Have you ever picked up a motor and discovered it had a few more wires then you were expecting? It could have been a stepper motor. Stepper motors are very commonly used in a number of products you might use every day. You can find them in everything from wrist watches to printers, IV pumps to gas pumps. Stepper motors are also popular in machine tools, process control systems, tape and disk drive systems, and programmable controllers.

While in school most technicians learned about motors, but as common as stepper motors are it is amazing, how few technicians know, or still remember, how stepper motors work, and how to troubleshoot and test them. There is much confusion about the differences between Unipolar and Bipolar types of stepper motors. There is confusion about how Constant Current and Constant Voltage types of stepper motor drives work. Why do I use a 5V motor when I have a 24V supply? But don’t feel bad. There are a number of engineers who do not understand either. This article will cover an overview of stepper motors; what they are and how they work. Then we will discuss how to troubleshoot them.

Stepper motors what they are and how they work

A stepper or stepping motor converts electronic pulses into mechanical movement. Each electronic pulse “step” causes the shaft to rotate a certain number of degrees (step angle). Thus, a stepper motor can be operated in an open loop application, which means it can be “told” to go a certain distance at a certain speed without the need for any type of feedback. Figure 1 illustrates a simple application for a stepper motor each time the controller receives an input signal, the paper is driven a certain incremental distance. Another important characteristic of the stepper motor is it can maintain the holding torque indefinitely when the rotor is stopped with out burning up the motor windings. When a stepper motor has a steady DC signal applied to one stator winding, the rotor will overcome the residual torque and line up with that stator field. The holding torque is defined as the amount of torque required to move the rotor one full step with the stator energized. When no power is applied to the windings, a small magnetic force is developed between the permanent magnet and the stator. This magnetic force is called the residual, or detent torque. The detent torque can be noticed by turning a stepper motor by hand and is generally about one-tenth of the holding torque.

How does a stepper motor work? Figure 2 illustrates a typical 1-phase step sequence for a 2-phase motor. In Step 1, phase A of a two-phase stator is energized. This magnetically locks the rotor in the position shown, since unlike poles attract. When phase A is turned off and phase B is turned on, the rotor rotates 90° clockwise. In Step 3, phase B is turned off and phase A is turned on but with the polarity reversed from Step 1. This causes another 90° rotation. In Step 4, phase A is turned off and phase B is turned on, with polarity reversed from Step 2. Repeating this sequence causes the rotor to rotate clockwise in 90° steps.

Nippon PulseYour Partner in Motion ControlNPM Basics of Stepper Motors

Figure 1

Figure 2

There are three main types of stepper motors: permanent-magnet (PM), variable reluctance (VR) and Hybrid. The permanent-magnet (PM) stepper motor operates on the reaction between a permanent-magnet rotor and an

electromagnetic field. One of the most common PM motors are of the Tin Can or claw-tooth type (Figure 3).

In these tin can steppers, the rotor shaft is surrounded by a magnet with radially opposing poles. It has no teeth. The stator is a series of poles with wound wire coils. Because of the magnet, the rotor will resist movement even when the motor is not energized.

Permanent magnet step motors are used in low-cost, low-power applications, the bill feeder inside vending machines is driven by a permanent magnet step motor.

The variable-reluctance (VR) stepper motor differs from the PM stepper in that it has no permanent-magnet rotor and no residual torque to hold the rotor at one position when turned off. This type of motor operates on the principle of minimizing the reluctance along the path of the applied magnetic field. One of the first uses for Variable-reluctance stepper motors was to move the direction indicator of torpedo tubes and guns on British warships in the 1920’s. Shortly thereafter they were employed by the US Navy for a similar purpose.

The hybrid step motor consists of two pieces of soft iron, as well as an axially magnetized, round rotor. The term hybrid is derived from the fact that the motor is operated under the combined principles of the permanent magnet and variable-reluctance stepper motors. The stator core structure of a hybrid motor is essentially the same as its VR counterpart. The main difference

is that in the VR motor, only one of the two coils of one phase is wound on one pole, while a typical hybrid motor will have coils of two different phases wound on the same pole. The two coils at a pole are wound in a configuration known as a bifilar connection. Each pole of a hybrid motor is covered with uniformly spaced teeth made of soft steel (Figure 4). The teeth on the two sections of each pole are misaligned with each other by a half-tooth pitch. Torque is created in the hybrid motor by the interaction of the magnetic field of the permanent magnet and the magnetic field

produced by the stator. Most hybrid steppers are NEMA size motors (Figure 5). The windings for steppers come in two types Bipolar and Unipolar. There are a number of advantages to each type of winding.

The two-phase stepping sequence described earlier utilizes a “bipolar coil winding.” Each phase consists of a single winding. This is referred to as a bipolar winding because the current flow, on the coils is reversed. By reversing the current in the windings, electromagnetic polarity is reversed.

Another common winding is the unipolar winding some times called four-phase steppers. This consists of two windings on a pole connected in such a way that when one winding is energized a magnetic north pole is created; when the other winding is energized, a south pole is created. This is referred to as a unipolar winding because the electrical polarity, i.e. current flow, from the drive to the coils is never reversed.

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Figure 3

Figure 4 Figure 5

Testing Stepper motorsTypically there are four things that can go wrong with a motor in general. 1. They burn up. 2. The brushes go bad. 3. The

bearings go bad. 4. The technician breaks them. Lets look at each of these in respect to the stepper motor. 1. They burn up.

An important characteristic of the stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. If a stepper motor stalls out it will not typically burn up as with most AC and DC motors. If the motor does burn up it typically indicates a driver problem. We will get more in why this is the case in the next article. Just replacing the motor will cause the motor to burn up again. This is a very uncommon problem with stepper motors unless there is a bad driver causing it. 2. The brushes go bad.

There are no brushes in a stepper motor. Therefore, this will never be a cause of failure. 3. The bearings go bad.

The cooler the motor stays the longer the bearings will last. But at times the bearings will go bad. Still this is not a very common problem. The bearings in most cheap motors are rated at 3K hours or more, and most high-end quality motors are rated at 90K to 100K hours.4. The technician breaks them.

OK, this is the most common cause of failure for stepper motors. When working with stepper motors be careful with them. They do not need to be handled like fine china but you cannot fix one with a hammer. Most cheap motors only use glue to hold the shaft to the rotor, and most quality steppers will use grooving along with adhesive.

TestingYou want to test the motor. First us an ohm meter. It will help you tell if a winding is burnt up and

what type of stepper motor you have. Most likely you will have a Bipolar or Unipolar motor. A Bipolar will always have 4 leads. A Unipolar will have 5 or 6 leads. If 5 leads, the two common wires are connected. A few motors you run into will have 8 leads, these can be wired as either a Unipolar or

Bipolar. Using the Ohmmeter, you can check the resistance of the windings. On a Bipolar the resistance for both winding should be the same in both directions (Figure 6). In a unipolar winding the resistance from each phase to com should be the same in both directions (Figure 7). After you have checked the motor with the ohmmeter, you can use a 9V battery to step the motor through its paces. This will confirm the motor windings are good.

Also turn the motor by hand while listening for bad bearings. All PM and hybrid stepper motors will have some detent torque. PM will have more than the hybrid steppers. If the leads of the stepper

motor are touching the detent torque will be greatly exaggerated. Be careful! Some technicians have falsely linked this to bad bearings. If the bearings are bad, there will usually be extra axial play in the motor. If possible check against a known good motor.

When replacing a motor many technicians wonder what the color code for the wires is. Remember the windings make up an electromagnet. So as long as you have the windings grouped correctly (Phase 1 and 3 together and Phase 2 and 4 together) the worst that will happen when you go to run the motor is that it will run backwards. If it does just swap one set of phases (1 and 3 or 2 and 4).

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Figure 6

Figure 7

The Nippon Pulse Advantage For 60 years, Nippon Pulse has built state-of-of-the-art products based on a solid foundation of advancing technology and thorough product research.

Nippon Pulse faithfully provides these high-quality products to a wide range of industries in North and South America and Europe. Nippon Pulse has established itself as a leader in stepper motor, driver, and controller technology while introducing innovative products such as the Linear Shaft Motor and Motionnet®. At Nippon Pulse, we believe by bringing products to market which not only meet customers’ requirements, but actually impress them, we contribute to the progression of technology and its positive impact on our society. We pride ourselves on the reputation of our high-quality products that provide that impact. A wholly owned subsidiary of Nippon Pulse Motor Co., Ltd., Nippon Pulse America is headquartered in Radford, Va.

Nippon Pulse has representatives throughout North and South America and Europe to directly assist customers. Limited quantities of stock on standard motors and electronics are available to allow faster response to customer needs. In addition, Nippon Pulse has a model shop in its Radford, Va. headquarters for quick turnaround on custom prototypes and special orders. Nippon Pulse’s mission is to faithfully create new products sought by its customers and to contribute to the development of society from a global viewpoint.

When you choose a Nippon Pulse motor, driver, controller, network or stage, you’re doing more than just buying a quality product. You’re benefitting from what we call the Nippon Pulse Advantage. This includes superior prototyping, complete system engineering, proper compliance and certification according to international guidelines, and exceptional tailoring to your needs. It also includes product support unmatched in the motion control industry.

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