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Posts from — April 2013

Stepper Vs Servo Motor Torque Curves Part IV

NEMA17 Torque CurvesNEMA17 Torque Curves

My final look is at NEMA17 motors.  Today’s contestants are:

  • In yellow, the IMS (now Schneider) MDI1PRL17C4x triple stack NEMA stepper motor with integrated driver and controller.  I’ve used these cute little motors before; they are a great fit for the right application.  As normal, the programming language sucks, but a CANOpen version is available.
  • In red and blue, a Panasonic MUMS011A1 NEMA 17 servo motor.  These motors have unfortunately been out of production for years; I loved their performance, encoder (10000 cpr), good looks, and price (about $250).
  • In green, a Quicksilver Controls QCI-A17H3 stepper motor with encoder that’s driven like a servo.  I’m including it to show how much improvement you can get from closed loop stepper control.


  1. The MDI1PRL17C4 shows typical stepper characteristics, with torque rapidly dropping off; it can’t even reach 2500 RPM.
  2. The MUMS011A1 shows typical servo performance, with a pretty flat torque curve, 3:1 peak to continuous torque, and compared to the steppers:  higher speeds, less continuous torque at low speeds, more continuous torque at high speeds, and much higher maximum torque.
  3. The QCI-A17H3 doesn’t turn the stepper into a servo, but compared to an open loop stepper, it has substantially higher maximum speed (4000 RPM – the highest of any stepper I’ve looked at), and offers significantly higher torque at higher speeds — in exchange for a higher price, of course.

Some General Stepper & Servo Notes

  1. Steppers are much simpler to drive: you can easily build a low cost drive using an integrated chip (e.g. from Allegro Microsystems, ST, or TI) or buy a commercial driver for $100-$150 (from Automation Direct, Gecko Drive, etc).  Many low cost PLC’s such as Panasonic’s FP0/FPG have built-in step/direction outputs.
  2. But open loop steppers are very annoying, because you have to figure out your needs before start: you have to specify fixed currents for holding and running torque.  If your current is too low, the motor will miss steps.  If your current and duty cycle are too high, the motor will get hot – and if a bit of extra torque is needed to overcome something unexpected, you’re out of luck.
  3. Closed loop steppers still aren’t servo motors.  However, I’m looking forward to affordable sensorless closed loop control (available or coming from Trinamic, TI, and probably others), which will improve stepper performance and minimize stepper heat generation.
  4. I think servo motors’ torque curve fits well with many applications: a lot of time you just need maximum torque for a short time period.  Servo motor system prices are also coming down (OK, not the basic motor, but there are affordable encoder, drive, and controller choices).


April 26, 2013   No Comments

Stepper Vs Servo Motor Torque Curves Part III

NEMA23 Single Stack Motors

In this post, I’m going to look at some fun NEMA23 single stack servo motors and a single stack stepper motor.  The entrants are:

  • In brown, the PacSci M21 with Sigmax technology.  The M21 has a lot more torque than many (most?) NEMA23 single stack steppers.
  • In blue and red, one of my favorite motors: the BH02300 servo motor.  I’ve spun one of mine at 20,000 RPM.  Like the Emoteq QB series, the BH series has great peak torque.
  • In yellow and green, the MCG (now Ametek) I2351014NC servo motor.  After all the high end motors, I decided it was time to feature a non-exotic and affordable (if you can get it) motor.


  1. The difference between the I2351014NC and the BH02300 is the peak torque (the BH02300 has much more) and maximum speed (20,000 RPM vs 12,000) and, of course, price; continuous torque is about the same.
  2. Both servos are can go much, much faster than the M21 stepper (although at 3000RPM, it’s pretty fast for a stepper motor).
  3. The M21’s continuous torque looks pretty good; it always has more continuous torque than either servo, and below about 2000 RPM has more continuous torque than the I2351014NC has peak torque.
  4. OK, it’s not really far to compare these motors — they obviously have their own areas (low end torque and low cost for the M21, high speed and moderate cost for the I2351014NC, and high speed, high peak torque, and high cost for the BH02300) but it’s fun to see big differences between motors that are all about the same size.


April 25, 2013   No Comments

Stepper Vs Servo Motor Torque Curves Part II

NEMA34 Torque Curves

NEMA34 Torque Curves

Today I am examining some interesting NEMA34 motors’ torque curves.  The graph is a bit complex because I want to show a variety of motors on one graph; to make it a bit simpler, I am using dashed lines for servo continuous torque, and solid lines for servo motor peak torque.

I chose motors that are all roughly the same size.

The motors and their colors are:

  • Burgandy Red – PacSci N32 PowerPac double stack NEMA34 stepper motor at 75V.  I choose this motor because it’s a high end stepper, and I own a couple.
  • Red – PacSci K32 PowerPac double stack NEMA34 stepper motor with Sigmax technology, also at 75V.
  • Yellow – Emoteq QB03402 double stack NEMA34 servo motor.  I own a similar motor (QB03403), plus the peak torque is very high.
  • Green – Parker Compumotor BE342H double stack NEMA34 servo motor at 170V.  Parker makes some really nice servo motors; the BE series has a lot of torque, and I want to look at the effects of voltage on torque curves.  The BE342H and BE342K have different windings.
  • Brown – Parker BE342H at 340V.
  • Dark Blue – Parker BE342K at 170V
  • Light Blue – Parker BE324K at 340V.


  1. The NEMA34 stepper curves are similar to the NEMA23 stepper curves; torque still drops off rapidly with increasing speed.  One quirk: maximum torque is around 120 RPM, not 0 RPM.
  2. PacSci’s Sigmax technology does provide significantly higher torque at all speeds, but does not change the shape of the torque curve.
  3. Overall stepper vs servo comparison is similar: the steppers have much more continuous torque at low speeds, less continuous torque at moderate speeds, less peak torque at all speeds, can’t handle high speeds, and cost significantly less than servos.
  4. The Emoteq BH03402 has exceptional peak torque, but you’ll have to provide a lot of current (e.g. 50A for the 130V C windings).
  5. The Parker BE342 shows the impact of voltage and winding.  When the servo motor does not get enough voltage, its torque can decrease like a stepper, but for a different reason: back EMF.
  6. The BE342K might seem better than the BE342H, since it has the “best” torque curve, but that comes at price: the same torque requires double the current of the BE342H.
  7. Stepper currents are much lower; the maximum current of any N32/K32 model is 10A.
  8. As always, it comes down to knowing your requirements: torque, speed, size, current, budget, etc.



April 24, 2013   No Comments