# How to accurately measure electric motor torque?

I'm currently doing an experiment on an AC motor to measure the torque output from a servo drive using fishing line wrapped around the servo shaft / gear while the other end is connected to a luggage scale.

The shaft is connected to a gear of 1/2'' radius. The fishing line is wrapped around the gear and the other end of the line is attached to a luggage scale. The motor is rated for 11 lb-in of continuous torque, corresponding to 22 lb-force for half-inch radius gear. However, I am not able measuring nearly that amount even at the maximum torque input current. My scale barely budges past 2lbs even at maximum torque (by applying the largest control input value). Any clues as to what I'm doing wrong? Thanks!

FYI I'm using an Applied Motion products Servo Drive model J0400-305.

• Can you verify the scale is calibrated with that arrangement and that the servo driver is at max applied voltage? – Tony Stewart EE75 Jun 9 '19 at 20:10
• @sunnySky EE75 yep those are confirmed – Seth Shill Jun 9 '19 at 21:19
• Maybe the servodrive is not configured to apply maximum torque. Check the current and torque limits programmed into the drive. – mkeith Jun 9 '19 at 21:32

You are mistaken.

• The torque controls the current.

Not vice versa.

That maximum torque your servo is specified at is the maximum you may apply without damaging it. This is most likely due to the limitations of the gearbox, because electrical motors are extremely forgiving. If you overload them, the only thing happening in first place is overheating, which is okay if you give the motor a rest to cool down afterwards. But the gears inside the gearbox may break mechanically if you apply too much torque, and they fail in an instant.

What you should not do is stalling a DC motor completely however, because the high current can damage the commutator surfaces if it's not moved.

• "The torque controls the current" is actually true, if you drive a bare DC motor with a constant current. If the motor is in a servo loop, then the servo will increase the current until it moves or until a current limit is hit -- but that's not a universal truth. – TimWescott Jun 9 '19 at 20:19
• Ah I see I made a mistake. My motor is actually AC type (I'm not sure induction or synchronous - it doesn't say). And I set the torque via a current setting on the motor. I've edited my post to more accurately reflect my setup, thanks for the comments! – Seth Shill Jun 9 '19 at 21:21
• The relationship between torque and current is the same for AC motors. However, the speed of maximum torque differs drastically for different motor types. For AC/DC universal motors, it's the stall torque, while for AC induction motors it may range from stall to 4/5 of synchronous speed, depending on the rotor design. – Janka Jun 9 '19 at 21:45
• From the datasheet of the servo, the motor is an AC/DC universal type. An AC motor couldn't reach more than 3600 RPM at 60Hz. – Janka Jun 9 '19 at 21:51
• Thanks @Janka. Where did you find that in the datasheet? I was looking for that info. – Seth Shill Jun 9 '19 at 22:37

The maximum torque occurs at the maximum current with a Torque Constant= 1.96 in-lb/A and Rated Current: 5.7 A rms, Peak Current: 17.1 A rms

There may be current limiting or driver losses but any position error ought to be amplified with enough proportional gain to drive it at maximum voltage until current or thermally limited.

Part Number: J0400-301-4-000
Frame Size: 60 mm
Continuous Torque: 11.23 in-lb
Peak Torque: 33.63 in-lb
Rated Voltage: 120 VAC
Rated Power: 400 watts
Rated Speed: 3000 rpm
Peak Speed: 6000 rpm
Rated Current: 5.7 A rms
Peak Current: 17.1 A rms
Torque Constant: 1.96 in-lb/A
Voltage Constant: 13.8 V/krpm
Armature Resistance:  0.85 Line-to-Line ohms

• Thanks @Sunnkyskyguy EE75! However I'm not sure how these statistics will be useful for me. Right now I'm not even getting close to the continuous torque (based on multiply force recorded with a luggage scale by radius) when applying the highest value for the control input signal. Is there something important about these values to use in my experiment? – Seth Shill Jun 9 '19 at 22:53
• If you aren't getting 5.7A rms out of your "invisible" controller with a locked rotor, you cannot get the rated torque. At 400W that would get hot yet rated for it and you should see 400/5.7A= 70V rms across the motor when stalled for max torque. When the servo is unlocked and reaches servo position, the torque and current reduces to 10% unless disturbed. You should try to understand every spec in this datasheet. – Tony Stewart EE75 Jun 10 '19 at 0:35