Motor kw to rpm? Am i mossing something?

[Benn]

I need a new motor for a dust extractor and it needs to be 3000 rpm

i had a 4kw motor but i see they go upto 7.5kw

i dont really understand
What is the benifit of getting a bigger kw motor but still only having 3000rpm?
it wont create more suction as still travelling at 3000 rpm?

or am i missing something ?
Motor will work less the bigger the kw?

 

[Mur Huwcun]

The KW is the power or HP. Same as a Ford 3000 would struggle to pump slurry at 1000 rpm but a T7 315 would play with it

 

[JockCroft]

I need a new motor for a dust extractor and it needs to be 3000 rpm

i had a 4kw motor but i see they go upto 7.5kw

i dont really understand
What is the benifit of getting a bigger kw motor but still only having 3000rpm?
it wont create more suction as still travelling at 3000 rpm?

or am i missing something ?
Motor will work less the bigger the kw?

I think you will find that Kw does not relate to speed. I have been looking at a 0.5Kw and the speed option is related to two pole or 4 pole. That was with MachineMart.
As I understand it Kw relates to horse power..

 

[Old apprentice]

It will not be has hard to drive.can't tel you depends if it is working hard on lower kw, it could use more power?

 

[Benn]

So it will help with motor longevity if get bigger kw,

But in theory getting a bigger 7.5kw still running at 3000rpm will not aid suction on dust extractor as its still powering the same fan at same rpm as my current 4kw motor,
But if i got a bigger motor in 7.5kw at 3000rpm i could power a bigger fan that would aid suction?
To work the extractor for my machines i cant exceed the 3000 rpm.

 

[Highland Mule]

I need a new motor for a dust extractor and it needs to be 3000 rpm

i had a 4kw motor but i see they go upto 7.5kw

i dont really understand
What is the benifit of getting a bigger kw motor but still only having 3000rpm?
it wont create more suction as still travelling at 3000 rpm?

or am i missing something ?
Motor will work less the bigger the kw?

There will be some restriction in the airflow - if you're pushing it through a narrow orifice then you'll need more power to sustain the fan speed. Think about the difference between stirring water and treacle.

 

[michael N123]

There will be some restriction in the airflow - if you're pushing it through a narrow orifice then you'll need more power to sustain the fan speed. Think about the difference between stirring water and treacle.

Or driving a shite car uphill compared to a powerful one

 

[Benn]

i will keep the pipe work the size of the extractor ports on each machine

I wonder how you tell what the happy medium is for pipe ducting size compared to the motor kw and rpm?
And guess there is also the legth of duct pipe work the longer it is the weaker the suction??

 

[Highland Mule]

i will keep the pipe work the size of the extractor ports on each machine

I wonder how you tell what the happy medium is for pipe ducting size compared to the motor kw and rpm?
And guess there is also the legth of duct pipe work the longer it is the weaker the suction??

As a general rule, better to blow air than suck it. Keep the fan at the starting end of the duct.

 

[010101]

Some of the duct fans have adjustable pitch on the fan blades. More cubic foot per minute for the same rpm, but more kw needed.

 

[Wisconsonian]

There will be some restriction in the airflow - if you're pushing it through a narrow orifice then you'll need more power to sustain the fan speed. Think about the difference between stirring water and treacle.

If it's fan then that will be true. A dust collector to me is a centrifugal blower wheel, so the load goes up with higher flow at lower restriction, and goes down at lower flow with higher restriction.

A 7KW motor instead of a 4KW at the same speed won't affect the blower side, the motor will run cooler and could last longer, but will use more electricity to do the same work. The blower is sized to the motor, all the rest will work better or worse depending on how the ductwork is sized.

 

[Moors Lad]

I don`t understand your question - is a 4kw motor not available now ? Are you being advised/ forced to use a 7kw motor? If a 4kw motor coped with the job before surely that`s what you need provided you can get one with the required 3000rpm speed.
With the price of electricity now you really do need to see if you can get a 4kw motor if it`s available and up to the job. 4 units an hour will be expensive enough but 7 units of electricity an hour will be not far off twice as much!!

 

[bravheart]

Psychometric charts for air flow quantities and sizes of fan. The flow is exponential bit like water pipe bore and quantities. But if ducting and fan hasn't changed then same again will do as long as it hasn't burnt out by overload.

 

[Benn]

No i was just curious the bigger motor size would improve the dust extractor
@Wisconsonian has explained what I thought.

anybody know how to work out how to get maximum suck from the right size ducting regarding the fan size ,
This dust extraction is for a small joinery shop which will now be expanding to twice as big so have oppertunity to re duct the whole shop to get the most out of the dust extractor.

 

[Lazy Sod]

0.75kw = 1hp, approximately.

 

[bravheart]

The motor and fan will be sized correctly for the ducting already fitted if you increase the ducting length you will reach a point where the fan will stall because of backpressure in the ducting. I would suggest taking professional advice before modifying the ducting.

 

[Ffermer Bach]

I think you will find that Kw does not relate to speed. I have been looking at a 0.5Kw and the speed option is related to two pole or 4 pole. That was with MachineMart.
As I understand it Kw relates to horse power..

I think power equals 2x pie X revs min/60 x torque

So power is a function of both revs/sec and torque

 

[bravheart]

I think power equals 2x pie X revs min/60 x torque

So power is a function of both revs/sec and torque

Don't have the answer for the O P and it's a long time since I was at School but you can have all the Power in the world but you won't increase the air flow after a certain point.
Every fan has different attributes blade angle, axial, centrifugal speed, air temperature etc.etc. it's a science all of its own.

 

[e3120]

A 7kW motor won't use 190 whatever % of a 4 kW one. Just like an engine, that's the possible power - the actual consumption will depend on the load. If the 4kW was on its knees, the bigger one might use less, but if the fan actually only needs a lot less, the smaller one will likely be a bit more efficient.

 

[Blue.]

Not been mentioned but the 7kw will probably have a bigger shaft,then your fan won’t fit.