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Speed Above 3000 Rpm


boby

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If motor is designed for 2880 rpm. at 415 volts 50 hertz supply.... and it is running with vfd. vfd max freaquency is 400 hertz. can we run the motor above 2880 rpm? if it is possible what are the corresponding changes in voltage and current? is it safe to run the motor beyond the rated rpm in load condition? the motor rating is 75 kw..
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Several issues involved with this.

 

1) "Safe" is a relative term. Safe for the motor means the bearings are designed to operate at an increased speed. Only the specific motor manufacturer can answer that question for you. Safe in terms of machine safety is another important issue. Consider that in all likelihood, a mechanical engineer was involved in the equipment design and based his component and material selection on a maximum speed. Exceed that and you are beyond the intended margin of safety. Shutdown time in an emergency is part of that consideration by the way.

 

2) Motor cooling is also a consideration. Check with the motor manufacturer to make sure that any cooling systems (fans etc.) will still function properly at increased speed.

 

3) You motor has a fixed kW rating, 75kW in this case. It is not capable of more than that. kW ratings are based upon an amount of torque at a given speed. Torque is produced by current flow, but is related directly to the ratio of Voltage and Frequency, called a V/Hz ratio. So in your motor at 415V 50Hz, the V/Hz ratio is 8.3 V/Hz. You cannot increase the voltage beyond what you have (and what the motor is rated for), so as you increase the frequency, you enter what is called a "constant power" mode, meaning the kW remains constant at 75. So with that in mind, your V/Hz ratio must go down meaning you will be losing torque even though the current will continue to rise. At some point you will no longer produce enough torque in that motor to make it spin itself let alone a load, yet you will overload it! This is especially important when you consider variable torque loads where the torque requirements will INCREASE with speed and flow. Increase of torque requirement + decrease in torque availability = rapid deterioration of viability.

 

So to answer your questions, yes you can increase speed as long as it is not beyond what the motor and equipment can safely tolerate, and as long as you understand that you will be losing torque as you go beyond the base speed.

"He's not dead, he's just pinin' for the fjords!"
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I suspect as your motor is rated at 415V on a 50Hz system it's a UK machine. Nowadays most induction motors are rated at both 50 and 60Hz for the continental market. You can usually check this on the nameplate where you will often find a 380 and 400V rating. I'm not sure this holds for older machines. If it is this type of machine we have been advised not to exceed 60Hz. I'm informed by my mechanical collegues that above 60Hz there is some sort of harmonic resonance issue (I don't understand this bit at all so I take it at face value). If you are taking about a 400Hz frequency on a machne rated at 50Hz then the answer is no you can't do it and it's not safe.
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boby,

your question cannot be answered only by words.

You have to ask a specialist to check your motor, or you have to ask the motor manufacturer.

My first reply is "not possible", but if rotor is ..... and bearings are ....... and cooling is ........ and frame is enough ridgid ..... and rotor balance is .... and magnetic steel sheets are ......... and winding are ........ and lubricant is ...... etc.etc. , it should be possible to reach high speed, i.e.: 10.000 rpm

 

Complete description of all variables andistuations is not possible in few rows, for this reason I wrote ...... and ......., sorry!

 

Regards

Mario

 

 

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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