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Old 19th July 2007, 12:02 PM   #1
mzzj is offline mzzj  Finland
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Default 20khz PWM @2kA, 24V

Electric go-kart project is getting out of hands and we need more current...

Has anyone done something similar or has nice ideas on output stage?

Single to-220 mosfet can take about 70-80A continuously, so I guess that around 30pcs of to-220 mosfets parallei should do the job..

Switching 2kA in 500nS or so is a another thing, I am affraid that layout is "intresting". 4oz copper is thickest PCB I can get, probably good enough for mounting decoupling caps but needs some stiffening with copper bars to carry 2000A.

Plan was to parallei 2 mosfets and 2 schottkys per "module" and run separate 6mm2 wire from each module to motor(half a meter maybe)
This way dynamic current sharing between mosfets should be better

Previous pwm-controllers can take 800A cont. and 1kA shortly(half a sec and bonding wires waporize)
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Old 19th July 2007, 12:20 PM   #2
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Wow, that is crazy.
A go cart need as much as 2000Amps?
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Old 19th July 2007, 02:53 PM   #3
acid_k2 is offline acid_k2  Italy
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2kA @ 24V = 48kW????? I hope you don't have a single electric motor....

for the controller: I think do you need a rigid copper bar (section at minumum 500 mm2) for a current of 2000A, so I think the controller must be fixed directly on the motor(s).

Modular controller is a great improvement, you can add as many modules do you need...
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Old 19th July 2007, 04:10 PM   #4
mzzj is offline mzzj  Finland
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Quote:
Originally posted by acid_k2

2kA @ 24V = 48kW????? I hope you don't have a single electric motor....

for the controller: I think do you need a rigid copper bar (section at minumum 500 mm2) for a current of 2000A, so I think the controller must be fixed directly on the motor(s).

Modular controller is a great improvement, you can add as many modules do you need...
Actually we have 2 motors and need 2 controllers


2kA is only for very brief perioids on acceleration, 2-5 seconds max. I think motor and copper busbars can be rated for average current(around 300-500A I guess) but it wont help if mosfets are vaporising every time I hit pedal to the floor.

Higher voltage would make things a bit easier, but unless we get better motors and batts we have to live with 24v
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Old 19th July 2007, 04:39 PM   #5
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Any chance to run the motors in series?

Then you'd have 48V @ 500 amps which may be more achievable.

The problem with low voltages is the I^2 * R losses. I remember trying to build an inverter for a MIG welder in my student days.

Are you just PWM'ing the battery direct to the motors? (no step up/down of voltage) ?

-Len.
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Old 19th July 2007, 05:45 PM   #6
mzzj is offline mzzj  Finland
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Quote:
Originally posted by len_scanlan
Any chance to run the motors in series?


Are you just PWM'ing the battery direct to the motors? (no step up/down of voltage) ?

-Len.
Series connection should be possible but we still need huge currents as plan was to use 2x2000A controllers, with series connection "only" one 2000A controller would be needed.

Yeap, it's direct pwm to motor, motor inductance takes care of smoothing the current. Previously we have used 1khz PWM frequency to ease on switching losses and current sharing problems but 20khz would result better low-power torque and get rid of that annoying 1khz whine.
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Old 19th July 2007, 06:00 PM   #7
djQUAN is offline djQUAN  Philippines
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have you considered using ISOTOP cased fets and buss bars?
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Old 19th July 2007, 06:54 PM   #8
BWRX is offline BWRX  United States
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Or IGBTs, especially since this is a low switching frequency and high current application.
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Old 19th July 2007, 08:44 PM   #9
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IGBTs will have too much forward voltage drop, the dissipation will be huge. I think you're limited to N type Mosfets

Some options:

55V, 120A, Rds=0.0035ohm
http://www.st.com/stonline/products/...tp185n55f3.pdf

40V, 75A (180 peak), Rds=0.0032ohm
http://www.irf.com/product-info/data...rl1404zpbf.pdf

40V, 80A, Rds=0.0027ohm
http://www.fairchildsemi.com/ds/FD%2FFDP8441.pdf

I didn't know Fets had come so far in terms of on resistance until I pulled down a few datasheets.

Even with the vanishingly small Rds(on) of these ones, you'll still need about 30 in parallel to make up the current rating. Mounting many TO-220 packages to a bus-bar will be challenging.

-Len.
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Old 19th July 2007, 09:16 PM   #10
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I used a frequency of between 200 and 300 Hz for my e-bike because it gave a satisfactory sounding whine.
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