Motor protection relay not reading current when PFCs are connected

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I have to admit that I skimmed through ths but one thing occurs to me.
If it's a motor protection relay it ought to be looking at just motor current. Not motor plus PFC. Strikes me that it is in the wrong place if it sees both.
 
Besoeker said:
I have to admit that I skimmed through ths but one thing occurs to me.
If it's a motor protection relay it ought to be looking at just motor current. Not motor plus PFC. Strikes me that it is in the wrong place if it sees both.
Click to expand...

True
but as the motor loads up and pf approaches 0.9 the real P and total power S converge

say running at full load 50 A
0.85 pf
S 360 kva
P 306 kw
Q 190 kvar

add cap
P remaims 306 kw
Q 190 - 75 = 115 kvar
S 327 kva
i 45 A
10% difference in i
new pf 0.935

could be significant depending on control method, duty cycle, load profile
 
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Besoeker said:
I have to admit that I skimmed through ths but one thing occurs to me.
If it's a motor protection relay it ought to be looking at just motor current. Not motor plus PFC. Strikes me that it is in the wrong place if it sees both.
Click to expand...

Why is that? Do you think a fault in the motor will potentially not be seen by the relay because of the PFC?
 
Bugman1400 said:
Why is that? Do you think a fault in the motor will potentially not be seen by the relay because of the PFC?
Click to expand...

The idea of a motor protection relay is to protect the motor. The motor will be damaged by a certain current level integrated over time, regardless of the power factor of that current.
It is a bit of a stretch, but consider a motor which is frequently started and intermittently overload for a longer period of time. An overcurrent device may allow the motor to continue in use based on the fact that it is cooling off between uses.
But if the current in the motor is 10A between uses it will not cool off as fast as if the current is 1A. The relay is operating with insufficient information.
It is not clear to me whether the relay in question is a full current over time overload device or just an overcurrent detection device with a long enough time delay to allow starting. If the latter, then there is still the problem that at full load with a PF of .9 the relay may read 90A when the actual current is 100A. Since losses are I2R dependent, a 10% difference in I makes a 20% difference in heat.

The lower the full load PF of the motor the worse the problem is, and PF compensation is more likely to be needed on a motor with low PF.
 
Run the motor under operating full load
rated 50
line actual 40
motor actual 44

set your ol based on that
say 46
the motor would see 44/40 x 46 = 51
still below 1.15 x 50 = 57.5

often it is not practical to locate the ct's at the motor or even past the caps

a motor this size & voltage needs custom settings imo, not always nameplate
 
GoldDigger said:
The idea of a motor protection relay is to protect the motor. The motor will be damaged by a certain current level integrated over time, regardless of the power factor of that current.
It is a bit of a stretch, but consider a motor which is frequently started and intermittently overload for a longer period of time. An overcurrent device may allow the motor to continue in use based on the fact that it is cooling off between uses.
But if the current in the motor is 10A between uses it will not cool off as fast as if the current is 1A. The relay is operating with insufficient information.
It is not clear to me whether the relay in question is a full current over time overload device or just an overcurrent detection device with a long enough time delay to allow starting. If the latter, then there is still the problem that at full load with a PF of .9 the relay may read 90A when the actual current is 100A. Since losses are I2R dependent, a 10% difference in I makes a 20% difference in heat.

The lower the full load PF of the motor the worse the problem is, and PF compensation is more likely to be needed on a motor with low PF.
Click to expand...

Good point. Do motor protection relays account for the PFC device? Seems like there would be a setting that would allow you input that info in since most them have thermal algorithms and the scenario you described would have a big influence.
 
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