I really didn’t want to get into a protracted debate. I was trying to offer some oversimplified generalities to help the average Joe gain a basic understanding. But, I will reply to your assertions.
The compressor applications I spoke of were recips, not screws. It was my understanding that screws run into wear issues when they don’t run at their favorite speeds & that is why they are often undersized a little, then backed up by recips that can start & stop more frequently without doing damage. I know little about unloaders. But hey, I’m not a compressor specialist, so my knowledge base there is thin. I do know a few things about drives, motors & controls.
I did not understand that reciprocating saws & punch presses were different from the kind of reciprocating equipment that you were referencing. Thank you for the clarification.
I was not aware that it was a fact that reciprocating compressors are not meant to run at varying speeds. I’ve been running 3 phase compressors off of drives, that run off of single phase, in places that don’t have 3 phase available, for more than 20 years. Sometimes, I ran at speeds other than nameplate. I hadn’t noticed any issues, but perhaps issues were present & I just don’t know what to look for.
It would be my expectation that the increase or decrease in wear based on change in speed would be variable depending on the specific piece of equipment being examined. I also believe that if you change the basis of judgment, you will be able to change the outcome. Eg, wear per hour, vs wear per cf, vs wear per Kwh, etc. But that is just an expectation on my part. I don’t have empirical data to back it up. Perhaps you do.
Yes, reduced oil pressure could be an issue at reduced speeds. Most equipment with forced lubrication has a range of oil pressure with which it can run properly. Equipment that runs with regulated oil pressure tends to be particularly insensitive to reduced primary pressure. At some reduced RPM, this will likely become an issue. I expect that the minimum acceptable RPM for proper oil pressure would tend to vary with the specific piece of equipment, it's age & it's maintenance history. But hey, there I go stating a silly expectation again.
Reducing speed also reduces motor cooling, unless the motor has a separate blower (as many inverter rated motors do). That is also that detail, which I probably chose to overlook before.
As speed increases over nameplate RPM, a motor running off of a drive changes from constant torque range to constant HP range & the torque available from the motor DOES reduce as speed continues to increase, due to current limiting from the drive. That IS a fact of physics. That is how drives are designed to work. Ask anyone who designs drives for a living.
I did not realize that his motor was 480. Since 480v 1 phase is not commonly available where 480 3ph is not present, & motors of that type probably don’t exist, than you are correct. The change is not as simple as adding a jumper. You got me on that one. Good catch.
I have run into harmonic issues in some of the more complex projects that I have been brought in on. I have had more than one instance in which adding a new drive into a system caused another drive to trip out, or sustain damage. Sometimes those two drives were in the same machine. Sometimes they were in different parts of the same building. Sometimes they were in different buildings in the same complex. This has happened to me with high end Japanese drives and top quality German drives. I don't have experience with the stuff that gets sold on Alibaba.
I normally spec my motors at or just above calculated load capacity & oversize my drive a bit above the motor size. I then limit the beating that the motor takes by programming the drive correctly. Correctly programmed drives generally still beat on motors in 3 ways. 1) The spike voltages are higher than nameplate voltage, which requires improved winding insulation to avoid Corona effect. 2) The carrier frequency is higher than nameplate frequency & causes increased mechanical cycling on the windings & accelerates stress fatigue. 3) when running a motor with an armature fan at reduced RPMs, you have reduced cooling. Drive rated motors are designed to handle these additional challenges. Standard motors, take it on the chin a little.
I have never ever seen a VFD used along with a soft start. Please let me know what kind of application used that combination. I would love to learn something new today.
I have added a VFD to an existing motor hundreds of times. It almost always works out, unless the application was marginal to begin with or the motor was really old or the motor was really beat. When I say old, I mean more than 50 years.
I think you misunderstood me. I DISallowed claims on improperly wired drives. I’ll have to go back & see if I was the victim of auto spell correct or if you misread what I wrote.
Of the drives I have seen, most that fail in the first few months, fail from improper usage. Most that don’t fail in the first few months, but do fail in the first 5 years, fail from overheating due to clogged fans. Most that fail after 10 years, fail from bad capacitors. It could be DC buss caps or the bypass caps inside the thyristor packs on the output stages. Buss caps are sometimes changed as a maintenance item. The bypass caps are usually potted inside the TRMs & are not accessible. That covers about ¾ of the failures that I have seen. I see drives from a wide variety of different industries, but mostly automated manufacturing equipment, elevators, robotics & machine tools. Compressor specific failures may be different.
If you’re in NJ, I’m going to guess that you probably get your drive repairs done by that place in Parsippany. I think it was either on Edwards Rd. or New Rd, but I haven’t dealt with them in many years, so I forget. I do remember their repairs generally being a little slower than the ones I did, back when I did that sort of thing as a sideline.
I agree that it is hard to justify loosing the benefits of 3 phase & changing to single phase.
….unless you have equipment that is already 3 phase & you need to use it someplace that only has single phase power available. I think that is what is being discussed here.
I think that I have sold a grand total of maybe 3 soft starts in my entire life. I mostly use drives. I sometimes use servos (which are actually just drives with feedback). I occasionally use phase inverters. I still use traditional contactors, when appropriate, too. My asking you to check on the constant frequency statement regarding the soft starts is based on that claim conflicting with the claims of a factory rep from S&S who later became a factory rep for Rockwell. He claimed that his soft starts were variable frequency. He twisted my arm to sell a couple of them many years ago. I’m not up on what is currently available. If you are confident of that claim, I’m ready to call my question answered, & thank you for confirming that information.
Regards,
the fish
