# Thread: oz/in to lb/ft to watts

1. ## oz/in to lb/ft to watts

Ok...stupid question...conversion factor for oz/in to lb/ft? 16 oz=1lb. 12 in=1ft.
ex. 360 oz/in = 360/16 = 22.5/12 = 1.875 lb/ft?
And how about a conversion factor for oz/in to watts? Or even lb/ft to watts? I know that lb/ft = hp@5252 rpm...so how do I convert oz/in to hp? Hp to watts? This would be helpful in sizing servos in place of existing stepper motors...

2. I'm not very knowlegable at this, but oz-in, lbs-ft, are torque values, not sure how/why you want/need to covert it hp, what are you tring to find with that number?

3. and google tells me that 1 HP = 745.699872 watts

4. well, doing little research i found this web site:

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

And this cought my eye:

Electrical power is rated in horsepower or watts. A horsepower is a unit of power equal to 746 watts or 33,0000 lb-ft per minute (550 lb-ft per second). A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. It is 1/746 of 1 horsepower. The watt is the base unit of electrical power. Motor power is rated in horsepower and watts.
Horsepower is used to measure the energy produced by an electric motor while doing work.

Also, it seems you would need rpms to calculate power from torque...

5. How do you size a servo replacement for a stepper? Steppers are listed in oz/in and servos in watts. Seems like you'd need to know the conversion formula in order to properly size your replacement servos...

6. yes, i guess you are correct, need some way to compare the two. Do you know normal operating rpms for stepper motor?

7. I'm going to assume that it's less than 1500 rpms. It seems that's the upper range for reliable stepper usage. Missed steps plague steppers run faster than that. It's also interesting to note that with geared reduction and belt reduction'ed servos they are spinning 3000-5000 rpms while the ballscrews see around 1500 rpms. Something like a 2-3.25:1 reduction ratio.

8. ok, if 1500rpm and using 495 oz-in stepper here is what i get:

495 oz-in = 3.4955 N-m

using online calculator i get 549.0714 W which =.736HP

9. maybe that should be my next question...what is the correct/appropriate speed to run a stepper? with that I think we'd have a definitive answer....

10. If a single number is given for a stepper motor's rated torque, it will be the holding torque (i.e. zero speed). A stepper motor's torque falls off with speed. Stepper motor manufacturers normally publish torque-speed curves for their motors (listing the drive conditions also--i.e. how much voltage, current, unipolar or bipolar, full step or half step or microstep.) In the absence of such a curve there generally isn't a way to predict a motor's performance at a given speed.

Best regards,

Randy

11. All the servo's I've seen are rated in in/oz., in/lb. and watts as well as continuous torque and peak torque.

12. Servo motors are fancy DC motors. You can push more current through them and get higher torque. Continuous torque is based on the long-term temperature rise in the coils. For short periods of time you can put a whole lot of current through the coils (peak torque) before they overheat. The current is always flowing the same way through the winding, and you just vary its magnitude slowly to accelerate and decelerate.

Stepper motors are operated at a constant current. At a standstill that also means constant voltage in the coils, and that is the most torque they will ever make. To step a stepper motor, you reverse the current in the coils in a specific sequence. As the RPM increases, the current reversals happen at a faster rate, and the inductance of the coils limits the rate of the voltage change (i.e. there is a time constant) so they are not developing as much torque as when at the standstill.

That's why, for the best high-speed performance of a stepper motor you want one with a low resistance (thus inductance since there are fewer turns of thicker wire) and then use a power supply of 20x or more of the motor's "rated voltage" since the driving circuit will keep the current at a constant level by pulsing the voltage going through the coils (at a very high frequency relative to the switching I mentioned above.)

Best regards,

Randy