Electrical motor circuits.

[lincs1sparky]

Hi to all,

When calculating electrical motor circuits is it necessary to take into account starting current as opposed to full load current. Some of the circuits I am trying to calculate only have an inrush current of milli-seconds and other's are for about 4 seconds.

Whilst I am aware of the type of mcb's and mccb's and their characteristics, is it possible to ignor the starting current as swa etc will be able to cope with temperature rise for short durations.

Many thanks.

Lincs1sparky.

 

[telectrix]

think you are right there. start up load can be handled by almost any cable. i would calculate for the full load

 

[lincs1sparky]

What about the supplies to the motors which are for 4-5 secs, I think my understanding is the starting current would be continually reducing for the 4-5 secs but not completely sure.

 

[Darkwood]

It comes down to volts drop the longer a motor takes to start up coupled with the length of run etc.... motors with quick start-ups can usually be worked out on their full load current but when loads are high inertia and slow to gain full speed then calculation need to be made so as not to create a volts drop on the supply and cable may need to be increase to account for this.

 

[IQ Electrical]

These starting currents are generally classed as 'inrush' and are the reason for selecting a type 'C' (small motors) or type 'D' (DOL motors, large star delta motors) device.

The full load current would normally be used as design current unless exceptionally high starting currents were encountered.