Bipolar electric motor
Encyclopedia
A bipolar electric motor is an electric motor
with only two (hence bi-) 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
.
The 'bipolar' term refers to the stationary field of the motor, not the rotor. The rotors often have more than two poles, three for a simple motor and potentially more for a high-power motor. A two-pole rotor has the disadvantage that it is not self-starting in all positions and so requires to be flicked to start.
of the 1870s onwards, used bipolar fields. These early machines used crudely designed field pole pieces with long magnetic circuits, wide pole gaps and narrow pole pieces that gave only a limited flux
through the armature. These fields were usually horseshoe-shaped, with either permanent horseshoe magnets or else either one or two field coils at some distance from the poles.
Early insulated wire was insulated, if at all,The first electromagnets were wound with bare copper wire, the only sort then available, and insulated with strips of cloth laid on the windings as they were wound. with wrappings of cotton thread. These coils could only handle a low temperature rise before overheating and burning out with a short circuit. The coils were thus long and shallow, sometimes of only a single layer of wire, which required a long core simply to contain their size. Single small coils could be mounted horizontally, but the most common arrangement used two tall coils side by side.
To improve the efficiency of the magnetic circuit, it was realised that multiple magnetic paths could be provided through the same armature. The two coils were now separated and placed at the sides of the motor, with their iron core as a sideways figure-8 circuit and the armature in a central pole gap. Flux from both coils passed through this gap. This gave a magnetic circuit that was shorter overall and thus had fewer magnetic losses. The more compact coil windings were made possible by the use of shellac
for impregnating the windings and improving the reliability of their insulation.
Later designs, from around 1900, became more compact with shorter, more efficient magnetic circuits. The field coils now moved into short, squat internal coils around the pole pieces themselves. The remainder of the magnetic circuit was a double-sided circular path around the casing of the motor. Whilst primarily designed to be more efficient, this also gave a far more compact layout in terms of space.
This circular layout also represented the end of the bipolar motor as an industrial power source. It was possible to place a second set of field coils and pole pieces within the same size of casing, giving a four pole arrangement. Because of the more efficient provision of field flux around the entire circumference of the armature, this give a motor of almost twice the power, for the same armature current. Armature current, and the associated commutator and brushgear, represented one of the most expensive parts of the motor to manufacture.
One of the last industrial uses for large bipolar motors was for the Milwaukee Road
's class EP-2
electric locomotives of 1917. The line had chosen to electrify its Coast Division route, using a voltage of 3,000V DC. These were not the first electric locomotive
s produced and incorporated lessons learned from previous practice. Many early locomotives had used one or two large motors mounted on the locomotive frame
, with drive to the wheels by traditional steam locomotive practice of coupling rod
s. Where AC motors were used, requiring many poles and thus large diameters, these frame-mounted motors appeared inevitable even though they required this maintenance-intensive mechanical drive to the wheels. An alternative system of nose-hung traction motor
s used small high-speed motors alongside each axle, driving through a reduction gearbox. This system would eventually predominate across both electric and diesel locomotives, but at this time it was difficult to produce a reliable high-power gearbox.
With the Milwaukee Road's choice of DC power and DC motors, it was decided to use axle-mounted motors, driving each wheel directly. The axle formed the spindle of not only the wheels, but also the motor armature itself. This obviously simple system had been used before, but only for low-powered locomotives with lightweight motors. As the wheels and axle, and in this case the motor too, are unsprung
by the suspension, any extra weight here would lead to poor riding qualities. To permit its use for these extremely powerful new locomotives, the motor was split in two. The armature was formed as part of the axle, but the much heavier field poles and coils were carried on the suspended frame of the locomotive. This gave an acceptable ride.
The complexity of this system was that the armature must now be free to move up and down relative to the field, as the suspension moves. With a contemporary four-pole motor, this would vary the pole gap at the upper and lower poles, probably to the extent that the armature hit the pole pieces (suspension travel being far larger than typical pole gaps). The solution was to return to the relatively antiquated bipolar motor. By placing the poles at the side of the armature and giving them flat vertical faces, the armature was free to move up and down between them. The motor design was relatively inefficient, even by the standards of the day, but these locomotives were designed for their power and haulage capacity with a generous supply of cheap hydro-electricity, rather than designed for efficiency.
The locomotives operated reliably and successfully for 35 years. They were eventually withdrawn owing to a general decline in US railroads in the late 1950s, the advent of cheap diesel power, and in particular to a rebuilding of the class that was poorly carried out and left the rebuilt locomotives with reliability problems.
s such as food mixers, vacuum cleaner
s and electric drills.
These motors are broadly the design of the brushed DC motor with series-connected field windings. They also work well on AC supplies and are now most commonly found on such. They offer greater torque and speed than induction motor
s and so have many applications where their capital cost and light weight are more important than their electrical efficiency.
The first such motors used a simple horseshoe permanent magnet. More modern 'can' motors, from the 1960s onwards, have remained bipolar but have, like the industrial motors, have used a more efficient pair of C-shaped magnets within a circular steel can case.
Owing to their additional cost and complexity, motors with field coils have only rarely been used for models. One well-known exception to this was the 'Taycol' range of motors, primarily aimed at larger model boats. These had their heyday in the 1950s and 1960s, becoming obsolete and uncompetitive in price as more powerful materials for permanent magnets, specifically ferrite
, became available.
Taycol began with simple horseshoe magnet motors, but their real speciality was with wound fields. Most of these used a single transverse field coil mounted above the rotor. Their larger 'Marine' and 'Double Special' ranges used a dual coil layout, with two vertical field coils mounted at the sides.
A similar, although smaller and far less powerful motor, was the Meccano
E15R motor.
Construction of a simple bipolar motor, usually with a bipolar rotor as well, remains a popular basic science project for children.
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
with only two (hence bi-) poles to its stationary field. They are an example of the simple brushed DC motor, with a commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
. This field may be generated by either a permanent magnet or a field coil
Field coil
A field coil is a component of an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. A current-carrying coil is used to generate a magnetic field....
.
The 'bipolar' term refers to the stationary field of the motor, not the rotor. The rotors often have more than two poles, three for a simple motor and potentially more for a high-power motor. A two-pole rotor has the disadvantage that it is not self-starting in all positions and so requires to be flicked to start.
Early motors
The first DC electrical motors, from the Gramme motorGramme machine
A Gramme machine, Gramme ring, Gramme magneto, or Gramme dynamo is an electrical generator which 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...
of the 1870s onwards, used bipolar fields. These early machines used crudely designed field pole pieces with long magnetic circuits, wide pole gaps and narrow pole pieces that gave only a limited flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
through the armature. These fields were usually horseshoe-shaped, with either permanent horseshoe magnets or else either one or two field coils at some distance from the poles.
Early insulated wire was insulated, if at all,The first electromagnets were wound with bare copper wire, the only sort then available, and insulated with strips of cloth laid on the windings as they were wound. with wrappings of cotton thread. These coils could only handle a low temperature rise before overheating and burning out with a short circuit. The coils were thus long and shallow, sometimes of only a single layer of wire, which required a long core simply to contain their size. Single small coils could be mounted horizontally, but the most common arrangement used two tall coils side by side.
To improve the efficiency of the magnetic circuit, it was realised that multiple magnetic paths could be provided through the same armature. The two coils were now separated and placed at the sides of the motor, with their iron core as a sideways figure-8 circuit and the armature in a central pole gap. Flux from both coils passed through this gap. This gave a magnetic circuit that was shorter overall and thus had fewer magnetic losses. The more compact coil windings were made possible by the use of shellac
Shellac
Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...
for impregnating the windings and improving the reliability of their insulation.
Later designs, from around 1900, became more compact with shorter, more efficient magnetic circuits. The field coils now moved into short, squat internal coils around the pole pieces themselves. The remainder of the magnetic circuit was a double-sided circular path around the casing of the motor. Whilst primarily designed to be more efficient, this also gave a far more compact layout in terms of space.
This circular layout also represented the end of the bipolar motor as an industrial power source. It was possible to place a second set of field coils and pole pieces within the same size of casing, giving a four pole arrangement. Because of the more efficient provision of field flux around the entire circumference of the armature, this give a motor of almost twice the power, for the same armature current. Armature current, and the associated commutator and brushgear, represented one of the most expensive parts of the motor to manufacture.
Electric railway locomotives
Chicago, Milwaukee, St. Paul and Pacific Railroad
The Milwaukee Road, officially the Chicago, Milwaukee, St. Paul and Pacific Railroad , was a Class I railroad that operated in the Midwest and Northwest of the United States from 1847 until its merger into the Soo Line Railroad on January 1, 1986. The company went through several official names...
's class EP-2
Milwaukee Road class EP-2
The Milwaukee Road's class EP-2 comprised five electric locomotives built by General Electric in 1919. They were often known as Bipolars, which referred to the bipolar electric motors they used. Among the most distinctive and powerful electric locomotives of their time, they epitomized the...
electric locomotives of 1917. The line had chosen to electrify its Coast Division route, using a voltage of 3,000V DC. These were not the first electric locomotive
Electric locomotive
An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or an on-board energy storage device...
s produced and incorporated lessons learned from previous practice. Many early locomotives had used one or two large motors mounted on the locomotive frame
Locomotive frame
A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure of some kind...
, with drive to the wheels by traditional steam locomotive practice of coupling rod
Coupling rod
right|thumb|connecting rod and coupling rods attached to a small locomotive driving wheelA coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have...
s. Where AC motors were used, requiring many poles and thus large diameters, these frame-mounted motors appeared inevitable even though they required this maintenance-intensive mechanical drive to the wheels. An alternative system of nose-hung traction motor
Traction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....
s used small high-speed motors alongside each axle, driving through a reduction gearbox. This system would eventually predominate across both electric and diesel locomotives, but at this time it was difficult to produce a reliable high-power gearbox.
With the Milwaukee Road's choice of DC power and DC motors, it was decided to use axle-mounted motors, driving each wheel directly. The axle formed the spindle of not only the wheels, but also the motor armature itself. This obviously simple system had been used before, but only for low-powered locomotives with lightweight motors. As the wheels and axle, and in this case the motor too, are unsprung
Unsprung weight
In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...
by the suspension, any extra weight here would lead to poor riding qualities. To permit its use for these extremely powerful new locomotives, the motor was split in two. The armature was formed as part of the axle, but the much heavier field poles and coils were carried on the suspended frame of the locomotive. This gave an acceptable ride.
The complexity of this system was that the armature must now be free to move up and down relative to the field, as the suspension moves. With a contemporary four-pole motor, this would vary the pole gap at the upper and lower poles, probably to the extent that the armature hit the pole pieces (suspension travel being far larger than typical pole gaps). The solution was to return to the relatively antiquated bipolar motor. By placing the poles at the side of the armature and giving them flat vertical faces, the armature was free to move up and down between them. The motor design was relatively inefficient, even by the standards of the day, but these locomotives were designed for their power and haulage capacity with a generous supply of cheap hydro-electricity, rather than designed for efficiency.
The locomotives operated reliably and successfully for 35 years. They were eventually withdrawn owing to a general decline in US railroads in the late 1950s, the advent of cheap diesel power, and in particular to a rebuilding of the class that was poorly carried out and left the rebuilt locomotives with reliability problems.
Modern bipolar motors
The bipolar motor is still in widespread use today, in medium power, low-cost applications such as the universal motors used in home applianceHome appliance
Home appliances are electrical/mechanical machines which accomplish some household functions, such as cooking or cleaning. Home appliances can be classified into:*Major appliances, or White goods*Small appliances, or Brown goods...
s such as food mixers, vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...
s and electric drills.
These motors are broadly the design of the brushed DC motor with series-connected field windings. They also work well on AC supplies and are now most commonly found on such. They offer greater torque and speed than induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
s and so have many applications where their capital cost and light weight are more important than their electrical efficiency.
Toy motors
The simple bipolar motor has been widely used in electric torys, since the early days of tinplate toys.The first such motors used a simple horseshoe permanent magnet. More modern 'can' motors, from the 1960s onwards, have remained bipolar but have, like the industrial motors, have used a more efficient pair of C-shaped magnets within a circular steel can case.
Owing to their additional cost and complexity, motors with field coils have only rarely been used for models. One well-known exception to this was the 'Taycol' range of motors, primarily aimed at larger model boats. These had their heyday in the 1950s and 1960s, becoming obsolete and uncompetitive in price as more powerful materials for permanent magnets, specifically ferrite
Ferrite
Ferrite may refer to:* Ferrite , iron or iron alloys with a body centred cubic crystal structure.* Ferrite , ferrimagnetic ceramic materials used in magnetic applications....
, became available.
Taycol began with simple horseshoe magnet motors, but their real speciality was with wound fields. Most of these used a single transverse field coil mounted above the rotor. Their larger 'Marine' and 'Double Special' ranges used a dual coil layout, with two vertical field coils mounted at the sides.
A similar, although smaller and far less powerful motor, was the Meccano
Meccano
Meccano is a model construction system comprising re-usable metal strips, plates, angle girders, wheels, axles and gears, with nuts and bolts to connect the pieces. It enables the building of working models and mechanical devices....
E15R motor.
Construction of a simple bipolar motor, usually with a bipolar rotor as well, remains a popular basic science project for children.