3phase (x3) motors in delta connection

Thread Starter

PAULSINCLAIR

Joined May 3, 2017
12
Hello,

I have (3x) 3phase motors (415V, 10A rms) in delta configuration. I'm unsure what the max current, line currents, phase currents, etc would be?
 

Marley

Joined Apr 4, 2016
502
Should have a plate on the side. Usually state voltage, current and power factor for both star and delta connection.
 

Thread Starter

PAULSINCLAIR

Joined May 3, 2017
12
i only have the wiring diagram and motor datasheet. The datasheet only says 10A AC RMS, 415V.

3, 3 phase motors in parallel, delta configuration.
 

Marley

Joined Apr 4, 2016
502
Oh I see. If each motor is 10A per phase then 3 motors in parallel will be 30A per phase. However, it is bad practice to connect motors directly in parallel. Each motor should have a separate motor rated thermal/magnetic circuit breaker that has all 3 phases linked so that an overload in one phase will trip all 3 phases. A motor circuit breaker usually has an adjuster so that the trip current can be exactly set.

Remember that peak current at switch-on will be at least 6 times the running current.
 

Dyslexicbloke

Joined Sep 4, 2010
566
If you dont have plate data, you can get a ballpark figure but it will be based on assumptions that are little more than a best guess.

Try to find a simmilar motor, size, shape, voltage & power online to check your assumptions...

400V, 10kW, 0.85 power factor, 0.97 efficiancy would be in the ballpark for an IE2

Assuming that...
10000/400 = 25 (This is the current a single phase resistive load would take)
25 / 0.85 = 29.41 (Extra reactive current based on power factor)
29.41 / 0.86 = 34.2 (Extra current because the unit is not 100% efficiant)
and we want this all expressed per phase on a 3 phase system so divide by root3
34.2 / 1.732 = 19.74 (Full load current per phase)
The reactive current is more or less constant so the power factor is worse as the load decreeses.

Power factor correction capacitors will reduce line current if they are installed near the motor.
Remember that the starting current will ber anywhere from 3 to 8 times the full load current depending on the mechanical load.
Even a simple fly wheel with little or no friction could have huge inertia which would translate into a very high current for several seconds.

If you use a soft start you must ensure that the PFC caps are not connected during the ramp...
if you go DOL or Star Delta they can be on all the time but you would need to significantly oversize the contactors.
switching the caps slightly after you switch the motor and using an AC6B contactor to do it is your best bet.

AC6B is a contactor with an early make aux block installed that pre-charges the caps for a fraction of a second via current limiting resistors, that are usually integral to the unit. DO NOT switch your motor load with one.

For inductive loads the AC3 rating of a given contactor is applicable not the AC1 which applies to none reactive loads.
AC3 is always significantly less than AC1, the point being you cant simply by a switch rated at 20A it must be at least 10kW AC3 and I always assume that W + VAR is a safer rating which would be 115% of the rated wattage for a unit with 0.85 power factor which in this case would be a minimum of 11.5kW AC3

PFC correction by the way is basically a cap, per phase, that matches the VAR of the motor at the applied voltage and freequency. In practice you would not correct to 1 so they will actually be smaller than that. I would suggest not correcting above 0.98
so if you start at 0.85 and want 0.98 you need caps that give you 0.13 of your VAR

PFC correction will reduce current and impact cable sizing and fusing but will not affect metered power.


Hope that helps...
Al
 
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