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Post up a thread for your EV. Progress pics, description and assorted alliteration
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Post by HeadsUp » Sat, 19 May 2012, 20:00


that $ 1 / m price is a bit more " trade "

$ 14 per meter on ' that other place ' is a bit on the high side

i would be surprised if 40 mm orange is more than $ 2.30 /m

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Post by T2 » Tue, 03 Jul 2012, 15:21

Nevilleh, I have a solution using the Logisys as series field controller or not, which you may wish to consider.

I have done this with a shunt field controller for a 400Amp ASG and it worked out well. Current was limited in that application to 400amps by having only two strings of Golf cart batteries. With the armature controller bypassed with a contactor of course the current is whatever your battery pack can support !

That said, to any one who may try this initially I would recommend through the testing phase that you do not bypass the controller and move only one of the series fields to the new field controller.

The circuit consists of an LM358 to amplify signals from 50mV current shunts and a Motorola MC34060A PWM chip. With the ASG being a SEPEX motor I had only to supply a maximum of 9 Amps to the shunt field. This relatively low current allowed the use of a single pnp darlington TIP147 as a high side switch @ $1-50 per piece. The output circuit in this case connected to the shunt field and terminated to ground via the field current feedback shunt resistor. This latter signal is needed to limit the max field current by comparing with a fixed reference which is adjusted during the setup procedure. The MC34060A has two op amp inputs to allow this. The second of these inputs does the actual work of controlling the field by progressively weakening it as the motors continue to accelerate which is accomplished by this input using the accel pedal input summed with the armature current feedback.

The series field version won't be as easy having to provide up to 300 Amp capability. However maybe not all this current will be needed as a glance at the torque characteristic of a series motor shows the field reaching its max value somewhat before max current is reached. As a security measure the addition of a PWM output pulse width limiter might be advisable I did not need that with a shunt field of course.

In use the operation of the circuit in FIELD mode is very stable, after tinkering with an RC feedback network. With the drive in neutral one person presses repeatedly on the accel pedal while another monitors the armature current feedback for a step response on the 'scope. At the extreme I was able to get low frequency oscillation, however the shunt field has a much longer time constant so may be for a series field a pulse generator cct may have to be added to the PCB to facilitate testing. As I wrote earlier you can sit the armature current wherever you want providing you are above base speed.
Regen Control was added later as it virtually comes free. A bias voltage in the analog circuitry moves the accel pedal voltage reference negatively so that in FIELD mode the pedal movement can move the current from -ve to +ve 400Amps. A centre zero indicator was used on the dash for monitoring purposes.

With two motors your system has tremendous flexibility. Looking forwards you could place both motor fields in series and accel to base speed at which point field weakening is enabled while the armature controller is retired. The system will see both machines as just ONE motor. A tachometer could measure the RPM. This is where a microcontroller comes in useful. At a predetermined speed the motors could be reconnected in parallel with full field restored and the system could resume with both motors once again being field weakened to accelerate. Some reading this won't be comfortable with weakening fields while accelerating. It is well to remember that while you have full voltage on the motors and max current then with max power the vehicle must continue to accelerate.

BTW Nevilleh what was the rpm when you began to notice the drop off in performance when using the two motors connected in series ?
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Post by Nevilleh » Tue, 03 Jul 2012, 18:49

You have raised some interesting ideas that might be worth playing around with.
Regen is one thought, but I have read somewhere that it is not a good idea with brushed motors because the brush angle is wrong (for generation)and you get a huge amount of arcing which reduces the brush and armature life drastically.
Haven't a clue how the motor performance would change with a separately excited field. The number of permutations becomes interesting!

The rpm at which the thing runs out of puff is about 3600.

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Post by T2 » Wed, 04 Jul 2012, 06:48

Hmm.. 3600 rpm is already fairly high feeding into a 4.1 : 1 rear diff.
Even so I would feel that your motors can be made to perform well even approaching 4500 rpm. It's possible that maybe one motor could be encouraged to shed enough voltage to enliven the system at the top end. I am sure you may have considered the Lo Tech trick of shunting one or even both of the series fields with a low value resistor in order to raise base speed so that more of the useful power band is moved to the upper reaches of the range. In a more up to date scenario a switch on the dash could initiate this at a particular speed say 3000 rpm. I also like the idea of driving in BYPASS mode on the open highway with a contactor shorting across the power switch when pedal is to the metal AND 100% mod is reached. Although for myself, I would prefer a solution that would avoid special training to new drivers wherever possible which means that more sophisticated methods in the line with what was proposed in a previous post are called for.      

Let's assume you start with control of just one field initially as I suggested with the armature controller at 100% modulation but acting as an active current limiter during the development phase.

The effect will be a decreasing armature voltage on the machine with the controlled field thus shifting more of the battery voltage towards the armature of the other motor allowing it to pull more current and allow the vehicle to accelerate more quickly. Not an optimal situation at that point but will provide an idea of whether moving the remaining series field 'out of the cold' later on is likely to be worthwhile.

I don't know, but the first problem needing solution is to get a ground referenced series field current feedback signal from an existing controller that uses a ground based switch. Possibly 95% of time current will be flowing in the freewheel diode loop which of course sits at the full battery pack voltage. I do have a discrete transistor circuit of a "current sense voltage translator", from an ancient copy of EDN magazine, I could dig out from my files if you want to explore that route, assuming you are even remotely interested in going in that direction.

OTOH I would suggest an optocoupled high side switch using N channel devices to be preferable and avoid the logisys altogether. I have never done this, so this is purely a suggestion that needs introspection on your part. At least with a low duty cycle thermal considerations should be the least of your worries here but it does put you more into a pcb design.

I can try to put up an image of my circuit board schematic if you want to investigate further.

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Post by Nevilleh » Fri, 06 Jul 2012, 14:34

Those rpms, with the 4.1 diff and the wheel and tyre size I have give a speed of 102 kph. I note that when the battery has just been charged and the voltage is up over 150, then I can get up to 110 kph which is going to be nearly 3900 rpm. When I had the two motors in parallel, the car would reach 130 kph and that means about 4600 rpm. My thinking since day one has been to just switch the motors from series to parallel above 80 kph, but I haven't bothered because 99% of my driving is in 50,60 and 70 zones. Also, with two motors the number of contactors needed to do anything at all is quite alarming.
I hadn't considered field weakening at all, even via the brute force method, but your idea of using a separate controller for the field current is interesting.
If I understand you correctly, you apply max current to the armature and then control the speed by varying the field current? Why do it this way? Doesn't that mean the brush current is at max all the time and hence brush wear also? I wonder what the motor curves would look like with that sort of drive?
How different would it be to running the field current (current limited) at max and controlling the speed by varying the armature current?
The Cougar controller that I'm using has a Hall sensor for the current and it wouldn't be hard to use a similar scheme for the field controller, so a "ground referenced" current feedback signal is easy enough.
I'd like to look at your schematic too. But only if you educate me as to what "OTOH" means!
(I'm not big on textspeak)

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Post by woody » Fri, 06 Jul 2012, 15:29

OTOH = On the Other Hand

right up there with AFAIK and IANAL as unguessable acronyms.
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Post by Johny » Fri, 06 Jul 2012, 15:39

Industrial DC controllers use field weakening only above a certain RPM. Up to that point they control the armature. Once they hit the speed limit, the armature control is abandoned and field weakening is used - with lower limits enforced.

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Post by Nevilleh » Fri, 06 Jul 2012, 17:55

woody wrote: OTOH = On the Other Hand

right up there with AFAIK and IANAL as unguessable acronyms.


Time was, everyone knew you were an expert because your language was liberally sprinkled with TLAs, but now you have to use ETLAs or you are a nobody.
Thanks Woody.

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Post by Nevilleh » Fri, 06 Jul 2012, 20:00

Johny wrote: Industrial DC controllers use field weakening only above a certain RPM. Up to that point they control the armature. Once they hit the speed limit, the armature control is abandoned and field weakening is used - with lower limits enforced.


So two controllers would keep the armature and field currents the same up to the max speed, then the field current would reduce? My two controllers would have to go to 500A, then further throttle opening would cause the field controller to reduce down to maybe 300A? Not sure how to organise a throttle pot to do that!

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Post by Johny » Fri, 06 Jul 2012, 20:35

Unfortunately, separately controlling the field of a series DC motor would be very lossy. Any system I can think of involves big contactors.
Remember that as the armature current reduces with speed increase, so does the field current. Series wound motor inherently do field weakening.
So Im not sure how much you would gain by controlling the fields separately anyway.

The next step really is to switch series/parallel. You know that, you're just trying to avoid it. Image

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Post by Nevilleh » Sat, 07 Jul 2012, 14:15

Yes Johny, you are quite right! I even have some spare contactors that I bought ages ago for that very purpose.

One of these days, maybe when the weather warms up and I don't have to freeze my extremities off working on the car.

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Post by T2 » Thu, 12 Jul 2012, 07:49

OTOH = On the...oh never mind

As you probably can guess, I had a whole post lined up which somehow got deleted, I should have saved it in Wordpad.

I am leaning towards a Lo tech solution in keeping with the maxim "Extreme Simplicity is an Essential Basis of EV Development".

At present I expect your existing circuit shows the two motor armatures connected in series but intertwined with reversing contactors. This leaves the two series field windings connected also in series at the other end of the string. The suggestion, made earlier, is to have these two field windings bridged by a low value resistor in series with a bypass contactor. A power resistor of some 10 - 20 milli-ohms with a 200W rating is required. You need to shunt 50~100 Amps from the series field. My knowledge of a series field ( a real life series motor not the fictitious idealized model they feed you in college/university) is that the field saturates fairly early so you need to divert quite a bit of current before it will make a noticeable effect.

Again it would be probably safer to bridge only the one field at the beginning to gauge the effect. Armature inductance is more than equal to that of the series fields so I do not expect switchmode behaviour within the controller to be affected by the sudden reduction in circuit inductance that the bridging will cause.
Of course I realise that it won't be the hard part to form this circuit.
The hard part is going to be finding something appropriate to form the resistor.
Hopefully going this route will save the onerous construction of a series field controller.
    
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Post by Nevilleh » Thu, 12 Jul 2012, 12:59

Yes, the present circuit is two motors connected in series ie field, armature, field, armature. The reversing contactors are connected to one motor only with a single contactor connected to short out the entire other motor when reverse is selected. Obviously it has only half the torque in reverse. It's done like this because the rear motor is very hard to get at and in the original parallel connection I simply disconnected the rear motor in reverse. Saves on contactors too!
At present, the toss-up is whether to switch to parallel motors at high speed - which I can do with the addition of one SPST contactor - or to try field weakening as has been suggested. Again, it would only be on the front motor.
In the original configuration with a 1000A controller and parallel motors, the starting torque was the same as now, as you'd expect because I had 500A flowing through each motor, and the top speed was some 130 kph as each motor saw the full battery voltage. With this 500A controller and series motors, the stress on the battery is halved, acceleration is good up to about 85 kph and top speed 100 - 110 kph depending on battery voltage ie top speed drops a bit as the charge level drops.
One advantage of field weakening to increase top speed would be that the battery stress is kept at the low level and this is not an insignificant one as I am finding that the battery voltage is dropping more now under load than it did a year or so ago. This may be due to some oxidation getting into my Cu link straps, but it is still a consideration. I hope it is not due to cell deterioration already!
I suppose that most of the voltage appears across the armature due to back emf and the drop across the field winding is pretty much due to that winding's dc resistance as its inductance is quite tiny, so a small value, high wattage resistor is needed. You are suggesting that the average V across the field is of the order of a volt or so.
I wonder where to find a suitable resistor?
It will be easy enough to try out if I can find such a thing.

OTOH it might not be! ONM....

BTW does anyone know what the field and armature resistance and inductance are for the L91 motor?
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Post by T2 » Thu, 12 Jul 2012, 20:05

It seems to me that your first job is to rewire your main circuit.

I would retire any plans to parallel armatures until the reason for suspected cell deterioration has been diagnosed.

My suggestion is to wire the two armatures directly to the controller output with no contactors in that path. In line with them is the series field circuit. The two series fields should be hooked up together in series of course. This allows both fields to be reversed at the same time, using the same reversing contactor as I am sure you are aware.

This will help reduce the peak current drawn on your battery pack for reverse. The ammeter shunt probably completes the circuit to the B+ ve terminal. I would try to keep armatures outside circuit switching since they are active generators compared to series fields which can be viewed as passive devices.

Finally for the resistor you might want to try shunting a silicon diode across a single series field to gauge its effect. A 200 amp diode mounted on a heatsink and thereby limiting the voltage drop across the series field to 600mV might provide something close to the field weakening you are looking for.
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Post by weber » Thu, 12 Jul 2012, 20:14

Nevilleh wrote:... I am finding that the battery voltage is dropping more now under load than it did a year or so ago. This may be due to some oxidation getting into my Cu link straps, but it is still a consideration. I hope it is not due to cell deterioration already
Surely you're not saying that your superior BMS can't tell you the voltage drops across your Cu link straps? Image

Not just cold weather?
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Post by coulomb » Thu, 12 Jul 2012, 22:30

T2 wrote: My suggestion is to wire the two armatures directly to the controller output with no contactors in that path. In line with them is the series field circuit. The two series fields should be hooked up together in series of course. This allows both fields to be reversed at the same time, using the same reversing contactor as I am sure you are aware.
I'm not seeing the detail of this without a diagram. I'd just add that I'd be very nervous about any series DC setup where you could get armature current without field current. Having the armature in series with the field guarantees that.
Finally for the resistor you might want to try shunting a silicon diode across a single series field to gauge its effect. A 200 amp diode mounted on a heatsink and thereby limiting the voltage drop across the series field to 600mV might ...

Might explode, I'd suggest. A diode conducting a lot of current has at least 700 mV across it, possibly over a volt. If you have 500-1000 amps through the field, I don't think the 200 A diode could be guaranteed of getting at most 200 A, and even if it did, as you said yourself, you'd want to make a big difference in the current to get the field out of saturation, and I don't think that 200 A would do it for much of the time.

In short, I think you'd need a much bigger diode, so it might be easier and cheaper to use a suitable resistor, which could be a length of suitably chosen cable. The resistor's share of the current will be more uniform with load (compared to the diode), which I think would work better.

[ Edit: over an amp -> over a volt. I think I'm getting old. Image ]
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Post by Nevilleh » Thu, 12 Jul 2012, 22:38

weber wrote:
Nevilleh wrote:... I am finding that the battery voltage is dropping more now under load than it did a year or so ago. This may be due to some oxidation getting into my Cu link straps, but it is still a consideration. I hope it is not due to cell deterioration already
Surely you're not saying that your superior BMS can't tell you the voltage drops across your Cu link straps? Image

Not just cold weather?


Yeah, could be. My superior bms now shows about a 7 volt drop ie the bms reports about 7 volts more than the battery voltmeter shows whereas it used to be only about 5 volts. The extra drop I surmise is in the cell interconnections. That's at full load of some 500A of course. The cell temperature sensors don't show any marked difference though. I'm just ignoring it for the moment, wait and see if it gets worse/better/goes away - unlikely! Individual cell voltages still about the same, which is why I'm thinking oxidation i n the joints. Sort of like arthritis?

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Post by Nevilleh » Thu, 12 Jul 2012, 22:45

T2 wrote: It seems to me that your first job is to rewire your main circuit.

I would retire any plans to parallel armatures until the reason for suspected cell deterioration has been diagnosed.

My suggestion is to wire the two armatures directly to the controller output with no contactors in that path. In line with them is the series field circuit. The two series fields should be hooked up together in series of course. This allows both fields to be reversed at the same time, using the same reversing contactor as I am sure you are aware.

This will help reduce the peak current drawn on your battery pack for reverse. The ammeter shunt probably completes the circuit to the B+ ve terminal. I would try to keep armatures outside circuit switching since they are active generators compared to series fields which can be viewed as passive devices.

Finally for the resistor you might want to try shunting a silicon diode across a single series field to gauge its effect. A 200 amp diode mounted on a heatsink and thereby limiting the voltage drop across the series field to 600mV might provide something close to the field weakening you are looking for.


Yes, but not easy to do as access to the rear motor requires me to disconnect the drive shaft and drop the whole motor mounting frame down out of the car. So I'll leave it as is. I don't need to reverse both fields, one motor is adequate for reverse.
There is no ammeter shunt, I am a Hall Effect sensor devotee.
The diode idea might be worth consideration, I'll see what I have lying around! (I bought quite a few when re-building my LogiSystems controller).
You can't wire the fields in series with the armatures without including a reversing contactor somewhere!

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Post by Johny » Fri, 13 Jul 2012, 05:13

I would be concerned about wasting power to get a higher top speed. That, and I have never seen anyone shut a series motor field before. Sounds like a recipe for a big bang.

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Post by Nevilleh » Fri, 13 Jul 2012, 11:21

Johny wrote: I would be concerned about wasting power to get a higher top speed. That, and I have never seen anyone shut a series motor field before. Sounds like a recipe for a big bang.


I don't know, field weakening is a well accepted technique for increasing the speed of a series wound motor. It makes sense when you think about it. If you reduce the field strength, the back emf produced by the armature must also reduce, so the speed of rotation can increase until the back emf again matches the supply voltage.
If you had say 500A flowing in the field winding and you shunt a couple of hundred A through a resistor, the total current is still the same so you aren't actually wasting any power, just diverting it from the field to the resistor.
I've read a bit about it in the last day or so and it seems a perfectly logical thing to me. If they can do it with the motors in electric trains and not blow up, I should be able to do it in my car! With no blowing up, that is. Image

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Post by weber » Fri, 13 Jul 2012, 14:20

Nevilleh wrote:If you had say 500A flowing in the field winding and you shunt a couple of hundred A through a resistor, the total current is still the same so you aren't actually wasting any power, just diverting it from the field to the resistor.
To "waste power" generally means to convert it from one form to another where the final form is of no use to anyone. So unless you're going to warm your feet on that resistor, I think Johny is correct.
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Post by Nevilleh » Fri, 13 Jul 2012, 14:29

Yes, but if the field is saturated at say 300A, then the additional 200A is wasted regardless and diverting it to a resistor is a difference that makes no difference.
The difficulty here is to discover at what current the field is saturated. And I have no real idea at this stage, can't even find any motor specs that give inductance and resistance.
And I could say that the power in the resistor enables a higher top speed, so it isn't wasted!
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Post by Johny » Fri, 13 Jul 2012, 14:52

It seems to me that the only reasonably safe way forward would be to measure the field DC resistance. Shunt with the same value of resistance thereby halving the field current. That should have a noticable effect.

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Post by Nevilleh » Fri, 13 Jul 2012, 16:45

Johny wrote: It seems to me that the only reasonably safe way forward would be to measure the field DC resistance. Shunt with the same value of resistance thereby halving the field current. That should have a noticable effect.


Yes, I had thought that would be a good starting point, although I'm inclined to use an R value of about triple the field initially. Easy to reduce a big, fat wire wound resistor than to increase it!
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Post by T2 » Fri, 13 Jul 2012, 17:44

I wrote :
Finally for the resistor you might want to try shunting a silicon diode across a single series field to gauge its effect. A 200 amp diode mounted on a heatsink and thereby limiting the voltage drop across the series field to 600mV might provide something close to the field weakening you are looking for.

and I have never seen anyone shunt a series motor field before

- Johny, I keep telling ya' " ya' gotta' get around more".


- Nevilleh, Scrub the diode idea I just read on this site about someone using an unravelled coat hanger (40milliohm) as a load resistor for the 30 second test of a 75Ahr 12volt Pb-acid at its 3C discharge rate.

-Nevilleh earlier you wrote:
"Yes, the present circuit is two motors connected in series ie field, armature, field, armature. The reversing contactors are connected to one motor only with a single contactor connected to short out the entire other motor when reverse is selected. Obviously it has only half the torque in reverse. It's done like this because the rear motor is very hard to get at and in the original parallel connection I simply disconnected the rear motor in reverse. Saves on contactors too !"

And I replied I would try to keep armatures outside circuit switching since they are active generators compared to series fields which can be viewed as passive devices.

Yes, it's difficult from where I stand to appreciate the amount of work involved in rewiring your armatures, and though we are suffering through the hottest heatwave in years- and the summer has hardly started - I am cognisant that it can be freezing cold where you are now and probably not the best time of the year to be working outside. However here is something new and depressing to think about - shorting out the series field (and armature, I assume) of the unused motor during the reverse procedure will not remove the remanent flux in that machine. When its shaft is rotated this machine will generate a voltage the effect of which will be to circulate a current through its own series field (if the shorting contactor is across them both) and generally oppose the torque of the motor which has been reversed.

That's one reason why I wrote :
I would try to keep armatures outside of circuit switching since they are active generators compared to series fields which can be viewed as passive devices.

As a safety measure I would strongly recommend you group the armatures together at the next opportunity and move both series fields inside the reversing contactor. There is no good reason you would not take advantage of having double the torque if it is no electrical problem to do so.

The speed boost contactor should connect a selected length of that coat hanger wire across either or both of the fields.

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