Stepper Motor Theory: how do stepper motors work and how do different specifications and factors affect their performance?

Stepper motor theory: the basics

A stepper motor is a motor that is designed to take steps rather than simply rotate immediately that power is applied. Different designed stepper motors will have a different number of steps but the two most common have 200 (1.8 degree) or 400 steps (0.9 degree).

As a result of their completely different design it is typically used in quite different applications to DC or Brushless DC motors (although there are some crossovers).

The only way to get a stepper motor to work in the manner for which it is intended is with a dedicated stepper motor driver or controller. If you try and connect the cables on a stepper motor to a power source (as you would do with a brushed DC motor) it will simply lock the motor into one position (and start to warm up if you leave it connected for a while!).

The Royal Navy pioneered the use of stepper motors in gun turrets in the 1930s.

A brief history of the stepper motor and its development

The question as to who actually invented the stepper motor is still open to some debate in part because original versions were not immediately known as stepper motors.

However, for most engineers (ourselves included) it is largely attributed to Frank W. Woods who patented a motor based on 5 stator coils which could be charged in various combinations to deliver step by step movement.

The first recorded example of a stepper motor being used in a practical application was by one of the biggest drivers of innovation in the 18th, 19th and 20th century, the British Royal Navy. The system was developed in the 1930s as a means of controlling gun turrets and cannons on large ships and similar systems remain in use today.

In the 1960s this type of basic stepper motor began to be superseded by large angle permanent magnet stepper motors similar to the types commonly in use today.

However, these motors suffered with a number of issues. Positional accuracy was limited due to the absence of accurate stepper motor controllers and resonance issues within the motor casings would often cause the motor to have to be stopped and restarted.

Throughout the 1970s and especially the 1980s and 1990s major advances were made in developing controllers which could address some of the resonance issues found in stepper motors, as well as manufacturing developments which reduced the cost of stepper motors. However, stepper motors at this time remained expensive and were typically used in defence and aerospace applications.

By the early 2000s these developments were so significant that the cost of stepper motors and stepper motor controllers began to fall, enabling them to be used in a range of applications where they were previously too expensive to use.

Why is taking steps useful for a motor?

By being able to take specific steps it is possible to accurately control the rotation of the motor down to percentages of a degree with incredible accuracy.

If you imagine a clock face with one hand, a ‘traditional’ DC motor would only be able to rotate at a consistent speed.

Any positioning would have to be controlled by time or by using a closed loop system with an encoder to determine position. However, with a stepper motor it is possible to quickly and simply move the hand from any position on the clock to any other position at whatever speed is required.

Depending on the stepper motor controller that you are using it is possible to manage the acceleration and deceleration of each movement and even programme specific sequences.

What is inefficiency?

The first thing to understand is the internal design of the stepper motor. The stepper motor is a type of brushless motor (only in the sense that it has no brushes) and it has the magnet directly attached to the shaft at the centre of the motor.

What makes this different from other motors is that the magnet has teeth around it, rather like the teeth on a cog. In fact it has 2 sets of teeth around the rotor which are offset and which have the north and south poles alternating.

The actual coils (which are powered on and off by the stepper motor controller) are mounted on the outside of the motor.

How a stepper motor turns

A typical stepper motor will have 2 sets of coils arranged opposite each other (180 degrees apart).

In order to get the motor to turn the coils are turned on, with one positive and the other negative. This creates a dual push/pull effect in the stepper motor which will move it around one step.

After one step is complete, the other pair do the same thing and the motor turns another step.

As this process is sped up by the stepper motor controller being used, the motor will start to turn more fluently (rather than a step, step, step, step process) and can reach speeds of up to 1000 rpm.

This process is then repeated through the four stages:

1. Coil 1 positive, coil 3 negative = 1 step

2. Coil 2 positive, coil 4 negative = 1 step

3. Coil 1 negative, coil 3 positive = 1 step

4. Coil 2 negative, coil 3 positive = 1 step

Depending on the type of controller that you have it is possible to include microstepping. Microstepping is a clever way of increasing the number of steps possible in a motor which only has 200 mechanical steps by introducing fractional control over the input electrical signal.

The best possible type of motor for applications such as dosing where positional control is essential

A typical stepper motor such as the NEMA 23 stepper motor in the ZD4N2318 actually has 200 possible steps in one complete 360 degree rotation. This is the most common stepper motor configuration, but there are other types which have more (for example the ZDSPN1709 has 400 steps).

With the standard 200 step stepper motor we therefore have 1.8 degrees per step (assuming we are operating in full step mode).

If you consider that a stepper motor controller such as the Zikodrive ZD2 can operate at up to 128 microstep resolution (meaning it has 128 individual ‘microsteps’ in 1 full step) then it is clear that a stepper motor can deliver exceptionally accurate positional accuracy.

This makes it hugely useful in applications such as pump applications or process control applications where highly accurate positioning can make the difference. Zikodrive stepper motor controllers are widely used in pump and process control applications.

The importance of the stepper motor controller in dictating the actual motor performance

As you can no doubt imagine from the description above, one of the most important factors in shaping stepper motor performance is the stepper motor controller which is used.

Quite simply, without a controller, a stepper motor will not be able to offer you anything in the way of mechanical performance apart from a locked shaft.

A very simple stepper motor driver will be able to turn a stepper motor but will not provide a major range of control and performance options which will help you really benefit from the performance features that a stepper motor can offer.

However, an advanced microstepping programmable controller such as the Zikodrive ZD4 will provide a comprehensive range of performance from the stepper motor of your choice. This type of controller can offer highly accurate positioning and can be set up to deliver the performance you need with a range of additional safety features such as over current protection, reverse polarity protection and more.

All of this can have a major bearing on the way in which the motor will perform, the life of the motor and controller and the efficiency of the entire system.

If you understand the basic principles outlined above and know that this is the right type of motor for your application you can start by browsing our range of stock stepper motor packages.

These will give you a good idea of the type of features available with our range and also the power and speed ratings you may be able to achieve.

If you have any additional questions on top of this you can always contact us to discuss it.

We try and maintain the online chat facility between 9 and 5 GMT (although we are not always able to do so if we are all busy) but if not you can always send us an email or give us a call to discuss your project and what is required to make it a success.