Power factor correction

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PlanB
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Power factor correction

Post by PlanB » Tue, 19 Jun 2012, 17:09

I stumbled across this today

Motor Power = 1.732 x motor voltage x motor current x power factor x efficiency

With motor efficiency percents in the low 90s & inverters in the high 90s I was troubled to find the power factor in the example listed as 0.77

Is power factor the elephant in the room with EVs? I know supply authorities will add PF correction capacitor banks to factories & I was wondering what the state of play is with EVs?

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Richo
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Power factor correction

Post by Richo » Tue, 19 Jun 2012, 20:37

The power factor only relates to the inverter/motor controller connected to the grid - or the motor directly connected to the grid.
For an EV the power factor issue will be in the battery charger.
Which you can already buy power corrected chargers.

So I would think the elephant is more of a mouse.
So the short answer is NO but the long answer is YES.
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Power factor correction

Post by coulomb » Tue, 19 Jun 2012, 22:00

Richo wrote: The power factor only relates to the inverter/motor controller connected to the grid ...

Err, I don't think that's quite correct. Some of the current the inverter produces has to go into providing magnetising (field) current. Just as with a series DC motor, some of the controller output voltage is dropped across the field winding, where it produces heat and doesn't contribute directly to mechanical power output. Of course, if it isn't there, then there is no field, and there is no output.

With an AC motor, a lot of the magnetising current circulates between the controller and the motor, and the battery only has to supply the copper losses. So in that sense, the series DC and induction motors are much the same.

The two currents are 90 degrees out of phase, so if the motor needs 50% field current, the resultant current is sqrt(1^2 + 0.5^2) = 112% of the actual drive (in-phase) current. That's a power factor of about 89%. (That seems wrong, so maybe my maths is wrong).

So all it means is that you need a bit more current capability from your AC controller to supply the field current. The power factor will vary depending on motor load and speed. For example, at higher speed, you need less field current (i.e. "field weakening" is employed), so I'd expect the power factor to be higher at higher speed.
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Power factor correction

Post by coulomb » Tue, 19 Jun 2012, 22:06

Btw, our MX-5 22 kW nominal motor (at 3000 rpm) has a nameplate power factor of 0.88. For EV motors, the power factor seems to vary from around 0.85 to 0.88 (on the nameplate; I suspect it varies quite a lot over an EVs power and speed range).

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Power factor correction

Post by PlanB » Wed, 20 Jun 2012, 00:03

The formula was from a Rinehart VFD data sheet, they make nuggety inverters not unlike the Wavesculptor. So why don't we see power factor correction in Ev drives if the effect is real?

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Power factor correction

Post by coulomb » Wed, 20 Jun 2012, 03:58

PlanB wrote: So why don't we see power factor correction in Ev drives if the effect is real?

Because "correcting" the power factor would remove the out-of-phase component of the motor current, and the field would not be excited.

[ Edit: I've since changed my mind a bit; see later post. ]


You can't get out of paying the field tax.

[ Edit: as Weber points out below, the "tax" is in terms of inverter required output current, not significantly more electricity from the wall. Sorry, I thought that required current was the focus of this discussion, but on re-reading I see it can be taken as low power factor causing an efficiency hit, which would be wrong. ]
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Power factor correction

Post by weber » Wed, 20 Jun 2012, 05:20

Coulomb, I think what you say is misleading. I'd say the VFD automatically gives you power factor correction. As you said, the magnetising current does not represent any power, since it is 90 degrees out of phase with the voltage. So it does not result in any extra battery current (except for the tiny extra bit due to resistive losses). So I wouldn't call it a "tax". Yes your inverter output current rating has to allow for the power factor, there's no way around that. The inverter doesn't have a kW rating it has a kVA rating. But your power bill for recharging your EV isn't affected by the power factor of the motor.
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Power factor correction

Post by coulomb » Wed, 20 Jun 2012, 13:43

Weber wrote:I'd say the VFD automatically gives you power factor correction.
Sorry, I find that misleading. To me, lower than unity power factor results when the product of voltage and current does not equal the real power output. This can occur due to the non-sinusoidal shape of the waveform (not relevant when driving an induction motor), or due to a phase difference between the voltage and current (very relevant for induction motors). It can't happen with DC, so I don't think it makes sense to say that the VFD/controller does any sort of automatic power factor correction.
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Power factor correction

Post by woody » Wed, 20 Jun 2012, 17:56

In a shunt DC motor, all of the current goes through both the field and stator windings.

In the controller, at low speed, the current "free wheel"s through the freewheel diodes which is kind of like low power factor. It's not a bad thing.

In a VFD / AC motor, the power factor is how much of the current is being used to run the motor (the rest is used to energise the field, but all of it returns to the controller minus some resistance losses).

In your formula, the power factor (cos theta) reduces the motor power available for a given current, but it's not an efficiency thing.

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Power factor correction

Post by Richo » Wed, 20 Jun 2012, 20:47

Weber wrote:I'd say the VFD automatically gives you power factor correction.
coulomb wrote: Sorry, I find that misleading... so I don't think it makes sense to say that the VFD/controller does any sort of automatic power factor correction.


So the power factor of a motor is ~0.80 giving U,V,W lines out of the controller the same pf.
What do you think the power factor on the DC bus line going into the controller then?

A VFD connected to the grid the AC->DC conversion is where the power factor correction is required due to the fact the DC current isn't continuous over the entire AC cycle.
On a motor directly connected to the grid the pf of the motor is directly passed to the grid.
So the short answer is NO but the long answer is YES.
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Power factor correction

Post by coulomb » Sat, 23 Jun 2012, 04:56

Richo wrote:So the power factor of a motor is ~0.80 giving U,V,W lines out of the controller the same pf.
What do you think the power factor on the DC bus line going into the controller then?

I would say it's "not applicable"; DC doesn't have a power factor, since it has no phase or frequency.

Ok, I think I see what you're trying to say; if you had to assign a power factor to DC it would be 100%, so in that sense, the controller "corrects" the power factor from the ~ 80% at the motor to a "designated" 100% at the battery.

Earlier I stated that if you corrected the power factor of the motor (you'd need external inductors, of course; capacitors across the PWM output of the controller would be like short circuits to the high frequency components of the output), that you'd eliminate the reactive power and hence the field excitation, so the motor would not work. That would only be true for a fixed power factor (so a fixed speed and load), and only if you had the exact capacitance to cancel all the reactive power. Weber has pointed out to me that you could "bias" the power factor, so that for example 100% from the controller led to 90% in the motor. Then the controller could run from 90% lagging to 90% leading, and the motor would see 80% lagging to 100%. This could save a minuscule amount of controller current requirement.

Finally, Weber pointed out that he was trying to counter a perceived suggestion from the original poster that power factor was somehow like an inefficiency. He's certainly right to point out that power factor isn't like a loss; it's just a limitation on the power you can get from a controller with a certain current limit. Put another way, it affects the kVA (controller output) to kW (motor power, electrical or mechanical) calculation, but not the battery to motor shaft efficiency.
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Power factor correction

Post by PlanB » Sat, 23 Jun 2012, 06:09

Sorry, I wasn't suggesting that motor PF<1 meant extra energy for the battery recharge more oversized infrastructure in terms of higher current VFD fets & batteries than otherwise would be needed if voltage & current were kept in phase.
I think this might have something to do with why Toyota are using variable voltage step up supplies between the batteries & the VFD? Sort of an impedance matching exercise?

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Post by coulomb » Sat, 23 Jun 2012, 14:56

PlanB wrote: I think this might have something to do with why Toyota are using variable voltage step up supplies between the batteries & the VFD?

No, I believe that's a straight efficiency measure; switching losses in the IGBTs are proportional to voltage, and at lower speeds, you don't need as much voltage. Power factor isn't affected by DC bus voltage.

It isn't usually practical to adjust the bus voltage, but Toyota wanted a lower voltage pack (fewer cells, smaller pack) and a higher voltage motor (more power and speed), and so added the step-up converter. Once you have that, it's easy to change the bus voltage for the motor IGBTs. The step-up converter is relatively small because it only handles the battery power (some 20 kW); the power transferred from one motor to the other can be of the order of 50 kW.
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Power factor correction

Post by weber » Sat, 23 Jun 2012, 15:15

PlanB wrote: Sorry, I wasn't suggesting that motor PF<1 meant extra energy for the battery recharge more oversized infrastructure in terms of higher current VFD fets & batteries than otherwise would be needed if voltage & current were kept in phase.
Yes, low motor PF requires higher current VFD fets.
No, low motor PF does _not_ require higher current from the batteries.

The effect of motor power factor does not go beyond the VFD (except as a very tiny second-order effect). I believe that's what Richo and I have been trying to get across, and felt Coulomb was not being sufficiently clear about.
I think this might have something to do with why Toyota are using variable voltage step up supplies between the batteries & the VFD? Sort of an impedance matching exercise?
Can't see it.

[Edit: To clarify, changed "power factor" to "motor power factor" as there is also the issue of charger power factor as mentioned by Richo.]
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Power factor correction

Post by Johny » Sat, 23 Jun 2012, 15:20

Where did it creep in that Toyota's step up converter has variable output?

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Post by coulomb » Sun, 24 Jun 2012, 01:24

Johny wrote: Where did it creep in that Toyota's step up converter has variable output?

I guess some time before 2003 when Toyota released the 2003 model.

Image

From "Evaluation of 2004 Toyota Prius Hybrid Electric Drive System"
Prepared by: Oak Ridge National Laboratory Mitch Olszewski, Program Manager 2006.

Apparently that part of the design surprised a few people, so you are in good company.

I believe that the 2010 Prius model is similar, but boosts to 650 V instead of 500 V DC.
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Power factor correction

Post by Johny » Sun, 24 Jun 2012, 04:24

Wow I didn't know that (variable voltage on step up DC-DC)! Weird response.
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Power factor correction

Post by bladecar » Sun, 24 Jun 2012, 16:04

Yes, they say 700000 kms for Cairns taxis and still going.
For those who love to drive on the least fuel, just touching the throttle at city speeds goes to glide mode, like slipping a manual car into neutral. Remove the foot and you gently regen and the car slows down, a little more throttle and the electric motor comes in (provided there is enough in the battery).
Even up hills, throttle position gives electric-only power, motor-only power or, with more throttle, regen-plus-motor-driving up hills.
Driving home the other night, no traffic behind me in my lane, I overtook an older holden at least 3 times because it rumbled away at the lights and stopped at the red ahead. Because no traffic behind, I either regened or glided along so as to hit the lights just after they changed. As a greenie, its a fun feeling.
The Courier in its car magazine and little articles is "showing" how it doesn't pay to buy an electric or hybrid because of lost resale value (they guess). The real story here is not to buy a new one (although I'm going to buy the first plug-in Prius) but to buy a 2nd hand prius (which are going for $20K. Great cars for 20K.
The only thing about it that hasn't won me over is the way the motor revs up and down according to throttle, in the style of an old car with a slipping clutch - just not what I'm used to.

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