What is the difference between a servo and a stepper motor?

According to engineering truism, there is nothing called a universal-solution-for-all just because you have got the best alternative for a specific problem in hand. The same rule holds particularly for the stepper and servo motors. Both of these motors are widely used in the industrial zones as both are equally effective in delivering high power, only if you know their utilization. However, in order to make the right choice for your application, the factors to be considered are: 

  • Budget
  • Speed, and
  • Acceleration 

Servo Motors

A servo motor consists of a fixed stator with stator windings and a rotating rotor including permanent magnets. As the current passes through stator windings, the magnetic flux generated acts on the magnetic field distribution of the rotor which develops a torque causing it to rotate. A servo motor has a lower number of poles compared to stepper motors. So, a servo must be run in a closed-loop system.

The diameter of a servo motor  typically ranges from NEMA (17 to 200)mm. The maximum torque it can deliver is about 250 foot-pounds.

The combination of higher speed and hence, higher torque enables a servo motor to deliver relatively higher acceleration than a stepper motor. Moreover, a servo motor ensures improved and precise positioning of closed-loop operation.

Stepper Motor

A stepper motor comprises a stationary stator that carries windings and a rotor containing  permanent magnets. When current passes through stator windings, the stator generates a distribution of magnetic flux interacting with the magnetic field of the rotor resulting in a turning force. Stepper motors have more pole-pair counts, typically 50 poles or more. The driver powers up each pole in a sequence so that the rotor steps up incrementally and moves in a consistent motion (due to its high pole counts).  

Stepper motors are generally available in moderate frame sizes, not greater than NEMA(National Electrical Manufacturers Association) 34, as most of the stepper motor applications fall within the range of NEMA (17 to 23) motor sizes. Resultantly, it is uncommon to find a stepper that is capable of generating torque, higher than (1,000 to 2,000) ounce inches.

Servo Motor Vs Stepper Motor in terms of Motion Control Applications

The overall number of poles in the two motors is the primary difference between them. Where a servo motor has a lower pole count of 2 to 7 pairs, a stepper motor has a much higher pole count, ranging from 25 to 50 pairs of poles. 

The difference in the number of the poles indicates that a servo motor needs an encoder for controlling position through pulse adjustment. Conversely, a stepper motor incrementally moves with a continued pulse within a closed loop system. 

Pros and Cons in Motor Control

Servo Motors 

Pros: Servo Motors provide high torques at high speed which is something that a stepper motor cannot do. A servo motor operates at an efficiency ranging from (80 to 90)%. It can work in both DC and AC drives, and does not get influenced by resonance issues or vibrations.  

Cons: A servo motor has a number of benefits, albeit there is a major setback in terms of its cost. A servo motor is costlier than a stepper motor. With the additional cost of a gearbox and an encoder included, the entire system becomes even costlier. Also, the requirement of both of these components makes the motor system mechanically more complicated, leading to frequent maintenance and hence, higher expenditure.

Stepper Motors 

Pros: A Stepper motor, for its higher number of poles, ensures accuracy in drive control for motion control applications. This type of motor  delivers high torque at low speeds. Stepper motors are relatively affordable and, hence,  easily available on the market.

Cons: However, there are certain limitations in 

a stepper motor. The motor loses its torque to about 80%, when applied at high speeds. They generate higher levels of vibrations and are susceptible to resonance issues. Moreover, the high amounts of heat produced by a stepper motor can be a problem in some applications.

Both the motor types have their respective set of benefits and setbacks, knowing which can help you decide which type of motor is right for your application. 

What to Consider While Settling for the Right Motor?

  • Budget
  • Load mass and inertia
  • Speed requirements
  • Torque requirements
  • Favorable acceleration
  • Size limitations

A servo motor guarantees undeniable advantages in its performance. However, a stepper motor is more competitive than a servo, when concerning the repeatability. The nature of the mass-spring of a stepper motor may cause a loss of a few steps. Since the drive is directing the stepper to move in an angular direction, the lost steps aren’t taken over from one rotation to the other, however, featuring  high repeatability.

Finishing: Which one is Better? 

Witn all considerations made above, the ultimate factor which sparks the further differentiation between a servo motor and a stepper motor is their cost. If you are running on a tighter budget, then there is no further looking for other considerations than choosing a stepper motor. A stepper uses inexpensive magnets and it seldom has the requirement of gearboxes in its system. 

The high pole count and ability to generate high torque at low speeds and consumption of less power makes the motor an ideal choice in terms of low expenditure. Moreover, it is a compact option and has its applications in biotech, medical, semiconductor manufacturing,  and security and defense sectors. 

Conversely, if cost is no bar to you and your primary requirements include high accuracy, low speed and low acceleration, then go for a servo motor. A servo motor is a good fit for systems typically utilized in web processing, converting, packaging, and similar other applications. Overall, if performance is the ultimate purpose behind the purchase, then consider availing a servo motor. 

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