One VFD two motors

[iant]

If you have a VSD supplying two motors, How would it know that there were two.
The 3 phase load would be presumed to be one motor and the drive would be sized accordingly.
Fluctuations would not be seen.

The alternative could be two Sevro motors and one controller.
These are now competitive in price
Then problem is solved

 

[leitmotif]

Iant
I think you are asking the same question I started with.
Especially so if you define servo motor as one with a speed feedback (or maybe anotehr feedback signal)

I dont have a good grasp of torque mode on VFD setups. Anybody refer me to good reading / study material please?

Dan Bentler

 

[iant]

yes Dan,
they are integral - function like a stepper motor
so do not use a VFD but a specific controller
lookup SEW eurodrive, Belkin, Omron, Mits, siemens simodrv etc
most manufacturers have them available.

 

[rdrast]

Why not just use a single motor, and a differential on the axle?

Put the encoder on the motor, and don't worry about the speed of the wheels individually.

 

[leitmotif]

Motor direct couple to driveline and standard rear axle is starting to look real good. That has been the plan all along.

The independent motors concept was a what if - wouldnt it be neat type of idea. Not completely out of the picture but now limited to realistic 4WD needs ie dirt, mud, snow and 20 mph or less.

Dan Bentler

 

[iant]

Eery trolly systemI have been involved with has had 1 motor and the wheels were on rail tracks - no problem

the other systems were AGV's totally different kettle of fish

 

[DickDV]

Just caught up with your post, Dan, but, it seems to me that using a single motor and a differential would be very compelling. However, with respect to the use of two motors on a single drive, I think the key to doing this is NOT tight speed control but rather unusually loose speed control.

So, with that, servos, encoders synchronous motors all go away. I think the suggestion of two 8% slip NEMA D motors on a single V/Hz drive makes the control issues all go away. You should be able to make turns successfully. The only thing that would happen on a turn is the inside motor would load more and the outside motor would load less or even regenerate. Actually, if the speed difference between the two motors on the tightest turn is 8%, you should be able to use 4% slip motors. With them the inside motor would go to 4% negative slip (full motoring) and the outside motor would go to 4% positive slip (full regenerative). The drive would only see the net between the two as it should be. You will need a regen drive anyway to deal with braking so any net imbalance would not be a problem.

Keeping a project like this simple is also a good way to make it cheaper and maintainable.

The only disavantage I see with the high slip motors is their slightly poorer inefficiency when both are motoring. But, in the interests of keeping the project manageable, I would make that tradeoff.

 

[OkiePC]

Motor direct couple to driveline and standard rear axle is starting to look real good. That has been the plan all along.

The independent motors concept was a what if - wouldnt it be neat type of idea. Not completely out of the picture but now limited to realistic 4WD needs ie dirt, mud, snow and 20 mph or less.

Dan Bentler

At such low speeds, I would skip the tranny and couple straight to the transfer case. One motor where your tranny used to be (the truck, right?)
One big bad VFD under the hood, with remote keypad on the dash, and optima yellow top batteries arranged alonside the drive to look like massive V8 cylinders, and the rest of the cheap industrial batteries hidden under the wood bed and bed sides. You could even make some cable raceways that looked like headers connecting the two battery arrays, and the connections to the DC bus would look like fuel lines or intake passages...I still have a picture of the blue truck...don't make me break out paint.net and give you some visual inspiration...

No need for a transfer case or driveshaft, really, unless you gotta front axle to drive and think you might want Hi range for some 40mph trips across town and back, but then you're looking at suspending a lot more weight if you skip the drive-shaft too. I would lean toward a clutch + 2 speed transfer case and keep the driveshaft to diff complete with pinion geometry that already works...

One VFD, One Motor, One Encoder, tight control, smoother regen braking.

 

[OkiePC]

At such low speeds, I would skip the tranny and couple straight to the transfer case. One motor where your tranny used to be (the truck, right?)
One big bad VFD under the hood, with remote keypad on the dash, and optima yellow top batteries arranged alonside the drive to look like massive V8 cylinders, and the rest of the cheap industrial batteries hidden under the wood bed and bed sides. You could even make some cable raceways that looked like headers connecting the two battery arrays, and the connections to the DC bus would look like fuel lines or intake passages...I still have a picture of the blue truck...don't make me break out paint.net and give you some visual inspiration...

No need for a transfer case or driveshaft, really, unless you gotta front axle to drive and think you might want Hi range for some 40mph trips across town and back, but then you're looking at suspending a lot more weight if you skip the drive-shaft too. I would lean toward a clutch + 2 speed transfer case and keep the driveshaft to diff complete with pinion geometry that already works...

One VFD, One Motor, One Encoder, tight control, smoother regen braking.

All the rotating mass leading up to the clutch will give you the ability to bark the tires too, increasing the peak torque capacity albeit very briefly. So the transfer case will become the piece that must be beefed up to deal with full torque at zero speed. I have a friend with an electric Miata, and a Prelude, both have snapped axles/trans shafts/etc.

Yes, 12 pole servo motors at each wheel with right angle gearboxes, positioner with automatic traction control would be sweet, with no limitations on steering synchronization, or suspensions dependendce plus 4wd, but that is a big budget project there buddy.

Skip the encoder, use the ABB sensorless mode (I finally saw your long post about the truck)...add it later when you are trying to get to 100 mile range...

 

[rdrast]

I suppose, one thing you might possibly do, if you must use two motors would be this:

Basics:
-- Both motors have encoders
-- One drive is the speed master, and runs in speed mode
-- Second drive runs in pure torque control

Now, configure the speed drive to generate it's total torque command to an analog output (directly after the speed regulator). Send that directly to the torque command of the slave drive.

This is cake to do with DC drives, and possible with most intelligent AC drives. There will be overall speed differences between inside and outside (from the speed axle's point of view) cornering, but it should act pretty much as you hope, even to the point of spinning one wheel out wildly if one is on pavement, and one is on slick ice.

 

[leitmotif]

Gentlemen

Thank you all for advice.
I think we can put this one to rest.

Only one question left unanswered
Anyone have any good referance material for me to read up on re torque control?

Dan Bentler

 

[rdrast]

Dan, I don't have any reading materials, but what do you want to know about torque control? It is a bit more complicated then with a DC system, but boils down to essentially the same thing where you just control the current in the armature, and don't care about the speed.

 

[leitmotif]

Dan, I don't have any reading materials, but what do you want to know about torque control? It is a bit more complicated then with a DC system, but boils down to essentially the same thing where you just control the current in the armature, and don't care about the speed.

The question came up in another web site for electric cars. I was not sure if it was really a good application and wondered about speed control - after all I never heard of a cop giving a torque ticket but for sure know that lack of speed control may get a speeding ticket.

What little I know about torque control mode I picked up here and understand it is commonly used in winding operations to maintain tension on material being wound.

Dan Bentler

 

[rdrast]

The question came up in another web site for electric cars. I was not sure if it was really a good application and wondered about speed control - after all I never heard of a cop giving a torque ticket but for sure know that lack of speed control may get a speeding ticket.

Dan Bentler

Well, you make that point in an interesting fashion, BUT, you are forgetting, that with a vehicle, there is always an outer loop... Either the driver, or a cruise control device which takes actual speed feedback from the driveshaft.

Even in IC engines, you really don't control the engine speed while you drive, you control its torque.

 

[leitmotif]

Dan, I don't have any reading materials, but what do you want to know about torque control? It is a bit more complicated then with a DC system, but boils down to essentially the same thing where you just control the current in the armature, and don't care about the speed.

BUT, you are forgetting, that with a vehicle, there is always an outer loop... Either the driver, or a cruise control device which takes actual speed feedback from the driveshaft.

OK so getting to vehicle with VFD drive
When I step on throttle increasing speed command VFD raises freq, motor torque rises to exceed torque demand of load thus speeding vehicle (load) up.

SO is torqe control of a drive accomplished by sensing say tension in a winding operation by say sensing a dancer position and converting that to voltage which is in essance a speed command signal? Is that all there was to torque control uhhhh greatly oversimplified??

Dan Bentler