over clocking 3 phase motors

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Subject

Author

Date

over clocking 3 phase motors

Andrew Bishop

06-15-2008

Re: over clocking 3 phase motors

Tony Jeffree

06-15-2008

Re: over clocking 3 phase motors

Robert Wilson

06-17-2008

Posted by Tony Jeffree on June 16, 2008, 5:46 am

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On Mon, 16 Jun 2008 01:14:48 -0700 (PDT), Charles Ping

>wrote:

>> > On Sun, 15 Jun 2008 15:16:22 -0700 (PDT), Andrew Bishop

>> > >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

>> > I have the VFD set for a max frequency of 120 Hz (so 2.4 times rated
>> > speed) on my ML-7; seems to work a treat. The motor hasn't died yet...

>> > Regards,
>> > Tony

>>
>> sounds good to me
>>
>> Thanks Andrew

>
>Frequency wise I'd agree - 120hz seems fine.
>However If you're doing that to a 2 pole (3000rpm) it'll be whirring
>along at 6000rpm. Personally I'd be frightened at that.
>A 4 pole (1500rpm) motor running at 3000rpm is probably OK since it's
>the same mechanical starting point as the 2 pole one.

Mine's a 4-pole.

Regards,
Tony

Posted by Andrew Bishop on June 16, 2008, 6:42 pm

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> On Mon, 16 Jun 2008 01:14:48 -0700 (PDT), Charles Ping
>
>
>

> >wrote:

> >> > On Sun, 15 Jun 2008 15:16:22 -0700 (PDT), Andrew Bishop

> >> > >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

> >> > I have the VFD set for a max frequency of 120 Hz (so 2.4 times rated
> >> > speed) on my ML-7; seems to work a treat. The motor hasn't died yet...

> >> > Regards,
> >> > Tony

> >> sounds good to me

> >> Thanks Andrew

> >Frequency wise I'd agree - 120hz seems fine.
> >However If you're doing that to a 2 pole (3000rpm) it'll be whirring
> >along at 6000rpm. Personally I'd be frightened at that.
> >A 4 pole (1500rpm) motor running at 3000rpm is probably OK since it's
> >the same mechanical starting point as the 2 pole one.

>
> Mine's a 4-pole.
>
> Regards,
> Tony

ok its a four pole and about 3 years old so I should be able to run at
120 hz whats the groups opinion on the low end 25hz

thanks for all the help
andrew bishop

Posted by Tony Jeffree on June 17, 2008, 2:17 am

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On Mon, 16 Jun 2008 15:42:56 -0700 (PDT), Andrew Bishop

>> >> > I have the VFD set for a max frequency of 120 Hz (so 2.4 times rated
>> >> > speed) on my ML-7; seems to work a treat. The motor hasn't died yet...

>> >> > Regards,
>> >> > Tony

>> >> sounds good to me

>> >> Thanks Andrew

>> >Frequency wise I'd agree - 120hz seems fine.
>> >However If you're doing that to a 2 pole (3000rpm) it'll be whirring
>> >along at 6000rpm. Personally I'd be frightened at that.
>> >A 4 pole (1500rpm) motor running at 3000rpm is probably OK since it's
>> >the same mechanical starting point as the 2 pole one.

>>
>> Mine's a 4-pole.
>>
>> Regards,
>> Tony

>
>ok its a four pole and about 3 years old so I should be able to run at
>120 hz whats the groups opinion on the low end 25hz

My setup is a Newton-Tesla conversion (but with the top frequency set
to 120Hz as a special). From the chart on their website, here:

http://www.newton-tesla.com/machinist/index.htm

it looks like the bottom frequency they are using is aboout 12.5Hz.

Regards,
Tony

Posted by John on June 18, 2008, 11:10 am

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> ok its a four pole and about 3 years old so I should be able to run at
> 120 hz whats the groups opinion on the low end 25hz
>
> thanks for all the help
> andrew bishop- Hide quoted text -

Low end speed is frequently limited by motor cooling. Most standard
motors have integral fans wich are designed to cool at their rated
speed. If you drop the speed cooling drops proportionally more. I
can't remember the relationship but think it's the cube of the speed.
Fit a seperate fan and you remove this limitation.
Of course if you overspeed, the fan absorbs more power so you're
getting diminishing returns. Eventually all the power gets taken by
the fan if it's still in one piece.

John

Posted by Robert Wilson on June 19, 2008, 6:41 am

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Hi,
Yes that's true because the current will be same to deliver the same
torque as at rated speed, although the freq and volts will be lower to
deliver the correct speed withouy saturating to rotor. So the I^2R
losses will be the same in the windings. The motor will also be
magetised to a similar level but I do seem to remember that motors tend
to be less efficient when run at low speed with a drive. Can't remember
exactly why though right now.

Thinking about it the core losses will be lower because of the reduced
fundamental frequency. This does not consider PWM frequency though
which can add extra heating to the rotor, hence why non-inverter rated
motors tend to require filtering with L and C components before delivery
to the motor.

I still think you would require extra cooling as John says.

Rob.

John wrote:

>> ok its a four pole and about 3 years old so I should be able to run at
>> 120 hz whats the groups opinion on the low end 25hz
>>
>> thanks for all the help
>> andrew bishop- Hide quoted text -

>
> Low end speed is frequently limited by motor cooling. Most standard
> motors have integral fans wich are designed to cool at their rated
> speed. If you drop the speed cooling drops proportionally more. I
> can't remember the relationship but think it's the cube of the speed.
> Fit a seperate fan and you remove this limitation.
> Of course if you overspeed, the fan absorbs more power so you're
> getting diminishing returns. Eventually all the power gets taken by
> the fan if it's still in one piece.
>
> John

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