# Sepex field current/voltage questions



## mechman600 (Oct 16, 2010)

Today was my very first commute to work with my very first conversion, a Mazda MX-3 super low budget conversion that came in at $2000 total, including the car. My blog is here if anyone is interested:
electric-booger.blogspot.com

I ended up getting a used Prestolite MVX-[something or other] from a guy who said it was series wound, but when it arrived, I quickly discovered it is a sepex motor. I already had my used Alltrax 7245 (series/PM) controller and decided to attempt to use it. It is 7" by 18".

I have tapped the pack for 12 and 24 volts for the field with a toggle switch on the gearshift lever. The field turns on with a relay when the idle microswitch clicks. I discovered today that 12V is not enough, as there is little start off torque and it has to rev much too high to get any power - shifting at 6000+ rpm. Plus at 12V, I can hear the brushes arcing at higher revs, and not when I switch it to 24V.

The big problem I am having is with the massive voltage spike from the field windings when the relay opens. I do have a 400V 15A RMS diode across the field terminals which helps, but there is still a spark. On 12V, the arcing lasts a split seconds but on 24V, the arcing lasts a few seconds inside the relay after each disconnect. After a few on and offs, the relay is smoking hot. Today I nearly cured the problem by relaying the field positive to ground on relay pin 87a when the field is shut off. But this evening I hooked the field to 36V to try it and when the relay went open sparks flew and the smell of burning filled the air. So question number one is - how can I completely eliminate this spike when the field relay opens? What diodes, capacitors, resistors, etc. do I need and how do I wire it up?

Question number two is - how much field amps can I give it? The field resistance is exactly 1 ohm, calculated by measuring the current. 12V = 12.5 amps, 24V = 25 amps, etc.

Thanks in advance.


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## EVfun (Mar 14, 2010)

You will need a properly rated relay. Perhaps an Albright SW-80 contactor (a smaller version of the SW200 used as the main contactor in many EVs) would be a better choice. You are switching a very inductive load and will need a diode and resistor across the relay to quench the turn-off. 

Others here are a lot more qualified than me to suggest values. I would suggest the biggest low ESR non-electrolytic capacitor rated for at least 100 volts in series with a resistor with a value about 1/2 the coil resistance.


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## mechman600 (Oct 16, 2010)

What about buying a tiny 48V 50A controller for $78 to do the job?
http://www.cloudelectric.com/product-p/co-kds48050e.htm


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## major (Apr 4, 2008)

mechman600 said:


> What about buying a tiny 48V 50A controller for $78 to do the job?
> http://www.cloudelectric.com/product-p/co-kds48050e.htm


Might be about the same $ as a contactor to handle the job and give you adjustment. I have to stop short of recommending a Kelly, but the concept is valid


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## mechman600 (Oct 16, 2010)

Thanks for the replies.
Another advantage to the small controller would be not having to tap my pack for the desired field voltage.
It works good now on 24V but I'd love to broaden the torque curve so I can be lazy and not shift gears as much!


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## gunnarhs (Apr 24, 2012)

mechman600 said:


> Thanks for the replies.
> Another advantage to the small controller would be not having to tap my pack for the desired field voltage.
> It works good now on 24V but I'd love to broaden the torque curve so I can be lazy and not shift gears as much!


Current (Amperage) is not the problem for the field - relay (even if it can spike up up to 10 times the nominal in your case 10X 1A), but the voltage is.
I would add a solid relay that supports DC-Voltage up to at least 24V.
If you only have access to shop which sell spare parts for normal cars, then buy the biggest relay possible (150A , 24V usually). But i recommend buying a special relay/contactor for this application.


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## mechman600 (Oct 16, 2010)

Yesterday I installed a larger relay to turn the field on and off - a relay that closely resembles a starter relay on some Ford vehicles. Now that the field power is being interupted with the larger relay, there is no arcing or funny relay noises even with 48 volts going to it. I seem to have solved this arcing issue for the time being. I am not sure the field voltage choice relay will last very long with the amount current I am forcing through it (it is merely a plastic, square 40A 5-pin relay) but I guess I will have to find out the hard way. I do have a spare relay in the glove box after all!

The other day I measured field current, which for power curve sakes is probably a more important figure than field voltage. But it turns out that the field winding resistance is exactly one ohm, so 12.5V applied = 12.5 amps, 25V = 25 amps, etc. Now I am at 48V for start off, and it makes a nice difference in torque. Most times I can take off from a stop in second gear.


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## major (Apr 4, 2008)

mechman600 said:


> The other day I measured field current, which for power curve sakes is probably a more important figure than field voltage. But it turns out that the field winding resistance is exactly one ohm, so 12.5V applied = 12.5 amps, 25V = 25 amps, etc. Now I am at 48V for start off, and it makes a nice difference in torque. Most times I can take off from a stop in second gear.


That's good for starting, but you had better reduce the field shortly thereafter. 48V * 48A = 2300W. Field coils will overheat. Get them back down to 24V at least.


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## mechman600 (Oct 16, 2010)

major said:


> That's good for starting, but you had better reduce the field shortly thereafter. 48V * 48A = 2300W. Field coils will overheat. Get them back down to 24V at least.


Good to know. I suppose this is especially true with my tiny motor (7"X18", 65 lbs). I am planning to change the field voltage toggle switch on the gear shift lever to a momentary push button style switch to eliminate any chance of accidentally leaving it switched to 48V.

Do you suggest that I stay with 36V, or is 48V safe for short applications, like 5-10 seconds long (which is what I have been doing)?


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## electro wrks (Mar 5, 2012)

Welding supply places have temp. sticks (crayons) that can be rubbed on the windings, or other parts,to tell the temp. Harbour Freight has a reasonably priced non-contact, laser aimed thermometer that I've had good luck with measuring temps. of line of sight parts. The laser can be offset from the area being measured. So, it needs to be moved around slightly to get a true high temp..

I'm guessing the motor nameplate insulation class rating is a theoretical absolute max number because it seemed absurdly high on some of the motors I've worked on. Does anyone have a max temp. number to stay below in EV applications?

Actually, I found a NEMA reference that clarifies the rating system: http://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html


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## major (Apr 4, 2008)

mechman600 said:


> Do you suggest that I stay with 36V, or is 48V safe for short applications, like 5-10 seconds long (which is what I have been doing)?


Hi mec,

The field coils are closely sinked (thermally) to the frame and can get nice air if you blow thru the motor. So running up at 50A is o.k. for a while. Hard for me to put a time on it, maybe 30-40 seconds. I'd take maybe 20 volts as a 30 minute rating on the coils.

One nice thing about copper is that it has an entirely predictable resistance change with temperature. And you can monitor V and I directly in real time and calculate the resistance. Look up the resistivity coefficient for sure, but from memory, it is like 120°C change for a 47% increase in resistance. Undoubtedly it is class H, so 120°C rise is your conservative limit.

About what field strength to use: You see that running 48A gives you good torque for accelerations. That is likely where you should be. Typically SepEx controllers "map" out a relationship for field current (If) vs armature current (Ia). So let's say you have a limit of Ia = 500A. Use 50A for If there. Use If = 10A minimum (Ia = 0). Draw a little graph. You'll see that If = 30A for Ia = 250A. And If = 20A for Ia = 125A. Try to stay near those ratios or a little less If if the field gets too hot.

You can do that with a series of contactors but it will be a jerky ride and uneven discharge if you're switching batteries in and out. The PWM field control would be better. 

Good luck,

major


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## mechman600 (Oct 16, 2010)

Thanks, major, that's good info.
I agree that a PWM controller would be ideal. A proper sepex controller would be even more ideal, but the Curtis one I want costs over half of what my entire build has cost me!


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## PaulS (Sep 11, 2012)

The best cure is a 200 amp "flyback" diode across the field - as long as you don't reverse the field for operation in reverse gears.

The arc you get is from the motor and not from the relay - the relay just has to handle the induced power once the current stops and induction takes over.


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## mechman600 (Oct 16, 2010)

I need to build a device that measures my armature current and gives a small PWM controller either a 5K-0 or 0-5VDC signal for field control. Unfortunately I do not have the skills for such a task. Googling has gotten me nowhere.
A shunt with some sort of circuit to amplify the 0-50mV signal to 0-5V? Any electronics gurus out there that could whip up a circuit diagram for me?


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## kennybobby (Aug 10, 2012)

*+1, Here's the answer.*



PaulS said:


> The best cure is a 200 amp "flyback" diode across the field - as long as you don't reverse the field for operation in reverse gears.
> 
> The arc you get is from the motor and not from the relay - the relay just has to handle the induced power once the current stops and induction takes over.


You beat me to it, but that is the correct electrical engineering solution. We also call that a "free-wheeling" diode. The diode on a relay is for the contactor coil of the relay, not the contacts. Free-wheeling current flows due to the collapsing magnetic field in a inductor. The size of the diode should be rated at least 2x the current in the coil, i.e. if you have 25 amps in the field coil, then the diode should be rated for at least 50A. if you have 1 amp in the relay contactor coil, then use a 2 amp diode, etc,.


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## major (Apr 4, 2008)

mechman600 said:


> I need to build a device that measures my armature current and gives a small PWM controller either a 5K-0 or 0-5VDC signal for field control. Unfortunately I do not have the skills for such a task. Googling has gotten me nowhere.
> A shunt with some sort of circuit to amplify the 0-50mV signal to 0-5V? Any electronics gurus out there that could whip up a circuit diagram for me?


Hi mech,

Using the current shunt, what you describe sounds like an amplifier circuit. 50mV to 5V would be a gain of 100, right?

I thought of something else you might try. Instead of a shunt, use a Hall effect current transformer (DCCT). These come is various sizes and configurations. Ones I use most often are rated at 4V output for rated current. But they are linear up to 10V. So a 4V/400A CT would be perfect. Check out http://www.lem.com/images/stories/files/Products/1-3_applications/CH24101.pdf I think you could pass your armature* supply wire through the sensor and connect the CT output to the field controller input. The DCCT would need its own isolated power supply of like 5 or 15V.

That might be pretty simple if it would work 

major


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## mechman600 (Oct 16, 2010)

That's great, major. Thanks!
I have been searching and searching for a +/-500A hall effect current transformer with a 0-5V signal and I am coming up short. In the link you provided (www.lem.com), they sell many of these but they all seem to have a signal voltage offset.
Example:








It makes sense for ECUs reading this signal. Having an offset voltage allows an ECU to detect faults if the signal goes open circuit. But I require a 0-5VDC signal.

Would this work?








Use a 500A 50mA shunt to feed this OP-AMP circuit to increase voltage to 0-5VDC? Maybe use a variable resistor in there somewhere for fine tuning. A lot of 10 CA741CE chips is $4 on eBay.

Unfortunately I know squat about electronics. But I do know how to Google very very well!


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## major (Apr 4, 2008)

The HAL 400-S should do the trick. http://www.lem.com/docs/products/hal standard_e.pdf


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## mechman600 (Oct 16, 2010)

I had found that one. But the supply voltage is 15V (+/-5%). I use an extra battery instead of a DC-DC converter, so I will be short on voltage to power this unit.


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## major (Apr 4, 2008)

mechman600 said:


> I had found that one. But the supply voltage is 15V (+/-5%). I use an extra battery instead of a DC-DC converter, so I will be short on voltage to power this unit.


I think it will work on a single sided supply for unidirectional current.


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## PStechPaul (May 1, 2012)

The LM741 op-amp requires +/- supplies. A better choice may be the LM358, which is a dual op-amp rated for single ended operation, and the input and the output will swing to GND. Here is the spec sheet:
http://www.ti.com/lit/ds/symlink/lm158-n.pdf

The formula for the gain of the non-inverting amplifier you show is confusing. The gain is (Rb+Ra)/Ra. There are lots of schematics for amplifiers and other designs, on the data sheet above.


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## mechman600 (Oct 16, 2010)

major said:


> single sided supply for unidirectional current.


I'm not sure what this means. Beyond my realm of knowledge. But I am willing to learn.
Will it work with a 12.6VDC supply, although maybe not as accurate or within design parameters? Accuracy may not matter too much, as there will be a great deal of adjustability with a PWM controller.
Or are you meaning a separate 15VDC voltage supply?


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## major (Apr 4, 2008)

mechman600 said:


> I'm not sure what this means. Beyond my realm of knowledge. But I am willing to learn.
> Will it work with a 12.6VDC supply, although maybe not as accurate or within design parameters? Accuracy may not matter too much, as there will be a great deal of adjustability with a PWM controller.
> Or are you meaning a separate 15VDC voltage supply?


http://www.lem.com/images/stories/files/Products/1-3_applications/CH24101.pdf See 3.2.12 Unipolar power supply. And you'll have a power supply for the field. Use that with a voltage divider or regulator.


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## mechman600 (Oct 16, 2010)

So I ended up purchasing this amp transducer...
http://www.lem.com/docs/products/htfs200_800p_e_rev09.pdf

...and I have questions regarding the wiring.

There are four terminals: +5, 0V, Output and Vref (IN/OUT). The specs say Vref is 2.5V. Is this a terminal that I need to feed 2.5V to? If so, will two equal resistors (2500 ohm), one from +5 to Vref and the other from Vref to 0V work?

Can someone please read the PDF and clarify how I need to wire this thing up?


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## PStechPaul (May 1, 2012)

I do not have direct experience with these transducers, but I found the following which may help (or possibly confuse you further) :
http://www.lem.com/images/stories/files/Products/P1_5_1_industry/CH24101E.pdf

It seems that you can just connect a 5VDC supply as indicated and read the output from the "output" terminal. It may be biased at 1/2 the supply voltage, so zero amps may read 2.5 volts. The "REF" pin apparently may be used as an output, so you might get a zero reading at zero amps if you read between it and the Output.

If you have a source of low voltage high current, you can test the coil. You can use a solder gun, which puts out as much as 100 amps at 1 or 2 volts, or you can use a battery and a headlamp bulb for about 20 amps. The output will probably be AC or DC depending on what you measure.


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## mechman600 (Oct 16, 2010)

Today I tested the transducer out. I first made the circuitry with the required capacitors set out by LEM in the datasheet above. I hooked up the transducer to +5V and common and the output was 2.47V. So was Vref. So PStechPaul, you were right about Vref.

I ran a 2/0 cable through the transducer from a 12V battery pack made of four group 31 batteries (wired in parallel) to a carbon pile and started loading them gradually up to 450 amps, which is my Alltrax controller's max output. At 450A, the transducer's output was at 3.87V and Vref was still at 2.47V. 3.87V is bang on for what it should be at that current according to the datasheet.

So it seems I have success so far. The next step is completing a bit more circuitry and installing the PWM controller into the car.

EDIT: I have successfully made this all work. I started a new thread to make it easily searchable:
http://www.diyelectriccar.com/forums/showthread.php?p=333959#post333959


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