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How to obtain torque constant for stepper motor?

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I want to measure the torque constant of a stepper motor but seems I am doing it wrong. I have an MST342C02 stepper motor (200 step/rev and 4 phases) with SMD42C2 Driver. I have a converter that gives 48VDC 12.5A from 110VAC. I have Teensy 4.1 as the microcontroller and use Accelstepper library. The motor is connected Bipolar Parallel. the motor has:

Holding Torque = 9 Nm

Phase Current (A) = 9.5 (parallel), 4.7 (series)

When the motor is stationary the current is 0.4 A. However, by looking at the driver manual, for a 4 phase motor in parallel, the max current is 1.41*(Nominal current per phase). So for my configuration, we would have 1.41 * 9.5 = 13.4 A. I searched the net for the torque constant and some said it’s (Holding Torque)/current = 9/13.4 = 0.67.

The motor datasheet graph has 7 Nm for 300 RPM. I run the motor at this speed with PWM 255 which gives the max torque. The Measured current at such speed is 1.48 A. Also, the motor datasheet for 600 RPM has 6.2 Nm. Now I set the motor to this speed with PWM 128 for half torque (motor jams at max torque, but I don’t know why) then the measured current is 0.83 A. Using the graph this should be 3.1 Nm. Now the torque constant is 7/1.48 = 4.73 or 3.1/0.83 = 3.73 which does not comply with the 0.67.

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edited 11 hours ago

Amirmkr

New contributor

• Your question is a bit confusing: (1) “motor is stationary the current is 0.4 A” – When you apply power with no load (free running), the not load current is usually small, perhaps 400mA. (2) But if load is more than maximum, or when you force it to “stall” (in your word “stationary”, than the stall current is very big, perhaps a couple of Amperes. (3) PWM255 means high number of steps per seconds, meaning high speed, but not necessary high torque. (4) To measure things, I would suggest to set full stepping mode (ie, no microstepping). – tlfong01 8 hours ago
• (5) Some useful references: (a) NEMA34 Step Motors MST340, 341 and 342 Product Data – JVL jvl.dk/files/pdf-1/datasheets/ld0042gb.pdf (b) SMD42C2 Driver Datasheet – Motion Control 247motioncontrol.com/media/1230/smd-42.pdf (c) Microstepping myths – MachineDesign 2003oct09 machinedesign.com/archive/article/21812154/microstepping-myths – tlfong01 8 hours ago
• 1(1) When the motor is stationary it has holding torque that can hold 9 Nm. (2) Stall happens when more than 9Nm is applied. (3) if you look at the driver manual we have a moving current pin that connects to the microcontroller. The manual says it gets 0-2.5 VDC using the PWM, but using the 3.3VDC digital pins on Teensy I got to have all values from 0-255. This moving current has nothing to do with the speed but the torque. The speed is another pin on the driver named stepclock. By setting 10 for the driver resolution, and knowing 200 step/rev for the motor, 2000 ministep/s equals 1 rev/s. – Amirmkr 6 hours ago
• Question: You said “if you look at the driver manual we have a moving current pin that connects to the microcontroller.” Are you referring to the driver manual of my (5) (b) Above? I cannot find the “moving current” pin! – tlfong01 6 hours ago
• 1That’s “move current” pin 12 on the driver, sorry I said moving current. (5)b is the motor datasheet. For the driver manual use the following link: jvl.dk/files/_2011clean/pdf/lb043gb.pdf – Amirmkr 5 hours ago
• Oh my goodness, so there is a driver user manual beside the driver datasheet, and this is the root cause of confusion. Updated reference list v0.1: (i) SMD42C2 Driver Datasheet (Manual) – Motion Control 247motioncontrol.com/media/1230/smd-42.pdf (ii) SMD41 and SMD42 Step Motor Ministep (MicroStepping) Drivers User Manual – JVL Industri Elektronik A/S 1998-2005 jvl.dk/files/_2011clean/pdf/lb043gb.pdf, /to continue, … – tlfong01 4 hours ago
• (iii) NEMA34 Step Motors MST340, 341 and 342 Product Data – JVL jvl.dk/files/pdf-1/datasheets/ld0042gb.pdf (iv) Microstepping myths – MachineDesign 2003oct09 machinedesign.com/archive/article/21812154/microstepping-myths / to continue, …. – tlfong01 4 hours ago
• You might like to let me know if you have a scope to display the waveforms of the stepper motor coil current. A cope is useful to explore the nature of the coil current, especially for microstepping mode operation. If you don’t have much experience in experimenting with microstepping, chopper drives, you may need to read some basic tutorials. You might also like to skim my chats with other forum news, to how to test microstepping/chopper mode operation currents and torques etc. / to continue, … – tlfong01 1 min ago  Edit
• tlfong01’s chats on microstepping motor current and torques: (a) chat.stackexchange.com/rooms/134387/… (b) chat.stackexchange.com/rooms/133950/… (c) chat.stackexchange.com/rooms/133829/… (d) chat.stackexchange.com/rooms/133206/… (e) chat.stackexchange.com/rooms/134572/… – tlfong01 just now  Edit

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