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Artwork: A simplified diagram of the parts in an electric motor. Animation: How it works in practice. Note how the commutator reverses the current each time the coil turns halfway. This means the force on each side of the coil is always pushing in the same direction, which keeps the coil rotating clockwise. A simple, experimental motor such as this isn't capable of making much power.
We can increase the turning force or torque that the motor can create in three ways: either we can have a more powerful permanent magnet, or we can increase the electric current flowing through the wire, or we can make the coil so it has many "turns" loops of very thin wire instead of one "turn" of thick wire. In practice, a motor also has the permanent magnet curved in a circular shape so it almost touches the coil of wire that rotates inside it. The closer together the magnet and the coil, the greater the force the motor can produce.
Although we've described a number of different parts, you can think of a motor as having just two essential components: There's a permanent magnet or magnets around the edge of the motor case that remains static, so it's called the stator of a motor. Inside the stator, there's the coil, mounted on an axle that spins around at high speed—and this is called the rotor. The rotor also includes the commutator. Universal motors DC motors like this are great for battery-powered toys things like model trains, radio-controlled cars, or electric shavers , but you don't find them in many household appliances.
Small appliances things like coffee grinders or electric food blenders tend to use what are called universal motors, which can be powered by either AC or DC. Unlike a simple DC motor, a universal motor has an electromagnet, instead of a permanent magnet, and it takes its power from the DC or AC power you feed in: When you feed in DC, the electromagnet works like a conventional permanent magnet and produces a magnetic field that's always pointing in the same direction.
The commutator reverses the coil current every time the coil flips over, just like in a simple DC motor, so the coil always spins in the same direction. When you feed in AC, however, the current flowing through the electromagnet and the current flowing through the coil both reverse, exactly in step, so the force on the coil is always in the same direction and the motor always spins either clockwise or counter-clockwise.
What about the commutator?
The frequency of the current changes much faster than the motor rotates and, because the field and the current are always in step, it doesn't actually matter what position the commutator is in at any given moment. Animation: How a universal motor works: The electricity supply powers both the magnetic field and the rotating coil.
With a DC supply, a universal motor works just like a conventional DC one, as above. With an AC supply, both the magnetic field and coil current change direction every time the supply current reverses.
A number of diagnostic tools, such as clamp-on ammeters, temperature sensors, a Megger or oscilloscope, can help illuminate the problem. Preliminary tests generally are done using the ubiquitous multimeter. This tester is capable of providing diagnostic information for all kinds of motors. Electrical measurements If the motor is completely unresponsive, no ac humming or false starts, take a voltage reading at the motor terminals. If there is no voltage or reduced voltage, work back upstream.
Take readings at accessible points including disconnects, the motor controller, any fuses or junction boxes, and so on, back to the over-current device output at the entrance panel. When there is no electrical load, the same voltage should appear at both ends of the branch circuit conductors.
In a three-phase hookup, all legs should have substantially equal voltage readings, with no dropped phase. If these readings vary by a few volts, it may be possible to equalize them by rolling the connections, taking care not to reverse rotation.
The idea is to match supply voltages and load impedances so as to balance the three legs.
If the electrical supply checks out, examine the motor itself. If possible, disengage the load. This may restore motor operation. With power disconnected and locked out, attempt to turn the motor by hand. In all but the largest motors the shaft should turn freely.
If not, there is an obstruction inside or a seized bearing. Fairly new bearings are prone to seizure because the tolerances are tighter.
This is especially true if there is ambient moisture or the motor has been unused for a while. Often good operation can be restored by oiling front and rear bearings without disassembling the motor. If the shaft turns freely, set the multimeter to its ohms function to check resistance.
The windings all three in a three-phase motor should read low but not zero ohms. The smaller the motor, the higher this reading will be, but it should not be open. A wide range of DMMs are available to measure voltage, current, and resistance, depending on the motor power ratings.
Small universal motors, such as those used in portable electric drills, can contain extensive circuitry including a switch and brushes. In the ohmmeter mode, connect the meter to the plug and monitor the resistance as you wiggle the cord where it enters the enclosure.
Move the switch from side to side and, with a trigger switch taped so it remains on, press on the brushes and turn the commutator by hand. Main article: 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. 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. BLDC motors are usually used in small equipment such as computers and are generally used in fans to get rid of unwanted heat.
They are also acoustically very quiet motors, which is an advantage if being used in equipment that is affected by vibrations. Modern BLDC motors range in power from a fraction of a watt to many kilowatts.
They also find significant use in high-performance electric model aircraft.
Switched reluctance motor[ edit ] Main article: Switched reluctance motor The SRM has no brushes or permanent magnets, and the rotor has no electric currents.