P
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
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