AM2 help Selection of Circuit breaker

[mcbarber]

Hi guys,

Need some help on selecting a circuit breaker for a 3 phase motor on a DOL starter for my AM2.

This is how far I have got let me know how to complete my calculation.

Information I know; Its a 1kw motor
Its wired in singles in trunking to the DOL starter, Then in 1.5mm SY
to the motor.


1000/(1.73x400) = 1.45A

To me 1.45A dose not seem correct, I know it would be a type C breaker as of the large current when starting. If anyone could help that would be great.

Cheers Matt

 

[rich.250]

Have another look at the equation......
See if you can spot the mistake.
You are on the right track


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[Lucien Nunes]

Consider:
What does the 1kW represent - input or output power?
What is the power factor of a typical 3-phase motor?
How should the MCB rating relate to the running current and the locked-rotor current?
What device is providing the overload protection?

 

[mcbarber]

hmm I'm getting a little confused, I haven't had any experience in this so trying to tech myself.
Is there any chance someone can break it down for me how to do it?

 

[rich.250]

As its for the AM2 I would assume that 1kw in the power input of the motor as your only concerned by that for cable sizing etc.
The calculation would be 400\1.73.
Try that and see if it makes more sense....


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[Simonslimline]

With the info provided the 1.443418014 is the current. The answer would change if you had power factor and efficiency in the calc etc. Or am I missing something?

 

[Lucien Nunes]

the 1.443418014 is the current

Given the limited precision of the input values, I would not try to quote the result to the nearest nanoamp! But yes, 1.4A is what you get disregarding efficiency and pf.

FWIW, a 1.1kW motor (1.5hp in old money) will have a line current in the order of 2.7A on 400V.

 

[mcbarber]

Ok so, I then try and select my circuit break from this info.

Iz >= In >= Ib
2.7A>= In >= 17.5A

does this mean I could use a 6A TYPE C MCB

 

[Darkwood]

Its a sticky area is this and I don't want to throw confusion into the works but what your taught about motors and inrush currents etc at college isn't really worth the paper its written on, its so brief and concise that it gives little prep' for the real world of motor control and protection. If this is a standard 400v 3ph induction motor with a normal starting duty on DOL then using Schneiders Tech' data sheets you can use a number of options for protection, in this particular case they state a 6(B), 6(C) or 4(D) will all be adequate and only your permitted zs would limit you but been so small an mcb then I don't see that even been an issue.

The misconception that type B, C and D are relative to certain inductive products is misleading, the tripping curves are different which allows for different inrush values and once you know this then you can determine if the inrush of the inductive load you have may require a higher breaker type or value, it is sometimes the case that a type (B) can be used like above but what you college is looking for maybe different and they maybe looking for you to spec' a type (C) as that's what they usually teach you a C is used for.