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ptjw7uk
03-27-2008, 06:17 AM
Hi All
Just a question on power output from the various pole motors in that if I get a 4 pole motor 1400 rpm 1/2 hp will I get more power at the chuck if I gear down a 2 pole 2800 rpm motor or is the more pole more powerful for same hp.
I cant get my head around this.
Peter

Benesesso
03-27-2008, 06:33 AM
The HP will be the same, or slightly less for the 2 pole because of friction loss thru the gearing. The 4 pole will have ~twice the torque (twisting effort), but 1/2 the rpm.

Lew Hartswick
03-27-2008, 10:50 AM
The HP will be the same, or slightly less for the 2 pole because of friction loss thru the gearing. The 4 pole will have ~twice the torque (twisting effort), but 1/2 the rpm.

Yep! People who ask the wrong question are doomed to receive the
...lew...

JCHannum
03-27-2008, 11:05 AM
The number of poles in a motor determine the speed of the motor.

Horsepower is horsepower.

ptjw7uk
03-27-2008, 11:09 AM
Yes I know but to my understanding the more poles the less effect load has on speed reason fan motors have oodles of poles, even 8 pole motors such that speed is almost constant regardless of load.
It is just that I am trying to decide whether to go 2 pole and gear down or 4 pole and leave as is.
In that the geared 2 pole will have more torque before lathe would slow down.
Peter

lazlo
03-27-2008, 01:41 PM
The number of poles in a motor determine the speed of the motor.
Horsepower is horsepower.

Yes, but like Benesesso said, torque is proportional to the number of poles, so a 4 pole motor has half the speed, and twice the torque, of a two pole motor.

macona
03-27-2008, 06:40 PM
That and many fans are not often ran at high speed so to eliminate belting you use a motor which runs at the optimal blade speed for your fan. This will often require a 6 or 8 pole motor.

JCHannum
03-27-2008, 08:25 PM
Electric motor torque is expressed as horsepower, one horsepower being 550 foot-pounds/second.

It is derived from the current consumed in watts, the basic formula being volts X amps/746=HP. RPM does not enter into the equation. There are, of course efficiency and other factors that do enter into the equation to determine final motor power available at the shaft.

One difference between a high speed and low speed motor is that a higher speed motor will be significantly smaller than a lower speed motor with the same HP rating.

Another difference is that a lower RPM motor will develop a higher starting torque than a comparable higher RPM motor with the same HP rating. This is why high speed motors are commonly used on fans and centrifugal pumps and similar applications with low starting torques, and low speed motors are used on PD pumps and other machines that require some grunt to start.

rantbot
03-27-2008, 10:32 PM
more powerful for same hp.
That's unlikely.

lazlo
03-27-2008, 10:36 PM
Electric motor torque is expressed as horsepower, one horsepower being 550 foot-pounds/second.

It is derived from the current consumed in watts, the basic formula being volts X amps/746=HP. RPM does not enter into the equation.

Horsepower = (Torque * RPM) / 5252

So the more poles a motor has, the lower the RPM, and the torque increases proportionally to keep the horsepower constant.

lazlo
03-27-2008, 10:40 PM
In that the geared 2 pole will have more torque before lathe would slow down.

If you gear a 2 pole, 3450 RPM motor down to 1725 RPM, it will have the same torque as a 4 pole, 1725 motor (ignoring efficiency losses).

Edit: sorry, I just noticed that Benesesso said the exact same thing in the first response on this thread, but ptjw7uk posted that quote afterwards :)

darryl
03-28-2008, 02:08 AM
The slower speed motor will have more torque than the higher speed one, and will be bigger for the same power rating. The torque won't necessarily be double or half, but close enough. The higher speed motor will probably run more smoothly because of the flywheel effect of the rotor, so it may have an advantage if it's geared down- providing the speed reduction method doesn't make things worse in this regard., and providing that it isn't unbalanced. If you're mostly concerned with torque, and motor size doesn't matter, go with the slower speed motor.

Personally, low vibration level would be a priority, so it would come down to actually powering up the motors and evaluating that factor from the start. If a high starting torque is a requirement, you'd have to choose the motor type as well as low or high speed.

J Tiers
03-28-2008, 08:13 AM
The slower speed motor will have more torque than the higher speed one, and will be bigger for the same power rating. The torque won't necessarily be double or half, but close enough.

Seems that the torque is pretty much fixed by the definitions of speed and HP.

So if the motor has half the speed of another of exact same HP, the relationship of the torques is absolutely an exact, fixed, ratio. otherwise the HP are not equal.

JCHannum
03-28-2008, 10:10 AM
We are kind of all saying the same thing, only differently. Electric motor horsepower is a product of current. It is the same regardless of motor speed. All 240 volt motors that draw 15 amps will have the same horsepower rating regardless of RPM.

Output torque will depend on final RPM, and will not change for a given current input. Barring other losses, a 1HP 3600 RPM motor geared down to 1800 RPM will produce the same torque as a 1HP 1800 RPM motor.

The answer to the original question is that it will essentially make no difference to the power at the chuck.