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motor speeds


krazyzay

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Can somebody tell me if running a 3 phase ac motor constantly @ 100hz will cause premature failure or what effect does it have on a 3 phase motor

 

There is a SEW 3 phase 1.5kw motor & gearbox running at 100hz via a danfoss 315 inverter. The current is fluctuating between 1.1a & 2.3a with no load but when the motor is running @ 50hz the current is stable @ 1.8a. The company which installed this says this is normal and this motor can run at 100Hz constantly. Is this true as I do not believe them.

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If the motor is designed for 50Hz and you are using the VFD do drive it at 100Hz , the motor is operating in a constant power mode, so torque has decreased by roughly 40%. If your load carachteristics have not changed, your motor is delivering only 60% of it's rated output torque at the higher speed. That means that as your load changes, the motor will see a greater slip change, and thus draw more current.
"He's not dead, he's just pinin' for the fjords!"
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How do you know that the load is staying exactly the same? Running at such a reduced torque would mean that even a very slight load change could induce a significant change in current.

 

If it is unloaded (as in open shaft) the current may be irratic as well. Check it again with the intended load.

 

Running at 100Hz may or may not shorten the motor life, it depends on the motor design. For instance if the motor base speed is 1450RPM, and you have bearings rated for 3500RPM, then there is no problem. But if the motor base speed is 3500RPM and the bearingas are rated only for that, then you are spinning at 7000RPM, your bearing will likely fail faster. Also, cooling fans may become ineffective at higher speeds, bearing lubrication may cease to function etc. etc. If SEW says it is OK however, it probably is because they have already considered these issues.

"He's not dead, he's just pinin' for the fjords!"
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