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### Watts to Torque

01/19/2009 4:44 AM

Hello All

I have an application - a vertical Turbine - that is expected to rotate at approx 500 rpm and produce around 50 watts. Can somene tell me how I work out the Torque Value from these figures? (approx value only req'd)

I know that 1 watt is equal to 1 Nm but presumably there is an equation that allows for the speed also?

scott

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#1

### Re: Watts to Torque

01/19/2009 6:58 AM

1 W = 1 Nm/s !

P(W)= T (Nm)* π*n(rpm)/30

If you know P and n you can compute T.

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#2

### Re: Watts to Torque

01/19/2009 7:40 AM

in this font pi() looke like n

P = τω = τ *(2* Pi())*rps = τ *2* Pi() *rpm /60 = τ* Pi() *rpm /30

However the turbine capacity looks to be too large

at a shaft torque of 0.955 Nm i don't know a few mm dia shaft may do it

What is the turbine for i wonder

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#3

### Re: Watts to Torque

01/19/2009 8:13 AM

Pretty low RPM for a turbine, unless it's a small water turbine? In a teeny tiny hydroelectric dam for gremlins maybe?

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#4

### Re: Watts to Torque

01/19/2009 12:28 PM

Even for a water turbine I don't know tap water as the source ? may be ? only 50W - OK may be my fan can run through the output - it is 60W i think. 500RPM at 50W -

But no fun for me. with a shaft dia of 0.5mm or so you don't need me to do anythig to it, a ant hit is likely to brak the turbine.

The water inlet is likey to require a 10 micron filter otherwise the dust in it is likely to create the pulsations and fatigue the shaft.

(may be accountant post ?)

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#5

### Re: Watts to Torque

01/19/2009 6:00 PM

A beer flowmeter, maybe? I know a few chaps who prolly expend 50 watts during a good chug!

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#11

### Re: Watts to Torque

01/20/2009 8:08 AM

I think a brilliant idea- while the beer is served , you get free power - but are you sure the head will be enough or we have to put the keg on third floor or so.

(again not calculating the Head or Q- just uncalculated remark)

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#8

### Re: Watts to Torque

01/20/2009 4:11 AM

sb - your figure 0.955 N.m assumes 100% generator efficiency.

But ignoring that, I can't agree with your shaft diameter. 0.5mm gives a torsional stress nearly 39000 MPa !

5 mm giving 39 MPa more like it, allowing for some bending forces from unbalanced loads, turbulence etc on the turbine.

Cheers.......Codey

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#10

### Re: Watts to Torque

01/20/2009 8:04 AM

1) it is the turbine output and no mention of generator connected -hence 100% efficiency

2) seeing the amount of stress - didn't stress myself to design the shaft- the size was given as just an off-calculation remark.

Hope don't mind.

By the way who has heard of a 500rpm hydro turbine generating as large as 50W ?

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#13

### Re: Watts to Torque

01/25/2009 2:55 PM

Agreed, 1 W = 1 Nm/s = 1 J/s

However: P (W) = T (Nm) * n (rpm) / 9.55, [9.55 = 30 / pi]

Or more usual: P (kW) = T (Nm) * n (rpm) / 9550. [9550 = 30,000 / pi]

Power is not Torque * n^2 (rpm)/30, Power rises with speed linearly!

You might be thinking of angular velocity in Rad/s, where

Angular velocity (Rad/s) = Pi * n/ 30. However, above formula for power in kW using rpm / 9550 is correct.

Power (W) = T (Nm) * angular velocity (R/s)

Or: Power (W) = T (Nm) * n (rpm) * pi/30 is also correct.

Regards

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#14

### Re: Watts to Torque

01/25/2009 5:47 PM

I am sorry but I think that you do not read the text or have the wrong spectacles. Please take a lens to see better.

You correct a right text assuming that it is wrong.

I used the symbol for "Pi" from the "Ω" file which is "π" and not "n" ( you see the differrences? dont you?) you should not assume that people are totally stupid and can write what you assume they wrote.

Any way thank you for the attention you gve to my text and to the many ways one can write same thing.

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#6

### Re: Watts to Torque

01/20/2009 1:14 AM

SB has given the correct answer P=Tω, where ω = 2∏(RPM/60)

He deserves a good answer (not an off topic)

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#7

### Re: Watts to Torque

01/20/2009 2:01 AM

Of course, NickName deserves a good answer too, and perhaps gets a bonus point for pointing out that 1W is definitely not equal to 1Nm.

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#9

### Re: Watts to Torque

01/20/2009 7:44 AM

Thank you for the positive comment. I am accustomed that most GA are given for not technical comments and my goal is not to gain GA but to help. I would not object to a GA since it gives the satisfaction of a good work done.

The shaft diameter is for a shear stress usually used around 3.5 mm, the 5mm suggested are correct.

The speed can be right if the turbine is a wind turbine or a hydraulic one not Pelton type. I think even that the value is too high for a possible application as mentioned above.

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#12

### Re: Watts to Torque

01/20/2009 11:26 AM

I have a great web site calculator that does all kinds of power calculations. It's always good to know the math, but this a quick and easy calculator.

http://www.magtrol.com/support/motorpower_calc.htm