NEMA MG1 Question

[charlie b]

We are reviewing a spec for a gantry crane that is used to move trash racks, set emergency closure gates, etc. It is calling for a constant torque speed ratio (CTSR) of 1000:1. I think what this means is that the motor can be operated anywhere between 100% of its design full speed and 1/1000th of that speed. We think that is overkill for the application (i.e., that fine a degree of speed control is more than is needed). We are being told that this is their standard product. We think that is an excuse for not wanting to do the engineering homework necessary to match the product with the need.

Questions:

1. Am I misunderstanding the nature of this spec requirement? Is there a significant difference in cost between a motor capable of 1000:1 control versus (for example) one capable of a 100:1 control?
2. Should the duty rating be Variable Speed per NEMA MG-1 par 31.3.8.1, or continuous per 31.3.8.2?

 

[Jraef]

With the 1000:1 reduction ratio on a VFD rated motor (which is where you will see it), you get a separately powered blower on the end of the motor to keep it cool, independent of the motor speed. TEFC gets to to 10:1, maybe 20:1, or TENV will only get you to 100:1. but the only way to get to 1000:1 is TEBC (Totally Enclosed Blower Cooled). But you can also get TEBC that is NOT good for 1000:1, so just calling for TEBC is not good enough by itself.

The reason they would typically want it on a crane motor is to attain the "holy grail" of VFD control; 100% torque at zero speed, which you can do with encoder feedback to a Flux Vector Control (FVC) capable drive. That provides the hoist motor the ability to take full control of the load PRIOR to releasing the mechanical holding brake, so that the load does not begin to drop before the motor and drive can react to it. But obviously if the motor is not turning, any fan attached to the shaft is not turning either.

If this is not for the hoist part of the crane, it might be that the intended use will involve a lot of creeping at very low speeds, without corresponding high speed movement or long rest times between, to where you again want that added blower for cooling.

 

[paulengr]

With the 1000:1 reduction ratio on a VFD rated motor (which is where you will see it), you get a separately powered blower on the end of the motor to keep it cool, independent of the motor speed. TEFC gets to to 10:1, maybe 20:1, or TENV will only get you to 100:1. but the only way to get to 1000:1 is TEBC (Totally Enclosed Blower Cooled). But you can also get TEBC that is NOT good for 1000:1, so just calling for TEBC is not good enough by itself.

The reason they would typically want it on a crane motor is to attain the "holy grail" of VFD control; 100% torque at zero speed, which you can do with encoder feedback to a Flux Vector Control (FVC) capable drive. That provides the hoist motor the ability to take full control of the load PRIOR to releasing the mechanical holding brake, so that the load does not begin to drop before the motor and drive can react to it. But obviously if the motor is not turning, any fan attached to the shaft is not turning either.

If this is not for the hoist part of the crane, it might be that the intended use will involve a lot of creeping at very low speeds, without corresponding high speed movement or long rest times between, to where you again want that added blower for cooling.

Close. Yes it is a thermal problem, not electrical. Fans and pump flow changes with the square of speed. So half the speed means one quarter of the CFM. Cranes are basically constant torque. So half speed is half power. The magic point where we reach maximum thermal capacity is at half speed so only 2:1 speed reduction. IF it was a pump/fan it follows the same curve so we only run into an issue when the heat generated by flux becomes an issue which gets us a 10:1 reduction with pumps and fans only. Also we need an encoder because at around 2-3 Hz the flux vector used for sensorless vector control disappears (becomes DC). So thus you need blower cooled or so-called “vector” motors or externally cooled ones using some other coolant (pipe ventilated, liquid cooled, etc.).