- #1

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What i dont get is the equation for torque is T=(2*p*N)/60

So increasing the rpm should only increase the torque right..

Im a little lost here..please help

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- #1

- 23

- 0

What i dont get is the equation for torque is T=(2*p*N)/60

So increasing the rpm should only increase the torque right..

Im a little lost here..please help

- #2

- 44

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So as rpm increases torque drops.

How much TORQUE is required to produce 300 HP at 2700 RPM?

TORQUE = HP x 5252 ÷ RPM

How much TORQUE is required to produce 300 HP at 4600 RPM?

I look at it as Work = Force X Distance

increase distance (RPM), reduce force

You ever ride a bicycle with gearing?

Hope this helps

- #3

billy_joule

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- #4

- 44

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Low gear, high ratio, a ton of torque.

High gear, low ratio, minimal torque.

If you have a small motor running with nothing on the shaft, speed is max, torque is low

grab the shaft with your fingers, speed drops, torque climbs to try to get the motor back to max speed.

Have you ever done any practical work with motors (as opposed to theoretical)?

- #5

cjl

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So as rpm increases torque drops.

How much TORQUE is required to produce 300 HP at 2700 RPM?

TORQUE = HP x 5252 ÷ RPM

Answer: TORQUE = 300 x 5252 ÷ 2700 = 584 lb-ft.

How much TORQUE is required to produce 300 HP at 4600 RPM?

TORQUE = 300 x 5252 ÷ 4600 = 343 lb-ft.

I look at it as Work = Force X Distance

increase distance (RPM), reduce force

You ever ride a bicycle with gearing?

Hope this helps

You're assuming fixed power, when the question seems to be asking about a DC motor.

- #6

- 44

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It has to be if he wants to vary rpm and watch torque vary.

"So increasing the rpm should only increase the torque right.."

- #7

cjl

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It has to be if he wants to vary rpm and watch torque vary.

"So increasing the rpm should only increase the torque right.."

No, power isn't fixed. He's talking about a DC motor.

"i was to decrease the rpm of a dc motor to increase the torque"

Look at the torque/power curves of a DC motor (often provided on the spec sheet) and this will make sense though.

- #8

russ_watters

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- #9

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If I change the speed on this curve, the resultant torque value change will keep hp rating constant, correct?

If I change my speed and my torque changes,(following the curve) the hp. rating changes?

That's what the curve is saying, vary the speed, varies the torque for a rated hp.

(That's how I always understood them)

If I change my speed and my torque changes,(following the curve) the hp. rating changes?

That's what the curve is saying, vary the speed, varies the torque for a rated hp.

(That's how I always understood them)

- #10

russ_watters

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Yes, but I don't think we are talking about ratings here, but rather actual output.If I change the speed on this curve, the resultant torque value change will keep hp rating constant, correct?

If I change my speed and my torque changes,(following the curve) the hp. rating changes?

That's what the curve is saying, vary the speed, varies the torque for a rated hp.

(That's how I always understood them)

- #11

jim hardy

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What i dont get is the equation for torque is T=(2*p*N)/60

From where comes that equation?

It describes something other than a DC motor.

I think that's the source of confusion.

As dennisc pointed out early on

torque follows power

- #12

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Torque and rpm are inversely related..but i dont understand how..

Forgive my lack of basics..

- #13

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No i havent. I am in the second year of my under grads, so its all theoretical for now. Thats the problem i guess.Have you ever done any practical work with motors (as opposed to theoretical)?

- #14

jim hardy

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Torque and rpm are inversely related..but i dont understand how..

in my book takes three variables to make an equation , with just two it's a definition.

Can you start from

Work = Force X Distance

and imagine a rope being pulled at a constant rate against force f

and what's pulling it is a pulley of radius 1 ft

so the torque t on that pulley is f ft-lb?

it'll help you to draw a picture, that'll give you a mnemonic mental image.

Next can you throw in time

and figure out the equation that relates

power, torque, and RPM ?

Where you are, do they use English or SI units?

horsepower = 2pi X torque X rpm / 33,000

hint what's 33,000/60 ?

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- #16

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They use SI units.Where you are, do they use English or SI units?

What is 33000/60??

Is it converting horsepower to joules?

- #17

rcgldr

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http://lancet.mit.edu/motors/motors4.html

- #18

jim hardy

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okay, so work that thought experiment using a pulley of radius 1 meter, and a force of f Newtons.They use SI units.

What is 33000/60??

Is it converting horsepower to joules?

Are you clear on the difference between power and work?

Power is rate of doing work, so suggestion to convert horsepower to joules gives me a "compile error".

33,000 / 60 = 550.

550 ft lbs /sec is one horsepower.

So 33,000 converts between ft-lbs per minute and horsepower.

So you use SI ?

What constant converts between newton-meters per minute and watts?

- #19

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I believe 100 nm / min = 1.6 watt (ish)

1 nm / min = .016 watt

We use 46,875 nm/min = 1 hp (750 wt = 1hp)

Will let Jim do the excellent job with calculations, I will try the practical side.

Have you ever driven a manual transmission in a car Vishal?

Ever ridden a bicycle with multiple gear ratios?

Your trading speed for torque. Can't do both at the same time.

1 nm / min = .016 watt

We use 46,875 nm/min = 1 hp (750 wt = 1hp)

Will let Jim do the excellent job with calculations, I will try the practical side.

Have you ever driven a manual transmission in a car Vishal?

Ever ridden a bicycle with multiple gear ratios?

Your trading speed for torque. Can't do both at the same time.

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- #20

russ_watters

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No, energy is energy. Torque is torque. They are two different things.

I like to think of such scenarios and equations as if-then statements:

- #21

jim hardy

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easy beginner's mistake to make though, both having same units force X distanceNo, energy is energy. Torque is torque. They are two different things.

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- #22

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I got confused with the units sorryAre you clear on the difference between power and work?

- #23

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i meant to say work. Arent work and energy the same thing?No, energy is energy. Torque is torque. They are two different things.

Torque again is work done, right??

- #24

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I drive a manual. But i dont get what you want to say.Have you ever driven a manual transmission in a car Vishal?

Ever ridden a bicycle with multiple gear ratios?

Your trading speed for torque. Can't do both at the same time.

When you say torque, the torque from what on what do you mean?

And how does changing the gear affect this?

- #25

jim hardy

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I

I got confused with the units sorry

]i meant to say work. Arent work and energy the same thing?

I drive a manual. But i dont get what you want to say.

When you say torque, the torque from what on what do you mean?

And how does changing the gear affect this?

I am in the second year of my under grads, so its all theoretical for now. Thats the problem i guess.

Torque again is work done, right??

I keep getting this feeling we've led you to water but you don't want to drink.

We'd like to help you along but i for one am unsure of your vocabulary.

Have you taken any physics at all?

No, torque is not work. Think of it as a force that twists instead of pushing in a straight line.

http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html

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