Inertia Matching with TWO or more motors

[hp13]

Been trying to find an answer to this all day. Would really appreciate any help!

Assuming the electric motors are perfectly synced-up, would using multiple motors effectively split the inertia loads evenly among the 2/3/4 motors?

Correct me if I'm wrong, but with the equation 1/2Mr^2 for inertia loads, each motor is essentially responsible for moving half the mass (or 1/3, 1/4 and so on and so forth), it seems the inertia would logically also be cut by the same ratio.

Thanks in advanced for any help!

 

[ScusaMe]

Might help if you defined the mechanical load or loads and how the motor or motors are mechanically connected to them.

 

[hp13]

The system is a basically a cart/car mounted on a rail with 4 wheels. The motors would be directly mounted on each separate wheel. Car would weigh about 70lbs depending on the number of motors used.

 

[waross]

Syncing up is only an issue with synchronous motors. Most motors will share the torque and by inference share the inertial load.
AC? DC? induction? universal? I'm not sure about brushless DC. A brushless DC is a type of synchronous motor and may have syncing issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hp13]

Thanks for the insight waross! I am currently looking at brushless DC motors, but no designs have been finalized and I am just looking at possible options. Inertia just seems to be a problem since I am hoping to get the cart up to speed rather quickly and top out at a high velocity (minimum 10ft/sec, with 2.3" wheels). Which means the motor (24VDC-48VDC) will need to run at 1000+RPM. Gearing can easily solve the inertia problem, but the motors and gearing I have found so far brings the output RPM to 800

Here is an example of what I was looking at: (

http://catalog.orientalmotor.com/it...&origin=keyword&by=prod&filter=0&pcat=mypage)

I'm hoping to make minimum changes to the requirements (10ft/s, 24-48VDC, 2.3" wheels) and find a solution. Inertia is just a pain because it seems that testing needs to be done to see if it will work or not.

 

[MiketheEngineer]

What if one wheel slips or looses traction??

 

[ScusaMe]

Or the motors don't have IDENTICAL speed-torque curves ?

Or the loads at each axle aren't IDENTICAL ?

 

[racookpe1978]

Straight line motion, or does the cart run on turning tracks so even speed needs to change on the 4 wheels? Can you use a simpler 2 motor setup with solid axles?

Loads on each wheel can never assumed to be the same: height of track will change a little bit, compression of the wheels and wear of the wheels will change over time, the position of the load itself or amount of the load, the friction in the axle bearings and between wheel and track (dirt/grime/grease/particles, etc.) will always cause a little difference in each of the four wheels.

 

[waross]

Slip motors such as the industry standard induction motor will share the torque quite well. Even different HP motors will work well together if the slip is the same.
Synchronous motors do not work and play well together. You may easily find one motor doing all the work as well as driving the other motor as a generator.
In a synchronous type motor the rotor angular position is tightly controlled. Consider two synchronous motors running side by side. A strobe light is used to compare the difference in shaft angular positions. One motor will run unloaded as a reference. As a load is applied to the second motor, the shaft will be observed to drop behind the other shaft by a few degrees. The more load the more angular displacement. Even at full load the difference is little. If the loaded motor is driven by an overhauling load, the difference will be in the opposite direction and the motor will be generating.
I have seen descriptions of synchronous motors that were solidly connected together. A micrometer adjusting device was applied to one of the couplings and the relative angle difference between the motors was adjusted to zero in order that the motors would share the load.
If you can find a driver for your brushless DC motors that will allow the frequency/speed to drop slightly as the load increases you may be successful. Brush type DC motors and induction motors driven by VFDs may be a better choice. It may be possible to drive two or more motors from the same drive.
The slight differences in load and ratio described by others will be accepted gracefully by induction motors and may be undetectable in normal operation.
The differences will be a killer if you try to use synchronous type motors even if you can get them to work under perfect conditions.
DC or induction motors; Simple control and possibly more than one motor per drive.
Brushless DC motors or other synchronous type motors; An individual, sophisticated control for each motor.
I don't know if suitable controls are available off the shelf or if you will have to develop a circuit to monitor the load and drop the speed set-point slightly as the load increases.
Good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter