High efficiency motor and soft starter problem 1

[tlona]

We have a fuel oil pumping system consisting of two centrifugal pumps each driven by 100HP motor. Both mother feeders are identical each with the exact same soft starter. One of the motors had to be replaced. The replacement motor is a high efficiency motor and now keeps tripping the soft starter. We swap circuits, motors, starters, etc and whatever configuration connects to the new motor, the starter trips. We changed to a different soft starter and still trips. Wires and motor meggered and all pass. Motor bench tests satisfactory. Has anyone had any issues using soft starters with high efficiency motors?

 

[itsmoked]

Your issue is fairly common. High efficiency motors are that way because they have done about the only thing they can to help motor's already pretty amazing efficiency. They reduce the copper losses. That is, they use larger wire. This can come back and haunt you because the initial starting current on motors is pretty much limited only by the copper losses or resistance. Once they drop that resistance with bigger wire the starting currents can climb considerably.

Your starter is pulsing the power to the motor on a cycle-by-cycle basis to get it going at a slower 'less shocking' rate. Unfortunately each cycle is allowing, or rather demanding, more current than the SS was either designed for or is set to allow and this is causing your trips.

Does your SS give an error code? It could be tripping because of over current or because your power supply cannot keep the voltage up due to the larger starting current demand. The SS should have some guidance in this with error codes.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Try a longer ramp time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Run the soft start in current limiting mode, not on a definite time ramp.

 

[tlona]

Thank tou for feedback. We initially had a longer ramp time but it still tripped. If we limit the current won't that limit the flux thus limiting starting torque. The error code was heat sink on circuit board. The corrective action was inadequate cabinet cooling but we are not in a cabinet

 

[123MB]

+1 on current limit start - this will effectively reduce the ramp time which will get the motor off quicker and use less thermal capacity of the starter

 

[LionelHutz]

Thinking that a longer ramp or a lower current limit will help may seem intuitive but it is wrong. You always use more thermal capacity of both the starter and motor as you current limit and stretch out the start. The thermal capacity is the lowest when you set a soft-starter to be as close to a full-voltage starter as possible. So, try a short ramp time and a relatively high current limit. If that doesn't work then the soft-starter is under sized for the application.

We have never had an issue unless the motor was undersized for the load which means the soft-starter picked to match the motor is also undersized. Never an issue on a centrifugal pump since they are one of the easier loads to start. But, the lowest rated units we build have over 2X the capacity compared to some of the competition.

I don't really agree with the motor wire size being the root cause of this issue. The power factor during a motor start is very low which means the motor inductance is the dominating impedance during a motor start. The issue is more likely to be caused by differences in the rotor design.

 

[waross]

A soft starter works by limiting the effective current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

What make and model of soft starter is it? Some manufacturers play a "numbers game" on ratings and design. One in particular has decided to use as little heat sink as possible (because it's a high cost item in a soft starter) and relies instead on a bypass contactor that closes very slowly and gently so that most people don't even realize it is no longer ramping (unless you have a meter hooked up to see that it is already at full voltage). They give you a flashing LED indicating "Ramping", but it's essentially a lie. If it is that one, and the motor starting current is now much higher, it might trip anyway because you are beyond the limits of even their bypass contacts too. I've seen that happen more than once.

The other possibility is that your soft starter has an initial torque setting that is too high now, and the higher magnetizing inrush of the energy efficient motor is allowing too much current at that setting. Some manufacturers call that a "Pedestal Start", others have different names for it. But that is typically only in play for the first 2-3 seconds. Check for some setting like that.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[tlona]

Thank-you all for the input. We will try some suggestions. We have a higher rated starter that we will try on Monday. As someone had mentioned it is as if the new motor does not have enough HP and it sometimes does seem that is true, but it is the same 100HP that we replaced (maybe the nameplate is incorrect). The initial starter we had was An Allen Bradly SMC-3. Sometimes it started and continue to run, but more often than not it would trip on starting. The motor vendor vendor sent us a WEG SSW-06. With this starter it will get past the starting sequence as you see the current drop off, it will run 'normal' for about 3-5 seconds then trip on an E04 error code. The book refers to this code as the heat sink switch (in internal circuit board switch) and the corrective action is that the cabinet enclosure ventilation is not adequate. WEG support has pointed to a defective starter but I am no convinced.

 

[jraef]

The SMC-3 and the Weg units are not the one I was referring to, but both of those are designed as inexpensive soft starters and will have similar issues to what I was describing; less capability and more self-protection features to prevent their being damaged by asking too much of them. I'm very familiar with the SMC-3 and although the it is the least expensive version of soft starter available from Rockwell, it's not a bad product. That makes me suspect you had the initial torque setting too high. The Weg unit on the other hand is, in my opinion, a marginal product from the outset. Still, check for that kind of feature or setting.

If that's not it and you have to replace it again, in your situation I think you should look for a soft starter that does NOT have an internal (built-in) bypass, because that's something the mfrs use as a "crutch" in their design so that they can reduce the heat sink. Using a version that is designed with NO integral bypass results, typically, in a larger unit but increased thermal capacity, something you need in order to avoid that nuisance tripping.

Unfortunately because the "market" is constantly demanding smaller-cheaper-faster everything, soft starters have been following that trend for so long now that some mfrs no longer even offer their older no-bypass versions. There are still a few however. Allen Bradley released a new one a few years ago called the SMC-50. ABB still offers some versions, Motortronispcs still did last time I looked (DXT series) and that is also sold by Toshiba, a company out of New Zealand called Aucom still has one design, which is also brand-labeled as Danfoss, and an Israeli company called Solcon still has them, also brand labeled by several others. Keep in mind that ALL of those mentioned ALSO have the types with integral bypass, and I think that's what you need to steer clear of here. So find a local supplier you trust, but be selective on what you buy. Then if you need to use it in an unventilated enclosure, add a separate fully rated contactor as the bypass, they will all be designed to be used that way.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

The company I work for could sell you a soft-starter with a built-in bypass contactor and it would work. Personally, you don't want one without a bypass because then you either have to install and wire an external one or you have to deal with the heat from the starter while it is running which would require venting the enclosure. But then I'm biased that way because we offer soft-starters with and without bypass contacts which have the same heavy duty ratings so you gain nothing by buying the non-bypassed units.

Have you checked the speed torque curve of the motor? You might have a motor designed for a VFD with a very low locked-rotor and pull-up torque. This type of design works fine on a VFD, but it requires high currents when starting via DOL or soft-starter. Often this type is called a design A motor, but it doesn't have to be.

 

[krisys]

In my view, the OP should obtain the motor data sheet and the starting curves to the soft starter vendor for the specialist to review. Based on that, the appropriate decision shall be taken.

Each soft starter is generally designed based on the motor starting characteristics and the data.

The guess works may result in un-expected failure. Though it is good to brainstorm like this for the better understanding and to help in decision making.

 

[LionelHutz]

If you're going to give the motor curves to the vendor then they also need the load data to make any useful determinations. They are not properly analyzing the system if they make a suggestion without the load data.

 

[tlona]

We tried a second starter this time a 150HP rated with all kinds of different settings as suggested. We get the same error code in that the electrics are overheating not the motor overload. Yes it looks as if this unit has an internal bypass. We have the initial starting torque setting at 50% as it would not start at the factory default of 30%. The LR starting torque for the motor is about 600ft-lbs then looks to drop off linearly to about 300ft-lbs at 1500 rpm (full speed is 1785 rpm) then spikes up. The motor, pump and drive vendors all coming on site tomorrow. At this point I thinking either there is an issue with the motor (i.e., lose rotor bar?) or it simply does not the proper motor to match the existing pump/system. Would there be any reason I need more HP with this type of motor? After all, there were no issues with this system until we had to replace the existing old school standard duty motor. Thank-you for all of your input.

 

[LionelHutz]

You're saying the lock rotor torque is about 2X the motor rated torque and then it drops to about 1X rated torque before the breakdown torque spike? That motor should have started with a 150hp starter from almost any vendor. I'd guess that either the motor is bad or there is a missing piece you have yet to post about.

 

[tlona]

More info in case I missed anything. The WEG starter gets the motor up and running but then trips shortly after running a at FLA.
The SMC-3 will sometimes start it and keep it running but most the time it fails during start-up as the LEAD pump. When using the SMC-3 starter, it will start every time as long as it is the LAG pump. It seems as long as the pump has some head pressure to pump against it will start the second pump is running. I tried duplicating this by choking down on the pump discharge isolation valve but could not get it to start. The pump is a two stage centrifugal. I believe that troubleshooting while utilizing the WEG starter just sent us on a wild goose chase. Their on-site tech support could not explain the error code which arose on two different starters each of which tripped both a standard efficiency and high efficiency motor with both in the coupled and uncoupled state. I am believe that it is still possible that the correct starter/VFD may get this motor running, but skeptical.

 

[X49]

When multiple centrifugal pumps are pumping into a common header, less power is normally required per pump, which will help. However, it is really odd that the 150hp starter will not start the pump. Is it possible that the motor is just overloaded when only one pump is operating? It is odd that the WEG starter trips once its up to speed. Have you measured running amps of the motor? Check all three phases to make sure your current balance is reasonable. A loose connection on the new motor could cause current imbalance. Normally this would cause a different error message, but it could conceivably overheat the starter first.

WEG starters sometimes have optional ventilation kits. Is this installed on the starter?

 

[tlona]

The 150HP WEG starter would start it but not keep it running. The WEG starter had more starting options so we were able to set higher starting voltage,torque,current, etc.. settings much higher than the SMC-3. Yes we checked 3-phases when running and all within 1A. Neither of our WEG starters had a ventilation kit. They come standard on the next higher rated WEG starter. We found that odd in that there was a very-very tiny fan (maybe the size of a half dollar) but it hardly did anything (At one point I was waving a cardboard box to cool it). Today we removed both motors and switched their installations so that Motor A (old motor) was driving Pump B (existing pump) and Motor B (new motor) was driving Pump A (existing pump). We had a dozen plus starts all successful. My only conclusion can be that for what ever reason the pump A train requires slightly less torque to start than the pump B train. So I believe the motor is good I am not 100% convince this is resolved but we shall see. So I believe the motor is good. If the problem arises then I think a different starter may do the trick. As far as the WEG starters, I think they would have worked but we just got a bad batch. Again, great website and thank-you all for all of the input.

 

[LionelHutz]

Am I correctly reading that the WEG starter tripped with two different uncoupled motors? That certainly does point to some issue with the starter.

We never found that fans did much during starting. During starting the SCR's are dumping a lot more heat into the heatsink than fans could possibly take away.