Running and older two speed motor with a VFD

[xJAyx]

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?

 

[jraef]

1) If you are shorting out one set of windings to go to high speed, that is a 2 speed 1 winding (2S1W) motor. There are several different TYPES of 2S1W starter though, so which type you have is going to be something to discover. 2S1W starters have 3 contactors, because of the need to short out one set of windings so if you have 3 contactors, that's what it is. 2S2W means there are the equivalent of two separate motors in one frame, so 2 separate motor starters, but only 2. When you go from low to high, you just open the motor starter for low.

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.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[xJAyx]

Thanks for the post.

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)?

 

[jraef]

Issues about starting it with a soft starter will depend on the soft starter you select. But your overall scheme is correct, you put the soft starter ahead of the 2S1W starter. In general, you need a soft starter with "dual ramps" and you set it up so that you have separate ramp profiles for each speed. The normal control system you have now selects and engages the speed, then the appropriate contactor aux contacts enable the soft starter AFTER they close using the appropriate ramp profile. You will have other logic considerations to contend with, such as speed change on the fly, or always starting from low and moving to high,, or speed change only after stop etc. etc.

"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

 

[DaveMeharg]

I had a similiar application with a 150 HP two speed fan used on a cooling tower. We took the two speed starter out and replace it with a VFD. We perminentely shorted the apporpriate wires together for the high speed configuration (need to be carful with overload setting coming off the VFD 3 phase legs). I would recommend contacting the motor manufacturer to discuss the requirement of whether load reactors are required. In our application given the distance we had from the VFD to the motor the manufacture indicated we were OK without line reactors (motor was only about 5 years old so insulation system was of a modern type). The futher distance between the drive and the motor the more issues you have with high voltage spikes impacting the motor. Also you can set up your switching frequency to be 2kHz to 4 kHz which will also lower the voltage spikes under long cable lengths. A lot of drives have the switching frequency set to 8kHz as standard but you can lower it. In terms of the line side we usually specify a line reactor with the VFD. This does not eliminate harmonics but helps to reduce the overall impact on your upstream system. Most drive manufacturers have free software on there web site to simulate the impact of the drive on your distribution system in terms of harmonics. The software allows you to select reactors to see what impact it has on lowering harmonics. I think the VFD does add some complexity but there is termendous energy savings in our case by being able to control the speed as required for the given conditions. You may also want to consider a drive with a built in bypass contactor in case you need in an emergency to start your blower across the line if the VFD fails.