One VFD two motors

leitmotif

Member
Join Date
Nov 2004
Location
Seattle Wa. USA
Posts
3,680
Want to use 3 phase motors to drive vehicle. One motor per wheel, each is independent of other except they are coupled by the road of course.

PROBLEM YOu want to turn at say 15 mph and it is a maximum turn. Speed is held constant at 15 mph. Speed differance between tires is 10%. One tire is 95% of speed and other is 105%.

May not be a problem in V/Hz mode but V/Hz is not an option because of starting difficulties. Let us assume torque boost is not an option.

So we are forced to do this in a speed feedback mode.
Vector (is it? I'm losing my term here) is not an option with two motors because VFD will not know which motor to believe.
Encoder would have to be used for speed feedback.
So in cornering if the motor with encoder is 105% VFD will keep trying to slow motors and if encodr is on motor with 95% speed VFD will try to speed both up.

Am I thinking this correctly?

CONCLUSION In this application control of two motors with one VFD just not a good idea.

Dan Bentler
 
want to use 3 phase motors to drive vehicle. One motor per wheel, each is independent of other except they are coupled by the road of course.

Problem you want to turn at say 15 mph and it is a maximum turn. Speed is held constant at 15 mph. Speed differance between tires is 10%. One tire is 95% of speed and other is 105%.

May not be a problem in v/hz mode but v/hz is not an option because of starting difficulties. Let us assume torque boost is not an option.

So we are forced to do this in a speed feedback mode.
Vector (is it? I'm losing my term here) is not an option with two motors because vfd will not know which motor to believe.
Encoder would have to be used for speed feedback.
So in cornering if the motor with encoder is 105% vfd will keep trying to slow motors and if encodr is on motor with 95% speed vfd will try to speed both up.

Am i thinking this correctly?

conclusion in this application control of two motors with one vfd just not a good idea.

dan bentler

+1
 
I forgot about this and should have asked.
What if encoder measures at another source that is completely independent of differing wheel RPM while in turn. Ie the encoder is measuring 15 RPM before during the turn and afterward - vehicle speed was constant and VFD speed input and speed feedback were all constant.

First off I am gonna have to think how to get a speed signal independent of the tires but let us assume I have it and it is "perfect"

Dan Bentler
 
I think i would explore the concept of a drive per motor, and a differential calculation. The NEED for difference between the two speed references to the two drives would first be "sensed" by a potentiometer on the steering linkage....converted to real world response with mathematics that include a calibrating multiplier, and calibrated to the exact vehicle. The forward speed "total" would be sensed by the gas pedal pot. Two analog inputs to your plc ("total vehicle speed request", and "differential request"), some linear conversions for the calibrated ratios....aaannnddd Whamo!!! The rear wheels of your truck actually HELP the steering achieve its objective....

:D

Stationmaster
 
Well I was pipe dreaming and trying for the simplest setup - sort of what if and could I?

It just may take two VFDs with speed input signals varying by steering wheel position.

Full independent wheels sounds neat but once again no free lunch.

Dan
 
Now you've got me thinking "traction control" and 4 X 4 ..... (y)


Uh oh have I created a monster by sharing crazy pipe dreams?

Have been thinking full independent 4WD with 3 phase. In addition all wheels can rotate 90 degrees to enable parking in those tight spaces. Worked at a place where material handling equipment could do that and turn a square corner.

Dan
 
Want to use 3 phase motors to drive vehicle. One motor per wheel, each is independent of other except they are coupled by the road of course.

PROBLEM YOu want to turn at say 15 mph and it is a maximum turn. Speed is held constant at 15 mph. Speed differance between tires is 10%. One tire is 95% of speed and other is 105%.

Uh oh have I created a monster by sharing crazy pipe dreams?

Have been thinking full independent 4WD with 3 phase. In addition all wheels can rotate 90 degrees to enable parking in those tight spaces. Worked at a place where material handling equipment could do that and turn a square corner.

Dan

Dan are you sure that those material handling vehicles are using speed control or are they using torque control?

When I envision how a conventional vehicle is controlled (presuming no gear ratio changes). It's done by torque control and wind resistance plus elevation difference.
 
Dan are you sure that those material handling vehicles are using speed control or are they using torque control?

When I envision how a conventional vehicle is controlled (presuming no gear ratio changes). It's done by torque control and wind resistance plus elevation difference.

Goody I will sort of agree with you in that torque demand on a vehicle is constant on a flat level road
PLUS you have to add wind resistance - can almost ignore at 35mph or less. On a grade rolling resistance increases dramatically.

I think I should not have mentioned the material handling equipment. For purposes of this discussion assume a standard pickup truck. Each rear wheel is driven by an independent motor.

Let us play with some numbers - chosen mostly to make the math as simple as possible. Mostly close to reality but not exact.

At 60 on flat ground
DISREGARD wind resistance
ASSUME perfect traction thus tires cannot slip

Total torque demand of vehicle is 60 ft lb. Cars are a constant torque load.
Wheel RPM is 900
Each wheel is 30 ft lb torque demand
Drive motors (two) are 8 pole 3 phase not coupled except for pavement
Synch or baseline speed is 900 RPM
Each motor is 5.2 HP continuous
Continuous torque per motor is 30 ft lb

OK now let us take a wheel locked turn at 15 mph 1/4 of 60 to keep math easy
ASSUME 10 % difference in wheel speed.
ASSUME Speed is kept constant at 15.
SO
Wheel RPM (straight ahead is 225 RPM
each motor is delivering 1.3 HP
Torque demand of car is still 60 ft lb

In turn
Inside wheel at 95% 213.8 Outside wheel is 236.3
Torque demand of car is still 60 ft lb (probably higher due to wheels turned but to keep simple we will ignore it)

What I do not know is whether torque demand at each wheel changes (inversely) to speed OR does HP change inversely with wheel RPM?

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
As an aside I intend to convert a 56 Chev pickup. Engine and transmission will be removed motor will couple direct to driveshaft. Have Leeson 3 phase 230 VAC 2 pole motor (surplus at $230) and ABB 40 HP 230 VAC drive (bit undersized I know) Rather make mistakes with a 40 HP drive at only $450.

To answer your question whether torque demand drive or speed demand right now I do not think I know enough to give a good answer. I THINK speed demand.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
I am beginning to think that independent drive motors (maybe even 4WD) is a great concept in dirt and at 20 mph or less. On pavement not so simple after all.

Dan Bentler
 
I am still trying to get my head around this one.
Why can't you have a lay shaft between the wheels and only one motor?

As the VSD monitors the motor currents and controls it by that - two motors can not give the correct info back to the drive.

Unless you use two vsd's and feed the dc Bus to the other Vsd maybe....

encoders are only going to feed back the info bt 1 vsd will not know what to do with that - which one needs to go faster and How does it control that?
 
I think torque mode would work, but the torque reference may need to have a non-linear relationship to the velocity drive command at low speeds.

I would go for solid axles and LS diffs, if I wanted quieter cornering, and had the option.
 
Hi!

Take a look on this NemaD motor torque curves! These might work, as the slip would compensate for the speed differences.

Kalle

The cutout is from this Siemens link:
http://sea.siemens.com/step/pdfs/ac_motors.pdf

I had to program 1336 Impact drives to run nema d motors in about 2000. It was a PITA to get everything working just right, and our motors were so old and had so many rewinds, each was more or less custom tuned. This was also the early days of Force Vector control for them, I do believe.

Those motors could take a lot of "abuse of duty cycle" We clutched in tough loads quickly and frequently, reverse direction sometimes, lots of starts and stops they were stout little motors, lots of umph, and jerk (axle snapping) power. pay attention to size rqmt changing in your speed range for the application.
 
Hi!

Take a look on this NemaD motor torque curves! These might work, as the slip would compensate for the speed differences.

Kalle

The cutout is from this Siemens link:
http://sea.siemens.com/step/pdfs/ac_motors.pdf

I had to program 1336 Impact drives to run nema d motors in about 2000. It was a PITA to get everything working just right, and our motors were so old and had so many rewinds, each was more or less custom tuned. This was also the early days of Force Vector control for them, I do believe.

Those motors could take a lot of "abuse of duty cycle" We clutched in tough loads quickly and frequently, reverse direction sometimes, lots of starts and stops they were stout little motors, lots of umph, and jerk (axle snapping) power. If you are upgrading an existing AVG pay attention to size rqmt changing in your speed range for the application.

If this is for your home EV, find the July issue of Petersen's 4Wheel Drive and Offroad magazine in print,. It is an excellent magazine to have under the phart fans at home It's aimed at off roaders, but the suspension tech articles, low buck how to-s, etc make it a great bargain...here's the editorial from that issue:
http://www.4wheeloffroad.com/departments/4xforward/131_1107_july_2011_4xforward_editorial/index.html

Worthwhile subscription, even if you don't off-road...tech-heavy mag...
http://www.4wheeloffroad.com/index.html[/soapbox]
 
Last edited:
This discussion got me thinking about motor efficiency which is very important in a battery supplied system. Why use one motor per wheel operating over a broad RPM range when you could couple an electric motor to the transmission that would give you maximum efficiency at your target speed? This would solve allot of your issues I think.

Shawn
 

Similar Topics

How many motors we operate from single drive? I have 4 motors of 1hp, all motors have same function. Is it possible to run all 4 motors from...
Replies
16
Views
5,158
I have 3 waist High Conveyors in a line that each have a 1.5 HP 460 motor each has a 140M-C2E Breaker/MOL & a contactor. Can I put a single VFD...
Replies
9
Views
3,578
Here is the situation: I want a VFD to control a Fan motor - but I have a redundant fan that should not be used unless I am servicing the main...
Replies
23
Views
8,489
Hey guys , I am involved in a equipment installation process wherein the customer wants me to run multiple motors with a single VFD. I...
Replies
3
Views
2,020
I have been looking at a couple of power tools that have a 220v 3 phase motor. I was wanting to avoid having to install a rotary converter. But I...
Replies
7
Views
2,133
Back
Top Bottom