Two identical drives are connected to a common load. What could be happen in case of difference in speed of the motors. In our case it result in breakage of shaft. so differential gear is used. Is the differential gear necessary in all the cases where two drives connected to a common load.

 

Depending on your physical arrangment and quality of your drives, you might be able to put encoders or resolvers on each shaft to syncronize the motors, eliminating the differential. This is highly dependent on the nature of the equipment.

 

>What could be happen in case of difference in >speed of the motors.

This is the key point. If the motors run at the same speed then you can direcly couple them. Getting the motors to run at the same speed can be tricky, of course.

There are lots of options, from virtually every drive manufacturer. Pick a supplier you're happy with and ask them about their solution. Don't get caught up with specifics. Servo, stepper, or vector doesn't really matter. SERCOS, SynchNet, Macro, SynchLink, FireWire, or Profibus-MC all do pretty much the same thing, and they all work. So does slaving a smart servo with a secondary encoder input, or using the traditional plus-or-minus 10V analog motion controller.

What you can't do is turn two motors on with motorstarters and expect them to run at the same speed, even if they are identical motors. You can't use a standard volts/hertz variable frequency drives, either. You need vector or servo.

-James Ingraham
Sage Automation, Inc.

 

This is an example of "load sharing". The drives are probably set up to share or balance the load between them. This has to be done anytime a rigid or coupled system exists-

 

The applications wich I've started up have one drive running in speed control and the other running in torque mode, a master follower setup. I've also started up two identicle motors with identicle power transmisions run by one ac drive. Since it is very dificult to run two drives and motors at the exact speeds. You need to do a load sharing between the two motors. ABB has a setup with the ACS600 that does this perfectly E-mail me directly if you want some help.
[email protected]
Mike

 

In most cases someone needs to spend some time tuning (pid loop)the 2 motors. The 2 motors must respond to the load symbioticly or you will possibly damage the shaft. Another situation is that you can set up torque limits or overcurrent limits that will force a responce from 1 or both of the units. For example if you exceed a 1 nm torque deviation between the 2 motors, 1 will free wheel. You can build the same response from current, velocity, position, or pretty much anything you can think of. Another key point is that on extremely touchy applications you have to worry about when the drives start and stop. Depending on which fieldbus or protocol is in use to synch the drives up a couple of millisecond delay, i.e. 1 sercos cycle could be enough to build up the unwanted loop error that could damage the shaft. Analog is a great way to do things when things are simple. Analog does not give you the best position,velocity, torque, or current resolution. I dont like using chart recorders anymore either. I guess I am spoiled. I tend to ramble sometimes but if i can be of help send me an email at [email protected]. I could go on for days but maybe I have offered enough help.

 

both drives get feed back from their individual techo.and this arrangement has been used in crane hoist operation.Now can i get come more light on this topic?

 

This looks like an application for master-follower. Decalre one drive as a master, run it in speed controlled motor. The second drive will be in torque-controlled mode, acting as a slave. The master will be parametrised so it sends a torque reference to the follower. The current controller will generate the torque required. Since both drives share the load equally, there will be no shaft breakage.
The load sharing can be done using AI-AO or in most of todays drives using fast communication links. This is a feature available with all major drive makes , AC & DC.

 

A shaft will break only if sudden braking occurs in one drive or the load. Other wise the motors should share the load or will overload one.
Differnce in speed will never happen since both motors are connected to the same load. Only the load sharing will differ and this will not break any shaft.
In any case, a good mechanical a design should shear the key or cotter pin and certainly not break the shaft.
Since you said shaft breakage, I suggest you to look into other reasons than the motors.
Best regards.
Sekar

 

Unless one or both drives are equipped with with a "slip" drive, such as a VFD, they can not be coupled, unless... well, that part of the story is in located in the AList Archives. Search for:

"Motors in tandem," or "Motors coupled to the same shaft," for analysis and experiences of this phenomenon!

Regards,
Phil Corso, PE
(Boca Raton, FL)
[[email protected]]

 

> Two identical drives are connected to a common load. What could be happen in case of difference in speed of the motors. In our case it result in breakage of shaft. so differential gear is used. Is the differential gear necessary in all the cases where two drives connected to a common load.

Answer: use torque motors and put them
electricly in series. The speed is proportional
to the current which is common to both motors.

DEC used this solution in DECTAPE years ago.

Rick
Merrill

 

I Hope reading my experience you will understand the exact reason for your failure.

I have faced a similar problem where due to differential gear box problem, Our mechanical people changed the gear box with a straight through shaft. The motors were driven by two indepenednt drives using tachometer as feedback. Though the input to the drive was same the feedback of individual drive was through the respective tachos.

Here we noticed that resticance box of a particular motor was getting frequently damaged. Even one of these motors got burnt. When we noticed the current it was seen that when the motors were put on lower notches due to higher torgue at lower speed it was observed that one motor was driving the second motor. The one of the motor was stalling and the second motor in order to increase the torqe was getting loaded. Also it was noted that once the drivewas put on the highest speed there was hardly any difference in their current and instead they were running smoothly.

Here the weak point was the resistance box and hence the same used to get burnt once the load sharing was taking palce.

Conclusion--Two Drives cannot work even if they have a common feedback for a single shaft motor. There has to be a differntial to take of these diffential speeds. Once the full load is achived one motor takes all the load while the other motor is just driven.

Here in your case the weak link is the shaft so it is ggetting broken. Best adive would be to drive both the motors using the same drive and a common feedback. This will work very well . This is how I am using my drive.