Need help interpreting stepper motor curve 1

[hmic21]

Hello all,
A recently recieved a curve from a stepper motor manufacturer. It is torque (oz-in) vs. speed (rps). It also has power vs. rps on the same graph, represented by a dotted line. I thought power was torque (N-M) * Velocity(rad/s)? I do these calculations, and the power i calculate is way more than the power shown on their curve. Is it possible that the motor is limited somehow and is the power on this curve also the output power if this motor is used as a generator?
Thank You,
hmic21(M.E.)

 

[cswilson]

Without seeing the actual plots, it is difficult to comment. A quick check of stepper motor plots in my possession shows that power does equal torque times speed in them (but I struggled with the conversion factors, so double-check yours).

One possibility for the discrepancy is the difference between the theoretically available torque at any speed (which occurs at a torque angle of 90 degrees) and the recommended maximum usable torque, which is typically about 50% of the maximum available, to provide a good safety margin for overcoming disturbances. It may be that the power curve reflects the recommended maximum usable torque, rather than the plotted available torque.

Curt

 

[dparker213]

The correct way to compute the power curve for this motor is Torque (Nm) * rps * (2*pi). The result is in watts.

 

[hmic21]

Yea dparker213 , thats what i did. I would post the curves but I'm not sure I know how to do that. But thanks a lot I'll look into what cswilson said.

 

[dparker213]

The other thing to keep in mind when you look at the torque curves for steppers is whether or not the curves where measured at full step mode or not. I am aware of some manufacturers that post curves based on full stepping the motors which show much better than the common usage which is microstepping.

I have a hunch that the solid lines represent the obtainable torque when full stepping, while their computed power obtainable is representative of the more realistic micro stepping mode. It would be interesting to see the chart.

 

[aolalde]

Power in Watts must be the product of Torque times speed.

P(watts) = T (N-m)*rpm / 9.549 = T(N-m)*rps*2Pi
1 (Oz-in) = 7.062 E-3 (N-m)

The power produced by 1 Oz-in @ 1000 rpm
P=7.062E-3*1000/9.549 = 0.7396 Watts

 

[hmic21]

Hi all,
Do you guys know how to post pictures on here, I would really like someone who knows this stuff to take a look at this curve.
HMIC21

 

[itsmoked]

Can you find a link to the document and cut and paste it here?

If you want to post a picture you must have it at a link that has just the jpg. <sad I know> Then you put the link in here like this:

 

[hmic21]

The curve is on this page.

http://www.geocities.com/xxzeus07xx/index.html

Thanks

 

[itsmoked]

Looks like yah got no power at zero RPM

Just as aolalde sez:
P(watts) = T (N-m)*rpm / 9.549 = T(N-m)*rps*2Pi

P = T x 0.00

Looks about what I'd expect.

 

[cswilson]

As I eyeball your plot and crunch the numbers, it does appear to me that the power values are only 50% of what straight torque-times-speed calculations would give you using the displayed torque curve. This re-enforces the speculation in my earlier post that the power curve reflects a "usable" torque value with the common 50% derating from maximum available torque, but the torque curve shown is maximum available torque.

Curt

 

[dparker213]

All the manufacturers that I have dealt with regarding stepper motors plot the torque curves as "maximum continuous" torque. Peak torque is given, but it would seem meaningless to do a torque curve plot using peak values, wouldn't it? Is it possible to hold a motor at peak torque long enough to do a curve plot?

My bet is that the plot was drawn in error. Probably a typo by the manufacturer.

The manufacturers that I deal with have stopped printing the "Power" plot on their charts. Frankly I never paid attention to that plot anyway. I suppose it does give a visual aid as to what speed the motor becomes constant power, which might be useful when scanning a whole sheet of these.