Inrush currents for a three phase induction motor

[Buzz1980]

Hi all, was wondering if anyone could help me with this. I have had a three phase induction motor rated at 85.8amps full load and voltage is 480volts connected in delta. My question is that when I was first starting up the motor it kept on tripping the main switch due to the inrush currents. The rated inrush current from the manufacture was 755amps so was set to that but was still tripping so I set it a bit higher and still tripped I kept on increasing until it never tripped but it is over the inrush current rating. My question is how high can you set your inrush current dials to? If there is belts connected to this motor at that moment in time will that increase the inrush currents to get the motor started? Also if that motor is pulling something will that increase the inrush currents aswell?
Any replies will be much appreciated?

Thanks trig

 

[Besoeker]

By inrush I assume you mean the starting current.
The magnitude does not depend on the load. Figures are typically six to eight times FLC.
What does vary considerably is the duration depending on the type of load and load inertia.
Overload protection needs to take account of that. I've known of DOL starters where run up time was in excess of one minute. Foe the majority of that period, the motor will be taking several times motor FLC.

You mention 480V. Doesn't sound like a UK application.
And a long shot..........should the connection be delta at 480V?

 

[Darkwood]

HI' Trig ... your profile states your an Handyman and without sounding abrupt the best advice is to get a competent person in, I would be giving the same advice to an Electrician who had strayed from his field of competence into motor control and supply.

Too many factors have to be considered here, when you say you are adjusting the dial are we talking on the Overload or a MCCB possibly you say main switch but main switch are normally just isolation devices but can be a combination (explain more)... we would need full motor plate details, starting duty of motor, protective device for short circuit, method of starting, type of work the motor is put to.... just to get us going --- if this looks a little confusing then you have your answer from me in my first line.

 

[Besoeker]

HI' Trig ... your profile states your an Handyman and without sounding abrupt the best advice is to get a competent person in, I would be giving the same advice to an Electrician who had strayed from his field of competence into motor control and supply.

Agreed.
I forget that this forum has a whole range of skills and competencies.
I totally missed the point about a handyman. No disrespect handymen - but horses for courses.
A motor rated at 86A at 480V would be in the region of 55kW.
Not really handyman territory.

The 480V is a bit of a concern too. Also not handyman territory. And not common in UK.

I think your advice is spot on.