Economical selection of 90KW Motor Starter Components 3

[luispereiraenvc]

Dear All,

I'm new in this forum, but I would like to have your opinions related to the following Motor Starter components selection.

Usually, when we need to dimension the contactors, circuit breakers, cables and so on, for a certain Pump, we consider the nominal power of the electric motor of such pump.
Let's see an example; for a Sea Water pump of 100m3/h, the manufacture provided the following information:

Pump Data
Capacity: 100m3/h
Power Consumption: 50,43KW
Non-Overload Power: 69,43KW

Motor Data
Power Supply: 3x440V/60Hz
Nominal Power: 90KW

So, we take the 90KW, as the manufacture didn't provided the Nominal Current we calculate it based on cos(phi)=0,90, so 130Amp.
Then we select the Overload Protection to be in the range of 130Amp.
The Contactors also (depending on the starting method), the cables from the starter to the motor, and from the Distribution Switchboard to the starter also consider 130Amp, and the Circuit Breaker on the Distribution Switchboard also!

Now with Economical concern on mind, my question is; the power consumption imposed by the pump will be in normal operation 50KW (and in the worst case 70KW if we double the Flow to 200m3/h), can we consider for the Calculations 70KW instead of 90KW? 20KW in cables can be a considerable amount of money! It can also allow the reduction of contactors and Circuit breakers range.
You can ask why the motor is 90KW instead of something close to 70KW. Well... the pump manufacture have some fixed range of Electric motors for that frame of pumps... and sometimes that can lead to such differences...

My first oppinion is that if we consider the cables to 70KW, the protections will have to be set to 70KW also, and an abnormal fault on the pump (bearings for example) will be protected by the Overload protection of 70KW. However if we set it to 90KW, some mechanical problem in the pump will be supported by the 20KW gap. That could be a problem!?!

What's your oppinion?
Did you know any regulation that can support this or other criteria?

Thank you for your support!
Luis Pereira

 

[ScottyUK]

Size the contactor based on a 90kW AC-3 rating. AC-3 is the normal DOL motor duty, and is good for most things unless you're reversing the motor, have very long acceleration times, plugging the motor, etc. The motor will still start like a 90kW motor, even if you only have 70kW of load. You can probably find a type-tested overload which will turn down to the current equating to 70kW of load if you really want to, but you should size the switching components based on the full 90kW.

60034 is a rotating machines standard. The standard for switching components such as breakers, contactors, etc is 60947.


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