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Since my AC motors article, ns have regularly been asked around how to turning back an AC induction motor.I didn"t cover exactly how induction motors start in any detail earlierbecause that"s comprehensive topic top top its own.The rotor of an induction engine is basically a permeable stole corewith one aluminium quick circuit winding cast in place. You have the right to seethe aluminium on both end of the rotor. The aluminium additionally goes throughlengthwise holes in the rotor to do a "squirrel cage" kind shortcircuit winding. You can barely watch lines, in ~ a slight edge on the rotorwhere the windings happen through.The short circuit winding causes the rotor to resist rapid alters in magneticfields, therefore if it"s subjected to a rotate magnetic field, it will certainly tryto follow it. (more on the here)In a 3 phase motor, 3 phases on three windings naturallycreate a rotate magnetic field. Yet for solitary phase AC motors, themagnetic field only alternates back and forth. Some trickery is neededto create a rotating field.

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Reversing a separation phase motor

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In this split phase motor, the main winding (label "M")is connected directly come 60 Hz AC power, when the other winding (label "O") is wired in collection with acapacitor (C). The interaction between the inductance of the motorwindings and also the capacitor"s capacitance reasons that winding to be about 90degrees the end of phase through the main winding.With the key winding producing a magnetic ar that alternates vertically,and the various other winding producing a magnetic field that alternates horizontallybut the end of phase, the sum of these is a rotate magnetic field.The rotor make the efforts to monitor it, causing it come spin.
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Reversing the motor is merely a issue of moving the power connectionso that the other winding is straight on AC. Essentially, movingone next of the power link from (A) to (B), resulting in winding (O)to be the main winding and winding (M) to be the step shifted one.On motors larger than 1/4 hp, the 2 windings commonly have differentnumbers the turns, for this reason this method of reversing may not be applicable.Check that the resistance of the two windings is the same first.If the windings are not the very same resistance, you deserve to still turning back itby reversing the polarity of one of the windings, detailed that thewindigs are not tied together inside the engine (as in, more than threewires coming out from the windings).

Starter windings on bigger motors

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Now, if us look inside a larger motor, like this 3/4 speech motor,the windings lookmuch much more complicated. The windings room spread across many slotsin the motor"s stator (C). The way, thereis much less of one abrupt change from one pole to the next. Thismakes because that a smoother magnetic field, which makes for a quieter, moreefficient motor.This motor has a thick main winding (M), add to a starter windingmade of diluent wire (S). The main winding creates a horizontalmagnetic field, if the starter winding create a vertical one.

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This starter winding is in collection with a capacitor (C), and also a centrifugalswitch (S). In this motor, the starter capacitor is mountedinside the main housing. An ext typically, the starter capacitor is mountedon height of the housing under a steel dome.The centrifugal switch (S) is placed to the back plateand is triggered by a decaying (P) that pushes against a tab ~ above theswitch (just left that the S in the photo).

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Removing the rotor and also looking in ~ the disk, you have the right to see two steel tabs.When the motor spins, centrifugal pressure pushes this outwards, whichin turn pulls the disk back. This release the plastic tab top top the switch,causing the switch to open and the starter winding to be disconnected.The disk pulls ago far enough that it"s no longer in contactwith the tab, minimizing friction and also wear. It"s a clever method toactivate a switch based upon centrifugal pressure without the need for theswitch chin to spin.The centrifugal switch arrangement makes a distinct "click"when that resets ~ the motor is closeup of the door off. The click that the switchengaging together it starts increase is much harder come discern.

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If the starter winding help the motor start, it would surely helpthe motor run as well. So why not just leave the starterwinding connected? Well, thewhole phase shift thing is no that elegant. The dimension of capacitor youneed really much relies on the motor"s load. To start the motor quickly,you need a bigger capacitance than you would for reliable continuousoperation. Also, the capacitor is an electrolytic capacitor, and also isn"tdesigned for continuous load. And because the starter winding is onlyused briefly, therefore it"s made of thinner wire to conserve money, becausecopper is expensive.There room some motors that use a big capacitor for beginning and asmaller capacitor for consistent operation. Such motors regularly havetwo externally an installed capacitors (C), as checked out on this one in mine table saw.These motors are called "capacitor begin capacitor run" motors.Capacitor begin capacitor run motors are typically much more than onehorsepower. This one is 1.75 horsepower.

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Motorscanbe do cheaper by instead of ther capacitor witha resistor. Though frequently no separate resistor is added. Instead,the starter winding is do from diluent (cheaper) copper wire soit has more resistance in the winding itself.This outcomes in lot lessphase shift than through a capacitor, yet enough to gain the engine running.The windings that the motor essentially kind an inductor, and also whenan AC sine wave (as in, AC power) is applied to one inductor, gift lags the voltage by 90 degrees. And also the magnetic fieldis strict a function of current.For a resistor, the current is in phase v the voltage. If we had a largeresistance and little inductance in series, the voltage drop, and also currentwould it is in largely figured out by the resistor. Therefore the current and also magneticfield would be largely in phase v the applied voltage. Withcurrent in the key winding lagging by 90 degrees, we would have a90-degree difference between the two, yet the starter windingwould be incredibly inefficient.In reality, the compromise is for much lessof a phase transition and an ext power. It"s enough to acquire the motor running.Regardless, the starter on these electric motors is rather inefficient, yet itdoesn"t matter much once the engine is running. However, the extra currentrequired for the starter could blow a circuit breaker, for this reason this methodis commonly only used for smaller sized motors, 1/4 hp come 1/2 hp.3/4 horsepower or larger motors typically use a beginning capacitor.If you are not acquainted with analog electronics, the above explanationis most likely inadequate, and also you might want to check out up an ext on inductionmotors if you don"t know it.With induction motors, the just things that wear out room the bearings, thestarter switch and also the capacitor. V no capacitor, there"s oneless thing to fail. Simply recently, ns accidentally jammed the starter move on a 1/4 hp resistive begin motor from a garments dryer(the one top top this blower) and also it only took around 15 seconds for the motor to tripits thermal protection circuit because the starter winding overheated.

Reversing a capacitor begin motor

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So just how do we reverse a capacitor begin motor? when started, a solitary phase inductionmotor will happily operation in either direction. To reverse it, we need tochange the direction the the rotating magnetic field created by the mainand starter windings. And this deserve to be completed by reversing thepolarity of the starter winding. Basically, we need to swap theconnections ~ above either end of the starter winding. Occasionally it"sjust the winding, periodically the winding, switch and also capacitor arereversed. The order of the switch and capacitor don"tmatter, as long as thy are in wired in series.

You could likewise reverse the engine by reversing the key winding(same effect).If you were to move the main and starter windings, as one doeswith a break-up phase motor, the motor will also reverse. However,it will not operation at full power and also is additionally likely come burn out. Thestarter winding is not suitable for constant operation.

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The label on this motor suggests "MOTOR IS NON-REVERSIBLE".If girlfriend look in ~ the previous pictures of this motor, you have the right to see there areonly three wires (red, yellow and blue) coming the end from the windings.One finish of the main and also starter windings is linked togetherright on the windings.To reverse the starter winding, I"d need to break that connectioninside the windings and also bring out the other end of the starterwinding. Yet I yes, really can"t obtain at it since of the way it"s within the motor. I"d have actually to cut a hole in theenclosure to even get in ~ the suggest where they space tied together. It"snot the this motor can"t be reversed, just that, as a cost savingmeasure, they do reversing the more difficult than is worth thetrouble.

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But on electric motors that space reversible, the label constantly indicates to swap 2 wires to reverse it.
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The wires to turning back are always the wires that lead to the starter winding.If you have a motor whereby the label is missing, the starter windingtypically has about three time the electrical resistance of the mainwinding and also is constantly in series with the starter switch and also capacitor(if there is one). If you have the right to isolate both end of this windingand swap them, you have the right to reverse the motor. If yet there room onlythree wires coming out of the windings, then the main and starter windingshave one end tied together and also the engine is not reversible.For a 1/2 hp 120 volt motor, the key winding will typically have about1.5 ohms, and the starter winding about 4 ohms. For 240 volt 1/2 hpmotors (240 volt only), you must expect abot 6 ohms ~ above the main winding, and 16 ohmson the starter winding. Mean the resistance the the windings to beinversely proportional through horsepower.A many motors will have actually a couple of extra wires coming off the windings.Often, a thermal move is attached to the windings, and also this switchmay be partly tied to among the windings. Also, if the motorcan it is in rewired because that 120 and 240 volt, the main winding will certainly consistof two 120 volt windings that have the right to be wired either in collection or parallel.So there can be quite a few wires coming off the windings. It can takea bit of time and probing approximately to figure it out.For electric motors that have the right to be wired at both 120 volts and 240 volts, the starterwinding is a 120 volt winding. As soon as these electric motors are wired for 240 volts,the key winding is provided as one autotransformer to makethe 120 volts for the starter winding. Otherwise, rewiring the motorfrom 120 to 240 volts would certainly be much more complex!