How Much HP Needed

[Newz2000]

Does anyone know how to calculate the amount of HP needed?

I'd like to make a lift capable of raising up to 100 lbs (45Kg) at a rate of 1 ft/sec (.3 m/sec) to a height of 12 feet.

I'm comfortable using pullies and/or gears on the motor.

I'm happy to do the math myself, but the only resources I can find are for hot-rodders. I'm looking more to figure out how feasable my idea is.

Also, is there any diff between an ac or a dc motor here? I can use either, but it seems like ac motors in the 1/4 - 1/2 hp range are pretty easy to come by.

[dyarker]

HP = 33,000 ft·lbf/min

[jwax]

You'll find motors specs interesting. A 1 HP DC motor may be over-powered to produce 2, 3, 4 or 5 HP, for awhile! Heat/duty cycle enters the overall equation here.
Your design will be very different for one-time-use vs. continuous duty.
Which is your application?

[Newz2000]

It will be used at most 20 times per day, but when it's used, it will likely be used several times in quick succession.

So 3300 ft-lb/min = 550 ft-lb/sec. Does that mean I can take 550/100 = 5.5 ft/sec from a 1 hp motor? That means I could get by with a 1/4 hp electric motor, which is cool, because I see them sitting near dumpsters all the time. Does the diameter of anything matter? Does it matter if I use pulleys? For example:

If I want to raise 12 feet, then I (think) this scenario changes the rope length to 48 feet but makes the lifting force only 25 lbs.
Does the motor care? This changes the formula to 550/25 = 22 ft/sec for a 1 hp motor.

Since many DC motors are rated in watts, not hp, is there a simple formula that can tell how to convert between the two?

If the above is correct, then I can see one benefit to the pulleys; they increase the resistance on the rope for when the load is being lowered so that the load doesn't plumit. Also, I think the two leads of the motor could be shorted to provide some breaking force further slowing descent. (does that work on AC motors like it does on DC?)

[Sambuchi]

Quick example..

550 foot pounds per second = 33,000 foot pounds per minute.

One horsepower provides the ability to do 550 ft-lbs of work every second.

Applying 55 lbs force over a distance of 10 feet is the same amount of HP as 11 lbs. of force applied over 50 feet.

Same idea go's for a lift.

A 10 HP motor would let you do 550 X 10 or 5500 ft-lbs. of work each second. It could lift 550 lbs. 10 feet in one second.

To lift more weight at the same rate, or the same weight more quickly, you would need more horsepower.

No gears were included for this example

[Sambuchi]

opps you just posted a picture.. hold on

[jollyrgr]

Since many DC motors are rated in watts, not hp, is there a simple formula that can tell how to convert between the two?

1 HP = 33,000 ft*lbf/min = 550 ft*lbf/sec = 746 Watts (electrical)

Per your specs you want to raise 100 lbs at the rate of 1 ft per second.

Or

100 ft*lbf/sec = (1HP/550ft*lbf/sec)* (100ft*lbf/sec) = 100/550 HP = 2/11 HP = 0.182 HP

A 1/5 HP would BARELY do it; quarter horse power motor should do it okay. The motor should have somewhere around 200 Watts.

[jwax]

In the ideal world, a 1/4 HP motor would work. Now you have to consider the transmission media- pulleys, gears or belts. They will have losses.
The motor is rated 1/4 HP at a specified RPM, so if .25 HP is needed, the motor must run at that speed. Your transmission has to do the conversion to your desired load lifting speed. Is there a necessity of reversing the motor direction, to lower the load? Do you need a mechanical brake to control the speed of descent?

[Chris Smith]

All things being equal, the math is divisible by any way you chose the pulley ratio.

However, keep in mind the theory and the actual don’t always match exactly.

If your math comes out to 1/4 hp doing the job, bump it up to ½ hp for efficiency, reliability, heat, wear and tear, and the fact that 1/4 hp motors suck on their start up ability. [except brush type DC motors]

Other than that, the math is straight forward.

Pulley size, speed, weight, all equal the final sum exactly like ohms law at the end of the day.

[gerty]

The hoists we had at work were 3/4 hp and rated for 1,000 lbs. Not a clue as to how many fet per second. Don't forget you'll need some sort of brake to keep the load in the up position when you let go of the button.