Running and older two speed motor with a VFD
Running and older two speed motor with a VFD
(OP)
I am looking for some advice on an application I am working on. I am a controls engineer, but I do not know much about application and selection of VFDs and line/load reactors.
I have a system I am working on upgrading. There is a 75hp blower motor that is a two speed two winding style. With the original design, I connect to lead set A to run at low speed. To run at high speed I short lead set A and energize lead set B. I would like to upgrade the controls to run this motor with a VFD. I only want to run on two speeds, so I am mostly doing this to limit inrush, and I have been told that a soft start will not work with this type of motor.
I have also been told by one vendor that with this type of motor I will need load reactors. My incoming power is clean, and I don't want to put any harmonics etc. back onto the line. What would be the best configuration for this system?
Also, is there a good resource for basic drive application knowledge?
I have a system I am working on upgrading. There is a 75hp blower motor that is a two speed two winding style. With the original design, I connect to lead set A to run at low speed. To run at high speed I short lead set A and energize lead set B. I would like to upgrade the controls to run this motor with a VFD. I only want to run on two speeds, so I am mostly doing this to limit inrush, and I have been told that a soft start will not work with this type of motor.
I have also been told by one vendor that with this type of motor I will need load reactors. My incoming power is clean, and I don't want to put any harmonics etc. back onto the line. What would be the best configuration for this system?
Also, is there a good resource for basic drive application knowledge?





RE: Running and older two speed motor with a VFD
2) If you are only doing this for soft starting, a VFD is a waste and a risk. The risk is from adding complexity unnecessarily. The waste is that you will consume more energy because of the inherent losses in the VFD. The losses aren't much, but they are unnecessary if there is an alternative. You ALREADY have a 2 speed motor, so you have an alternative.
3) Load reactors on VFDs are a good idea for all sorts of reasons, the age of the motor being one. But there is nothing about using a 2S1W (or 2S2W) motor that REQUIRES you to use a load reactor. Still a good idea, but not an absolute. So this was erroneous information.
4) If you are concerned about putting harmonics back onto the line, that's another reason to try to avoid a VFD. There are plenty of ways to mitigate the harmonics, but nothing will totally eliminate them.
5) Whomever told you that you cannot use a soft starter does not know much about soft starters, or the only brand they have to sell you is incapable (usually meaning too cheap) to accomplish this. So again, erroneous information. Running a 2 speed motor from a soft starter is done all the time, not a problem if you use the right products in the hands of someone who knows what they are doing.
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RE: Running and older two speed motor with a VFD
Good to know this is a 2S1W motor. I know how to control this type of motor in the "traditional" sense using three contactors. The motor isn't run with an enclosed starter or an MCC, it's a starter built into a control cabinet as part of a control system, so there is no mystery. Some logic to run the contactors which I understand well, and three contactors, which I knew their logic but didn't know how it played in with the motor, other than what is obvious from the schematic.
So according to you I can run this motor with a soft start. Would I accomplish that by simply controlling the motor speeds as normal with three contactors, and placing a soft start in the branch line feeding the motor (before the contactors of curse)? Obviously I would select a soft start rated for the high speed FLA, which is higher than the slow speed FLA. Is there any issues with running the softstart at a fraction of it's rated current (ie when running the motor a slow speed)?
RE: Running and older two speed motor with a VFD
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Running and older two speed motor with a VFD