Power Factor Correction Add-On Products Viability

[RumRunner]

Efficiency of a motor isn't necessarily directly related to the power factor.

Efficiency of a motror is simply the ratio of power in to power out. Low power factor indirectly leads to possible increased losses in the supply lines but doesn't change overall efficiency of the motor itself.

Well, that is a rudimentary knowledge on how inductive loads provide useful work to our benefit.

Low power factor and its effects are more noticeable on the energy distribution sector.

This low power factor increases the non-useable energy that is turned to heat. Heat in distribution systems is the consequence of low power factor.

This heat build up that occurs in the distribution lines increases resistance of conductors and thereby cause voltage drop. This voltage drop propagate to the system being served.

Motors running on LPF also run in elevated temperature. . .a consequence of LPF. ( some modern motor design can compensate for this only to a certain degree. )

This doesn’t save you however from the pitfall of LPF because it (load) draws higher internal current.

Power supply running outside unity PF reduces efficiency of motors connected as mentioned in my previous posts.

 

[Besoeker3]

Power supply running outside unity PF reduces efficiency of motors connected as mentioned in my previous posts.

That's simply incorrect.

 

[Jraef]

That's simply incorrect.

Agreed.

Also:

Motors running on LPF also run in elevated temperature. . .a consequence of LPF. ( some modern motor design can compensate for this only to a certain degree. )

Motors are DESIGNED to operate safely and at rated efficiency at the PF they are tested to have. So the motor temperature is not "elevated" in the sense that it will be higher than "normal" because of "low" power factor, the motor efficiency and temperature rating is DEFINED based on the rated PF. Then if the PF is lower than rated, it means the motor is unloaded, ergo the unloaded state by itself lowers the motor temperature, so the lower PF is irrelevant from a thermal standpoint.

You (myspark) are absolutely correct though that the BURDEN of low PF is felt by the distribution system as a whole, which is why the utilities charge penalties for having low PF to large commercial and industrial users. It's just not significant from the end user standpoint, which is why they use penalties; they are incentivizing those large contributors to correct their PF in order to save the UTILITY from the losses in THEIR equipment, such as having to over size transformers.

 

[kwired]

I agree with Besoeker and Jraef.

power factor,high or low, is going to cause losses in the distribution, require larger transformers, conductors, etc.

But you don't correct the PF of a motor, it was designed to operate the way it is. Even if you connect PF capacitors right inside the motor terminal box, all you have corrected is from that point to the source, the motor windings themselves still operate at or very near the PF they were designed for, and they need to for the magnetic properties to be within design parameters or it will effect output torque. All that happens with PF correction is the reactive current of the motor windings gets exchanged between the winding and the capacitor instead of being exchanged between motor winding and the source, if sized for adjustment to 1.0 PF then there is no reactive current between the point of correction and the source. Even VFD's have approximately same power factor between the drive and motor as would be with a motor connected directly to a true three phase source, but any power factor on the drive input is distortion power factor from the solid state switching and not displacement power factor from running an inductive load.

 

[Sahib]

But you don't correct the PF of a motor, it was designed to operate the way it is. Even if you connect PF capacitors right inside the motor terminal box, all you have corrected is from that point to the source, the motor windings themselves still operate at or very near the PF they were designed for, and they need to for the magnetic properties to be within design parameters or it will effect output torque. All that happens with PF correction is the reactive current of the motor windings gets exchanged between the winding and the capacitor instead of being exchanged between motor winding and the source, if sized for adjustment to 1.0 PF then there is no reactive current between the point of correction and the source.

But the PF of an induction motor depends on its load. On light load, the motor PF is poor.

 

[synchro]

But the PF of an induction motor depends on its load. On light load, the motor PF is poor.

That is correct, but the PF is worse at light loads because the real power decreases by a significantly larger factor than the reactive power does under these conditions. So the smaller PF at light loads will not result in higher losses from reactive power, but instead these losses will actually decrease somewhat.

 

[Sahib]

That is correct, but the PF is worse at light loads because the real power decreases by a significantly larger factor than the reactive power does under these conditions. So the smaller PF at light loads will not result in higher losses from reactive power, but instead these losses will actually decrease somewhat.

But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor. So motor PF and efficiency are correlated for standard induction motors.

 

[retirede]

But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor. So motor PF and efficiency are correlated for standard induction motors.

They are correlated with the load and by coincidence therefore, each other.

The point is that applying external PF correction will in no way measurably affect the efficiency of the motor.

 

[kwired]

But the PF of an induction motor depends on its load. On light load, the motor PF is poor.

And that PF at the motor still remains the same. Capacitor only effects the PF measured on the supply side of wherever it is connected.

Even if you put the capacitors inside the motor housing, the motor coils still operate at same PF as they would if there were no capacitors in the circuit. The reactive current either flows between the motor coils and the source, or the motor coils and the PF capacitors - at the rating of the capacitor, any reactive current above the capacitor rating still goes to the source.This reactive current is necessary for the motor to do what it does.

Yes light loads have lower PF but the load is lower also, so net kvar can still be lower than when motor is loaded, which means resulting losses from having power factor are still lower than they are at full load without any correction.

 

[Sahib]

Thank you for the replies. I hardly ever deal with PF in my day to day duties. The problem is somehow these vendors get access to those near the top of the command chain and then we have to deal with it. Another issue as I see it is the devices in question are not switched out after the motor loads stop. Then the power factor goes the other way----now we are capacitive.

You may have switching transient overvoltages, when you are capacitive. Consider providing surge arresters.

 

[Besoeker3]

But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor. So motor PF and efficiency are correlated for standard induction motors.

But that is not why supply PFC is generally used.

 

[Sahib]

But that is not why supply PFC is generally used.

But motor load is generally predominant load in a power system.

 

[Besoeker3]

But motor load is generally predominant load in a power system.

Your point?

Some motors can be cage machines, others would rotor, some subsychronous converter cascades, some sychronous, Unity Power Factor DC systems, some active frond VFDs and some like large anodising plants where the load is predominantly high current rectifiers.

 

[Sahib]

Your point?

Most use over sized motors operating at poor PF. So supply PFC is generally aimed to ameliorate it.

 

[Sahib]

Not in my experience - but what do I know??
We did several UPF drive systems around the several MW range for paper mills for example.

It has relevance to OP: he may check his predominant load is motor load. A field survey reveals motors rarely operate at full load and so in OP case also at lagging PF most of time. So PF improvement may be beneficial to him.

 

[GoldDigger]

It has relevance to OP: he may check his predominant load is motor load. A field survey reveals motors rarely operate at full load and so in OP case also at lagging PF most of time. So PF improvement may be beneficial to him.

Being beneficial, to the extent of being cost effective, is only likely if the customer pays a hefty PF penalty on their utility bill, and not necessarily even then.

 

[Sahib]

Being beneficial, to the extent of being cost effective, is only likely if the customer pays a hefty PF penalty on their utility bill, and not necessarily even then.

The OP is not proposing for PF correction; it is rather imposed on him by his superiors. There may be small saving due to reduction in current in cables if he puts the PF correction box at the load center rather at the main disconnect.

 

[kwired]

The OP is not proposing for PF correction; it is rather imposed on him by his superiors. There may be small saving due to reduction in current in cables if he puts the PF correction box at the load center rather at the main disconnect.

The longer the circuit run the more might be gained as it will lower watt losses in the circuit run. You still have watt losses in the run, just a little less when the reactive power component has been (mostly) eliminated from that run.

Most the power factor correction that is done (probably in most of the US) is done to avoid penalties from the power supplier, which usually are much more severe financially than the watt loss would be if that were the only cost to not correcting the PF.

And yes it is true that at low load level most the time the PF will be lower, but remember at low load level you also have lower kW and lower net kVA even though the PF is low to contribute to lower overall line losses than you would have at full load.

 

[Besoeker3]

I, too, work at a gov't installation. Our installation owns the MV distribution which consists of two very large subs. We have switched Cap banks at each sub. An entity recently approached us wanting to do PF correction "at the load". Their angle was that by correcting at the load, the caps would switch on and off with the load itself. Not a bad idea....however, in our case we are charged a penalty based on PF lower than .9 at the sub and the automatically switched cap banks take care of that. So, there is really no economic benefit. I am totally sold on the concept that PF correction "at the load" is by far the best approach scientifically, but economically it just doesn't work for us here.

At the installation stage of a piece of electrical kit, an economic benefit is that it may reduce the size the size, and thus cost, of the cable feeding it. And the labour to install that cable.
70mm² XPLE is cheaper than 95mm² XPLE for example. Without PFC, the 70mm² XPLE might be just too small so you'd have to use the next standard size up, the 95mm² .

With the PFC you may well get by with the smaller cable. It's just an example. You'd have to factor in the cost and installation of the PFC. Each case needs to be taken on its own merits.

 

[GeorgeB]

But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor.

My limited experience has shown that an "energy efficient motor" is optimized for (near) full load and the efficiency at say 25% or 50% load is worse than that of a "standard" design. Check with your motor supplier.