Today’s blog is written by Routeco’s Drives and Motion PSS Team Leader Kevin Johnson which is aimed to answer any questions you may have about Rockwell Automation’s variable speed drives.
Variable speed drives have traditionally been used for continuous speed applications where the speed has to change infrequently. Recently however due to advances in drive technology they also offer position control as an option. A few design aspects need to be considered before applying a Drive for this feature. Below are some frequently asked questions (and answers) when considering the choice of which drive system to use.
1) Is the system as accurate as a servo?
Yes it can be in theory. This will however mainly depend on the torque performance of the motor at zero speed and the resolution of the feedback device.
2) Is there a special motor requirement?
The induction motor can be used as standard to provide the torque required for the application. If the application requires the motor to be run at a low speed (less than 50% of nominal) then the integral fan on the motor is not effective and the motor may get hot for this reason we would recommend that the motor is a force cooled design.
3) We have a gearbox so it’s better to put the feedback after the gearbox?
No it is not! If the gearbox has any backlash the motor will try to move to compensate for any error this means that the motor will oscillate across the lost motion of the gearbox. As a further benefit if the feedback comes from the back of the motor the resolution is increased by a factor equal to the gearbox ratio i.e. a 1000ppr encoder with a 100:1 gearbox will provide 100000 pulses per output rev.
4) Will I be able to replace my servos with standard drives?
It would depend on the dynamics required in the application, an induction motor will not perform (accelerate) as well as a servo, however if a slow response is acceptable then this can be a considered path bearing in mind point 2) above then an Induction motor may be possible.
5) Which Rockwell Automation drives support ‘position’ mode?
From the current range the minimum would be a Powerflex 525 but the 527 and 750 range offer increased performance, all the drives would require a feedback device and a compatible feedback card fitted in the drive.
Routeco Drives and Motion specialists can advise you on the relative strengths and weaknesses of the solutions available and to book a appointment please contact your local Routeco branch.
For more information, contact your local Routeco office on 0370 607 1000 or email info@routeco.com
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