# 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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asked 3 hours ago

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- 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 2 mins ago Edit
- (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 9 secs ago Edit

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