How to I tell an OK stepper from a really good one ?

[jaybee101]

Hi All,

Most things in life you get what you pay for...

I know what torque steppers I need, but there are several that would do the job in a whole range of prices.

So how do I decide which is the best one ?

Any particular manufacturers to look out for (or avoid)

Anything on the datasheets I should be looking for ?

This purchase is'nt particularly price sensitive (about time I spent some money one new toys), but I dont want to pay a higher price for no benefit.

Thanks

John

[erase42]

an OK stepper motor will be holding a beer.
a REALLY good stepper motor will be holding two.

[pminmo]

Electrically, lower coil inductance, lower coil voltage. It's easier to get better speed/torque performance.

[mcpltd]

As pminmo said low inductance is good, but this also means that the current will be higher, and means that you will need a higher power driver.
As for quality, they all look the same now, and there are only a few manufactures that i would say are better than the rest, but you will pay lot more for them (3 X more) as an example pacific Scientific power max motors are very good steppers, but it is unlikely a CNC application would gain that much from using them.
I would say spend no more than £20 for a 23 frame stepper and £70 for a 34.

[MikeUK]

Originally Posted by mcpltd As pminmo said low inductance is good, but this also means that the current will be higher, and means that you will need a higher power driver. As for quality, they all look the same now, and there are only a few manufactures that i would say are better than the rest, but you will pay lot more for them (3 X more) as an example pacific Scientific power max motors are very good steppers, but it is unlikely a CNC application would gain that much from using them. I would say spend no more than £20 for a 23 frame stepper and £70 for a 34.

Hi,

Does that mean these aren’t worth the money?

http://motioncontrolproducts.co.uk/p...products_id/81

Mike

[Mariss Freimanis]

Primary is power and smoothness. It takes a dynamometer and a 25,000 line encoder to measure each accurately.

It takes a pair of channel-lock pliers, a moist paper towel and a multimeter to measure power to a +/-5% accuracy, a laser pointer, pencil, paper, super-glue and a sliver of a mirror to measure microstep smoothness to a +/-1% accuracy. I posted both these methods but I can't remember the forums or message numbers.

Mariss

[vacpress]

Mariss,

How does the moist paper towel trick work? Is there a conductor of some sort on the other side to make a variable resistor?

What are the 'known' values that are used to make usefull calculations from this? Are they just relative to one another?

Thanks for posting this. Sorry you cannot find your previous posts.

[ger21]

All I can remember is you wrap the paper towel around the shaft and grab it with the channel locks.

[Mariss Freimanis]

I knew that would get curiosity going.:-) The paper towel is moistened to keep it from catching on fire.

This is the low-tech method I came up with some time ago to measure motor power directly using just a paper towel, channel lock pliers and a multimeter:

1) Run your motor up to a speed of interest. It must be more than 2 to 3 revs per second.

2) Set your multimeter to 'DC Amps' and put it in series with the power supply. You will be measuring power supply current to the drive.

3) Measure the supply current with no load on the motor.

4) Fold a full paper towel over and over until it is 1" by 3" in size. You want it nice and thick. Moisten it and fold it over the motor shaft in a "U" shape. Grab the motor shaft with your channel lock pliers with the towel as a 'brake pad'.

5) Start slowly clamping down with the pliers while watching the Ammeter. More pressure is more amps. At some pressure the motor will stall. Note the meter reading at the instant of stall. Repeat a few times until you sneak up on the stall point and get the highest ammeter reading. This takes a little skill and a steady hand.

6) Subtract (3) from (5) to get the difference between the two current readings: Delta I = (I stall) - (I no-load).

Multiply 'Delta I' times your measured power supply voltage to get Watts: Watts = (Delta I) * (V supply)

That is your motor's power output power in Watts at the speed you are running it at. To calculate motor torque at that speed use:

In-oz torque = Watts * 1351 / RPM

That's all there's to it. A big NEMA-23 or any NEMA-34 motor will put enough power into the towel to catch it on fire. Moistening it and keeping moist will turn that power into steam and keep the motor shaft from getting heat damaged. I have a video I made somewhere where the shaft glows a dull cherry red when a dry towel is used. It ruins the motor shaft and you have a paper towel on fire, so keep it wet with bigger motors.

The reason for folding the towel over a million times is to make it thick. Otherwise the channel lock pliers will ruin the motor shaft if it can bite thru the paper.

Done carefully (accurately measuring current at instant of stall) will give an answer within +/-5% of a genuine dynomometer result. This method falls apart at slow speeds because the motor generates very little power at low speeds (Power = torque times RPM). The difference between no-load current and stall current becomes too small to measure accurately.

The results from this method have been compared to dynomometer results (yes, we a good calibrated one and I use it all the time) hundreds of times with all sorts of motors. The results agree within the stated +/-5% tolerance.

Mariss

[mcpltd]

This is taking the consumed power not the actual mechanical power.
If the driver and motor were 100% efficient this would be an accurate way to measure torque, but they are not 100% efficient.
If you know the efficiency then that would make it more accurate.
It would be interesting to see this procedure being done if the motor had a flat or a keyway on the shaft

Originally Posted by Mariss Freimanis I knew that would get curiosity going.:-) The paper towel is moistened to keep it from catching on fire. This is the low-tech method I came up with some time ago to measure motor power directly using just a paper towel, channel lock pliers and a multimeter: 1) Run your motor up to a speed of interest. It must be more than 2 to 3 revs per second. 2) Set your multimeter to 'DC Amps' and put it in series with the power supply. You will be measuring power supply current to the drive. 3) Measure the supply current with no load on the motor. 4) Fold a full paper towel over and over until it is 1" by 3" in size. You want it nice and thick. Moisten it and fold it over the motor shaft in a "U" shape. Grab the motor shaft with your channel lock pliers with the towel as a 'brake pad'. 5) Start slowly clamping down with the pliers while watching the Ammeter. More pressure is more amps. At some pressure the motor will stall. Note the meter reading at the instant of stall. Repeat a few times until you sneak up on the stall point and get the highest ammeter reading. This takes a little skill and a steady hand. 6) Subtract (3) from (5) to get the difference between the two current readings: Delta I = (I stall) - (I no-load). Multiply 'Delta I' times your measured power supply voltage to get Watts: Watts = (Delta I) * (V supply) That is your motor's power output power in Watts at the speed you are running it at. To calculate motor torque at that speed use: In-oz torque = Watts * 1351 / RPM That's all there's to it. A big NEMA-23 or any NEMA-34 motor will put enough power into the towel to catch it on fire. Moistening it and keeping moist will turn that power into steam and keep the motor shaft from getting heat damaged. I have a video I made somewhere where the shaft glows a dull cherry red when a dry towel is used. It ruins the motor shaft and you have a paper towel on fire, so keep it wet with bigger motors. The reason for folding the towel over a million times is to make it thick. Otherwise the channel lock pliers will ruin the motor shaft if it can bite thru the paper. Done carefully (accurately measuring current at instant of stall) will give an answer within +/-5% of a genuine dynomometer result. This method falls apart at slow speeds because the motor generates very little power at low speeds (Power = torque times RPM). The difference between no-load current and stall current becomes too small to measure accurately. The results from this method have been compared to dynomometer results (yes, we a good calibrated one and I use it all the time) hundreds of times with all sorts of motors. The results agree within the stated +/-5% tolerance. Mariss

[Mariss Freimanis]

1) Have you tried it?
2) A brilliant solution for a flat or keyway is to use a coupling.

Mariss

[mcpltd]

Not tried it, I don’t think it will give a true reading of actual mechanical torque.

How efficient is a typical stepper drive and motor? Because you are measuring consumed power and translating that to torque, efficiency has to be taken into account.
Definitely wont be accurate to plus or minus 5%. How can it be minus when there are losses that have not been taken into account?

Originally Posted by Mariss Freimanis 1) Have you tried it? 2) A brilliant solution for a flat or keyway is to use a coupling. Mariss