Rpm vs Max safe speed 1

[Isaacwjp]

Hi all, I'm in the cooling tower industries and deal with motors occasionally. Attached is the data from a permanent magnet motor.

Could someone help to clarify the difference between 227 rpm and 3600 Maximum safe speed?

 

[waross]

227 is the design speed.
3600 RPM is a mechanical limit, not to be exceeded by either over frequency or by over driving the motor.
One example of over driving may be backspin after the power ISS cut on a deep well pump with no check valve.
I have heard urban legends about progressive cavity pump motors being damaged by backspin.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Frequency is mentioned as 11.35 Hz (on the nameplate attached by you)??
Should it not be 60 Hz??

R Raghunath

 

[jraef]

These are very special purpose built cooling tower motors designed to be run from VFDs, but optimized for the slow speed operation of a cooling tower fan. So the BASE motor speed is the 227RPM when run at 21.35Hz and 400V.

The “maximum safe speed” is just telling you that the bearings in the motor are rated for 3600RPM max. You can’t actually get this motor to that speed with any appreciable torque and overloading it to the extreme.

You would have to take it up to 180Hz, but assuming a 400V feed, the V/Hz ratio would drop so precipitously that there would be very little torque remaining, so the mass of the fan blades alone would likely stall it, let alone that at almost 16x the design speed it would be trying to handle a load that is now 3896 (15.85 cubed) times larger than the motor is capable of. Bottom line, don’t concern yourself with that number.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[FacEngrPE]

3600 RPM may be "safe" for the motor, but not for the fan which is intended to spin at 227 rpm. As per fan law power required to drive the fan is proportional to the cube of the fan speed, the actual speed limit will be fixed by the VFD max amps setting. That could still be faster than a prudent engineer will spin the fan.

 

[Isaacwjp]

Thank you all for your input. I understand it now

 

[Gr8blu]

Base speed = speed at which the machine stops being able to deliver continuous torque.
Top speed = speed at which machine stops being able to deliver continuous power.
Max safe speed = an overspeed (i.e. above top speed) condition where the mechanical construction of the rotating element(s) are designed with sufficient safety margin to prevent destruction such that nearby personnel are injured. NOTE: It does NOT mean the machine will continue to successfully operate as intended after the incident.

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