Motor Overload sizing with power factor correction capacitors

[megloff11x]

I stumbled over this question in a study guide for another test, but haven't seen it in any NEC published study guides. This other guide claimed that you should reduce the motor overload size if the motor uses power factor correction capacitors at the motor after after the overload. It seemed to indicate that if given a motor power (VA or W), efficiency, and power factor, you should:

Calculate the new motor & capacitor current with the appropriate efficiency, power factor, and VARs of correction, using the usual techniques.

Use this corrected current and multiply it by the 1.15 or other specified factor under Article 430 (I forget the subsection and left my book at home) to get the new lower motor overload current setting.

The PFC caps reduce the phase angle between the current and Voltage, and for a given Voltage, tends to reduce current through the overload, which is upstream of both. Hence an overload of the motor would occur at a proportionally lower current than it would have without the capacitors.

I've never stumbled across any NEC provided reference that went through this drill. It makes sense but I haven't found official sanction for the method.

Has anyone seen this, and is this an official NEC approved method?

Matt

 

[Keri_WW]

Sounds pretty much correct. Read through Article 460 of the NEC (460.9 to be specific).

 

[Besoeker]

I stumbled over this question in a study guide for another test, but haven't seen it in any NEC published study guides. This other guide claimed that you should reduce the motor overload size if the motor uses power factor correction capacitors at the motor after after the overload. It seemed to indicate that if given a motor power (VA or W), efficiency, and power factor, you should:

Calculate the new motor & capacitor current with the appropriate efficiency, power factor, and VARs of correction, using the usual techniques.

Use this corrected current and multiply it by the 1.15 or other specified factor under Article 430 (I forget the subsection and left my book at home) to get the new lower motor overload current setting.

Personally, I think that's not a good way to go. Motor overload protection should be there to protect the motor and should thus sense just motor current. There should be no other load between the protection device and the motor.
Even if the code allows it, I still think it's a bad idea.

 

[weressl]

I stumbled over this question in a study guide for another test, but haven't seen it in any NEC published study guides. This other guide claimed that you should reduce the motor overload size if the motor uses power factor correction capacitors at the motor after after the overload. It seemed to indicate that if given a motor power (VA or W), efficiency, and power factor, you should:

Calculate the new motor & capacitor current with the appropriate efficiency, power factor, and VARs of correction, using the usual techniques.

Use this corrected current and multiply it by the 1.15 or other specified factor under Article 430 (I forget the subsection and left my book at home) to get the new lower motor overload current setting.

The PFC caps reduce the phase angle between the current and Voltage, and for a given Voltage, tends to reduce current through the overload, which is upstream of both. Hence an overload of the motor would occur at a proportionally lower current than it would have without the capacitors.

I've never stumbled across any NEC provided reference that went through this drill. It makes sense but I haven't found official sanction for the method.

Has anyone seen this, and is this an official NEC approved method?

Matt

460.8 Conductors.

(A) Ampacity.​

​

​

The ampacity of capacitor circuit conductors
shall not be less than 135 percent of the rated current of
the capacitor. The ampacity of conductors that connect a
capacitor to the terminals of a motor or to motor circuit
conductors shall not be less than one-third the ampacity of
the motor circuit conductors and in no case less than
135 percent of the rated current of the capacitor.
​

(B) Overcurrent Protection.​

​

​

An overcurrent device shall
be provided in each ungrounded conductor for each capacitor
bank. The rating or setting of the overcurrent device
shall be as low as practicable.​

Exception: A separate overcurrent device shall not be required
for a capacitor connected on the load side of a
motor overload protective device.
​

(C) Disconnecting Means.​

​

​

A disconnecting means shall
be provided in each ungrounded conductor for each capacitor
bank and shall meet the following requirements:

(1) The disconnecting means shall open all ungrounded
conductors simultaneously.

(2) The disconnecting means shall be permitted to disconnect
the capacitor from the line as a regular operating
procedure.

(3) The rating of the disconnecting means shall not be less
than 135 percent of the rated current of the capacitor.​

Exception: A separate disconnecting means shall not be
required where a capacitor is connected on the load side of
a motor controller.​

460.9 Rating or Setting of Motor Overload Device.​

Where a motor installation includes a capacitor connected
on the load side of the motor overload device, the rating or
setting of the motor overload device shall be based on the
improved power factor of the motor circuit.
The effect of the capacitor shall be disregarded in determining
the motor circuit conductor rating in accordance​

with 430.22.

 

[Besoeker]

Exception: A separate overcurrent device shall not be required for a capacitor connected on the load side of a
motor overload protective device.

Thanks for that.
That's why I said, even if the code allows it, I still think it's a bad idea.
Bad why?
The current is likely to be less or very much less than motor current.
Case in point...
A project we currently have is for a 550kW, 690V single motor drive.
Motor current is 538A. The required PFC to get to 0.95 takes 75A.
A common protection device for both circuits makes no sense.

 

[weressl]

Thanks for that.
That's why I said, even if the code allows it, I still think it's a bad idea.
Bad why?
The current is likely to be less or very much less than motor current.
Case in point...
A project we currently have is for a 550kW, 690V single motor drive.
Motor current is 538A. The required PFC to get to 0.95 takes 75A.
A common protection device for both circuits makes no sense.

Really? So how would a capacitor develop an overload?

Capacitors on large motors sometimes equipped with an integrally mounted coordinated fuse to protect against short circuit, so that the short does not shut down the motor. The loss of the capacitor would of course increase the current that could cause trouble for a tightly calibrated overload setting, but does not necessarily mena immediate shutdown, leaving time for the operator ot make some adjustments.

 

[Besoeker]

Really? So how would a capacitor develop an overload?

Dielectric degradation, open circuiting of one phase, tracking between terminals, increased supply voltage distortion.......to name but a few.
:smile:

 

[weressl]

Dielectric degradation, open circuiting of one phase, tracking between terminals, increased supply voltage distortion.......to name but a few.
:smile:

Overload? Are you for real?

 

[Besoeker]

Overload? Are you for real?

Absolutely for real.
I have seen PFC failures.
Overheating, the enclosure expanding, feed cables getting so hot that they were brittle.
Where we have supplied PFC with a system, we have always put in detuning reactors to cope with supply voltage distortion.
Don't know how well this will display but I did this for our guys to assist is selection of PFC:

It's just science, not rocket science.
So, yes, I am serious about the topic.
If you don't agree with my calculations, that's OK.

 

[weressl]

Absolutely for real.
I have seen PFC failures.
Overheating, the enclosure expanding, feed cables getting so hot that they were brittle.
Where we have supplied PFC with a system, we have always put in detuning reactors to cope with supply voltage distortion.
Don't know how well this will display but I did this for our guys to assist is selection of PFC:

It's just science, not rocket science.
So, yes, I am serious about the topic.
If you don't agree with my calculations, that's OK.

Where is the overload that would be protected by a thermal overload relay?

Why are you trying to cloud the issue?

 

[Besoeker]

Where is the overload that would be protected by a thermal overload relay?

The overload wouldn't be protected by a thermal overload relay designed to protect the motor.
That's exactly my point.

 

[mull982]

Usually when I have pf capacitors on the load side of the motor overload, I caculate what the newcurrent through the overload will be with the reactive power subtracted and set the motor overload per this value.

I can see what others have said however that if the capacitor fuses blow, then this overload will likely trip because it would then see the full current through the overload.

 

[weressl]

The overload wouldn't be protected by a thermal overload relay designed to protect the motor.
That's exactly my point.

The capacitor is a non-overloading device. A resistance heater, a lightbulb are other examples of non-overloading devices.

 

[Besoeker]

The capacitor is a non-overloading device. A resistance heater, a lightbulb are other examples of non-overloading devices.

They may be defined that way in code. I don't know.
Well, I can say from both practical experience and theoretical analysis that capacitors can take above their rated current for a number of reasons. I have seen it happen.

Apart from any other consideration, the cabling that feeds a 75A PFC load won't be adequately protected by an overcurrent protection device rated for a 500A motor. The PFC, its contactor and cabling needs its own protection.
Fuses are fine as a minimum for catastrophic failure but pretty much useless for sustained overload.

 

[ptonsparky]

FLA of motor is 120 by nameplate.
Actual motor amperage with no cap is 110.
Amp with cap included is 100.
9% reduction in current through heaters. Reduce the heaters to 109.

Don't worry about the capacitors if connected on the load side of the starter with the motor. Buy the higher dollar ones that include fuses and idiot lights if you are really concerned about the smoke they may suddenly let out.

High voltage on the caps does cause them to draw more current which will shorten the life.

 

[weressl]

They may be defined that way in code. I don't know.
Well, I can say from both practical experience and theoretical analysis that capacitors can take above their rated current for a number of reasons. I have seen it happen.

Apart from any other consideration, the cabling that feeds a 75A PFC load won't be adequately protected by an overcurrent protection device rated for a 500A motor. The PFC, its contactor and cabling needs its own protection.
Fuses are fine as a minimum for catastrophic failure but pretty much useless for sustained overload.

Overload is usually defined as 150% and above. Conventiaonal overloads don't even start to pant until 125% and it takes a LOOOONG time for them to trip. Any thermal overload protective device, sized for the capacitors will be ineffective to protect against any of your theoretical 'above their rated current' occurences. The overloads are designed to protect MOTORS and designed for their overloading profile.

The POFC utilized for motors is normally connected to the motor terminals. It is allowed to be sized for the PFC FLA as per tap rule. The PFC therefore is switched on/off together with the motor.