At first I am a bit confused…why it does not trip?
So I ask:- Ok help me out here…
What is the overload set at?
Mr. C: 132A
And what current is the motor full load current rating?
Mr. C : “132Amp”
And what current was the motor physically pulling?
Mr. C: “139Amp”.
And for how long was the motor pulling this 139Amp?
Mr. C: “What does time have to do with it?” he asks? He even mumbled something to the tune of a swear word in the background.
This was my explanation:
I ask: Should an overload trip at the setpoint it is set at?
“Yes” he says. “It must trip because that is what the overload is set for”.
For quite a few moments I just sat there thinking:-
So the motor is designed to run at 100%FLC but you want it to trip? Why?
And I can feel the tension building between us.
Mr. C: “Lets say it does not trip at FLC – I told you it was pulling more the overload setting and that is why I called you. This is a rubbish product”.
I thought for a moment what would be the best approach to explain this.
Ok, so if your machine went through a temporary glitch and it pulled 120% of the overload trip point setting for 10 seconds – which will not damage the motor or your machine – will you be a happy chappy if it tripped, stopping all production…I don’t think so?
But if it continuously pulled 120% over current which – as the latent heat build up in the motor – will ONLY damage the motor over time, we do want the overload to eventually trip.
But if your machine pulled 500% of the O/L setting for 10 second would you want it to trip – Most definitely I would think.
So, that is the way the overload is designed – the higher the load current the quicker it trips and at less than 115% for NEC and 105% for IEC trip setting it will not trip at all…this is referred to as the Proportional Inverse Tripping curve.
Mr. C: “Ohhhh…I did not know”. And a soft voice says: “Thank you, bye”.
So the bottom-line is this:
A overload will not trip at the set rating but as the overload current increases it will progressively trip for a shorter trip time.
For a more technical approach: