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In , the repair and maintenance of electric motors, generators and transformers is worth nearly € million alone in the UK, and over € billion within the. Most electric motors are designed to run at 50% to % of rated load. Maximum major working motors as part of your preventative maintenance and energy Figure 3 Relationships Between Power, Current, Power Factor and Motor Load. WATTS New is one of the few blogs in the industry that covers issues with electric motors, generators, gearbox, VFDs, and more! Subscribe today to receive an.
The force between the two magnetic fields tends to rotate the motor shaft. The commutator switches power to the coils as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, so that the rotor never stops like a compass needle doesbut rather keeps rotating as long as power is applied.
Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. Sparks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections—and hence coil ends—momentarily while crossing the gaps.
Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivitylimits the output of the motor.
The making and breaking of electric contact also generates electrical noise ; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance on larger motors or replacement on small motors. The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.
While most commutators are cylindrical, some are flat discs consisting of several segments typically, at least three mounted on an insulator. Large brushes are desired for a larger brush contact area to maximize motor output, but small brushes are desired for low mass to maximize the speed at which the motor can run without the brushes excessively bouncing and sparking.
Small brushes are also desirable for lower cost. Stiffer brush springs can also be used to make brushes of a given mass work at a higher speed, but at the cost of greater friction losses lower efficiency and accelerated brush and commutator wear.
DC machines are defined as follows: Field circuit — A set of windings that produces a magnetic field so that the electromagnetic induction can take place in electric machines.
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A mechanical technique in which rectification can be achieved, or from which DC can be derived, in DC machines. DC shunt-wound motor DC compound motor two configurations: Permanent-magnet electric motor A PM permanent magnet motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque.
Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control.
PM fields stators are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings.
Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux.
However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines. To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy.
With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly-fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles to ensure starting, regardless of rotor position and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets. In this motor, the mechanical "rotating switch" or commutator is replaced by an external electronic switch synchronised to the rotor's position.
Efficiency for a BLDC motor of up to The BLDC motor's characteristic trapezoidal counter-electromotive force CEMF waveform is derived partly from the stator windings being evenly distributed, and partly from the placement of the rotor's permanent magnets.
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Also known as electronically commutated DC or inside out DC motors, the stator windings of trapezoidal BLDC motors can be with single-phase, two-phase or three-phase and use Hall effect sensors mounted on their windings for rotor position sensing and low cost closed-loop control of the electronic commutator. They have several advantages over conventional motors: Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors.
This cool operation leads to much-improved life of the fan's bearings. Without a commutator to wear out, the life of a BLDC motor can be significantly longer compared to a DC motor using brushes and a commutator.
Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a BLDC motor may be used in electrically sensitive devices like audio equipment or computers. The same Hall effect sensors that provide the commutation can also provide a convenient tachometer signal for closed-loop control servo-controlled applications. In fans, the tachometer signal can be used to derive a "fan OK" signal as well as provide running speed feedback. The motor can be easily synchronized to an internal or external clock, leading to precise speed control.
BLDC motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels. Also, sparking generates ozone, which can accumulate in poorly ventilated buildings risking harm to occupants' health. In a series wound motorthe field coils are connected electrically in series with the armature coils via the brushes.
In a shunt wound motor, the field coils are connected in parallel, or "shunted" to the armature coils. In a separately excited sepex motor the field coils are supplied from an independent source, such as a motor-generator and the field current is unaffected by changes in the armature current. The sepex system was sometimes used in DC traction motors to facilitate control of wheelslip.
Permanent-magnet motors[ edit ] Permanent-magnet types have some performance advantages over direct-current, excited, synchronous types, and have become predominant in fractional horsepower applications.
They are smaller, lighter, more efficient and reliable than other singly-fed electric machines. Permanent magnets have traditionally only been useful on small motors because it was difficult to find a material capable of retaining a high-strength field. Only recently have advances in materials technology allowed the creation of high-intensity permanent magnets, such as neodymium magnetsallowing the development of compact, high-power motors without the extra real-estate of field coils and excitation means.
But as these high performance permanent magnets become more applied in electric motor or generator systems, other problems are realized see Permanent magnet synchronous generator.
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Axial field motors[ edit ] Traditionally, the field has been applied radially—in and away from the rotation axis of the motor. However some designs have the field flowing along the axis of the motor, with the rotor cutting the field lines as it rotates. This allows for much stronger magnetic fields, particularly if halbach arrays are employed.
This, in turn, gives power to the motor at lower speeds. However, the focused flux density cannot rise about the limited residual flux density of the permanent magnet despite high coercivity and like all electric machines, the flux density of magnetic core saturation is the design constraint.
Speed control can be achieved by variable battery tappings, variable supply voltage, resistors or electronic controls. A simulation example can be found here  and . The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both. This is commonly done with a special set of contactors direction contactors.
The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device made of thyristorstransistorsor, formerly, mercury arc rectifiers.
An electric locomotive or train would typically have four motors which could be grouped in three different ways: All four in series each motor receives one quarter of the line voltage Two parallel groups of two in series each motor receives half the line voltage All four in parallel each motor receives the full line voltage This provided three running speeds with minimal resistance losses. For starting and acceleration, additional control was provided by resistances.
This system has been superseded by electronic control systems. Field weakening[ edit ] The speed of a DC motor can be increased by field weakening. Reducing the field strength is done by inserting resistance in series with a shunt field, or inserting resistances around a series-connected field winding, to reduce current in the field winding. When the field is weakened, the back-emf reduces, so a larger current flows through the armature winding and this increases the speed.
Field weakening is not used on its own but in combination with other methods, such as series-parallel control. Chopper[ edit ] In a circuit known as a chopperthe average voltage applied to the motor is varied by switching the supply voltage very rapidly.
As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies.