Controlling separate DC traction motors

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Glenn Brooks
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Controlling separate DC traction motors

Post by Glenn Brooks »

Hello all,

Iam on to the next stage in planning my old yard goat center cab electric train conversion.

Wondering how to synchronize two separate DC series wound motors and controllers to operate with one speed control/throttle? Can anybody recommend the best way to wire two separate DC series wound motors(Iam using Curtis 1209 b controllers) so they both produce the same torque/speed with one throttle? and possibly a wiring diagram??

Note: each motor will power one truck.

Note: Curtis publishes a diagram with one motor and controller, but doesn’t seem to reference how to control two separate motors with one speed control...

Thanks
Glenn
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tornitore45
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Re: Controlling separate DC traction motors

Post by tornitore45 »

Just a stab. The current controls the torque. So you control one motor and sense the current. Then make a feedback loop that "tweaks" the second controller (the pot or the reference) to "mirror" the current. The loop sense the error between the 2 current sensor outputs and react to adjust one motor drive to match the other.
Do not think you will find all of what you need, some electronic circuit design and construction will be required.
The term mirror has a specific meaning in electronic but is not what I intend here.
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Re: Controlling separate DC traction motors

Post by Wichita Narrow Gauge »

What you are attempting to do is referred to as Drive Slaving
You want the long answer? $$$$$$$$
Or the cheep answer? less $

I would first try the cheep answer, you can always spend "ALL YOUR MONEY" later

this "ONLY WORKS" if
1) both motors are the same MFG, the same model number and both are in the same condition
2) the drives are the same MFG, model #, same age and (tuned) the same
3) the load and gear ratio are the same for both motors


All the DC, AC VFW Drives have a low voltage "Velocity Control Signal" input
Typically three wires. One Voltage 5-12 Volts DC, One common and the signal
The three wires connect to the Potentiometer on the control panel
If you install a two deck Potentiometer of the same value and taper, "one wired to each drive" you now have a separate electronic circuit that is mechanical coupled between the two drive controllers
I would install a trimmer Potentiometer in one of the circuits just for tuning
You will never achieve a 100% Slaving condition wit this method but it may just work
rkcarguy
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Re: Controlling separate DC traction motors

Post by rkcarguy »

Glenn,

As Wichita mentioned, the motors need to be well "matched" to do this. I have experience with this on a smaller scale (RC car racing plus some play with R&D for the same), and it often meant picking through several motors and dyno testing them to get a pair that were close enough to run as twins. The variance was startling, torque and RPM varied as much as 15% between the same motors, all how well the Chinamen wound the armature on that particular day lol.
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Re: Controlling separate DC traction motors

Post by John Hasler »

These motors are likely to match much more closely than your RC motors, especially if the RC motors were PM.

It's still a concern, though. You could sometimes have one axle slip and the other not.

The ideal arrangement would be to install a tach on each axle and have a microcomputer manage the motors via the open loop controllers you have.

Or you could put the motors in series and obtain a 96V controller...
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Re: Controlling separate DC traction motors

Post by Wichita Narrow Gauge »

As I stated above
There will always be an imbalance somewhere in the Synchronization of a Multi Motor drive system and that imbalance will be exacerbated by variations of temperature, humidity and mechanical slippage
.
The best way to address this problem is to design slippage into the system to negate the imbalance.

1) Some type of mechanical slip clutch
2) de-tuning the electronic motor drives for a softer response to the Velocity Command Signal will go a long ways to balance the system

Fortunately we are not designing Rockets, mars rover and Bullet Trains, we can live with a little instability
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Re: Controlling separate DC traction motors

Post by Steggy »

John Hasler wrote: Tue Jan 19, 2021 8:43 pmThese motors are likely to match much more closely than your RC motors, especially if the RC motors were PM.
I'll second John on that.

Wound-field DC motors, whether series, shunt or compound, can be produced to a remarkable level of consistency—far better than motors with a permanent magnet field. The process of winding armatures and stators is a precise one that has been refined by more than 140 years of motor development. It would not surprise me in the least if those two forklift motors, when simultaneously powered from the same source and subjected to the same amount of loading, will run within a fraction of a percent with each other.

In years past when I was involved with nascent efforts to develop reliable wheel slip control packages for freight locomotives, I often configured and observed data acquisition on locomotives being used as test beds. Part of that process was the use of transducers to get an accurate current flow reading for each traction motor. It was consistently noted that as long as wheel slip did not occur, all motors would draw about the same amount of current, usually within 10 to 20 amperes of each other with the loco at full throttle, an operating condition that could push motor current to as high as 1500 amperes at initial startup.

I think if anything is going to adversely affect the ability of your traction motors to be consistent in performance it is going to be the control system and cabling, not the motors themselves. In particular, power semiconductors' characteristics are affected by operating temperature, and will vary from one part to another, even those whose dice were produced from the same wafer. It's a limitation of solid state technology that has yet to be surmounted.
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Re: Controlling separate DC traction motors

Post by Glenn Brooks »

Thanks guys, the motors appear physically identical, so initially plan to go with the crossed fingers approach in this first iteration (KISS and hope for the best) and see how balanced they are in operation. Also the 1209b controller has a secondary voltage adjustment screw which will allow some minor tuning of the controller output to each motor, to help synch with the other controller. I plan to monitor power draws and measure shaft RPM’s on both motors, then decide if they need to be balanced, or further controlled, after first operation.

Here are my proposed circuits for 12v motor control and 48 v traction. Would like to ask for comments /concerns with this approach.

This is the control circuit. Curtis makes this electronic pot box for the 1209 controller. So thinking it could/should send the same signal to both controllers.
A0D095A8-931B-41C6-A472-13E27D4C9161.jpeg
Diagram below depicts the 48volt traction circuit. Wondering if it helps to have separate 400 amp fuses for each hood end?

Iam not sure how each motor will work as a branch of the primary 48v circuit.
655DFDA1-DD25-4EE4-A134-261C2E5E5F42.jpeg
Final sketch shows the complete wiring harness (minus miscellaneous gauges and lights etc)
B0B447BF-1889-4CAF-804B-9AE4B21B69B3.jpeg

This design raises a couple of questions, which I so far haven’t really found answers ... For example:

1) should I install one 800 amp primary contractor to control both motors, or, use two separate 400 amp controllers (one for each motor)

2) should I use separate forward /reverse contactors for each motor/controller? (to maintain independent drive systems for each hood end), or one for both? Does it matter?

3) can the one electronic pot box (throttle control) in this design likely control the two controllers?

4) should each hood end have its own 400 amp fuse, in addition to an 800 amp fuse at the battery box?

Thanks very much for any comments or suggestions !

Glenn
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Steggy
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Re: Controlling separate DC traction motors

Post by Steggy »

Glenn Brooks wrote: Wed Jan 20, 2021 12:54 am2) should I use separate forward /reverse contactors for each motor/controller? (to maintain independent drive systems for each hood end), or one for both? Does it matter?
If you can, I recommend you use a single reversing contactor to control both motors. Using multiple contactors opens the door to a malfunction that could result in the motors running in opposite directions, unless you can mechanically interlock them to prevent that possibility.

That aside, I'm questioning why you think each end should be independent. Both trucks are bolted to the same structure and unless one truck detaches, both are going to be applying draft force to the same frame and carbody. Hence the two motors will be mechanically as one in this arrangement. Given that, you should also treat the motors electrically as one, same as is done on a real locomotive.

If this were my project, I'd wire both motors in parallel to a single controller that has the capacity to handle the combined load. Using two controllers as you are planning to do cheerfully assumes they will be identical in performance, which I think is unlikely to be the case, especially considering the thermal characteristics of power semiconductors. Unless the controllers are very closely matched through the entire operating range, one motor is going to be working harder than the other, will run hotter than the other (which will change the motor's characteristics) and its truck will be more prone to wheelslip.
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Re: Controlling separate DC traction motors

Post by John Hasler »

> If this were my project, I'd wire both motors in parallel to a single controller that has the capacity to handle the combined load.

I'd consider putting the motors in series, but then I'd be building my own controller, too. He's got the controllers he's got.
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Re: Controlling separate DC traction motors

Post by Glenn Brooks »

BigDumbDinosaur wrote: Wed Jan 20, 2021 7:17 pm
If you can, I recommend you use a single reversing contactor to control both motors. Using multiple contactors opens the door to a malfunction that could result in the motors running in opposite directions, unless you can mechanically interlock them to prevent that possibility.

If this were my project, I'd wire both motors in parallel to a single controller that has the capacity to handle the combined load.
BDD, any idea what my current requirements would be with one reversing contractor for both motors. 1209b controllers are rated as 400 amp max output, but reportedly operate routinely in the 100-200 amp range...

800 amp reversing contactors also are rare to non existent birds, and very pricey ($2500). Hence, I assumed I would need two 400 amp contractors for the two motors.

If one 400 amp contactor could work, Id be happy to go for it. :D


John, all the initial recommendations I received about using two series wound motors basically pointed toward using one controller for each motor. At least for the two I have on hand. Apparently the PWM stuff and some new programmable models allow multiple motors. But the older style 1209b controllers basically are wired individually to each motor. basicly plug and play, given my limited knowledge set...

Glenn
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Re: Controlling separate DC traction motors

Post by John Hasler »

Putting the motors in series would be an elegant solution but it would require a single 96 V controller which you don't have so it's not really an option.

For reversing you need a single 4 pole double-throw contactor rated for 400 A. You could use one double pole double throw contactor for each motor but I agree with BigDumbDinosaur that this would be best avoided. If you do go that route I suggest using contactors with 24 V coils and connecting the coils in series. That way if one coil goes open you just can't reverse.
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