What do you think the CT is measuring?
at the motor
or the controller, ie line
Pretty much the crux of the op's ???
w/o cap 10
with zilch
Motor protection relay not reading current when PFCs are connected
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What do you think the CT is measuring?
at the motor
or the controller, ie line
Pretty much the crux of the op's ???
w/o cap 10
with zilch
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
Last edited:
Bugman1400
Senior Member
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Why is that? Do you think a fault in the motor will potentially not be seen by the relay because of the PFC?
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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
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
Bugman1400
Senior Member
- Location
- Charlotte, NC
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.
Or the motor protection realy might register a fault on the PFC but the motor is fine.
Nailed it in one!
You might find this interesting.
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf
Page 16 shows the choices of where to put the caps, replace the overloads with a Multilin & associated CT's.
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf
Page 16 shows the choices of where to put the caps, replace the overloads with a Multilin & associated CT's.
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