Universal motor

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Modern low-cost universal motor, from a vacuum cleaner. Field windings are of copper wire, toward the back on both sides. The rotor's laminated metallic core is gray, with darker slots for winding the coils having high efficiency. The similarly shaped metallic commutator (partly hidden toward the front) has become dark from use. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing. Universalmotor 3.JPG
Modern low-cost universal motor, from a vacuum cleaner. Field windings are of copper wire, toward the back on both sides. The rotor's laminated metallic core is gray, with darker slots for winding the coils having high efficiency. The similarly shaped metallic commutator (partly hidden toward the front) has become dark from use. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing.

The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. [1] It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator. It is often referred to as an AC series motor. The universal motor is very similar to a DC series motor in construction, but is modified slightly to allow the motor to operate properly on AC power. This type of electric motor can operate well on AC because the current in both the field coils and the armature (and the resultant magnetic fields) will alternate (reverse polarity) synchronously with the supply. Hence the resulting mechanical force will occur in a consistent direction of rotation, independent of the direction of applied voltage, but determined by the commutator and polarity of the field coils. [2]

Contents

Universal motors have high starting torque, can run at high speed, and are lightweight and compact. They are commonly used in portable power tools and equipment, as well as many household appliances. They're also relatively easy to control, electromechanically using tapped coils, or electronically. However, the commutator has brushes that wear, so they are much less often used for equipment that is in continuous use. In addition, partly because of the commutator, universal motors are typically very noisy, both acoustically and electromagnetically. [3]

Working

Universal motors' field coils are series wound with the rotor coils and commutator. Universeelmotor.svg
Universal motors' field coils are series wound with the rotor coils and commutator.
Equivalent circuit ESB Einphasen-Reihenschlussmotor.svg
Equivalent circuit

Not all series-wound motors operate well on AC current. [4] [note 1] If an ordinary series-wound DC motor were connected to an AC supply, it would run very poorly. The universal motor is modified in several ways to allow for proper AC supply operation. There is a compensating winding typically added, along with laminated pole pieces, as opposed to the solid pole pieces found in DC motors. [2] A universal motor's armature typically has far more coils and plates than a DC motor, and hence fewer windings per coil. This reduces the inductance. [5]

Efficiency

Even when used with AC power these types of motors are able to run at a rotation frequency well above that of the mains supply, and because most electric motor properties improve with speed, this means they can be lightweight and powerful. [5] However, universal motors are usually relatively inefficient: around 30% for smaller motors and up to 70–75% for larger ones. [5]

Torque–speed characteristics

Series-wound electric motors respond to increased load by slowing down; the current increases and the torque rises in proportion to the square of the current because the same current flows in both the armature and the field windings. If the motor is stalled, the current is limited only by the total resistance of the windings and the torque can be very high, and there is a danger of the windings becoming overheated. The counter-EMF aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate. At that instant, the counter-EMF is zero and the only factor limiting the armature current is the armature resistance. Usually the armature resistance of a motor is low; therefore the current through the armature would be very large when the power is applied. Therefore the need can arise for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter-EMF. As the motor rotation builds up, the resistance is gradually cut out. The speed-torque characteristic is an almost perfectly straight line between the stall torque and the no-load speed. This suits large inertial loads as the speed will drop until the motor slowly starts to rotate and these motors have a very high stalling torque. [6]

As the speed increases, the inductance of the rotor means that the ideal commutating point changes. Small motors typically have fixed commutation. While some larger universal motors have rotatable commutation, this is rare. Instead larger universal motors often have compensation windings in series with the motor, or sometimes inductively coupled, and placed at ninety electrical degrees to the main field axis. These reduce the reactance of the armature, and improve the commutation. [5]

One useful property of having the field windings in series with the armature winding is that as the speed increases the counter EMF naturally reduces the voltage across, and current through the field windings, giving field weakening at high speeds. This means that the motor has no theoretical maximum speed for any particular applied voltage. Universal motors can be and are generally run at high speeds, 4000–16000 RPM, and can go over 20,000 RPM. [5] By way of contrast, AC synchronous and squirrel-cage induction motors cannot turn a shaft faster than allowed by the power line frequency. In countries with 60 Hz AC supply, this speed is limited to 3600 RPM. [7]

Motor damage may occur from over-speeding (running at a rotational speed in excess of design limits) if the unit is operated with no significant mechanical load. On larger motors, sudden loss of load is to be avoided, and the possibility of such an occurrence is incorporated into the motor's protection and control schemes. In some smaller applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a safe level, as well as a means to circulate cooling airflow over the armature and field windings. If there were no mechanical limits placed on a universal motor it could theoretically speed out of control in the same way any series-wound DC motor can. [3]

An advantage of the universal motor is that AC supplies may be used on motors which have some characteristics more common in DC motors, specifically high starting torque and very compact design if high running speeds are used. [3]

Disadvantages

A negative aspect is the maintenance and short life problems caused by the commutator, as well as electromagnetic interference (EMI) issues due to any sparking. Because of the relatively high maintenance commutator brushes, universal motors are best-suited for devices such as food mixers and power tools which are used only intermittently, and often have high starting-torque demands.

Another negative aspect is that these motors may only be used where mostly-clean air is present at all times. Due to the dramatically increased risk of overheating, totally-enclosed fan-cooled universal motors would be impractical, though some have been made. Such a motor would need a large fan to circulate enough air, decreasing efficiency since the motor must use more energy to cool itself. The impracticality comes from the resulting size, weight, and thermal management issues which open motors have none of.

Universal motors are also very noisy compared to other types of AC and DC motors.

Speed control

Continuous speed control of a universal motor running on AC is easily obtained by use of a thyristor circuit, while multiple taps on the field coil provide (imprecise) stepped speed control. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC).

Variations

Shunt winding

Universal motors are series wound. Shunt winding was used experimentally, in the late 19th century, [8] but was impractical owing to problems with commutation. Various schemes of embedded resistance, inductance, and antiphase cross-coupling were attempted to reduce this. Universal motors, including shunt wound, were favoured as AC motors at this time as they were self-starting. [4] When self-starting induction motors and automatic starters became available, these replaced the larger universal motors (above 1 hp) and the shunt wound.

Repulsion-start

In the past, repulsion-start wound-rotor motors provided high starting torque, but with added complexity. Their rotors were similar to those of universal motors, but their brushes were connected only to each other. Transformer action induced current into the rotor. Brush position relative to field poles meant that starting torque was developed by rotor repulsion from the field poles. A centrifugal mechanism, when close to running speed, connected all commutator bars together to create the equivalent of a squirrel-cage rotor. As well, when close to approximately 80 per cent of its run speed, these motors can run as induction motors. [9]

Applications

Domestic appliances

Operating at normal power line frequencies, universal motors are not often found in a range less than 1000 watts. Their high speed makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high speed and light weight are desirable. They are also commonly used in portable power tools, such as drills, sanders, circular saws, and jigsaws, where the motor's characteristics work well. An added benefit for power tools used by welders is that classic engine-driven welding machines may be a pure DC generator, and their auxiliary power receptacles will still be DC, even though a typical NEMA 5-15 household configuration. The DC power is fine for typical jobsite (outmoded) incandescent lighting and the universal motors in some drills and grinders. Many vacuum cleaner and weed trimmer motors exceed 10,000 RPM , while many Dremel and similar miniature grinders exceed 30,000 RPM.

Universal motors also lend themselves to electronic speed control and, as such, were an ideal choice for domestic washing machines. The motor can be used to agitate the drum (both forward and in reverse) by switching the field winding with respect to the armature. The motor can also be run up to the high speeds required for the spin cycle. Nowadays, variable-frequency drive motors are more commonly used instead.

Rail traction

Universal motors also formed the basis of the traditional railway traction motor in electric railways. In this application, the use of AC to power a motor originally designed to run on DC would lead to efficiency losses due to eddy current heating of their magnetic components, particularly the motor field pole-pieces that, for DC, would have used solid (un-laminated) iron. Although the heating effects are reduced by using laminated pole-pieces, as used for the cores of transformers and by the use of laminations of high-permeability electrical steel, one solution available at the start of the 20th century was for the motors to be operated from very-low-frequency AC supplies, with 25 Hz and 16+2⁄3 Hz operation being common.

Starter motor

Starters of combustion engines are usually universal motors, with the advantage of being small and having high torque at low speed. Some starters have permanent magnets, others have one of the four poles wound with a shunt coil rather than series-wound coils.

Related Research Articles

<span class="mw-page-title-main">Electric motor</span> Machine that converts electrical energy into mechanical energy

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. An electric generator is mechanically identical to an electric motor, but operates in reverse, converting mechanical energy into electrical energy.

<span class="mw-page-title-main">Commutator (electric)</span> Device for changing direction of current

A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings on the armature are connected to the commutator segments.

<span class="mw-page-title-main">Alternator</span> Device converting mechanical into electrical energy

An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature. Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually the term refers to small rotating machines driven by automotive and other internal combustion engines.

<span class="mw-page-title-main">Stator</span> Stationary part of a system

The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors, which is typically contrasted with rotor. Energy flows through a stator to or from the rotating component of the system. In an electric motor, the stator provides a magnetic field that drives the rotating armature; in a generator, the stator converts the rotating magnetic field to electric current. In fluid powered devices, the stator guides the flow of fluid to or from the rotating part of the system.

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

<span class="mw-page-title-main">Brushless DC electric motor</span> Synchronous electric motor powered by an inverter

A brushless DC electric motor (BLDC), also known as an electronically commutated motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic controller to switch DC currents to the motor windings producing magnetic fields that effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

<span class="mw-page-title-main">DC motor</span> Motor which works on direct current

A DC motor is an electrical motor that uses direct current (DC) to produce mechanical force. The most common types rely on magnetic forces produced by currents in the coils. Nearly all types of DC motors have some internal mechanism, either electromechanical or electronic, to periodically change the direction of current in part of the motor.

<span class="mw-page-title-main">Squirrel-cage rotor</span> Rotating part of the common squirrel-cage induction motor

A squirrel-cage rotor is the rotating part of the common squirrel-cage induction motor. It consists of a cylinder of steel laminations, with aluminum or copper conductors embedded in its surface. In operation, the non-rotating stator winding is connected to an alternating current power source; the alternating current in the stator produces a rotating magnetic field. The rotor winding has current induced in it by the stator field, like a transformer except that the current in the rotor is varying at the stator field rotation rate minus the physical rotation rate. The interaction of the magnetic fields in the stator and the currents in the rotor produce a torque on the rotor.

<span class="mw-page-title-main">Traction motor</span> An electric motor for vehicle propulsion

A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, or electric multiple unit trains.

<span class="mw-page-title-main">Armature (electrical)</span> Power-producing component of an electric machine

In electrical engineering, the armature is the winding of an electric machine which carries alternating current. The armature windings conduct AC even on DC machines, due to the commutator action or due to electronic commutation, as in brushless DC motors. The armature can be on either the rotor or the stator, depending on the type of electric machine.

<span class="mw-page-title-main">Field coil</span> Electromagnet used to generate a magnetic field in an electro-magnetic machine

A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.

<span class="mw-page-title-main">Repulsion motor</span> Type of AC electric motor

A repulsion motor is a type of electric motor which runs on alternating current (AC). It was formerly used as a traction motor for electric trains but has been superseded by other types of motors. Repulsion motors are classified as single phase motors.

<span class="mw-page-title-main">Gramme machine</span> Electrical generator that produces direct current

A Gramme machine, Gramme ring, Gramme magneto, or Gramme dynamo is an electrical generator that produces direct current, named for its Belgian inventor, Zénobe Gramme, and was built as either a dynamo or a magneto. It was the first generator to produce power on a commercial scale for industry. Inspired by a machine invented by Antonio Pacinotti in 1860, Gramme was the developer of a new induced rotor in form of a wire-wrapped ring and demonstrated this apparatus to the Academy of Sciences in Paris in 1871. Although popular in 19th century electrical machines, the Gramme winding principle is no longer used since it makes inefficient use of the conductors. The portion of the winding on the interior of the ring cuts no flux and does not contribute to energy conversion in the machine. The winding requires twice the number of turns and twice the number of commutator bars as an equivalent drum-wound armature.

<span class="mw-page-title-main">Reluctance motor</span> Type of electric motor

A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. The rotor does not have any windings. It generates torque through magnetic reluctance.

<span class="mw-page-title-main">AC motor</span> Electric motor driven by an AC electrical input

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

<span class="mw-page-title-main">Rotor (electric)</span> Non-stationary part of a rotary electric motor

The rotor is a moving component of an electromagnetic system in the electric motor, electric generator, or alternator. Its rotation is due to the interaction between the windings and magnetic fields which produces a torque around the rotor's axis.

A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact.

In electrical engineering, electric machine is a general term for machines using electromagnetic forces, such as electric motors, electric generators, and others. They are electromechanical energy converters: an electric motor converts electricity to mechanical power while an electric generator converts mechanical power to electricity. The moving parts in a machine can be rotating or linear. While transformers are occasionally called "static electric machines", since they do not have moving parts, generally they are not considered "machines", but as electrical devices "closely related" to the electrical machines.

<span class="mw-page-title-main">Switched reluctance motor</span> Externally controlled electric motor that runs by reluctance torque

The switched reluctance motor (SRM) is an electric motor that runs by reluctance torque and thus is a subgroup in reluctance motors. Unlike common brushed DC motor types, power is delivered to windings in the stator (case) rather than the rotor. This greatly simplifies mechanical design as power does not have to be delivered to a moving part which eliminates the need for a commutator, but it complicates the electrical design as some sort of switching system needs to be used to deliver power to the different windings. Electronic devices can precisely time the switching of currents, facilitating SRM configurations. Its main drawback is torque ripple. Controller technology that limits torque ripple at low speeds has been demonstrated. Sources disagree on whether it is a type of stepper motor.

<span class="mw-page-title-main">Bipolar electric motor</span> Electric motor with only two poles to its stationary field

A bipolar electric motor is an electric motor with only two poles to its stationary field. They are an example of the simple brushed DC motor, with a commutator. This field may be generated by either a permanent magnet or a field coil.

References

  1. ↑ Motors for DC will anyway require laminated rotors, owing to commutation
  1. ↑ "Electric Motors - Dietz Electric". dietzelectric.com. Archived from the original on 2018-07-11. Retrieved 2018-07-10.
  2. 1 2 Herman, Stephen L. Delmar's Standard Textbook of Electricity, 3rd Edition. Clifton Park, NY: Delmar Learning, 2004. p.998
  3. 1 2 3 Herman, Stephen L. Delmer's Standard Textbook of Electricity, 3rd Edition. Clifton Park, NY: Delmar Learning, 2004. p.1001
  4. 1 2 Kennedy, Rankin (1915). The Book of Electrical Installations. Vol. II. Caxton. p. 152.
  5. 1 2 3 4 5 Transformers and Motors, by George Patrick Shultz
  6. ↑ Herman, Stephen L. Delmar's Standard Textbook of Electricity, 3rd Edition. Clifton Park, NY: Delmar Learning, 2004. p.850
  7. ↑ Herman, Stephen L. Delmar's Standard Textbook of Electricity, 3rd Edition. Clifton Park, NY: Delmar Learning, 2004. p.905
  8. ↑ GB 18847,H F Joel,published 1892
  9. ↑ "Repulsion-start induction-run motor | HVAC Troubleshooting". hvacspecialists.info. Archived from the original on 2018-07-09. Retrieved 2018-07-10.
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