By definition, Eccentricity is a measure of how much a roll deviates from being perfectly circular. Ideally, eccentricity should be zero, but in reality, it is never zero. Practically not a single roll is a perfect circle because it is produced using a machine which itself is prone to some machining errors (since it is product of some other machine and so on).
Eccentricity in the roll gives rise to wobble when rotated using a motor. Wobble is the angular deviation of the axis of rotation over one revolution.
Now let's understand how eccentricity can affect the machine performance.
Now consider two rolls in contact, one having half the diameter of the other.
With an induction motor, it expects to deliver a fixed torque at a fixed/constant speed. But as the load varies (due to eccentricity), the torque starts varying. The result is higher acceleration of the motor.
So change in the torque gives rise to change in acceleration & hence change in speed.
If the motor is to deliver a fixed torque, then due to the load variation, the speed profile will be as shown below:
To maintain constant speed, the motor has to deliver torque as per the load variation.
Typically, the motor has to respond to changes in torque every 100 ms (e.g. for roll running at 600 rpm).Typically for induction motor, torque rise time is in the tune of 350-400 ms. So despite having superior drive control, a normal induction motor cannot respond in the required time frame. The more connected rolls, the greater the load variation. In production machines, there are many rolls rotating together at very high speeds.
How can Servo system help here?
1. Conventional systems driven by induction motors are not dynamic enough to respond to such quick changes (to the tune of 100 ms).
2. A permanent magnet synchronous servo motor (e.g. AKM from Kollmorgen) has torque rise times of 80-100 ms. Therefore, the response is faster.
3. A servo drive (e.g. AKD from Kollmorgen) has current controller cycle time of 670 ns (fastest in the industry) which helps the servo motor achieve the desired response.
So in order to take care of varying torque demands due to eccentricity issues in the mechanical systems, servo based motion control systems can perform much better as compared to conventional systems. What kind of eccentricity issues have you had to deal with recently?
FAQ
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Q: At what point should I consider upgrading from an induction motor to a servo system?
A: If your process involves multiple rolls running at high speed — such as in printing, converting, or web-handling applications — and you're seeing speed inconsistencies, tension fluctuations, or product quality issues, eccentricity-related load variation is a likely culprit. A servo system becomes especially worthwhile when the number of connected rolls increases, since each additional roll compounds the total load variation that the drive must manage.
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Q: Can a servo system compensate for eccentricity without mechanically correcting the rolls themselves?
A: Yes — a servo system can effectively compensate for the torque and speed disturbances caused by eccentric rolls without requiring you to rework the mechanical components. By continuously adjusting current output to match real-time load demands, the servo drive absorbs the variation that the imperfect roll geometry introduces. That said, reducing eccentricity at the source through tighter machining tolerances is still good practice, as it reduces the corrective burden on the motion system.
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Q. In what types of applications is controlling eccentricity wobble most important?
A: Controlling eccentricity wobble is especially important in machines with multiple rotating rolls, such as printing, converting, packaging, and web-handling equipment. These systems operate at high speeds and rely on consistent motion control to maintain product quality. Using a servo system helps stabilize the process and reduce performance issues caused by mechanical imperfections
