How To Understand the Difference Between Servo & Stepper Systems

Hi, Adam Ring here and I’d like to talk a little bit about the differences between stepper motors and servo motors. There is a difference in the way they are structured and controlled. Stepper is typically 50 – 100 pole brushless motor where a servo is basically a 4 – 12 pole brushless motor. The stepper typically doesn’t have any kind of encoder for feedback, sometimes you can add it later on as an option where a servo is always built with some sort of feedback device whether it be an encoder or a resolver in order to tell its drive the position of its motor shaft.

Now if we take a look at the speed torque curve for each of these motors they’re very different. A stepper motor has a lot of torque at slower speeds. In fact quite a bit of holding torque because of so many pulls it can have a lot of holding torque at low speed and then the faster that you run, that torque actually starts to drop off. But you look at the speed torque curve of a servo motor it’s actually very, very flat and you get a constant torque regardless of what speed you’re traveling up until its maximum speed.

If we look at the amount of current that a particular motor draws, a stepper motor typically is going to draw full current whether it’s standing still or running. In some cases drives that you use with them may have a feature where you can reduce the current if they sit still for a long enough time, but other than that they are basically just drawing full current all of the time which also produces extra heating, as you can imagine. Servo motors on the other hand use the amount of current that’s required in order to get the motor to hold or move as demanded by the application. So typically they are going to run a lot cooler and they also have a neat little feature that enables them to produce on occasion 2 – 3 times their rated torque for short periods of time as a peak rating.

Let’s look at the cost of installing a stepper or a servo motor. As you can imagine, a stepper motor is typically going to cost less. The motor construction is simpler, you don’t typically have the feedback and the electronics to control the motor are typically simpler and less expensive. With a servo motor obviously they are going to have some type of feedback device built into the motor and then the drives that we use to control them typically are a little more involved when it comes to how they are designed and built and typically costs a little bit more.

A little bit about the application these might be used in: the stepper motor is used in applications requiring about 2000 rpm or less where you need a lot of torque at the low end, whereas a servo motor is typically used for your higher speed applications that are more dynamic and require more acceleration and deceleration typically 2000 rpm and higher.

So depending on whether you’re doing indexing moves with a light load or really dynamic moves with higher loads you can choose whether to use the stepper or the servo motor. Whether you’re in the market to purchase a stepper motor or a servo motor or not really sure which one is the best choice, visit innovativeidm.com we have an excellent knowledge center where you can come and research differences, catalogs, white papers, videos and learn more. After all we are Innovative-IDM, Home of the Legendary Customer Experience.

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