# Synchronous motors

What is the working principle of synchronous motors

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Electro magnets located around the perimeter of a rotating armature, the electro magnets energized in sequence to act on the armature causing it to rotate... not dependent on an alternating current cycle.
Value is where very low speed and high horsepower is required. prime example in slow speed large reciprocating compressors... 200 RPM several thousand HP.
Phil Scott

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Synchronous motors use AC stator windings and have DC supplied to the rotor coils. This arrangement allows for a "slipless" motor that operates at an exact RPM. Also prevents lagging on the AC distribution caused by normal AC motors. Synchronous motors can also be over excited for use as a rotating condensor to use for power factor correction.
Fred

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What has all this DC and Ward (Leonard) stuff got to do with the original question?
"What is the working principle of synchronous motors???"
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John G

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Look up rotating field. Speed is dependent on frequency
This is the basis of both synchronous and induction motors. The stator windings are arranged so that, when they are supplied with 3 phase (usually), a rotating magnetic field is produced ( low speed motors look like a ring of electromagnets but they are not switched on and off in sequence but supplied with 3 phase). This field, has constant magnitude and a frequency dependent speed (Hence the use of the word "synchronous").
Sequentially switched motors exist- stepper motors - but not where real muscle is needed. The rotor of a synchronous machine is a magnet which "locks in" with the rotating field and rotates with it. The speed is determined by the windings and the number of poles per phase. At 60 Hz, the speeds can be 3600,1800,1200,900,.......rpm.(120*frequency*poles/2 always even number of poles). The induction motor is more common and is simpler to build- it has the same stator construction but the rotor can be anything conductive (voltages are induced in the rotor and these cause currents which produce a rotating field at synchronous speed) and operates more like an automatic transmission -requiring some slippage to produce torque. It's speed will always be below synchronous speed (say 3550 rpm).
The induction motor also inherently develops starting torque while a synchronous motor requires some starting mechanism such as an added induction motor winding. Hence induction motors are more common than synchronous motors. The use of synchronous motors for such things as large compressors is not that they are particularly suited to that any more than an induction motor, but that the power factor can be controlled easily as needed.
Essentially all our AC power is produced by synchronous machines driven as generators (which, electrically, can be run as motors) although induction generators do exist.
The following may be of interest http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee11/bdeee11_8.aspx
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Don Kelly @shawcross.ca
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... LOL. No, look up "synchronous motor" on Google. Much better hits; here's a typical one: http://www.tpub.com/content/neets/14177/css/14177_92.htm
Pop
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Well, and most electric clocks <g>.
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the ones at the Montgomery Ward store, turn of the century as a rough guess were flat with the perimeter magnets looking a lot like the cylinders on a 1930's radial aircraft engine. But smaller, with a much larger rotor. these were brushed armatures.
The modern versions I have seen looked to be simiilar, not the barrel or squirrel cage shape shown at one of your links although I am sure there are those types also...these were not not that sort of a layout. Those with the squirrel cage layout would have been much higher speed.
it is the large flat pancake layout that would have provided the ultra low speed high torque operation.
the 2000 hp motors at Anheuser Busch micro breweries were 8' to 10'' in diameter and maybe 18" to 24" thick through the armature... max... but I wasnt paying close attention to the motors myself at the time...in these cases I was doing mechanical engineering functions.
The clock motors I see mostly around plug into the 110 volt house current....look like shaded pole gear motors to me...and of course miniscule torque... but yes of course with ac motors the field and armature currents are 'synchronous'.
Phil Scott

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Motors can generally be classified as a three terminal device, two terminals are electrical and one terminal is mechanical. For a motor, the output is mechanical and the two inputs are electrical. For generators, the output is electrical and one input is mechanical and one is electrical. In general, we think of the two electrical terminals as in terms of real power and reactive power. If you want to, you can make the analogy to DC machines and think in terms of a field and armature input. The field coil of a DC machine sets up the magnetic field with which the armature current interacts with to created work or torque. F=iLXB. So the field supplies the B and the armature supplies the i.
In the case fo the synchronous machine, the B takes the form of permanent magnets or electromagnets via the DC winding. In the case of the induction motor, the field component comes into the motor as a part of the current, so part of the current supplies the i and part of it is used to set up the field. So in this case, the rotating airgap flux set up by the stator windings locks in step with the DC field on the rotor and the rotor rotates in locked step with the stator field, hence the motor is synchronous.
In the case of the induction machine, the rotor current is induced in the rotor shorted windings by the airgap flux created by the stator winding. The induced current then sets up a rotor field, which corresponds to the field in the DC machine. This field is always trying to catch up with the stator field but can never be at the same speed because the rotor field is created by the induced rotor currents, and rotor currents exists only if the rotor is not at the same speed as the stator airgap flux. So if the rotor is rotating at the same speed as the stator field, there are no induced field in the rotor, therefore there are no i cross B interaction, therefore no torque.
Another type of synchronous motor is the synchronous reluctance motor, which ratains the stator winding configuration but the rotor does not contain windings or magnets, the rotor does have saliency, or pole piece structure without the windings. What the saliency creates is a preferred path for the airgap flux to travel through in order to complete the magnetic circuit, so the air gap flux now flows through the point with the least airgap length, which then locks the rotor in step with the stator field.
Another synchrnous motor is relatively new, it uses the basic topology of a stepper motor, both stators and rotors are salient. The double saliency drives the motor deeply into saturation and the designers actually uses the saturation to their advantage. The stator poles are wound with single tooth indings and the these windings are fed with DC currents. The crititcal thing here are twofold: 1) The stator and rotor pole numbers must be different and optimised to create the most torque and 2) The DC current is switched on and off in sequence to optimise the steady state torque of the machine. This is known as switched reluctance motors. The main drawbacks are that the torque is pulsating, and the nature of the motor excitation creates a lot of noise by the flexing of the motor backiron, and an electronic drive is needed to start and operate the motor. The advantages are that these motors create very high low speed torques, the rotor is extremely robust, and the motor dynamics are excellent, making these motors very useful for high permance applications.
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A nit pick.

The rotor field of an induction motor moves at synchronous speed with
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Multi-phase AC applied to a properly wound stator creates a constantly rotating magnetic field.
Stick a magnet in the field and it will follow it around.
Replace the magnet with an electro-magnet and you have a synchronous motor.
daestrom
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A synchronous motor is one where a fixed magnetic field rotates in exact synchronism or lock to a rotating magnetic field. The rotating field is generated by energizing the coils with alternating current and is usually but not always the stator or non-moving part. These coils are always driven with two or more phases of AC current to get magnetic rotation. Three phases are common. Single phase motors use capacitors or other methods to create the necessary phase split for rotation.
The fixed magnetic field that locks onto the rotating field is usually in the rotor. They can be permanent or electromagnets. The larger motors are most always use electromagnets while small motors use permanent magnets. A third type called hysterisis sychcronous uses a non-magnetized iron rotor that becomes a "soft" permanent magnet when the motor is energized.
All synchronous motors provide precise rotation locked to the AC line frequency. They are found in everything from electric clocks to tape recorders, phongraphs and other equipment where speed regulation is important. They also provide a capacitive phase angle and in the larger sizes are useful for power factor correction. Bob
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