over clocking 3 phase motors

>>> wrote:
>>>> Andrew Bishop wrote:
>>>>> what sort of maximum frequency can I run a 3 phase 3 hp motor at
>>>>> running 240 3 phase without killing motor
>>>>> Thanks Andrew Bishop
>>>> Hi,
>>>> Sorry to top post, but frankly those if you who are overspeeding a motor
>>>> on a VSD are insane. Motors are designed to operate at the frequency on
>>>> their rating plate,no higher. You are risking a motor explosion. I
>>>> have seen the after effects of a motor letting go and I promise you it
>>>> is like a bomb going off. You don't have to go a lot higher to be in
>>>> the dangerous region.
>>>>
>>>> Just so you know my background is motor speed controllers and control
>>>> systems and have seen the results of endeavours like this.
>>>>
>>>> Most sincerely,
>>>>
>>>> Robert.
>>> Well Robert I do a lot of work on electric motors, in fact three of my
>>> biggest 4 customers are rewind and motor people.
>>> Presumably you have never heard of VDF rated motors ? Why do VDF
>>> manufacturers make them able to go to 120 Hz on the normal range
>>> because although I have seen special motors that run on 200 Hz and 400
>>> Hz I have never seen a 120 Hz motor.
>>> [ Quick phone call to one of the motor people and he's not got a clue
>>> what I'm talking about ]
>>>
>>> The rotor of a 2 pole motor is identical to a 4 pole rotor, it's the
>>> stator that's different so if a 4 pole runs at 1425 and 2 pole runs at
>>> 2800 how do you differentiate what rotor of the two identical ones to
>>> fit.
>>>
>>> Last item have you seen a rotor off a modern metric framed motor
>>> lately ?
>>> Gone are the helical grooves filled with cast alloy. They are still
>>> there but it's all internal, the outside is one complete smooth
>>> surface with nothing to snag or cause centrifugal force to act on it.
>>>
>>> Want proof ?
>>>
>>> I have converted 6 or 7 3/4Hp 2 pole motors to run at 12,000 with
>>> the expert help of the motor people.
>>> These run 8 hours a day 4 -1/2 days a week continuous and have done
>>> for the last 4 or 5 years.
>>> Only one has given problems and that was caused by crashing the head
>>> into the frame of the machine which bent the spindle.
>>>
>>> I have seen motors that have exploded but not for many years and it's
>>> usually been another fault that caused the problem. Like someone
>>> dropping a length of chain into a fork truck motor so they can get off
>>> work early [ true ]
>>>
>>> Most applications that will come into use here is on machine tools
>>> which also need a low speed so the norm is to use 4 pole motors to get
>>> the low speed [ many shaper etc even use 6 pole motors for the low
>>> speed application ] and over speed to get higher which still doesn't
>>> exceed fitting a 2 pole motor anyway.
>>>
>>> Personally and from many years hands on experience I think you are
>>> over reacting.
>>>
>>> John S.