Overspeed Motor - High Inertia Load Scenario

[W@ttson]

Has anyone here ran a motor in an overspeed condition in an application that typically is considered high inertia load - say in a crane and hoist application?

As in ran the motor in the "Constant Power Region or Flux weakening region" as shown below:



For example, say your load profile is the following:

At rest: You need 150% of motor Full Load torque to get the load moving from stand still. You need this for 15 seconds.
Your ramp time from Stand still to full speed (base speed) is 10 seconds.
Once you are at base speed (really you are at the slip speed) you only need 40% of FLT.

The time to run your application is important so since you need to reduce the total operation time, and your FLT in the constant velocity is relatively low, you think to "overspeed the motor" so you want to run it over the base speed.

If I recall correctly, when you cross over into the Constant power region your torque trails off by the ratio of 1/n^2 where n is the speed.

Has anyone here effectively done this?

Thank you

 

[Jraef]

Run motors at beyond their base speed? Yes, many times. I don’t think I have ever run into the torque situation you described however. Generally the motor torque requirement is fairly constant, so the final torque requirement at the over speed condition is what determines the size of motor required. When you start there, you end up with a motor that is basically over sized at the base speed, so attaining “150%” torque kind of comes with the territory.

 

[W@ttson]

Run motors at beyond their base speed? Yes, many times. I don’t think I have ever run into the torque situation you described however. Generally the motor torque requirement is fairly constant, so the final torque requirement at the over speed condition is what determines the size of motor required. When you start there, you end up with a motor that is basically over sized at the base speed, so attaining “150%” torque kind of comes with the territory.

the issue arises if things were designed / sized for base speed and now are being required to overspeed. If you had spare capacity in the base speed calculation such that say you bumped up the hp rating to the next standard size or something like that, I assume that as long as the amount your torque reduces in the constant hp range is more than your load profile, this should be OK.