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Interesting Controller Behavior

1819 Views 14 Replies 3 Participants Last post by  Sylent Green
I gave up on building my own controller after setting a few things on fire and having a few EE's scratch their heads at my IGBT's staying on (after being asked to do so)...no matter what.

Which brings me to where I am now. In order to try to get a better understanding of things beyond the books, I purchased a few Curtis 1253 (80V) controllers and the associated software to make my system (electric motorcycle 72V nom) functional enough to finish the battery charger design--just for the challenge, I know I can buy one of them too--and I ran in to some very interesting behavior from the controller.

System worked OK, top speed about 70mph, but the motor ran hot (270F with NCT) re geared, now top speed of 45mph and motor is cooler.

Details:
Controller is designed for series wound DC motors: 80V , 350A with 600A peak.
Motor is wired shunt (actually a SepEx motor which, if I understand correctly, is more of a control strategy than a motor type)
Motor measures very close to 1 Ohm on both the field and the armature, close to 0.5 ohm when wired parallel (shunt)
This should give approx 150A max current which is very close to the measured value when operating at 100% duty cycle and max voltage from the controller measured at the motor.

The monitoring software agrees with the empirical data.

The locked rotor torque is very low, which is what was anticipated with the shunt wound setup, but it is so low that I can easily hold it still with my feet.

Here is the interesting (to me) part.

The controller--with no feedback--will not go over 10V at the motor (and the predictable 50A) also confirmed both by meter and software. When the armature is not moving

Changed a few parameters and the entire controller (with same part number) and get the same result.

I am of the opinion that if I could get max voltage, or 100% duty cycle, I would have adequate torque on starts.

I have a series wound motor on the way so I can continue testing, but what gives?

I realize a series wound motor might very well run 500A at 10V, but how does the controller "know" the armature is not moving?

Thoughts?
Please educate me.

Kendrick
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I realize a series wound motor might very well run 500A at 10V, but how does the controller "know" the armature is not moving?
I'll proffer that controller controls current, it doesn't really need to "know" anything except motor current and throttle position. It adjusts the duty cycle to move it in the right direction for the requested torque.
 

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Discussion Starter · #3 ·
I'll proffer that controller controls current, it doesn't really need to "know" anything except motor current and throttle position. It adjusts the duty cycle to move it in the right direction for the requested torque.
I agree, and the output of the controller responds to the throttle going as high as 150A, limited by the motor winding resistance and BEMF, but it does not react that way until the armature is moving.
This is easily repeatable in the shop. I have even gone as far as to get it to max duty cycle and max amps then slow the motor down with the brake.
I have to brake quite hard to get it to slow down, but once it does, the current and voltage at the motor both drop to 10V, 50A, ( according to software monitor)15% pwm.

The controller appears to be responding internally to some kind of reactance from the field or armature.
It doesn't hurt anything or get hot (anywhere--wiring, connections controller or motor) it just sits there.
If I let off the throttle it goes to 0, but it stops responding to throttle opening beyond 15% until the armature starts to move.
 

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I don't really know the answer, but it is somewhat trivial to read motor voltage & bus voltage. motor voltage=0=not spinning. So some function of current and voltages and throttle easily enough, i.e. limit to 50A under 10 volts.

edit, you should hook up a 12v battery and measure current/voltage.
 

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Discussion Starter · #5 ·
I actually did hook the motor to 12V back when I got it, but I don't recall the numbers.

How would this information help me to understand my controller behavior?

I can understand the controller having built in preset limitations, but that still does not explain why the pulse width, and its associated voltage, does not increase with throttle opening (yes the throttle voltage is changing appropriately-- from 0.2V to 4.5V in a linear fashion) beyond 15% until the armature starts moving

Once the armature is moving, both the pulse width and associated output power behave as expected.

Why would a 50A limit be imposed on a controller that easily handles 350-600A?
Or a 10V limit on an 80V system?

Just curious

Kendrick
 

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Hi Kendrick,

Are you measuring battery current or motor current?
The armature resistance is most likely on the order of 10's of milliohms not one ohm. You do not get a valid reading at the terminals with an ohmmeter. Typically requires a bridge with probes on the comm.
Stall torque will suck when you wire as a shunt motor. Controller output voltage is low at launch so the shunt field is underexcited causing low flux and resulting crappy torque.

You need a series motor or a sepex controller.

Regards,

major
 

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Discussion Starter · #7 ·
Hi Major,

I am measuring the current on the negative battery cable, and was assuming Kirchoff applied here.
Limiting voltage on launch makes sense to me on a controller that is designed for series wound, but I am still amazed--fascinated--intrigued--by the controllers ability to seemingly "know" what is happening.

It is an open loop system.

Do I need to put my current sensor on the motor out leg of the controller, and will the information be different than just efficiency losses?
Total power must remain the same--no?

I just got my new (to me--read: good deal on EBay) series wound motor that is roughly the same dimensionally (slightly larger 7.5" x10"), and when I make my adapters to install it, I will have new information, and maybe a little more knowledge.

Thanks
 

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Kirrchoff's laws always apply. The motor controller is not a 2 terminal device so current in does not (but could) = current out. It is basically a buck converter. Current out is always greater than current in except when full on or off.

The current limit on motor controllers used in these large EVs is motor current. So the 50A battery current at stall could be 100's of amps in the motor circuit. Most like to monitor battery current as it relates easier to range. I like to see both battery and motor current.

Neglecting a few % loss, on the controller, power in = power out. Vb * Ib = Vm * Im. So if Vm < Vb, Im > Ib.

major
 

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Discussion Starter · #9 ·
I get it now

The behavior could be compared to that of an AC transformer (not to over simplify) from a bottom line point of view.

Cool

I too will be monitoring both currents now because I would expect to see efficiency "sweet spots" as found in many other systems.

Thank you for taking the time to explain this to me.

Now if I can figure out how to broach an internal spline for my DC series wound motor sprocket adapter, I will be up and running in a few days.

This is interesting, but I find few people in my circles that I can share my enjoyment with. They just kind of glaze over after 2 or 3 minutes of talking.

Kendrick
 

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Discussion Starter · #10 ·
Got my adapters made and mounted my series wound motor in the bike.

Wired everything and took it for a ride this AM before coming to work.

Only voltage I checked was battery to make sure it was fully charged and will follow up with detailed info after I get home and can measure some things.

Initial impressions:

Motor is smoother and quieter
No load (wheel and chain only) rpm is higher, did not go too far as I know this is a characteristic of series wound motors and did not take it past 1000 wheel rpm (approx. 75 mph) or 3000 motor rpm.
It does make more low rpm torque, and will climb a shallow hill from a dead stop with no problem.
It is not, however, very impressive.
Once rolling above 10 mph or so throttle response is very sluggish by comparison to the shunt wound setup.
Top speed appears to be lower (40 mph ish with high speed gearing), but the lower acceleration rate means I ran out of road way before it flattened out, so there is still hope.
Motor did not even get warm, no visible arcing at the comm.under any circumstances
Battery ran down quickly, but I was putting it through its paces and it is comparable to the shunt.

The adapters I made for both motors are pretty cool and I will add pics later.
Gotta work now.

Kendrick
 

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3 : 1 reduction ratio, motor to wheel?? Probably need twice that. Get a voltmeter and ammeter.

Show us what you have. Bike, battery, wiring, etc. The more we csn see, the better the chance we can help. Also, where do you have parameters set. Can you get a screen shot?

major
 

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Discussion Starter · #12 ·
I have all the things you suggest, and I will buy things that help educate. I can get a screenshot of parameters, and the bike build (more or less) is in my profile album which I have yet to find an easy way to link.
I can easily change the gearing to 4.5:1, but I will have to be creative to go further.
That is the kind of challenge I enjoy.

try this
http://www.diyelectriccar.com/forums/album.php?albumid=254

I am going to put an amp clamp on in the am and check a few spots.
Voltages too.

I will be testing my charger (version 3.2) tonight. It is a 2 stage charger with a relay for the pass element. The one(s) I built using the TI3906 chip killed the Darlington BJT every time I went to charge at the high voltage.

This is not boring

Kendrick
 

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Discussion Starter · #14 ·
New info with new motor

Locked rotor amps (1 sec) 380A @ 9V measured at the motor (actually measured the current between the field and armature, and the voltage at the controllers motor connections)

Batt V 75.8 (no load) Batt current 74A (1 sec) locked rotor load

I can see 350 400A while riding any time I go to WOT--do we have a different term for that on an EV since there is no actual throttle?

No connection or component got over 100 deg F during testing (ambient temp 87 deg F)
I have to modify my pics to attach them, they will be coming later.

Kendrick
 
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