Skin effect and other considerations for VFD to ACIM power connections

[P E Schoen]

As a result of the discussion of Litz wire and skin effect in my previous
post, I wondered if it may apply to VFD motor controllers, and particularly
in EVs, where AC induction motors are often "overclocked" to 180 Hz or even
higher. Most DIY EVs use welding cable for motor and battery connection, and
there was recently a discussion about proximity of the controller to the
motor or battery pack:
http://www.diyelectriccar.com/forum...-battery-motor-91282.html?p=372159#post372159

My post stated:

"I have been looking into Litz wire for a DC-DC transformer I am building
that will operate at 50-100 kHz or so, and it does seem that the skin effect
is quite significant. Here is some theory:
http://newenglandwire.com/products/litz-and-formed-cables/theory

"According to my calculations, #1/0 cable at 350 CM/Amp is good for 300
amps, and has a DC resistance of 100 uOhms/foot. So 10 feet of cable would
have a loss of 90 watts at 300 amps. However, the AC resistance at 20 kHz is
12.4 times that, so it may be very significant (1100 kW). However, that is
just the carrier frequency and the effective waveform will usually be less
than 200 Hz, at which the cable has about 1.24 times that at DC, and the
losses will be 112 watts."

"It might be good to use 10 parallel strands of #10 AWG wire which has a
skin factor multiplier of 3.9 at 20 kHz, and negligible effect at the
effective frequencies of 10-200 Hz. It would be interesting to run a
temperature rise test on a #1-0 cable and a 10 strand bundle of #10 to see
if there is any significant temperature difference."

"This is actually AC resistance and not inductive reactance, so it is
actual power and not VA. "

"The DC cables may benefit from having some inductance, which may help
reduce the ripple on the DC bus link. I don't think the capacitor size is
affected by the distance from the battery, and in fact may be reduced for a
long DC run. The battery pack probably has a very high impedance at 10-20
kHz. And the controller may benefit by having a long run to the motor, as it
may increase the load inductance and minimize high current spikes."

I also found some interesting information on VFD cables, although it did not
mention skin effect.
http://www.escmotors.com/public/pdfs/vfd_whitepaper_driveflex.pdf

Paul
http://www.pstech-inc.com

 

[Maynard A. Philbrook Jr.]

As a result of the discussion of Litz wire and skin effect in my previous
post, I wondered if it may apply to VFD motor controllers, and particularly
in EVs, where AC induction motors are often "overclocked" to 180 Hz or even
higher. Most DIY EVs use welding cable for motor and battery connection, and
there was recently a discussion about proximity of the controller to the
motor or battery pack:
http://www.diyelectriccar.com/forum...-battery-motor-91282.html?p=372159#post372159

My post stated:

"I have been looking into Litz wire for a DC-DC transformer I am building
that will operate at 50-100 kHz or so, and it does seem that the skin effect
is quite significant. Here is some theory:
http://newenglandwire.com/products/litz-and-formed-cables/theory

"According to my calculations, #1/0 cable at 350 CM/Amp is good for 300
amps, and has a DC resistance of 100 uOhms/foot. So 10 feet of cable would
have a loss of 90 watts at 300 amps. However, the AC resistance at 20 kHz is
12.4 times that, so it may be very significant (1100 kW). However, that is
just the carrier frequency and the effective waveform will usually be less
than 200 Hz, at which the cable has about 1.24 times that at DC, and the
losses will be 112 watts."

"It might be good to use 10 parallel strands of #10 AWG wire which has a
skin factor multiplier of 3.9 at 20 kHz, and negligible effect at the
effective frequencies of 10-200 Hz. It would be interesting to run a
temperature rise test on a #1-0 cable and a 10 strand bundle of #10 to see
if there is any significant temperature difference."

"This is actually AC resistance and not inductive reactance, so it is
actual power and not VA. "

"The DC cables may benefit from having some inductance, which may help
reduce the ripple on the DC bus link. I don't think the capacitor size is
affected by the distance from the battery, and in fact may be reduced for a
long DC run. The battery pack probably has a very high impedance at 10-20
kHz. And the controller may benefit by having a long run to the motor, as it
may increase the load inductance and minimize high current spikes."

I also found some interesting information on VFD cables, although it did not
mention skin effect.
http://www.escmotors.com/public/pdfs/vfd_whitepaper_driveflex.pdf

Paul
http://www.pstech-inc.com

If it is any interest, we run inverters up in the 8KHz region and
normally for short runs to the motor, use standard rope wire.

We also use line reactors in cases of longer runs on the output side.

In case where we had some concern, we made a braided lead wires out of
a series of small PVC coated smaller gage and that works well.

Also, you can use insulated tape/ribbon wire, which works wonders for
lowering the eddy currents to virtually 0. you can even stack it with
no dialectic, just bond the ends to the terminator. Some people just a
can of insulation varnish and spray the inside layers after assembly.

Jamie

 

[Tim Williams]

1. You'd have to know the motor impedance at frequency to figure out load
current, and thus cable losses. It could be measured. Knowing impedance
(rather than just inductance) also provides an estimate for motor losses,
which won't be small.

If a core has ideal Steinmetz losses (a reasonable approximation for
laminated iron), the impedance above the eddy current cutoff frequency is
a constant resistance. In effect, it's a transformer coupling into the
shorted turn formed by the eddy current paths.

In the cutoff region, losses are independent of frequency, so you have no
advantage switching at a higher or lower frequency -- the ripple remains
constant, because the ripple current is a squarewave (or a little tilted
from what inductance remains), not triangular.

Personally, I would bet the ripple current at the switching frequency is
sufficiently small to ignore, and therefore, Litz'd cables hold no
advantage. Do note, however, that very large applications will benefit,
since 180Hz doesn't fit inside #4-0 very well, let alone anything bigger.


2. DC link cap depends on the system, so I won't make any sweeping
generalizations. It needs to be big enough to keep inverter ripple happy,
while avoiding resonance with the battery wiring and anything else.
Speaking of;

3. AFAIK, batteries are more or less massive nonlinear capacitors. Viewed
over time, a battery is firstly:
- Electrolyte resistance, ESR (temperature dependent)
- Inductor: lead inductance (time constant ~us)
- Capacitor: two plates surrounded by dielectric fluid or ionic double
layers; depends on type and charge condition; lossy (low ms to sec?)
- Ionic diffusion: time and charge dependent effects: weird battery stuff;
lossy (secs to mins)
- Charge storage: normal battery behavior, a near-constant-voltage charge
reservoir with variable ESR (~mins to... well, years)

The only direct inductance is lead inductance, and the only direct
resistance is electrolyte ESR. These limit the peak short circuit current
under any charge or ionic condition. But actual short circuit current may
be lower due to other effects (most notably the increase in ESR at low
charge), and may vary up and down over time due to capacitance and
diffusion and convection and... who knows. All these other effects could
be modeled with various lumped RLC equivalents, but the magnitude and
arrangement all depends, and you'd have to measure your own battery pack
for which parameters are most significant.

(It seems, few people have any idea what a battery looks like,
electrically, let alone are willing to put it into writing. The above is
based on what I've heard, plus physical intuition, but isn't informed by
actual representative measurements. This is more of a framework than a
reference, anyway, because the magnitude of each effect varies, and you'll
have to measure your own batteries to figure it out.)

4. In short, I doubt the batteries themselves have a high impedance at
~10kHz, but more than a few feet of cables will have noticeable
inductance, so that you wouldn't care what the battery actually is at that
frequency. Enough link cap to smooth out the lead inductance and battery
capacitance (and maybe take the edge off some of the ionic junk), with
enough ESR to avoid resonances, is what's called for. Whether that value
is more or less than provided, who knows.

(Yes, I know, I'm replying to the bits you quoted, not your actual
question anymore.. hopefully it will still be useful.)

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

As a result of the discussion of Litz wire and skin effect in my previous
post, I wondered if it may apply to VFD motor controllers, and
particularly
in EVs, where AC induction motors are often "overclocked" to 180 Hz or
even
higher. Most DIY EVs use welding cable for motor and battery connection,
and
there was recently a discussion about proximity of the controller to the
motor or battery pack:
http://www.diyelectriccar.com/forum...-battery-motor-91282.html?p=372159#post372159

My post stated:

"I have been looking into Litz wire for a DC-DC transformer I am
building
that will operate at 50-100 kHz or so, and it does seem that the skin
effect
is quite significant. Here is some theory:
http://newenglandwire.com/products/litz-and-formed-cables/theory

"According to my calculations, #1/0 cable at 350 CM/Amp is good for 300
amps, and has a DC resistance of 100 uOhms/foot. So 10 feet of cable would
have a loss of 90 watts at 300 amps. However, the AC resistance at 20 kHz
is
12.4 times that, so it may be very significant (1100 kW). However, that is
just the carrier frequency and the effective waveform will usually be less
than 200 Hz, at which the cable has about 1.24 times that at DC, and the
losses will be 112 watts."

"It might be good to use 10 parallel strands of #10 AWG wire which has a
skin factor multiplier of 3.9 at 20 kHz, and negligible effect at the
effective frequencies of 10-200 Hz. It would be interesting to run a
temperature rise test on a #1-0 cable and a 10 strand bundle of #10 to see
if there is any significant temperature difference."

"This is actually AC resistance and not inductive reactance, so it is
actual power and not VA. "

"The DC cables may benefit from having some inductance, which may help
reduce the ripple on the DC bus link. I don't think the capacitor size is
affected by the distance from the battery, and in fact may be reduced for
a
long DC run. The battery pack probably has a very high impedance at 10-20
kHz. And the controller may benefit by having a long run to the motor, as
it
may increase the load inductance and minimize high current spikes."

I also found some interesting information on VFD cables, although it did
not
mention skin effect.
http://www.escmotors.com/public/pdfs/vfd_whitepaper_driveflex.pdf

Paul
http://www.pstech-inc.com

 

[P E Schoen]

Just FYI, I deleted my posts in the thread previously linked (by request),
and moved it to its own thread:
http://www.diyelectriccar.com/forums/showthread.php/litz-wire-and-vfd-cable-acim-91411.html

Thanks,

Paul
http://www.pstech-inc.com

 

[RobertMacy]

...snip excellent information, to keep Aioe happy

I also found some interesting information on VFD cables, although it did
not mention skin effect.
http://www.escmotors.com/public/pdfs/vfd_whitepaper_driveflex.pdf

Paul
http://www.pstech-inc.com

Years ago while working on an induction furnace 150kW+ at 50kHz for use at
Bethlehem Steel, we ran 1000Apk 50kHz through simple 3/8 copper tubing
with water running through it. There were 7 turns around a carbon 'sink'
for melting zinc. Sink was over 20 by 30 inch [from memory] In trying to
get good efficiency, didn't notice much loss in the cabling. But a few kW
probably would not have shown up.

The Converter System was about 8 feet tall 5 ft wide, sat near the sink.
Most of the cabling that was possible used the copper tubing. Small straps
[very WIDE] were used where voltage isolation, number of contacts, etc
dictated. It was probably around 8 ft between the sink and the system.
However, exit water ran very cool. didn't even notice a rise in temp.

Wouldn't it be ironic to move from radiator water in a gasoline engine
system to requiring radiator water in an electric system?

 

[RobertMacy]

Also, you can use insulated tape/ribbon wire, which works wonders for
lowering the eddy currents to virtually 0. you can even stack it with
no dialectic, just bond the ends to the terminator. Some people just a
can of insulation varnish and spray the inside layers after assembly.

Jamie

Do you have data comparisons? Using simulations, I looked at separate
parallel paths connected at each end and came to the conclusion there was
no real improvement. [However, reality is ALWAYS in the empirical.] The
results show the center conductors being in a strong field become starved
for current thus the outside conductors still carried most of the current.

Plotting current distribution across a 'fixed' multiple conductor cable
made up of insulated conductors, attached in parallel at the ends looked
almost EXACTLY like the distribution of current inside the equivalent
solid cable.

The Project mentioned here refers to trying to increase Q of a high
frequency coil and just could NEVER lower total resistance losses [which
included series AND eddy current losses] In other words, it was almost a
'wash' between using a solid cable and a cable made of insulated strips of
same size.

Still, do you have any data comparisons?

 

[RobertMacy]

(It seems, few people have any idea what a battery looks like,
electrically, let alone are willing to put it into writing. The above is
based on what I've heard, plus physical intuition, but isn't informed by
actual representative measurements. This is more of a framework than a
reference, anyway, because the magnitude of each effect varies, and
you'll
have to measure your own batteries to figure it out.)

Batteries are tricky to characterize.

However, lead-acid batteries in a security system back up: 12Vdc with
4A-hr, mesured using small signals around quiescent point did NOT look
capacitive, rather VERY resistive! Surprised me, I was counting on the
aditional bypass, just wasn't there.

 

[Tim Williams]

Years ago while working on an induction furnace 150kW+ at 50kHz for use
at Bethlehem Steel, we ran 1000Apk 50kHz through simple 3/8 copper
tubing with water running through it. There were 7 turns around a
carbon 'sink' for melting zinc. Sink was over 20 by 30 inch [from
memory] In trying to get good efficiency, didn't notice much loss in
the cabling. But a few kW probably would not have shown up.

Yeah, typical designs might aim for 80% efficiency on interconnects. You
only need pipe or hose a couple of inches across to carry megawatts at
that rate. Yeah, you burn a hundred kilowatts here or there, but as long
as they're drenched in water, who cares? (Seems to me, designers should
aim for better efficiency in an industry so 'green' in nature, but go
figure.)

The Converter System was about 8 feet tall 5 ft wide, sat near the sink.
Most of the cabling that was possible used the copper tubing. Small
straps [very WIDE] were used where voltage isolation, number of
contacts, etc dictated. It was probably around 8 ft between the sink
and the system. However, exit water ran very cool. didn't even notice
a rise in temp.

Sounds about right.

Excessively large pipes and flows are common too, probably in part for
overhead -- double or triple the flow required means the end user can be
that much more careless with water quality. A 150kW system might run 30
GPM at 60 PSI, which is...around a few HP in water already?

Wouldn't it be ironic to move from radiator water in a gasoline engine
system to requiring radiator water in an electric system?

Although I wouldn't mind water-cooled motors, inverters and batteries in
EV applications, say. The power density is so much better.

And then there's LN2... which is being tried, for power distribution!
Now, it'd be pretty neat if they can find a superconductor that'll work
on, say, circulating dry ice + acetone, which is well within the range of
two or three stage refrigerators rather than air liquifiers, practically
room temperature (depending on which mountain top or polar region you
ask). Still expensive, but better insulation and cooling efficiency means
still better operating value, even if the critical field of the stuff is
total crap.

Which brings us back to skin effect. Superconductors are all skin, and
it takes a whole lot of thin superwires to carry much useful current, let
alone build a magnet of sizable field strength -- the contribution per
strand per turn in a 10T magnet can't be much! Should be some
Litz-related action going on in those things.

Tim

 

[[email protected]]

"There is another theory, which states that this has already happened."

Although I wouldn't mind water-cooled motors, inverters and batteries
in EV applications, say. The power density is so much better.

Toyota hybrids have had water-cooled motors and inverters since 1997;
the motors also probably get a little help from the transmission oil.
The motor/inverter has its own little water pump and radiator, separate
from the one for the gasoline engine; it uses water and ethylene glycol
just like the engine. Their batteries (as far as I know) are air-cooled.
Honda IMA (Insight circa 2000) probably didn't have a water-cooled motor,
but newer Honda hybrids probably do. I have seen a 288 V NiMH battery
that was water-cooled, and I think Tesla's cars and the Chevy Volt use
water-cooled batteries.

The thing I don't like about water-cooled batteries is that it's harder
to turn off the juice if you think something has gone wrong, unlike a
motor or inverter. You can open up the external circuit, but you can't
stop the individual cells/modules, which can be interesting if the
battery is mechanically damaged.

Which brings us back to skin effect.

I am not sure what frequencies hybrid cars and EVs use. I suspect it's
high audio because I can still hear the inverter noise sometimes. I
know they use finely-stranded cable for the traction motors (closer to
welding cable than THHN), but I think that's as much for flexibility
under high vibration as it is for electrical reasons.

Matt Roberds

 

[Maynard A. Philbrook Jr.]

Also, you can use insulated tape/ribbon wire, which works wonders for
lowering the eddy currents to virtually 0. you can even stack it with
no dialectic, just bond the ends to the terminator. Some people just a
can of insulation varnish and spray the inside layers after assembly.

Jamie

Do you have data comparisons? Using simulations, I looked at separate
parallel paths connected at each end and came to the conclusion there was
no real improvement. [However, reality is ALWAYS in the empirical.] The
results show the center conductors being in a strong field become starved
for current thus the outside conductors still carried most of the current.

If this thread is still going when I get back to work from vacation, I
think I can get my hands on test data.

Plotting current distribution across a 'fixed' multiple conductor cable
made up of insulated conductors, attached in parallel at the ends looked
almost EXACTLY like the distribution of current inside the equivalent
solid cable.

We are talking about high freq AC ?

The Project mentioned here refers to trying to increase Q of a high
frequency coil and just could NEVER lower total resistance losses [which
included series AND eddy current losses] In other words, it was almost a
'wash' between using a solid cable and a cable made of insulated strips of
same size.

You need to use thin wall tubing for coils. Also you must remember that
SR on a coil can act like eddy currents. The Geometry and core material
matters.

Still, do you have any data comparisons?

When I return to work, I'll see the LAB guys and get some data
.. They're always coming up with new product ideas. I won't say they're
the smartest tool in the shed but they get buy

I know they were doing test comparisons on different power wire at 1kHz
and lots of amps a couple of weeks ago.. We have a real generator with
many poles driven from a high HP motor.

Jamie

 

[RobertMacy]

I am not sure what frequencies hybrid cars and EVs use. I suspect it's
high audio because I can still hear the inverter noise sometimes. I
know they use finely-stranded cable for the traction motors (closer to
welding cable than THHN), but I think that's as much for flexibility
under high vibration as it is for electrical reasons.

Matt Roberds

Since this is kW of power, even reducing the harmonics down 70 to100 dB
would be required to avoid radiating like gangbusters. The fundamental
would be huge, so the choice could be below 10kHz to avoid ANY EMC
Regulatory agency [well almost]

The next step is up to 150kHz, in US not so bad, but that's fraught with
peril in Europe because Regulatory Agencies block a range in that band to
reserve communications for submarines. Somewhere in the US [Wisconsin?}
there is a huge grid of antennas for submarine communication/monitoring.
100kHz *is* a looong wavelength.

 

[RobertMacy]

We are talking about high freq AC ?

The 150kW induction furnace ran at 50kHz. Another design was trying to
improve Q in an inductor at 1MHz, and another was trying to lower losses
in a motor design that ran at 3MHz stepping frequency. [albeit was a small
motor ]

You need to use thin wall tubing for coils. Also you must remember that
SR on a coil can act like eddy currents. The Geometry and core material
matters.

I think standard 'plumbing' tubing from a hardware store was used. Also
interesting note. It was cheaper for the company [not civilization] to run
water through the system and into an open drain, than to buy, set up,
maintain a closed system requiring a heat exchanger etc. So, for over a
year, 24/7 [except extended holidays] water ran into the drain from the
exit flow from all that wiring starting at standard inlet pressure of
something like 60psi Looked like a garden hose running.

When I return to work, I'll see the LAB guys and get some data
. They're always coming up with new product ideas. I won't say they're
the smartest tool in the shed but they get buy

I know they were doing test comparisons on different power wire at 1kHz
and lots of amps a couple of weeks ago.. We have a real generator with
many poles driven from a high HP motor.

Jamie

By the way, I was always disappointed with the easy/cheap/viable ways to
reduce loss. Seems like loss just is.

 

[RobertMacy]

We are talking about high freq AC ?

The 150kW induction furnace ran at 50kHz. Another design was trying to
improve Q in an inductor at 1MHz, and another was trying to lower losses
in a motor design that ran at 3MHz stepping frequency. [albeit was a small
motor ]

You need to use thin wall tubing for coils. Also you must remember that
SR on a coil can act like eddy currents. The Geometry and core material
matters.

I think standard 'plumbing' tubing from a hardware store was used. Also
interesting note. It was cheaper for the company [not civilization] to run
water through the system and into an open drain, than to buy, set up,
maintain a closed system requiring a heat exchanger etc. So, for over a
year, 24/7 [except extended holidays] water ran into the drain from the
exit flow from all that wiring starting at standard inlet pressure of
something like 60psi Looked like a garden hose running.

When I return to work, I'll see the LAB guys and get some data
. They're always coming up with new product ideas. I won't say they're
the smartest tool in the shed but they get buy

I know they were doing test comparisons on different power wire at 1kHz
and lots of amps a couple of weeks ago.. We have a real generator with
many poles driven from a high HP motor.

Jamie

By the way, I was always disappointed with the easy/cheap/viable ways to
reduce loss. Seems like loss just is.

 

[Maynard A. Philbrook Jr.]

I am not sure what frequencies hybrid cars and EVs use. I suspect it's
high audio because I can still hear the inverter noise sometimes. I
know they use finely-stranded cable for the traction motors (closer to
welding cable than THHN), but I think that's as much for flexibility
under high vibration as it is for electrical reasons.

Matt Roberds

Since this is kW of power, even reducing the harmonics down 70 to100 dB
would be required to avoid radiating like gangbusters. The fundamental
would be huge, so the choice could be below 10kHz to avoid ANY EMC
Regulatory agency [well almost]

The next step is up to 150kHz, in US not so bad, but that's fraught with
peril in Europe because Regulatory Agencies block a range in that band to
reserve communications for submarines. Somewhere in the US [Wisconsin?}
there is a huge grid of antennas for submarine communication/monitoring.
100kHz *is* a looong wavelength

We make coax feed line for subs that spools out from a compartment in
the sub and floats itself up on the surface. of course, antennas are
attached to this before it is released. We also make the long spools of
wire they drag behind them

Incase of an emergency exit, they simply cut and go, leaving it
floating around on the water for that unsuspecting boat coming along to
get it wrapped up in their blades and shafts

Jamie

 

[Maynard A. Philbrook Jr.]

We are talking about high freq AC ?

The 150kW induction furnace ran at 50kHz. Another design was trying to
improve Q in an inductor at 1MHz, and another was trying to lower losses
in a motor design that ran at 3MHz stepping frequency. [albeit was a small
motor ]

You need to use thin wall tubing for coils. Also you must remember that
SR on a coil can act like eddy currents. The Geometry and core material
matters.

I think standard 'plumbing' tubing from a hardware store was used. Also
interesting note. It was cheaper for the company [not civilization] to run
water through the system and into an open drain, than to buy, set up,
maintain a closed system requiring a heat exchanger etc. So, for over a
year, 24/7 [except extended holidays] water ran into the drain from the
exit flow from all that wiring starting at standard inlet pressure of
something like 60psi Looked like a garden hose running.

When I return to work, I'll see the LAB guys and get some data
. They're always coming up with new product ideas. I won't say they're
the smartest tool in the shed but they get buy

I know they were doing test comparisons on different power wire at 1kHz
and lots of amps a couple of weeks ago.. We have a real generator with
many poles driven from a high HP motor.

Jamie

By the way, I was always disappointed with the easy/cheap/viable ways to
reduce loss. Seems like loss just is.

One thing to remember, as you lower the R, you raise the Q, wire
behaves like induction. I've seen ringing that causes losses.
You can't win So using some form of PF correction helps a great deal.

One trick I've seen done by a guy working on an EV, to remove the
switching from the power leads, which causes loss, he populated the
large wire he was using with lots of torides, no loops, just threaded
the power wire through them. that suppressed a lot of the magnetic
rippling from the switching.

Jamie

 

[RobertMacy]

One trick I've seen done by a guy working on an EV, to remove the
switching from the power leads, which causes loss, he populated the
large wire he was using with lots of torides, no loops, just threaded
the power wire through them. that suppressed a lot of the magnetic
rippling from the switching.

Jamie

good idea blocks all those higher frequency harmonics.