High efficiency motor and soft starter problem
High efficiency motor and soft starter problem
(OP)
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?





RE: High efficiency motor and soft starter problem
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
RE: High efficiency motor and soft starter problem
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
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.
RE: High efficiency motor and soft starter problem
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: High efficiency motor and soft starter problem
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
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
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
RE: High efficiency motor and soft starter problem
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.
RE: High efficiency motor and soft starter problem
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.
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
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.
RE: High efficiency motor and soft starter problem
WEG starters sometimes have optional ventilation kits. Is this installed on the starter?
RE: High efficiency motor and soft starter problem
RE: High efficiency motor and soft starter problem
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.
RE: High efficiency motor and soft starter problem