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Single Phase Motor Control

This type of motor is designed to provide strong starting torque and strong running for applications such as large water pumps.

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Unlike a circular induction motor, a linear induction motor shows 'end effects'. These end effects include losses in performance and efficiency that are believed to be caused by magnetic energy being carried away and lost at the end of the primary by the relative movement of the primary and secondary. With a short secondary, the behaviour is almost identical to a rotary machine, provided it is at least two poles long but with a short primary reduction in thrust that occurs at low slip below about 0.

However, because of end effects, linear motors cannot 'run light' -- normal induction motors are able to run the motor with a near synchronous field under low load conditions. In contrast, end effects create much more significant losses with linear motors.

This occurs in single sided motors, and levitation will not usually occur when an iron backing plate is used on the secondary, since this causes an attraction that overwhelms the lifting force. Linear induction motors are often less efficient than conventional rotary induction motors; the end effects and the relatively large air gap that is often present will typically reduce the forces produced for the same electrical power.

However, linear induction motors can avoid the need for gearboxes and similar drivetrains, and these have their own losses; and working knowledge of the importance of the goodness factor can minimise the effects of the larger air gap.

In any case power use is not always the most important consideration. For example, in many cases linear induction motors have far fewer moving parts, and have very low maintenance.

Because of these properties, linear motors are often used in maglev propulsion, as in the Japanese Linimo magnetic levitation train line near Nagoya. The world's first commercial automated maglev system was a low-speed maglev shuttle that ran from the airport terminal of Birmingham International Airport to the nearby Birmingham International railway station between — One of the original cars is now on display at Railworld in Peterborough, together with the RTV31 hover train vehicle.

Linear induction motor technology is also used in some launched roller coasters. At present it is still impractical on street running trams , although this, in theory, could be done by burying it in a slotted conduit. Outside of public transportation, vertical linear motors have been proposed as lifting mechanisms in deep mines , and the use of linear motors is growing in motion control applications. They are also often used on sliding doors, such as those of low floor trams such as the Citadis and the Eurotram.

Dual axis linear motors also exist. These specialized devices have been used to provide direct X - Y motion for precision laser cutting of cloth and sheet metal, automated drafting , and cable forming.

Also, linear induction motors with a cylindrical secondary have been used to provide simultaneous linear and rotating motion for mounting electronic devices on printed circuit boards. Linear induction motors have also been used for launching aircraft, the Westinghouse Electropult [7] system in was an early example and the Electromagnetic Aircraft Launch System EMALS was due to be delivered in Linear induction motors are also used in looms, magnetic levitation enable bobbins to float between the fibers without direct contact.

The first ropeless elevator invented by ThyssenKrupp uses a linear induction drive power. From Wikipedia, the free encyclopedia. Retrieved 24 December This motor has a start type capacitor in series with the auxiliary winding like the capacitor start motor for high starting torque. Like a PSC motor, it also has a run type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit.

This allows high overload torque. It is able to handle applications too demanding for any other kind of single-phase motor. These include woodworking machinery, air compressors, high-pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp.

A modified version of the capacitor start motor is the resistance start motor. In this motor type, the starting capacitor is replaced by a resistor. This motor also has a starting winding in addition to the main winding. It is switched in and out of the circuit just as it was in the capacitor-start motor. The starting winding is positioned at right angles to the main winding. The electrical phase shift between the currents in the two windings is obtained by making the impedance of the windings unequal.

The main winding has a high inductance and a low resistance. The current, therefore, lags the voltage by a large angle. The starting winding is designed to have a fairly low inductance and a high resistance. Here the current lags the voltage by a smaller angle. The magnetic fields are out of phase by the same amount. This supplies enough torque to start the motor. When the motor comes up to speed, a speed-controlled switch disconnects the starting winding from the line, and the motor continues to run as an induction motor.

The starting torque is not as great as it is in the capacitor-start. Applications, Advantages and disadvantages: The resistance start motor is used in applications where the starting torque requirement is less than that provided by the capacitor start motor. Apart from the cost, this motor does not offer any major advantage over the capacitor start motor.

A comparison for the popular types of a split phase motors is shown in the below image. Have a look at the next animation. The coil, split ring, brushes and magnet are exactly the same hardware as the motor above, but the coil is being turned, which generates an emf.

The animation above would be called a DC generator. As in the DC motor, the ends of the coil connect to a split ring, whose two halves are contacted by the brushes. Note that the brushes and split ring 'rectify' the emf produced: An alternator If we want AC, we don't need recification, so we don't need split rings. This is good news, because the split rings cause sparks, ozone, radio interference and extra wear.

If you want DC, it is often better to use an alternator and rectify with diodes. In the next animation, the two brushes contact two continuous rings, so the two external terminals are always connected to the same ends of the coil.

This is an AC generator. The advantages of AC and DC generators are compared in a section below. We saw above that a DC motor is also a DC generator. Similarly, an alternator is also an AC motor. However, it is a rather inflexible one. See How real electric motors work for more details. Back emf Now, as the first two animations show, DC motors and generators may be the same thing.

For example, the motors of trains become generators when the train is slowing down: Recently, a few manufacturers have begun making motor cars rationally. In such cars, the electric motors used to drive the car are also used to charge the batteries when the car is stopped - it is called regenerative braking.

So here is an interesting corollary. Every motor is a generator. This is true, in a sense, even when it functions as a motor. The emf that a motor generates is called the back emf.

The back emf increases with the speed, because of Faraday's law. So, if the motor has no load, it turns very quickly and speeds up until the back emf, plus the voltage drop due to losses, equal the supply voltage. The back emf can be thought of as a 'regulator': When the motor is loaded, then the phase of the voltage becomes closer to that of the current it starts to look resistive and this apparent resistance gives a voltage.

So the back emf required is smaller, and the motor turns more slowly. To add the back emf, which is inductive, to the resistive component, you need to add voltages that are out of phase. Coils usually have cores In practice, and unlike the diagrams we have drawn , generators and DC motors often have a high permeability core inside the coil, so that large magnetic fields are produced by modest currents. This is shown at left in the figure below in which the stators the magnets which are stat-ionary are permanent magnets.

The two stators are wound in the same direction so as to give a field in the same direction and the rotor has a field which reverses twice per cycle because it is connected to brushes, which are omitted here. One advantage of having wound stators in a motor is that one can make a motor that runs on AC or DC, a so called universal motor. When you drive such a motor with AC, the current in the coil changes twice in each cycle in addition to changes from the brushes , but the polarity of the stators changes at the same time, so these changes cancel out.

Unfortunatly, however, there are still brushes, even though I've hidden them in this sketch. For advantages and disadvantages of permanent magnet versus wound stators, see below. Also see more on universal motors. Build a simple motor To build this simple but strange motor, you need two fairly strong magnets rare earth magnets about 10 mm diameter would be fine, as would larger bar magnets , some stiff copper wire at least 50 cm , two wires with crocodile clips on either end, a six volt lantern battery, two soft drink cans, two blocks of wood, some sticky tape and a sharp nail.

Make the coil out of stiff copper wire, so it doesn't need any external support. Wind 5 to 20 turns in a circle about 20 mm in diameter, and have the two ends point radially outwards in opposite directions.

These ends will be both the axle and the contacts. If the wire has lacquer or plastic insulation, strip it off at the ends. The supports for the axle can be made of aluminium, so that they make electrical contact. For example poke holes in a soft drink cans with a nail as shown.

Position the two magnets, north to south, so that the magnetic field passes through the coil at right angles to the axles. Tape or glue the magnets onto the wooden blocks not shown in the diagram to keep them at the right height, then move the blocks to put them in position, rather close to the coil.

Rotate the coil initially so that the magnetic flux through the coil is zero, as shown in the diagram. Now get a battery, and two wires with crocodile clips. Connect the two terminals of the battery to the two metal supports for the coil and it should turn. Note that this motor has at least one 'dead spot': It often stops at the position where there is no torque on the coil.

Don't leave it on too long: The optimum number of turns in the coil depends on the internal resistance of the battery, the quality of the support contacts and the type of wire, so you should experiment with different values. As mentioned above, this is also a generator, but it is a very inefficient one. To make a larger emf, use more turns you may need to use finer wire and a frame upon which to wind it. You could use eg an electric drill to turn it quickly, as shown in the sketch above.

Use an oscilloscope to look at the emf generated. Is it AC or DC? This motor has no split ring, so why does it work on DC? Simply put, if it were exactly symmetrical, it wouldn't work. However, if the current is slightly less in one half cycle than the other, then the average torque will not be zero and, because it spins reasonably rapidly, the angular momentum acquired during the half cycle with greater current carries it through the half cycle when the torque is in the opposite direction.

At least two effects can cause an asymmetry. Even if the wires are perfectly stripped and the wires clean, the contact resistance is unlikely to be exactly equal, even at rest. Also, the rotation itself causes the contact to be intermittent so, if there are longer bounces during one phase, this asymmetry is sufficient. In principle, you could partially strip the wires in such a way that the current would be zero in one half cycle.

AC motors With AC currents, we can reverse field directions without having to use brushes. This is good news, because we can avoid the arcing, the ozone production and the ohmic loss of energy that brushes can entail. Further, because brushes make contact between moving surfaces, they wear out.

The first thing to do in an AC motor is to create a rotating field. Note that the Earth wire doesn't carry a current except in the event of electrical faults.

With single phase AC, one can produce a rotating field by generating two currents that are out of phase using for example a capacitor. This gives a field rotating counterclockwise. In a capacitor, the voltage is a maximum when the charge has finished flowing onto the capacitor, and is about to start flowing off. Thus the voltage is behind the current. In a purely inductive coil, the voltage drop is greatest when the current is changing most rapidly, which is also when the current is zero.

The voltage drop is ahead of the current. In this animation, the graphs show the variation in time of the currents in the vertical and horizontal coils. The plot of the field components B x and B y shows that the vector sum of these two fields is a rotating field. The main picture shows the rotating field. It also shows the polarity of the magnets: If we put a permanent magnet in this area of rotating field, or if we put in a coil whose current always runs in the same direction, then this becomes a synchronous motor.

Under a wide range of conditions, the motor will turn at the speed of the magnetic field. If we have a lot of stators, instead of just the two pairs shown here, then we could consider it as a stepper motor: Please remember my warning about the idealised geometry: Induction motors Now, since we have a time varying magnetic field, we can use the induced emf in a coil — or even just the eddy currents in a conductor — to make the rotor a magnet. That's right, once you have a rotating magnetic field, you can just put in a conductor and it turns.

This gives several of the advantages of induction motors: Below left is a schematic of an induction motor. For photos of real induction motors and more details, see Induction motors.

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