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Old 03-26-2012, 09:09 AM   #41 (permalink)
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Originally Posted by mechman600 View Post
Paul at EPC Corp is sending me a "field control module"....... Honestly, I am a bit skeptical, but he says it works.
This worry's me...

EPC has not been known for high quality motor controllers, Im sure this piece isnt any better....

Here is some info on their controllers. EPC Controllers..... - DIY Electric Car Forums

As for running the controller with the field powered separately shouldn't be a problem. The motor will look like a permanent magnet motor to the controller.

If its a SEPEX motor, it should have field current rating on the name plate.

-Adam

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Old 03-27-2012, 05:57 AM   #42 (permalink)
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Crap. I read the whole thread you linked to above. I'm scared. I'm scared because Paul told me that the motor he sold me would work great in my conversion, but maybe he sold me some old piece of crap he had lying around.

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Old 03-27-2012, 09:37 AM   #43 (permalink)
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It turns out there is WAY more room behind the back seat than I ever would have imagined. Way more than a civic hatchback. Even way more than my wife's Toyota Matrix. Good news for battery storage!
The MX-3 is a bloody monster truck by ecomodder standards.

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Old 03-27-2012, 03:33 PM   #44 (permalink)
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Sepex motor research nerd

Needless to say, I have done a whole lot of research on sepex motors over the last week or so. Sepex = separately exited field. The field is wound much finer than a series wound motor, requires much less current and it separately controlled from the armature.

Sepex motors have many advantages:

-Much broader RPM range. With a constant armature current, the more current you feed the field, the more torque will be produce but at a slower speed. The less current you feed the field, the faster the motor will spin but with less torque. A proper sepex motor controller with proper field mapping will give the field more amps at low speed for more torque, and as the motor speeds up it will reduce the field strength for more motor speed, thus broadening the "power band."
-Regen braking. Many controllers can simply reverse the armature while keeping the field the same (or vice-versa). However, this only works well in lower voltage applications where the brush timing is neutral or close to it.
-Reversing. Basically the same as regen braking.

Sepex motor disadvantages:

-Proper motor controllers are more expensive
-Controllers of high voltage are hard to find. Generally, 72V is as high as you can go.

So where does that leave me? We'll see what this "field control module" looks like from EPC corporation. I hope it can handle a wide range of voltages, not just 12V. What I envision is controlling the field through two "taps" in my pack (maybe 12V and 24V or 24V and 48V) through two relays and a double throw switch. This would be my electric "gear shift". The higher field voltage would be used to take off from a start and the lower voltage once it revs up past a certain speed. All in conjunction with my PM/series motor controller.

Even if this field control module is junk, I only need it to suppress the giant voltage spike when I open the field circuit. Could this not be accomplished by simply wiring a large capacitor with the field?

All this might have ended up being a good thing.
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Old 03-27-2012, 04:05 PM   #45 (permalink)
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For supressing voltage spikes off of motors I believe you want to use something called a transil diode. They also go by other names (which I forget), but they're a really fast acting diode.
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Old 03-27-2012, 05:17 PM   #46 (permalink)
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Originally Posted by mechman600 View Post
Even if this field control module is junk, I only need it to suppress the giant voltage spike when I open the field circuit. Could this not be accomplished by simply wiring a large capacitor with the field?

All this might have ended up being a good thing.
Hi mechman,
A capacitor will make the field winding - capacitor into an oscillator, which will be worse than a single spike.
To suppress the high voltage from coil collapse a diode is placed across the coil so that it conducts the reverse spike and a diode can be placed to conduct the forward spike into a higher voltage, like the battery. For most applications a standard silicon rectifier with high enough voltage and current ratings works fine. You can't reverse the field if you use diodes.
-mort
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Old 03-27-2012, 05:59 PM   #47 (permalink)
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Thanks, Doax and Mort. The whole oscillator thing is beyond me. But the diode makes perfect sense. Exactly like how a diode is often placed across pins 85 + 86 (solenoid) inside a relay to prevent spikes from frying the controlling ECU when the circuit goes open.

In my field circuit, am I correct in assuming that the voltage spike is very low amperage? I am basically electronics illiterate. Can you give me the part number of a suitable diode?

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...and a diode can be placed to conduct the forward spike into a higher voltage, like the battery.
OOOO....regen spike suppression! Ok, maybe not.

EDIT: I'm thinking a 5A 100V diode would do - just put it between the field terminals. Is Santa real (could it be so simple?)?
http://www.ebay.com/itm/10-x-SR5100-...item3a5b1744e0

Last edited by mechman600; 03-27-2012 at 06:26 PM..
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Old 03-27-2012, 07:06 PM   #48 (permalink)
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In my field circuit, am I correct in assuming that the voltage spike is very low amperage? I am basically electronics illiterate. Can you give me the part number of a suitable diode?
Hi mechman,
The field winding is an inductor. A voltage across the inductor causes a current to flow and the inductor stores energy in a magnetic field, it's like a flywheel all spun up. When the voltage is removed, opening the field switch for instance, the current continues to flow, at a falling rate depending on the total amount of energy stored. If the voltage is removed abruptly the full current that was flowing now sees a very high resistance, and E=I*R so the voltage spikes. If the circuit was hooked up with the battery positive and then a switch and then the field coil and then the battery negative, just when the switch is opened the current was flowing through the coil to the battery negative. That current continues at the same rate and direction, so the voltage at the switch end of the coil goes way negative (compared to the battery positive voltage that it was when the switch was closed). The diode needs to conduct the full current that the coil carries. The voltage spike won't be high because the diode resistance is low (and so the spike is suppressed). The highest voltage the diode sees is full battery voltage. For up to 144 volts, a 200 volt diode with a forward current rating higher than (like twice) the maximum field current will do.

Do you know the field resistance or maximum current and voltage?

The diodes you link to are Schottke, which are low forward voltage and fast, both things you don't need and cost extra. Schottkes aren't bad, just unnecessary. I'd imagine the maximum field current is more than 5 amps, so something like this might be better. But you don't need 10. Also you can find cheaper diodes locally, I bet.

Also, as an aside, Daox refers to transient suppression diodes which are made exactly for this job, except they are expecting electronics type voltage, like 12 or 15 volts.

-mort
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Old 03-27-2012, 08:02 PM   #49 (permalink)
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The diodes you link to are Schottke, which are low forward voltage and fast, both things you don't need and cost extra.
You definitely need faster-than-fast diodes. Regular "fast" diodes are quite slow, and usually 'snappy'. These work fine rectifying a slow sine wave, where the zero crossing is a gentle slope. Horrible things happen when they are faced with the sharp cut-off of a semiconductor switch. They briefly continue to pass current after the voltage reverses and then "snap", which results in localized melting and cascading failure.

You want "ultra-fast" diodes with soft recovery. Schottky diodes do nicely, if you are working with low enough voltages. They cost more to fabricate, but this is usually offset by their lower forward voltage drop and thus higher current capacity in the same package type.

If you are designing a new circuit, using a MOSFET switched to emulate an ideal diode ("active rectification" / synchronous rectification) is an even more efficient solution.

Last edited by DJBecker; 03-27-2012 at 08:09 PM..
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Old 03-27-2012, 08:35 PM   #50 (permalink)
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Horrible things happen when they are faced with the sharp cut-off of a semiconductor switch.
I believe we are talking about a electromechanical relay. Release bounce times for relays capable of switching a few amps are on the order of 5 ms and longer. Silicon is fine.
-m

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