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  #1  
Old Sun 22 October 2006, 10:16
Dirk Hazeleger
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Load/torque testing a stepper motor - dynamometers

I started this thread in hopes of finding simple setups to test motor performance. Torque curves are non existant for some motors (Chinese especially) and the ones available rarely are at the power levels we would run, or may not be the optimum for that motor. The only scientific test I've come up with is winch type setup that you would increase the weight and speed to get different stall points and chart where you would loose steps. Any one have any other ideas?
Dirk
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  #2  
Old Sun 22 October 2006, 11:30
Gerald_D
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Dirk, you are talking of a "Prony Brake" or a "dynamometer" - googling those terms turns up a lot of stuff. Here is a nice one.
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  #3  
Old Sun 22 October 2006, 15:45
Dirk Hazeleger
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I'm sure a machine like that is out of my price range. When I said simple I also meant inexpensive. I was looking more for ideas with a MacGyver touch , basically a poorman's dynamometer.
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  #4  
Old Sun 22 October 2006, 16:26
Mike Richards
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Dirk,
I use a hanging 'fish scale' that reads up to 25 lbs. To use it, I tie one end of 1/8th inch nylon cord to the scale, loop the cord around a V-belt pulley that is fastened to the shaft of the stepper, ramp the motor to a set speed, and then gradually pull on the cord until the motor stalls. The 25-lb scale is a little light for the larger steppers. A 50-lb scale would have been better.

If you use my method, be careful. There is often no warning that the stepper is ready to stall. Also, there can be significant heat developed as the cord tightens around the pulley. Nylon melts easily, so repeated tests would probably require sisal or other non-synthetic cord.

Remember that most motors are measured in oz*inches. Unless my math is messed up, that means the radius of the pulley is the inches part of the equation and the scale * 16 is the oz part of the equation.
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  #5  
Old Sun 22 October 2006, 17:57
Dirk Hazeleger
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Hey Mike
I remembered your posting about your cord test. My winch idea was a variation of your test. Increasing speed on a certain amount of weight until stall. I've done quite a bit of web searching since my first post. I'm thinking about hooking up a generator (brushed dc motor) to the steppers shaft (direct couple the shafts). The step frequency going to the driver would give Shaft RPM. The load could be a bank of light bulbs or resistors hooked to the generator. This seems like a rather easy solution as the current produced by the generator can be calculated to HP or torque. It could also be interfaced with Data Aqusition software on a computer to graph the torque curves.
Here's a pic showing one I ran across on the web

What do you think?
Dirk

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  #6  
Old Sun 22 October 2006, 21:28
Gerald_D
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Using a motor as a generator as a dynamometer is fine as long you don't want to measure very low speed (near or at stall or startup). The pic above doesn't show a torque measuring system.

If you looked at some Pronies, you will have seen that the wrapped cord/belt type system (as used on exercise bikes) should officially have a fish scale at each end of the cord, and the difference in readings is the one used for the torque calc.

Modern dyno's have a torque arm resting on a strain-gauge loadcell.
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  #7  
Old Mon 23 October 2006, 01:23
Gerald_D
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After thinking about it in the traffic this morning, you need 3 devices:

a. A braking unit: The device that applies load or resistance

b. A torque measuring system.

c. A speed measuring system.

Starting from the bottom, the speed measuring on a stepper is from the input signal, but maybe you want to test servo motors one day.....?

For torque measuring, I reckon a torque arm pressing on a digital kitchen scale would do the trick nicely....

But the braking unit needs the most thought. You probably want to measure steady readings only, so you can have a fairly high inertia. In fact, to smooth the pulses of the stepper, you probably want a fairly high inertia, or "flywheel" effect. If you don't smooth the pulses, your torque reading could be difficult.

A DC brake motor brake with resistors is good for the higher speed readings. If that motor is big enough to stay cool at a locked rotor torque equal to where your stepper might "fight it", then you simply need a driver (Gecko 320?) to power the DC motor adequately. In other words, dispense with the passive light bulbs and actively drive the brake motor instead.

To mount the brake motor, get one with shafts out both ends. Put a U-cradle under the motor with its shaft going through (frictionless) bearings in the arms of the U. The torque arm attaches to the motor case and stops it from turning.

(For my sins, I've run diesel engines against dyno's like this - Arthur Schenk AG was the dyno producer)
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  #8  
Old Mon 23 October 2006, 01:41
Dirk Hazeleger
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Gerald
You may be right as there may be a rpm that this method becomes ineffective. The unit pictured is a teaching aide but still runs aroud 4000.00. I ran across this link
http://www.innovatia.com/dyno_design.htm
and I haven't plunked down the 50 bucks but it seems a good way to start. It mentions limitations so that may be low rpm measurements. The fish scale cord wrap might be a good idea for low speed.
Dirk
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  #9  
Old Mon 23 October 2006, 01:55
Dirk Hazeleger
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I missed your post as I was writing mine. I like your idea of a braking motor for the load. An encoder would probably be better for rpm as when you lost steps it would show up but the pulse signal would keep on going.
Dirk
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  #11  
Old Mon 23 October 2006, 07:39
Dirk Hazeleger
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After giving this more thought. A standard motor or engine will loose RPM as a increased load is put on it. A stepper will not loose rpm because the driver dictates the speed of the motor, it will instead loose steps. I'm thinking at the moment that the only test that can be made is stall torque at different rpms. Am I missing something
Dirk
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  #12  
Old Mon 23 October 2006, 08:04
Gerald_D
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As you said, the stepper will lose steps if overloaded, and that does mean a loss of speed - rather severely

I guess you would also keep an eye on current/voltage at the point just before it loses steps/speed to make a judgement of whether the motor can produce those speeds/torques continuously?
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  #13  
Old Mon 23 October 2006, 08:32
Dirk Hazeleger
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My thinking is to apply the load at a given rpm until you loose steps and then back off the load until the motor will run for a given period with no loss of steps. I'm not sure you can reliably measure the current, but it's worth a try.
Dirk
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  #14  
Old Mon 23 October 2006, 09:09
Gerald_D
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Dirk, a multimeter's "DC current" is apparently the true current in a stepper motor. When I tried to verify this snippet in my hairy memory, I found this thread that should interest you - a lot!.....

http://finance.groups.yahoo.com/grou...ve/message/567
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  #15  
Old Mon 23 October 2006, 09:35
Dirk Hazeleger
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Wow
That is copied and stored for safe keeping. Very good info indeed.
Dirk
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  #16  
Old Mon 23 October 2006, 21:13
Dick van Randen
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Re the brake for the Prony.

How about using the disc brake from a mountain bike? Mount the metal disc on the motor shaft and the brake caliper on the arm to the scale? You'll need some kind of alignment jig to keep it all from going in orbit but thats details , details , details...
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  #17  
Old Tue 04 September 2007, 17:15
FPV_GTp
Just call me: Ivan
 
Melbourne
Australia
hi

I know this is a old thread but interesting read

have a look at some of this guys stuff http://www.sportdevices.com/

the best would be a eddy-current load cell , like the one you guys posted before http://www.cs.wright.edu/people/facu...as/stand2.html


If you yahoo in images section " telma retarder " the end result is http://images.search.yahoo.com/searc...Ki.dLQFQAAAA@@

Commonly used on heavy vehicle application busses trucks so on , to aid in brake stopping
http://www.monroetruck.com/Mod_ASR.asp
http://www.industrialautomatic.com/html/telma1.htm
http://www.netikka.net/ville.vieri/dyno/eindex.htm
http://telma.com.pl/
http://www.telmausa.com/telma_htm/page2.htm

Telma retarder is a better option apply a electrical load depending on the windings more volts and or more amps and the rotors are slowed down and so on. there is no physical contact like a caliper to a disc rotor.

With the aid of a load cell sensor and a speed sensor and some nice electronics and hardware one could build a speed/load controller for such a dynamometer to operated the system , even if the software could detect missing steps under load and adjust accodingly the laod applied.

Remember these telma retarters are huge and store large amounts of energy.

so one would have to build something smaller to handle 1 to 5kwatts@ what rpm


SP-4 Dynamometer cell automation with built-in Closed-loop dynamometer controller. www.sportdevices.com/sp4/



I'm sure it would be easy to make a eddy-current retarter . A few bearings a shaft a several magnetic coils and two disc rotors.

If higher speeds are required better bearings with oil mist lubrication system and insted of using cast iron rotors like on the telma retarter some nice quality steel that has a good magnetic quality machine up and vented for cooling purposes. But tesing stepper motors will be short periods so this might not be a issue.

Even using a aircondition electrical clutch of a vehicle would do the job , nice and small and very cheap to obtain runs on a 12volt DC power source but a large amount of ampares ( car battery has 450 CCA available at all times depending on the rating of the battery in question )

The RPM of a Telma retarder is a restriction but but 4000 rpm to 5000rpm is not a issue testing stepper motors.

cheers
Attached Images
File Type: jpg telma1.jpg (9.2 KB, 355 views)
File Type: bmp telma2.bmp (71.3 KB, 361 views)
File Type: jpg telma3.jpg (69.9 KB, 361 views)
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  #18  
Old Tue 04 September 2007, 17:40
FPV_GTp
Just call me: Ivan
 
Melbourne
Australia
hi

again have a look at this dyno manufacturer , nice small units

http://www.land-and-sea.com/eddy-cur...ynamometer.htm

http://www.land-and-sea.com/eddy-cur...r-drawings.htm


and one of the worlds best dyno manufactures and test systems is superflow
http://www.superflow.com/home/index.cfm


cheers
Attached Images
File Type: jpg micro1.jpg (52.2 KB, 361 views)
File Type: jpg micro2.jpg (129.7 KB, 362 views)
File Type: jpg micro3_min.JPG (13.8 KB, 357 views)
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  #19  
Old Tue 04 September 2007, 22:45
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Ivan, for testing stepper motors, or any "servo" motors for that matter, we want to get near to zero (stall) speeds and holding torque at standstill. Eddy-current brakes can't work at zero speed - they need speed to generate the eddy-currents.
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  #20  
Old Thu 06 September 2007, 01:39
FPV_GTp
Just call me: Ivan
 
Melbourne
Australia
Quote:
Originally Posted by Gerald D View Post
Ivan, for testing stepper motors, or any "servo" motors for that matter, we want to get near to zero (stall) speeds and holding torque at standstill. Eddy-current brakes can't work at zero speed - they need speed to generate the eddy-currents.

Hi

Eddy current dyno can stall and motor at zero and so can a water brake dyno

a eddy current dyno doesn't need speed to generate the eddy current. whether a eddy current ( telma Retarder ) dyno be suitable for testing mmmmmmmmmmmmmm thats a another question??????

The word eddy current is misleading in the dynos discription , a eddy current dyno u supply a external power source to make a magnetic field to stop the disc rotors from turning.

I have a eddy current dyno at work and i can stall any motor the dyno is rated at 650kwatts retarder.

dissipating heat will be a issue

To test a electric motor / stepper motor / servo motor at near zero to obtain max torque is not a true indicater how well a motor performs over a rpm range.


http://www.dynatorch.com/stepper.htm here are a few graph examples
Attached Images
File Type: jpg servo1.jpg (14.5 KB, 324 views)
File Type: jpg servo2.jpg (14.7 KB, 325 views)

Last edited by FPV_GTp; Thu 06 September 2007 at 02:05..
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  #21  
Old Thu 06 September 2007, 01:50
FPV_GTp
Just call me: Ivan
 
Melbourne
Australia
here is a Testing Stepper Motors http://www.magtrol.com/motortesting/...per-motors.htm

Quote:
Complete Test System: Fully equipped with a custom dynamometer, supporting electronics, stepper motor testing software and computer hardware; all built into a heavy-duty, portable table
6 Different Motor Parameters Tested: Measures holding torque, detent torque, pull-in torque, pull-out torque, angle torque and step response
Motor Drive Control: Provides enable, pulse (step), direction and power relay signals
Precise Torque and Angle Measurement: Torque data achieves ±0.1% accuracy and angle data is ±0.01%
Optional Power Measurement: with Magtrol’s 6510e Single-Phase and/or 6530 Three-Phase Power Analyzer

cheers
Attached Images
File Type: jpg step_dyno.jpg (57.2 KB, 324 views)
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  #22  
Old Thu 06 September 2007, 02:00
FPV_GTp
Just call me: Ivan
 
Melbourne
Australia
http://www.magtrol.com/motortesting/dynamometers.htm

Quote:
Hysteresis Brake Dynamometer
Hysteresis Brake Dynamometers (HD series) are versatile and ideal for testing in the low to middle power range (max. 14 kW intermittent duty). Hysteresis Brakes do not require speed to create torque, and therefore can provide a full motor ramp from free-run to locked rotor. Brake cooling is provided by convection (no external source) or by air (compressed air or dedicated blower) depending on the model. Hysteresis Dynamometers have both continuous and intermittent power ratings where the dynamometer is capable of dissipating more power for shorter periods of time. All Hysteresis Dynamometers have accuracy ratings of ±0.25% to ±0.5% full scale, depending on size and system configuration. Also available are special designs for high-speed motor testing.



Eddy-Current Brake Dynamometer
Eddy-Current Brake Dynamometers (WB series) are ideal for applications requiring high speeds and also when operating in the middle to high power range. Eddy-Current Brakes provide increasing torque as the speed increases, reaching peak torque at rated speed. The Dynamometers have low inertia as a result of small rotor diameter. Brake cooling is provided by a water circulation system, which passes inside the stator to dissipate heat generated by the braking power. The water cooling in the WB provides high continuous power ratings (max. 140 kW). The WB Dynamometers have typical accuracy ratings of ±0.3% to ±0.5% full scale, depending on size and system configuration.


Powder Brake Dynamometer
Powder Brake Dynamometers (PB series) are ideal for applications operating in the low to middle speed range or when operating in the middle to high torque range. Like Hysteresis Brakes, Powder Brakes provide full torque at zero speed. Like the Eddy-Current Brake Dynamometers, the PB series is water-cooled, allowing for power ratings up to 48 kW. The PB Dynamometers have typical accuracy ratings of ±0.3% to ±0.5% (±2% for PB 2.7) full scale, depending on size and system configuration.

Tandem Dynamometer

Magtrol offers Eddy-Current and Powder Dynamometers mounted in tandem. In tandem, the unique features of each type of dynamometer brake are utilized, allowing nominal braking torque to be applied to the unit under test from zero speed to maximum rotation. The Tandem Dynamometers are available on a number of PB/WB combinations
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  #23  
Old Thu 06 September 2007, 03:41
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
"Eddy current dyno can stall and motor at zero and so can a water brake dyno"

I agree that an eddy current ("Telma"-type) and a water brake (Froude-type) can stall fuel engines (diesels, etc). I have done it myself with Schenk dynamometers. But, if there was no engine on the dynamometer, we could turn the brakes (eddy & water) slowly even when full power (or water) was given to the brake.

Then we got another electrical Schenk that was basically a generator, and when we tested 1000HP V12 diesels (MTU) for 3 days we could sell electricty back to the city. With this system we could "lock" the rotor electrically - we could even drive the rotor slowly backwards and forwards to line up the bolts.
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