Skin effect and other considerations for VFD to ACIM power connections

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

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. :)

1. 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;

1. 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.)

1. 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

Just FYI, I deleted my posts in the thread previously linked (by request), and moved it to its own thread:

Thanks,

Paul

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?

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?

Batteries are tricky to characterize.

However, lead-acid batteries in a security system back up: 12Vdc with

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.)

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?

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

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

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.

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

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

We are talking about high freq AC ?

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.

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

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.

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 ;) ]

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.

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

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 ;) ]

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.

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

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

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

good idea blocks all those higher frequency harmonics.