Calculating maximum speed and torque requirement of stepper motor

[bobdxcool]

The bipolar geared stepper motor I am planning to use has a resistance/ phase of 8.5 ohms per phase, inductance of 2.14mH per phase , normal current per phase is 2 amps, holding torque of motor is 0.8Nm, gearbox ratio is 1:8, holding torque of gearbox is 6.4Nm. The maximum permissible RPM of gearbox shaft is 200 rpm. The input voltage rating of the stepper motor is 24-48 volts. I am planning to use a 24 volts power supply and DRV8825 driver to drive this motor. Can anyone help me on how to calculate the maximum RPM of the stepper motor ? And also usually how long can the motor run at this maximum speed ?

Also, I intend to use this motor for a motorized cord reel application, where I would retract a cable using this motor and a chain mechanism. The total weight of the cable is around 12 kg. Would this torque of 64kgcm (6.4Nm) be sufficient for this application?
The diameter of the cable drum is 100mm, and the diameter of the cable is 18mm, with the length of the cable being 29 feet
Also attached is stepper motor datasheet.

 

[bobdxcool]

Attached below is a drawing of the motor with the gear.

 

[itsmoked]

You state 8:1 and a maximum speed of 200RPM.

Assuming that's the output shaft speed limit then the motor speed limit is simply 8x200 or 1,600RPM.

The motor should be able to run at that speed indefinitely.

Half the voltage probably will result in about 1/2 the torque.

You need to know the outer weakest torque diameter of the hose once it';s wrapped around a full reel to get an idea about the torque requirements.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

I can't see a stepper running 1600 rpm at only 200 pct of rated voltage.


Mike Halloran
Pembroke Pines, FL, USA

 

[bobdxcool]

I would be using a 48 volts power supply itself since that would give me full torque. Also, around 100-120rpm at the shaft of the gearbox is good for my application which would need the stepper motor to run at 800-960rpm or let's keep it at 1000rpm.

 

[itsmoked]

I'd load up the reel with hose and hang it from the highest height you would expect it to be mounted. Then, with a charged hose I'd use a bender-bar torque wrench to turn your reel for the ENTIRE length of the hose. Somewhere there will a clear maximum torque requirement demand. It is impossible to tell with math. It could be the fattest reel point with the remaining hose just clearing the floor or it could be the fully loaded reel with 14.67" inches of hose hanging off, or even the reel empty but dragging hose across the floor. No telling.

On finding the highest torque demand point you then test around that point with the wrench turning the reel at the speed you desire making sure the torque-at-speed is understood.

When you've used the torque wrench to measure the actual required torque you have a quantitative number to work with.

Take that number and DOUBLE it for an adequate result or TRIPLE it for a conservative number.

From that final number and the estimated rotational speed of the gearbox output shaft you'd need you can guarantee a functional result and can calculate the actual motor power and torque requirements.

Any other method is just so much baseless speculation. Certainly trying to work this whole thing only from the data sheets of a motor and gearbox will NEVER get this working right.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

Itsmoked gave you an elegant method for estimating the torque demand.

Further to my point about speed, check the current risetime for a 2+mH/8+ohm coil. I don't think you'll get far beyond ~500 steps/sec even with a fancy driver. How many steps/rev for your motor?





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Have you considered a torque motor. Torque motors have been driving take-up reels for decades. A torque motor will deliver a constant torque, and in some applications such as travelling bridges or should your hose become caught on something,, can remain stalled indefinitely.

Bill
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"Why not the best?"
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