Have a question regaring a recent project completed. (come in Jraef)
snipped to make room....
Hmmm... you are going to get me in trouble with somebody somewhere...
Hang on, this is going to be a long one...
The PF400 design is "optimized" to be a cost effective solution for applying drives to centrifugal loads such as pumps and fans, often referred to as "Variable Torque" loads. Centrifugal machines are called VT because the load, in this case the flow through the pump volute or fan blades, does not change at a linear rate with the speed. The motor load, and by definition the required TORQUE from the motor, varies at the CUBE of the change in speed; it's called "the Affinity Law" in physics. In a "normal" or what is considered a "Constant Torque" load, the torque requirement for moving the load remains the same regardless of speed, so the HP, being defined as XX torque at YY speed, varies with speed at a linear rate, i.e. 1/2 speed = 1/2 HP. But in a VT load since the load is not coupling with the machine as much at lower speeds, the load, and by definition the torque required from the motor, varies at that cube rate. That means for example at 1/2 speed the load on the motor becomes 1/2
3, so .5 x .5 x .5 or .125, so
1/8th of the HP at 1/2 speed.
How that relates to drives is twofold.
- Since reducing speed increases the percentage of power going through the VFD that is lost as switching losses, the toughest thing for a VFD to do is to start a load from a dead stop, then power a fully loaded motor at constant torque at a reduced speed. It has likely had to go into overload briefly to accelerate the load, then is generating the most heat INSIDE of the drive components (Watts loss per amp) afterward. So when designing a drive, that is your worst case scenario and you size the components and cooling to accommodate it. But if you KNOW in advance that the load is VT, then you do NOT need to size the components for that same worst case scenario, it can never happen. Therefore you can use smaller (by comparison) power devices for the same maximum current rating. So a "VT drive" is basically just a de-rated version. Conversely then, if you have an over sized VT drive, it can in theory be the same as a CT drive. In the PowerFlex line, all drives EXCEPT the PF400 are actually like that, you order them as "Normal Duty" or "heavy Duty", which equate to VT and CT respectively. A-B, and most other VFD mfrs now, have started using this terminology because it more accurately describes what you are asking of the drive, most people were unaware of what I just explained about VT and CT loads. Since over 60% of all AC motors used in the world are on pumps, and more on fans, "Normal" is defined as pumps and fans.
- The other issue when designing a VT rated drive has to do with the V/Hz ratio. You can save ADDITIONAL energy on an AC motor running at light loading by reducing the amount of current used in creating the magnetic flux inside of the rotor. This is a portion of the motor power consumption that contains a relatively significant amount of losses, and is related to the voltage applied to the windings. If you reduce the voltage, you reduce the magnetization losses. it isn't much, but it is something. This is the same basic concept that is over blown by the "energy saver" scammers. But inside of a VFD, it is inherently easy to accomplish, you don't have to add any hardware, you just tweak the software. The drawback to doing this is a loss of torque, so on a CT type of load, you do NOT want to do this, otherwise you are increasing the slip in the motor and making it work harder, which can increase the current unnecessarily for the work it is performing. you might think, "But wait, you said the HP is going down with speed, so current is going down as well, so the losses are going down too and the motor will run cooler." Generally true, but at the SAME time, the cooling fans are moving less air and the motor heat is not being removed as effectively. So any INCREASE in motor heat relative to applied load can get trapped inside the motor and cause damage. If however it is a VT load again, then the rate at which the motor loading is dropping is so dramatically higher than the motor's capacity anyway, you can once again safely apply this reduced flux concept to save a bit more energy in pump and fan applications. So how this is accomplished is via tweaking the ratio of voltage and frequency, the "V/Hz ratio" that the drive sends to the motor. In a CT application, this is fixed in order for the drive to maintain full torque (hence the "constant" torque name). For example a 460V motor puts out 60Hz for full speed, so the ratio is 460/60 or 7.6 V/Hz. That ratio is maintained through the speed changes, so at 50% speed, you are at 230V (230/7.6 = 30Hz). But in a VT application, you can LOWER that V/Hz ratio when the speed is reduced in order to save a little more energy by reducing losses in the motor. Sure you will lose torque, but remember, in a VT application you don't NEED that torque as the speed goes down, because the load is dropping off at that cubed rate! So why not take advantage!
Now we come to that earlier statement of mine "
all drives EXCEPT the PF400". BECAUSE the PF400's purpose was to directly compete with OTHER lower cost drives that were dedicated to VT loads, the PF400 is doing two things related to the above issues. It is lower cost BECAUSE it is not expected to have to handle the rigors of a CT load, meaning the components are not sized the same, AND it comes pre-programmed for VT loads in that the V/Hz ratio is NOT linear as it would need to be in a CT/Heavy Duty drive.
So when you called the Tech Support line and they told you to change the setting in A170, they were taking care of issue #2, returning the V/Hz ratio to being linear so that the motor can produce Constant Torque at all speeds as it needs to in what I assume is a PD (Positive Displacement) blower.
Now to answer your other question, could the fact that it is a VT drive have caused damage to the motor?
Maybe.
If they left the drive settings at the factory default (as must have been the case if you changed it), then when you reduced the speed to 20% of rated, you were ALSO reducing the magnetic flux in the motor at a higher rate. That then would reduce the torque that the motor put out. But the PD blower CONNECTED to that motor was still requiring the SAME torque that it did at full speed. So what happens? The loss of torque becomes an increase in motor slip, which means, as I described above, the motor is pulling MORE amps per HP than it needs to, so more watts losses INSIDE of the motor relative to loading. But even though the load (HP) is reduced, because the motor's ability to cool itself is at the SAME TIME reduced with speed, it is possible that this caused the motor to cook itself.
By the way, you MAY have a potential problem with the VFD as well. Now that you have tweaked the V/Hz ratio back to being for CT loads, the losses inside of the VFD will be going up as well. Remember, this drive was not DESIGNED to handle that. If the VFD was over sized with respect to the motor, then that's OK. But guess what, 50HP at 230V is the LARGEST size for the PF400, so there is no way this drive could have been over sized. In fact, given that is it 208V, and small margin of safety that you may have had is ALREADY consumed in that issue. This drive is rated for 145A, with virtually no overload capability (110% for 60 seconds, again because it was designed only for VT loads) at 45C (104F inside of the enclosure), only 130A if it is 50C (122F). A 208V 50HP motor FLA is 143A per the NEC, yours hopefully is lower. But because it is a CT load connected to it, this may be a problem. How this can relate to potential motor damage is not clear, but it is possible that they knew this at commissioning and compensated for it. Check the programming of Function A182, the Drive Overload Settings. This function tells the VFD what to do if it detects that it's transistors are getting too hot because of loading. The factory default is "0", meaning take no action, because (again)
as a VT drive this should never happen. If they had an issue with this at commissioning, they may have set A182 to 2, 3 or 4 as a way to protect the drive. What that then means is that if the drive detects that the transistors are overloaded, it will fold back the Current Limit setting, fold back the speed reference, or both, depending on those settings. That then, combined with the issue of the improper V/Hz setting, can exacerbate the problems by artificially reducing the output even more than you thought it was.
