guldkorn från CNCZone, lågbudget torque-mätning av stegmo
Postat: 14 mars 2007, 13:16:52
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Geckodrive-Mariss:
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.
Geckodrive-Mariss:
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.
