Knowledgebase: Drives
Average Starting Torque vs Required Breakaway Torque
Posted by Mark Wilbur on 27 October 2011 04:06 PM
Since Belt Analyst is a static analysis program, it can only provide an approximate analysis of starting and stopping which are dynamic conditions. As such, many of the calculations for starting are only indications of a possible problem. For instance, the Req Breakaway Torque is based on the entire belt breaking away at the same time which does NOT happen. Especially when the starting time is longer. And the Ave Starting Torque is also an oversimplification based on F=MA. So when you input a longer start time, the Ave Starting Torque actual goes down. That doesn't mean the maximum starting torque of the motor changes but the program does not know that. If you are starting "Direct online" or with a "Reduced Voltage Starter" you might want to make sure the Ave Starting Torque is greater than the breakaway torque but for most drives that utilize fluid couplings, VFDs, etc, it doesn't matter what the average starting torque requirement is as long as the maximum starting torque is adequate. Because of the unknowns of doing a static analysis, we only turn the drive tab yellow to indicate a caution so the user can make a decision whether this is a problem or not based on the actual drive type. If you are using VFD or Fluid couplings or some type of drive that allows the motor to get to full speed before the load is applied, many people simply change the "Breakaway Friction" multiplier under "Edit", "Customized Defaults", Drives from the default 2.0 to maybe 1.25 which reduces the breakaway torque required and eliminates the yellow tab. This is not a good way to calculate starting requirements but it is the standard used by CEMA and DIN and all international practices when only doing a static analysis. We use our Dynamic Analyst module to calculate the actual starting torques required.

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