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Overvoltage and regenerative failure of 4 Quadrant servo drives

Friday, October 14, 2016

In applications where the drive chain is direct drive in order to avoid unwanted gear backlash and lost motion there is a higher tendency for the “Overvoltage fault” to be observed on a 4Q servo drives. Ignoring this fault will result in the failure of the controller.

In a production environment, where cycle times and reliability are of the upmost importance, sluggishness and downtime are to be avoided if productivity is to be increased. As such, profiles are kept as sharp as possible; with quick reversing of direction and stopping motions to make the cycle as short as possible.

During deceleration, stopping or changing direction, modern servo controllers have the ability to dynamically control this part of the motion profile. They are able to do this because they are of 4 quadrant design. A by-product of this control topology is that the kinetic energy that is overcome in the moving mass is “absorbed” electrically by the DC bus capacitor in the controller. Typically the controller can “absorb” some of this energy, but frequently in dynamic direct drive applications it is too much. Once the driver is saturated, the supply voltage increases causing the controller to fault on “overvoltage” and the power supply to “crowbar” and protect itself from the rising potential. As there is nowhere for this energy to go it will cause damage to the controller, blowing the DC bus capacitor and possibly the MOSFETs—a costly failure, which would be most unwelcome.

What can be done to avoid this problem?

The power supply can be used in conjunction with a shunt regulator to dissipate this energy as heat. This way the “absorbed” kinetic energy from the vigorous motion can be handled safely without the motor drive faulting or more importantly blowing up! The Shunt regulator will be specified to the power supply, such that they work in harmony with the drive, giving you the full applied voltage but the required protection for your servo drive.

For further details about 4 quadrant drives and why this issue occurs please read our technical article “Power Supplies for Robotic Systems”

The 4 quadrants of motor operation:

Quadrants

About the author

Mark Gibbons has worked in automation and control for over 13 years, gathering experience from a wide variety of applications and equipment. Mark has an honours degree in Integrated Engineering from Cardiff University of Wales and has shared this knowledge and experience with the next generation of engineers with his lecturing. Mark is also an associate member if the Institute of Engineering Technology (IET).


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