Motor overload is not tripping.
Motor overload is not tripping.
(OP)
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?
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?





RE: Motor overload is not tripping.
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
RE: Motor overload is not tripping.
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
RE: Motor overload is not tripping.
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)' ?
RE: Motor overload is not tripping.
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.
RE: Motor overload is not tripping.
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.
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RE: Motor overload is not tripping.
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(2B)+(2B)' ?
RE: Motor overload is not tripping.
RE: Motor overload is not tripping.
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(2B)+(2B)' ?
RE: Motor overload is not tripping.
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.
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RE: Motor overload is not tripping.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor overload is not tripping.
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.