characteristics:-

high dynamics…

wide speed control range ….

compact dimensions due to low power/weight ratio ….

optional low-cost motors with barium ferrite magnets or motors with very high performance density thanks to samarium cobalt magnets …….

long brush life due to generously dimensioned collector and long brushes ……..

ball-bearings greased for life …..

optional with tachogenerator and/or incremental encoder

smooth running ….

insulation according to insulation class F, DIN VDE 0530, optional withstanding tropical conditions …..

easy connection of the motor with standard connection cable, optional connectors or terminal box …

self-cooling …

design according to DIN 42 677, installation position as required ….

bearing plates and housings made of high-quality light metal alloy …….

rotor dynamically balanced according to vibration severity grade N ….

standard shaft end without groove, special version possible, e. g. with keyway ……

special motors, e. g. short motors, hollow-shaft design available ….

holding brake optional ……….

a domestic light dimmer.

DC servo motors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these servo motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens servo motor response to command signals. Servo motors include permanent-magnetic, printed-circuit, and moving-coil (or shell) dc servo motors. The rotor of a shell dc servo motor consists of a cylindrical shell of copper or aluminum wire coils which rotate in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core. The servo motor features a field, which is provided by cast AlNiCo magnets whose magnetic axis is radial. Servo motors usually have two, four, or six poles.

Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.

DC Servo Motor Selection: The first selection approach is to choose a servo motor large enough for a machine that has already been designed; the second is to select the best available servo motor with a specific feature and then build the system around it; and the third is to study servo motor performance and system requirements and mate the two.

The final servo motor system design is usually the least sophisticated that meets the performance specifications reliably. Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances. This says that the servo designer must define the system carefully, establish the servo motor’s performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servosystem and choose a servo motor type.

universal motor

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The Universal Motor is a motor with a wound armature and a wound stator. The armature is fed via brushes on a commutator, and is essentially the same as a D.C. motor. The universal motor will operate off a single phase A.C. supply and accelerates until the load torque equals the output torque. Domestic appliances, such as vacuum cleaners, and small hand tools such as electric drills use this technology. The speed is changed by reducing the voltage applied to the motor. This is often a triac based voltage controller similar to

Electric motors convert electrical energy into mechanical motion and are broadly classified into two different categories: DC (Direct Current) and AC (Alternating Current). Within these categories are numerous types, each offering unique abilities that suit them well for specific applications. In most cases, regardless of type, electric motors consist of a stator (stationary field) and a rotor (the rotating field or armature) and operate through the interaction of magnetic flux and electric current to produce rotational speed and torque.

Page Contents

Brush DC Motors

BLDC Motors

AC Motors

Stepper Motors

Reluctance Motors

Universal Motors

In the United States, for both AC and DC motors, the Institute for Electrical and Electronic Engineers (IEEE) establishes the standards for motor testing and test methodologies, while the National Electrical Manufacturers Association (NEMA) prepares the standards for motor performance and classifications.

Basically, electric motors are divided into three broad horsepower (hp) categories: small, medium and large. The most common motors are considered to be fractional-horsepower motors with ratings from 1/20 to 1 hp and are categorized as small motors. Also included in the small category are motors with smaller ratings, which are commonly classified as sub-fractional or miniature. Medium size motors are considered to be in the range of 1 hp through 100 hp, with large motors occupying the 100 hp to 50,000 hp range. Categories can vary due to motor type (AC or DC), speed, number of poles and maximum ratings, so the categories listed should be viewed with this in mind. Since medium-sized motors are also used in large quantities in industrial and consumer applications, NEMA’s MG1 standard permits motor comparison between manufacturers. With large motors, however, there is no standardization, since motors in this category are normally designed for specific applications.

This paper discusses the most common and popular motors in today’s market and offers insight into their strengths, weaknesses and suitability for various applications.

Brush DC Motors

One of the earliest of all motor types, the brush DC motor employs carbon brushes and highly-conductive, slotted slip rings to form a mechanical switch (commutator) that creates current in the armature (rotor) to produce a magnetic field. An interacting magnetic field generated by the stator causes the armature to rotate. The term commutating machine is used to describe any type of motor using a commutator ring, including the universal motor type described in a later paragraph.

Two types of brush DC motors are commonly available today: separately excited and series-wound. A third type, the shunt-connected motor in which the armature and field windings have the same DC source, is not widely used because of the lack of speed control capabilities imposed by the winding connection.

Separately Excited DC Motor

In many traction applications where both armature voltage and stator current are needed to control the speed and torque of the motor from “no load” to “full load”, the separately excited DC motor is used for its high torque capability at low speed achieved by separately generating a high stator field current and enough armature voltage to produce the required rotor torque current. As torque decreases and speed increases, the stator field current requirement decreases and the armature voltage increases. Without a load (known as “zero torque speed”), the speed of the separately excited motor is strictly limited by the armature voltage and stator field current. Separately excited DC motors are the first type of motor to use closed-loop control and can also be used in servo systems for control of speed and/or position.

Series-Wound DC Motor

In many simple brush DC motor applications used in appliances, a series-wound brush DC motor is chosen because of its low-cost speed control possibilities. Series-wound DC motors are also found in high-power traction drives. In this motor, the stator and rotor are connected in series across the voltage source, producing equal operating current in both. By using a simple chopper or phase-controlled rectifier circuit to control the applied voltage, the DC voltage and speed of the motor can be controlled. The major drawback of this motor type is that if a “no load” condition occurs (“zero torque speed”), the motor could accelerate beyond its mechanical design limit and fail. This is particularly true because the material used to create the commutator is highly-conductive and relatively heavy and has an inherently weak mechanical attachment structure.

While beginning to be displaced by newer motor technologies (i.e., “brushless” DC, permanent magnet, and electronically controlled induction and reluctance motors), brush DC motors are still very popular due to their relatively low cost, particularly in the smaller sizes, rugged construction and simple design. These motor types can be found in almost all wattage and horsepower ratings and are used primarily in variable speed and torque control applications. The only real drawback with this motor is that the brushes and commutator need periodic replacement due to wear caused by friction, heat and current flow. In recent years, these drawbacks have led to the development of the “brushless” DC motor, which is actually an AC motor and will be described later.

Brushless DC (BLDC) Motors

Synchronous motors are similar in design to the synchronous AC motors previously presented but are powered from a DC source and require electronic control to operate. Because of the DC source, the BLDC motor can be controlled like a brush DC motor. However, unlike the brush DC motor in which the armature windings reside on the rotor, the “brushless” DC motor employs permanent-magnetic field excitation similar to squirrel cage or field wound AC induction motors, eliminating the need for the commutator ring and carbon brushes.

In “brushless” DC motors, an electronic switching circuit produces commutating currents in the stator windings based on the position of the magnetic poles on the rotor. As with other synchronous motors, the rotor of the “brushless” DC motor rotates at the same speed as the stator commutation.

AC Motors

Invented in the same era as the brush DC motor, the AC motor itself is divided into two major categories: asynchronous (induction) and synchronous. When driven by a fixed-frequency AC source with a constant load, the induction motor operates near the frequency of the input source (or multiple thereof), while the synchronous motor will operate at the input source frequency (or multiple thereof). As the frequency of the source is varied, both motor classes will accordingly change rotational speed. However, as the load changes, the difference between the input line frequency and the rotational speed of the rotor for the induction machine will be greater than that of the synchronous machine because of magnetic slip (the difference in rotor speed versus stator speed in a motor) caused by induction.

AC motors rotate by producing a rotating magnetic field pattern in the stator that causes the rotor to follow the rotation of this field pattern. While induction machines produce rotor movement by inducing a magnetic field in the rotor, the rotation tends to lag and be asynchronous to the movement of the stator magnetic field. However, synchronous AC motors produce magnetic fields that cause the rotor to synchronize to the rotation of the stator magnetic field and tend to be more efficient than induction motors in applications requiring more than several hundred horsepower. In addition, synchronous motors are often employed with asynchronous motors in large industrial applications to stabilize voltage and improve overall power factor performance due to the synchronous motor’s ability to provide leading power factor.

Single-phase AC motors are extremely low cost and usually operate at a multiple of line speed for single speed operation. Poly-phase AC machines are the choice for higher-performance drives requiring more torque in smaller motor frames, a reduction in phase current demands and/or higher reliability (or “no stop”) operation. With the emergence of digital signal processors (DSP) and microcontrollers (MCU) combined with new power electronic devices, closed-loop control systems employing vector, direct torque and adaptive controls methods can be used to expand the low cost capabilities of AC motors into many new applications.

Asynchronous AC Motors

Asynchronous AC motors, also known as AC induction motors (ACIM), are probably the most widely used motor of the day. Induction motors are simple, rugged, reliable and easy-to-manufacture. They include single- and poly-phase designs developing power levels from fractional to thousands of horsepower. ACIM includes squirrel-cage and wound-rotor induction designs.

Squirrel-Cage – In squirrel-cage motors, the rotor consists of permeable metal containing embedded strips of magnetic material. As the stator magnetic field rotates, the field interacts with the magnetic field established by the magnetic poles of the rotor, causing the rotor to turn at nearly the speed of the rotating stator magnetic field.

Wound-Rotor – In wound-rotor designs, the permanent magnets used in the squirrel-cage motor are replaced by a rotor having windings. In this motor, when the stator magnetic field rotates and the rotor windings are shorted, the stator magnetic field motion induces a field into the wound-rotor, once again causing the rotor to turn at nearly the speed of the rotating stator magnetic field. However, if the rotor windings are connected externally through slip rings on the shaft, the winding current can be controlled to increase or decrease the slip speed of the rotor.

Synchronous AC Motors

Synchronous AC motors are not as widely used as ACIM because of having a more complex and expensive rotor design. Synchronous AC motor types include: permanent magnet, field wound, stepper and reluctance. Of these, both permanent magnet and field wound synchronous motors are used in “brushless” DC motor implementations.

Stepper Motors

Stepper motors permit rotation in small angle increments (steps) in response to individual control pulses applied to its windings. At low speeds (normally below 300-350 rpm), motor rotation is synchronized to the input control pulses. As the input pulse rate increases, motor slew caused by rotational slip prevents the motor from stopping after each pulse. Stepper motors are categorized as permanent-magnet (PM), variable reluctance (VR) or hybrid (a combination of PM and VR).

Reluctance Motors

The reluctance motor, commonly referred to as either a variable or switch reluctance (VR or SR), offers the simplest, lowest cost motor available to date. This motor consists of a shaped, highly permeable material for the rotor and two- or more-phase windings in the stator. The shape of the rotor concentrates magnetic flux lines into “poles” on the rotor which then interact with the rotating field being developed in the stator windings. The VR and SR motors must be electronically controlled to start turning and to produce the torque needed to continue rotation. Torque pulsations generated by this design tend to produce undesirable audible noise, leaving much research still to be performed. The potential for its low-cost application into many areas, however, makes it an extremely desirable motor to use in a broad range of products.

Universal Motors

The universal motor presents a special case for a small-series motor that can be driven by either an AC or DC current source. This motor type, which represents another class of the commutating machine, consists of a series-wound fractional-horsepower motor. The universal motor’s maximum speed is developed without load, while its greatest torque is developed at lowest speed. Universal motors are often found in vacuum cleaners, portable power tools, food processors, mixers and other small devices operating over a speed range of 3,000 to 10,000 rpm.

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It could be a universal motor or a DC motor. I have seen some 240V hair driers with a permanent magnet DC motor and a single half wave rectifier diode. Some tests: If there is a rectifier diode in series with the motor it is almost certainly DC only (keeping in mind it could only be used to reduce the average voltage by 50%). If it has permanent magnets for the field it is DC only. There would be a rectifier somewhere. If it has a laminated core for the field winding that is “universal” meaning AC/DC. It has to be able to reverse the field and the armature at the AC rate. Generally the armature (rotating part) will look similar for all types.

Servo Motor Characteristics

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The hair dryer motor is universal, ie it will run on AC or DC. The main difference between the two is that the AC motor has laminated field poles and is always a series connected motor. the only disadvantage of running a universal motor on AC is that you will get some sparking at the brushes.