Thread Starter

Double 3

In my application I must drive two drums with two
75kw motors, and I want to use two certain frequence-converter to do this job.

In this system, two drums must synchronize almost absolutely, because there is so big friction between them that their velocity are the same. I think it's the same like this: two motors have one shaft.

But for two close-loop speed controlled motors, any speed error will promote them fight against one another, and the result will be BLOCK UP both!

Can anybody who have experience tell me how can I
resolve this problem? By the way,can you give me suggestions which frequence-converter I should select, motors also.

thanks a lot!

Steve Bailey

I've achieved close synchronization (within less than 1/4 of a motor revolution) by putting an encoder on each motor and bringing the signals into a PLC where I calculate the difference in cumulative pulses between the two and use that difference to modify the speed reference to one of the VFDs.

Neal Keakney

The best solution is to run one motor as a velocity master and the other as a velocity slave. Using the current of the second motor to adjust the ratio between to the two drives. Since the diameters of both cylinders can never be exact the two will eventually fight each other. AS the current increases, due to the diameter difference, the motion controller will recalculate the ratio to adjust the slave relative to the master.

We have used our PMAC multi axis controller to accomplish this type of control successfully.


Neal Kearney
Manager of Sales and Marketing
Delta Tau Data Systems
18410 Harborside Drive
Cornelius, NC 28031
704 895-9696
Fax 895-4903
[email protected]

Crucius, Wesley

You need a motion controller capable of master-slave control. There are many to choose from (without knowing anything about your control
hardware platform, it's impossible to make a recommendation). Put an encoder on both drums, or if you have tight mechanical coupling between
motor and drum, you may be able to use the encoder emulation provided by most modern drives. Pick one drum (either one, although there is probably a particular one that makes more sense) as the master and then the other
one is the slave. The application is similar to line shaft control, yet a little simpler because you have a 1:1 speed ratio for master and slave.
A typical control technique for two motors running one shaft is a Master controlling the speed and a Slave sharing the torque i.e. torque helper. DC drives do this very well. Any frequency drives must be vector type with
excellent torque control option. Angular synchronization may also work but this is usually to synchronize mechanically independent axes i.e. electronic lineshaft.

D. C. Pittendrigh

Hi All

Normally this type of application requires a master slave setup of the 2 drives. The master has a speed controller and the slave must be made to follow the masters current setpoint or torque setpoint depending on the type of drive, however a lot depends on the speed and acceleration required as to whether this will work, also the mechanical coupling of both drums to the load i.e. if the load does not couple the drums to each other the method doesn't work. It would then be neccessary to have 2 speed controllers one on
each drive and they have to be perfectly matched, this is a science in itself and is out of the scope of my experience however it seems that you
will not have this problem

Donald Pittendrigh
Use position feedback and a position loop on each axis and command the two loops in parallel. If one starts to lag more, the error will be greater and it will force that one to catch up in position. It is really the positions of the two drums that must be synchronized, so work on position directly.
Thomas B. Bullock, President
Bull's Eye Marketing Incorporated
Industrial Controls Consulting Division
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Johan Bengtsson

As you see hou have got two types of answers

both are reffering to a master/slave type of setup, that is the similar part, the difference is what the slave should do

some answers effectively say "electonic gearing"
"control the speeds to be the same" "control the
position to be the same"

other answers effectively say put the slave in torque control mode.

Now well, both are valid answers, but generally to
slightly different problems.

If you effectively have one axis (ie the motors are mechanically forced to have the same speed. They are mecanically connected in a way that would allow you to use one as a motor and the other as a generator (not that you want to but anyway). Then the "best" answer is the second, use torque control for the slave. Both motors
will help each other with the same torque, the master will control the speed. If the axis for example is running to slowly then the master control will increase torque, on both motors.
The first approach, electrical gearing could work, but would probably me harder to set up properly.

If you have two separate axis, that are not so tightly coupled, or not coupled at all, you have to use the first approach since the torque version will not work here at all.

I got the feeling that you have a case where slave torque mode would fit you best.

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]

milind kshirsagar

Hi all
According to me the best possible solution is
to use the master slave synchronisation and the
syncronisation must be of the position type.
That is the position of both the drives must match
exactly. You can see the B&R plc site with this
plc there are various motion cards are available
and with simple software (in ANSI C) you can do
the synchronisation.

Best Regards
Milind kshirsagar
Application Eng.
B&R India
[email protected]

G. Hossein Mazarei

A master_ slave drive system with the use of angular synchronizer may work correctly if the angular synchronizer is tuned perfectly. I think the above comments are dealing with this statement. However, cause in some circumstances one of motors may go to generator mode I think it would be better to use the same DC link for two drives. In this condition, the energy which is produced by one motor is used by another one and the whole system forbid an over voltage error. But noticed that your drives must have a very good speed control system and use correct shaft encoders in this regard.
If you wanted to use AC motors for your project, Siemens’s Master drives with the use of T300 angular synchronizer would be a good selection in this regard.
G. Hossein Mazarei
[email protected]
Yaskawa Drives have a special software built-in their AC drives for electronic gearing. This eliminates the need for an external device such as a PLC. One of your drums will be a master and the other one will follow it as of 2-3 pulses of the encoder on the master drum. If you need details off-list you can contact me.
Ahmet Dogan
[email protected]
Most all of the aforementioned information,that I have read is correct. It is evident that all of the reply's contain accurate, discriptions,as well as valuable information. I have a current application in development at this time. My choice for the solution to my motion timing was solved by Rockwell Automation Solutions (ie; Allen Bradley contrologix 5555 controller, as well as two ultra 300 servo drives, master ,salve capable of absolute encoder or resolver inputs) These units are utilized for the specific type of synchronization as you have discribed. Rockwell also has available some larger drives for the 75 Kva applications. try there website

I hope that this might be of some assitance for you.
Best Regards

The correct Equipment to use in this sort of application would have to be Control Techniques Drives.

I recommend the following Equipment:
2x 75 Kw Control Techniques Unidrive
2x 1024 PPR CT Encoders ( For POSITION Feedback)
2x 2nd Encoder Module

Now to do this application is very simple on CT drives. I have previously done this application with great results.

Basically what happens is the following.

One Drive is set up as a master and the other a slave.

Both Drives are run in CLOSED LOOP FLUX VECTOR control. The encoders on the motors are used as feedback to the drive for position monitoring.

The encoder input is four Quad so we are actually getting a resolution of 4096 PPR.

The "2nd Encoder" modules are used to provide feedback from the master drive to the slave drive. Note: This is an digital lock, the motor shafts will be "locked" together. The accuracy of this system is within 2-3 Pulses (Less than 1 Degree on the motor Shaft).

I also did a reverse digital lock application (The motors were on opposite sides of the load).
Before we got the job though we had to first demonstrate the application to the customer, so we took 2 motors (4 kw) and fixed wooden discs to the shafts. We then drilled holes in the discs in which we put a match. Needless to say the match did not break, even at speeds of about 3000 RPM, while changing the direction of the motors.

I hope this info helps you.

Should You need any further info contact me.

Frantz Miles
CT. Dynamics (SA).
P.O. Box 7780
Newton Park
Port Elizabeth
South Africa

TEL : +27 41 3734971
FAX : +27 41 3734978
email: [email protected] (AH)
[email protected] (W)