Motor overload is not tripping. 2

[rottenrik]

I have set of pumps with soft starters just to control the ramp up. The lines got clogged up and the motor never hit FLA to trip the OL and burned up the motor.
I have a Danfoss VLT Compact Starter MCD 201 installed with motor protection circuit breaker and line contactor. Why is FLC not being reached to trip the motor protector?

 

[rbulsara]

Is there an actual O/L relay or just the motor protector? The motor protector will not trip on overload or if oversized. MCP does not have thermal overload element, just the instantaneous or magnetic.

And did the motor completely stall or was rotating slowly?

Post the motor and protective device ratings and measured or seen currents.

An unlikely scenario could be : If motor is oversized and it slows down, not stall, and yet the current does not reach FLA or O/L setting, its cooling will be affected as motor is not rotating. This will overhead the winding and damage it and will not be picked up by the upstream protection. This sounds unlikely though.

Post some more data.



Rafiq Bulsara

http://www.srengineersct.com

 

[DRWeig]

I think Rafiq's "unlikely scenario" is actually probably your culprit.

Pump power is maximum at its rated flow and pressure. As flow goes down, power goes down. At deadhead, pump motor is generally nowhere near overloaded. However, it can no longer cool itself since there is no flow. It'll get too hot and die, but the overloads don't come into play since current is actually below motor FLA.

Options: 1-flow switch or DP switch that kills pump below minimum flow rate (check pump manufacturer, they can tell you minimum). 2-install high temperature limit switch.

Seen it happen a couple times...

Good on ya,

Goober Dave

 

[electricpete]

Fluid conditions have some role - it is not obvious to me that clogged necessarily means higher torque. My frame of reference is fixed speed radial flow centrifugal pumps in steady state, where clogged up system would generally mean lower power demand from the pump. But with mixed flow it can be more complicated and changing speed during startup is another complicating factor.

Since clogging was mentioned, was there possibility of mechanical jamming of the pump?

Was the pump/motor ever observed to move?

In addition to pump conditions, I'd think the design of soft starter (how low is voltage reduced and what speed is it ramped) can be important.

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(2B)+(2B)' ?

 

[rottenrik]

I do not have much info here because I was asked to look into this after the fact. I do not know if there was any movement. However it was deadheading. This system is not used regularly and when it is running it usually is one hour without stopping.
This is centrifugal pump. The motor is a 480v 12.5Kw 21.5a. The motor protector was set at 22a. The soft starter is set with minimal initial torque, ramp up of 10s and no ramp down. I am not monitoring flow or pressure. It is just ramping up to full speed.
Thanks to all.

 

[jraef]

Generally, if the OL was set to 22A and the FLA of the motor is 421.5A, almost any overload condition should have tripped that OL relay.

Almost, but here is another far fetched scenario for you.

That Danfoss soft starter (actually made by Aucom in New Zealand and brand-labeled) is what is called a "2-phase" soft starter. In order to save money, they only put SCRs on 2 of the 3 phases, the 3rd is just a piece of bus bar. This works OK to soft start a motor, but there are caveats. One of them is, the ramp must be kept short because while ramping, the starting current is severely unbalanced. Current imbalance in a 3 phase motor creates negative sequence currents that create negative torque. So the per-unit current creating output shaft torque is higher. In other words 300% current limit in a 2-phase soft starter produces less net shaft torque than 300% current in a regular 3 phase soft starter. In a pump system, that translates to less head. At the same time, that torque battle inside the motor is creating rotor heat disproportional to the normal amount of heating during start-up, so the heat rise is faster than it would otherwise be.

Now if your motor dead-heads, i.e. no flow, that means the current is actually going to be lower than expected. If then the soft starter is looking for the current to exceed a specific value before determining the motor has finished accelerating, and the motor never gets to that value because of the above, then it could conceivably get stuck in ramp mode indefinitely. This is always a potential issue with soft starters, but one that rarely results in anything bad happening because the motor OL protection will trip, making someone just adjust the start ramp settings to prevent the overload from tripping.

But in the case of a 2-phase soft starter, it is possible to have the motor getting an amount of current that does not exceed the motor overload trip threshold, but is creating more heat in the motor than it should, which can eventually burn up the motor before tripping the OL relay.

Far fetched? I have seen it happen more than once. What some companies do, such as Siemens who also use the 2-phase method on some of their starters, is to incorporate a solid state OL into the soft starter and bias the OL protection scheme to account for this added motor heating. That Danfoss unit indicates it does as well, but it could be that since someone used a motor protector with a built-in OL in front of the soft starter, SO they disabled the OL protection in the soft starter itself.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[electricpete]

Wow - great info Jeff.

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(2B)+(2B)' ?

 

[jraef]

Please forgive all of my typing / editing errors in the above. I'm not a stupid as I made myself look...

 

[electricpete]

the only typo I saw was FLA. But I never realized some soft-starters may create unbalanced voltage as part of economic design, and also didn't think about the fact that softstarters might sense current as indication of motor speed. I'm sure that's common knowledge to others but it's a learning experience for me.

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(2B)+(2B)' ?

 

[jraef]

You have to know when to quit ramping if the motor accelerates ahead of schedule, especially if the load is light on a centrifugal, otherwise you get an oscillation that can damage the motor, pump etc. So one way to overcome that is to override the ramp time if you detect acceleration. You can detect early acceleration by monitoring current, looking for an increase over XXX%, followed by a drop to less than XXX-YY%, and therein lies the magic that differs from engineer to engineer. Some people also add a hard limit of the ramp time setting, some do not, especially if you want current limit. That's because current limit is usually a necessity and if you quit ramping before the motor is at about 90% speed, you can get a big starting surge anyway, defeating the purpose of having soft starting in the first place.

The 2-phase issue was tried a long time ago with bad results and abandoned, but was resurrected again in the age of digital firing and protection systems because the issues could be mitigated. But if, as I suspect, someone defeated those mitigating features, the bad can come back into play. There are also a couple of bottom-feeder soft starters out there that don't even bother, knowing that the issues are rare. They must think they can get away with "blaming the victim" when it happens.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Thanks jraef. Another star. What typos? 4grin

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

 

[Shozza]

Jraef,
I think you may be confusing the MCD201 with the MCD202.
The MCD201 is the basic model which does not have an inbuilt overload which is probably why the motor protection circuit breaker was used.
Also the MCD201 does not contain current transformers for current sensing (unlike the MCD202) as it is strictly a timed voltage ramp starter.
It is however a 2-phase controlled starter and your description of the current imbalance problems is right on the money.

Rottenrik,
Are you certain that the motor current did not increase beyond motor FLA? Do you have the remote keypad for the soft starter or do you have some other way of monitoring current? A possible scenario is that the motor current did increase beyond FLA but the motor protection circuit breaker is actually faulty and therefore didn't trip? I'd consider replacing the breaker with a new one.