Stepper Vs Servo Motor Torque Curves Part I
While I was working on my upcoming project, I got sidetracked by the issue of servo motors versus stepper motors.Â Since I’ve never seen direct stepper and servo torque curve comparisons, I created four comparison charts using a variety of interesting motors.
Disclaimer:Â I graphed all the torque curves myself, and their shapes should be pretty accurate; however, I didn’t have very good data for some motors (e.g. Emoteq peak torque).Â Motor pricing can very quite aÂ bit, depending on the exact model and quantity ordered, so any pricing is a rough guide.
If you click on the images, you will get a much bigger version.Â To avoid a megapost, I’ve split this topic into multiple posts.
Now that the preliminaries are out of the way, let’s start with the first contestants.
First up is a pair of motors: the Kollmorgen AKM21G single stack NEMA23 servo motor with 75V winding and the Pacific Scientific P22 double stack NEMA23 stepper motor.
I picked these motors because 1) they are both from the same company (Danaher), 2) I’ve used both, 3) they’re about the same size, 4) are designed for similar voltages, and 5) both are high end motors, so they should show off the best of their technologies.
The colors are as follows:
- Red – AKM21G peak torque
- Blue – AKM21GÂ continuous torque
- Green – P22 at 72V
- Yellow – P22 at 24V
- Notice how quickly the P22’s torque falls off at 24v – at slow speeds, torque is about 150 oz-in, but it’sÂ <50 oz-in by 1000 RPM (substantially less than the AKM21’s continuous torque).
- The P22 is considerably better at 72V – the torque curve is much flatter, and it has more continuous torque than the AKM21G up to about 2100 RPM.Â The higher drive voltages reduces the effects of the stepper motor’s inductance.
- Servo drives are the only option for higher speeds.Â The AKM21G is rated at 7800 RPM max, while the P22 torque curve only goes to 3000 RPM.
- The AKM21G has a much flatter torque curve, especially continuous torque.
- The servo’s peak torque is a big advantage: the peak torque available from the single stack AKM21G is ~50% greater than the maximum P22 stepper torque.Â The peak torque advantages gets much bigger as the speed increases.
- On the other hand, the servo is much more expensive: an AKM21G will run $400 or more, while the P22 is around $100.