the VFD and jumpered) and the 1200 amp breaker that would normally supply the VFD was closed
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Most of the ones that we install for industrial installations will have contactors installed to be able to bypass the VFD. That way the plant can still run if the VFD it self fails.
Don
 
Yes, most of ours do and the last one I had to troubleshoot because of not starting. Kept faulting out. Fault code said to check Line Voltage. Had 480V on each phase. They were in a hurry to get exhaust going so that they could begin production and we decided to start in with across the line starter.

That didn't start it either, but after hearing the bus duct blow we knew where to look for the problem. It took us about 4 hours to replace 2 sections of the bus duct.

VFD worked fine when we finished.
 
You guys are discussing some massive motors. I wrote a little post which I hope will allow brother to do some more internet research into how a VFD operates. Variable Frequency Drive, as opposed to a simple motor starter, will allow the user to set the motor speed. These have become better at their job, through SCR to IGBT technologies and now with very impressive 'read my motor' functions. For most applications, this means a four pole motor which turns 1800 RPM at 60 Hz:
RPM=(120)(Hz)/#poles; you need to know this to read the display.

Now we're talking about the synthesized sine wave, which is sent through the stator windings. It generates a magnetic field which energizes the rotor, then it moves around at so many cycles per second (Hz.) The rotor has currents running through it, and attempts to react to the 'new' move. Since it is constrained by the motor bearings, all it can do is turn. I needed to explain this (you're probably very familiar with it) to make sure we're on the same page as far as how this sine wave is generated.

An inverter used in a VFD has inputs, R S T. This is used to power the control logic and generate a DC buss. Most VFDs can be programmed and tested using single phase to operate the logic, but using single phase to actually run three phase motors works best using a rotary inverter made for it. In any case, the DC buss is used by a three phase output module (six transistors in an H bridge) to generate PWM'ed sine wave output on terminals U V W. An output which excites reverse currents will turn the motor into a generator, causing regenerative braking and raising the DC buss voltage. This can then be dumped into regerative resistors or (costly equipment here) switched back into the incoming AC feed.

What happens is that the gate (ON) signals to the output module are slammed on; the transistor would be destroyed by running it in the linear region. It is slammed on and off very quickly, but the pulse goes through an inductive load as opposed to being shorted out. How long it stays on is the pulse width. These pulse widths are modulated according to where on our sine wave they're supposed to be, short near 0 and 180 degrees and long near 90 or 270. Of course, a different transistor is used for 0-180 and 180-360.

In the real world, a motor is brought up to speed. At below synchronous speed, we do not excite the motor with full voltage. So the motor's internal characteristics (speed, torque, current); the application; and the desired constraints (energy savings, inputs, current limits) are used to program a pwm'ed sine wave for various speeds. These are called V/F curves, and chosen for various installations. A linear V/F curve is good for just turning a motor on the floor, but an S curve might be chosen to reduce energy or peak input current. It's easy to learn the basics about PWM control of induction motors. :)
 
Personally I think it is irresponsible to install a full voltage bypass of a VFD. It makes more sense to purchase a spare VFD or for those very few critical operations a parallel one.

Most industrial systems are no longer capable of running only at 60hz.
The last hospital I dealt with required 4 hours of adjusting the balancing dampers to accept the full speed operation of the fans. Of course once the VFD was repaired it took another several hours to put them back where they belonged.
 
Hey, Bro. Thought I'd add my 2 cents for what it's worth. Most of my experience is in industrial maintenance and VFD?s (or inverters) have definitely taken over. Here are just a couple of tips I have if you're new to this part of the biz:

* Adjustable-Speed Drives have earned themselves a new section in Article 430 for the 2005 NEC. You can find it at 430.120 to 128. New for this revision is a requirement for Motor Overtemperature protection (430.126). I thought this was kindof odd when I first read it because the motors on VFD's last MUCH longer than the others. Most of the newer drives come with "ramped" over-load protection meaning it changes with the speed of the drive. Check with the manufacturer or the manual to see if the drive has this feature in it.

* The operator's manual will also have all the start/stop/reset info in it plus instructions for setting up OL protection and speed control, etc. These vary from drive to drive even within the same manuf'r. I know it goes against your instinct but reading the instructions WILL pay off.

* Some of the posts recommended putting an across the line by-pass in the circuit. I have not seen this in any of my applications. iWire mentioned that his customer was very displeased with these results. Back-up's are nice but if they won't fit the application, don't bother. I recommend stocking a spare drive. That's where a little planning will go a long way: You want to try to find a line/brand of drives and stick with it. Easier for maint. staff to learn and replace them in the future. Allen-Bradley made smaller drives they boasted made for a direct replacement for their contactor/overload assemblies for about the same cost. These drives aren't cheap but you do get what you pay for.

* Lastly, I have heard of at least one incident where the motor used on a VFD had to be rated for use on an Adjustable Freq. Drive. The first one they used (on a large fan) wasn?t and it was pulling high amps. They swapped it for one that was rated for such use and brought the amps way down. This is not the norm but something to look-out for

BTW, I have run into a situation where the drive did run on single phase to produce 3-phase. It wasn't by design. This was one of those smaller drives (3 hp) that we used Buss LP-CC-12 fuses on. In our application, we were running a little too close to the limits of the drive for it to function properly. It tripped the fuses and I was called in to replace them (gotta get the plant runnin'! We're lossin' money! blah, blah, blah. . .) Well, I did. And before I left, I checked the amps on the line side. 10 Amps on 1 & 3, nothing on phase 2. Later that week, we upped the size of the drive and haven't had any problems since.

Best of luck and we?re always happy to help a brother out!
:wink:
 
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