MSO Allen Bradley

Hello everyone, I hope you can help me. I have a VFD 527 this kind of VFD can be programmed with the motion commands like MSO, MSF, etc... Every time I am sending a MSO (Motion Servo On) the motor starts moving, How can I avoid that? I just wanted to send the MSO and the motor has to be stopped until I send the Jog.

Thank you in advance for your help.

Does the motor run at a specific speed, or does it just "creep" at low speed ?

There is an RA Knowledgebase document (#QA54233, Access Level: Everyone) that describes low-speed motor "creep" after you enable the servo loop with an MSO on the PowerFlex 527 and 755.

https://rockwellautomation.custhelp.com/app/answers/answer_view/a_id/559540/loc/en_US

My understanding is that this only happens when the drive is set up for Volts/Hz operation (the least sophisticated operating mode), because the Slip Frequency setting causes the motor to spin trying to create torque. You can set Slip Hz = 0, or configure the drive for Sensorless Vector (or, of course, use a motor with an encoder) to resolve this issue.

Hi,

Thanks you for your answer, the motor just creep at low speed. I am going to look for that parameter Slip Hz.

Best regards

Ken Roach said:

Does the motor run at a specific speed, or does it just "creep" at low speed ?

There is an RA Knowledgebase document (#QA54233, Access Level: Everyone) that describes low-speed motor "creep" after you enable the servo loop with an MSO on the PowerFlex 527 and 755.

https://rockwellautomation.custhelp.com/app/answers/answer_view/a_id/559540/loc/en_US

My understanding is that this only happens when the drive is set up for Volts/Hz operation (the least sophisticated operating mode), because the Slip Frequency setting causes the motor to spin trying to create torque. You can set Slip Hz = 0, or configure the drive for Sensorless Vector (or, of course, use a motor with an encoder) to resolve this issue.

Click to expand...

Hi Ken,

I have already changed the Speed Slip Hz to 0 and it works. If I want to change the configuration of the drive to Sensorless Vector, What do I have to take into consideration? Also I am having trouble to stop the motor because the time it takes to stop at full speed is too large. Best Regards

The drive needs to know a little more about the motor in order to do Sensorless Vector mode, so as that Knowledgebase document says, you need to at least run the Motor Analyzer test.

I don't know anything about your application so I can't tell you if the motor and drive are sized correctly for its dynamics.

If you need to decelerate a heavy load using the motor only, your options are a drive with sufficient excess capacity on its own DC bus, or a dynamic brake resistor feature, or a drive that allows regeneration onto the AC mains (an active-front-end drive), or a DC Common Bus drive system.

Since you have already selected and installed the PowerFlex 527, the most likely option is to install a dynamic brake system. The PowerFlex 527 has a built-in braking transistor to bleed off power from the DC bus, so adding a resistor pack is literally a two-wire connection.

There is an Application Technique publication that can help you calculate the proper size of dynamic brake resistor for your duty cycle and load:

https://literature.rockwellautomation.com/idc/groups/literature/documents/at/pflex-at001_-en-p.pdf