# types and applications of single phase induction motor

A motor partnersuche kostenlos grafschaft bentheim may be run from a single phase power source. (Figure ) However, it will not self-start. It may be hand started in either direction, coming up to speed in a few seconds. It will only develop 2/3 of the partnersuche kostenlos grafschaft bentheim 3-φ power rating because one winding is not used.

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3-φmotor runs from 1-φ power, but does not start.

The single coil of a single phase induction partnersuche kostenlos grafschaft bentheim motor does not produce a rotating, but a pulsating field reaching maximum intensity at 0o and 180o electrical. (Figure )

Single phase stator produces a nonrotating, pulsating magnetic field.

Another view is that the single coil excited by partnersuche kostenlos grafschaft bentheim a single phase current produces two counter rotating magnetic field phasors, coinciding twice per revolution at 0o (Figure -a) and 180o (figure e). When the phasors rotate to 90o and -90o they cancel in figure b. At 45o and -45o (figure c) they are partially additive along the +x axis and cancel along the y axis. An analogous situation exists in figure d. The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed.

However, if the rotor is rotated forward at a bit less than the synchronous speed, It will develop maximum torque at 10% slip with respect to the forward rotating phasor. Less torque will be developed above or below 10% slip. The rotor will see 200% - 10% slip with respect to the counter rotating magnetic field phasor. Little torque (see torque vs slip curve) other than a double freqency ripple is developed from the counter rotating phasor. Thus, the single phase coil will develop torque, once the rotor is started. If the rotor is started in the reverse direction, it will develop a similar large torque as it nears the speed of the backward rotating phasor.

Single phase induction motors have a copper or aluminum squirrel cage embedded in a cylinder of steel laminations, typical of poly-phase induction motors.

### Permanent-split capacitor motor

One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split motor in Figure.

Permanent-split capacitor induction motor.

This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor. This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter.

Single phase induction motor with embedded stator coils.

Single phase induction motors may have coils embedded into the stator as shown in Figure for larger size motors. Though, the smaller sizes use less complex to build concentrated windings with salient poles.

### Capacitor-start induction motor

In Figure a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes.

Capacitor-start induction motor.

### Capacitor-run motor induction motor

A variation of the capacitor-start motor (Figure ) is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

Capacitor-run motor induction motor.

A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

### Resistance split-phase motor induction motor

If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. (Figure ) With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.

Resistance split-phase motor induction motor.

This motor has more starting torque than a shaded pole motor (next section), but not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.

### Nola power factor corrrector

Frank Nola of NASA proposed a power factor corrector for improving the efficiency of AC induction motors in the mid 1970’s. It is based on the premise that induction motors are inefficient at less than full load. This inefficiency correlates with a low power factor. The less than unity power factor is due to magnetizing current required by the stator. This fixed current is a larger proportion of total motor current as motor load is decreased. At light load, the full magnetizing current is not required. It could be reduced by decreasing the applied voltage, improving the power factor and efficiency. The power factor corrector senses power factor, and decreases motor voltage, thus restoring a higher power factor and decreasing losses.

Since single-phase motors are about 2 to 4 times as inefficient as three-phase motors, there is potential energy savings for 1-φ motors. There is no savings for a fully loaded motor since all the stator magnetizing current is required. The voltage cannot be reduced. But there is potential savings from a less than fully loaded motor. A nominal 117 VAC motor is designed to work at as high as 127 VAC, as low as 104 VAC. That means that it is not fully loaded when operated at greater than 104 VAC, for example, a 117 VAC refrigerator. It is safe for the power factor controller to lower the line voltage to 104-110 VAC. The higher the initial line voltage, the greater the potential savings. Of course, if the power company delivers closer to 110 VAC, the motor will operate more efficiently without any add-on device.

Any substantially idle, 25% FLC or less, single phase induction motor is a candidate for a PFC. Though, it needs to operate a large number of hours per year. And the more time it idles, as in a lumber saw, punch press, or conveyor, the greater the possibility of paying for the controller in a few years operation. It should be easier to pay for it by a factor of three as compared to the more efficient 3-φ-motor. The cost of a PFC cannot be recovered for a motor operating only a few hours per day.

Summary: Single-phase induction motors

• Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an out of phase current of near 90o. Once started the auxiliary winding is optional.
• The auxiliary winding of a permanent-split capacitor motor has a capacitor in series with it during starting and running.
• A capacitor-start induction motoronly has a capacitor in series with the auxiliary winding during starting.
• A capacitor-run motor typically has a large non-polarized electrolytic capacitor in series with the auxiliary winding for starting, then a smaller non-electrolytic capacitor during running.
• The auxiliary winding of a resistance split-phase motor develops a phase difference versus the main winding during starting by virtue of the difference in resistance.
Three-phase totally enclosed fan-cooled () induction motor with end cover on the left, and without end cover to show cooling fan. In TEFC motors, interior heat losses are dissipated indirectly through enclosure fins, mostly by forced air convection.
Cutaway view through stator of induction motor, showing rotor with internal air circulation vanes. Many such motors have a symmetric armature, and the frame may be reversed to place the electrical connection box (not shown) on the opposite side.

An induction motor or asynchronous motor is an in which the in the needed to produce torque is obtained by from the of the winding. An induction motor can therefore be made without electrical connections to the rotor. An induction motor's rotor can be either or

induction motors are widely used as industrial drives because they are rugged, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with (VFDs) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque fan, pump and compressor load applications. Squirrel cage induction motors are very widely used in both fixed-speed and (VFD) applications.

## History[]

A model of Tesla's first induction motor, in Tesla Museum, Belgrade
Squirrel cage rotor construction, showing only the center three laminations

In 1824, the French physicist formulated the existence of, termed. By manually turning switches on and off, Walter Baily demonstrated this in 1879, effectively the first primitive induction motor.

The first commutator-free two phase AC induction motor was invented by Hungarian engineer ; he used the two phase motor to propel his invention, the.

The first AC three-phase induction motors were independently invented by and, a working motor model having been demonstrated by the former in 1885 and by the latter in 1887. Tesla applied for in October and November 1887 and was granted some of these patents in May 1888. In April 1888, the Royal Academy of Science of Turin published Ferraris's research on his AC polyphase motor detailing the foundations of motor operation. In May 1888 Tesla presented the technical paper A New System for Alternating Current Motors and Transformers to the (AIEE) describing three four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting, another with a wound rotor forming a self-starting induction motor, and the third a true with separately excited DC supply to rotor winding.

, who was developing an system at that time, licensed Tesla’s patents in 1888 and purchased a US patent option on Ferraris' induction motor concept. Tesla was also employed for one year as a consultant. Westinghouse employee was assigned to assist Tesla and later took over development of the induction motor at Westinghouse. Steadfast in his promotion of three-phase development, invented the cage-rotor induction motor in 1889 and the three-limb transformer in 1890. Furthermore, he claimed that Tesla's motor was not practical because of two-phase pulsations, which prompted him to persist in his three-phase work. Although Westinghouse achieved its first practical induction motor in 1892 and developed a line of polyphase 60 induction motors in 1893, these early Westinghouse motors were with wound rotors until developed a rotating bar winding rotor.

The (GE) began developing three-phase induction motors in 1891. By 1896, General Electric and Westinghouse signed a cross-licensing agreement for the bar-winding-rotor design, later called the squirrel-cage rotor. was the first to bring out the full significance of (using j to represent the square root of minus one) to designate the 90º operator in analysis of AC problems. GE's greatly developed application of AC complex quantities including an analysis model now commonly known as the induction motor.

Induction motor improvements flowing from these inventions and innovations were such that a 100- induction motor currently has the same mounting dimensions as a 7.5-horsepower motor in 1897.

## Principle of operation[]

A three-phase power supply provides a rotating magnetic field in an induction motor
Inherent slip - unequal rotation frequency of stator field and the rotor

In both induction and, the AC power supplied to the motor's creates a that rotates in synchronism with the AC oscillations. Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a somewhat slower speed than the stator field. The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited or closed through an external impedance. The rotating induces currents in the windings of the rotor; in a manner similar to currents induced in a 's secondary winding(s).

The induced currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. Due to, the direction of the magnetic field created will be such as to oppose the change in current through the rotor windings. The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor current and torque balances the applied mechanical load on the rotation of the rotor. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slightly slower than synchronous speed. The difference, or "slip," between actual and synchronous speed varies from about 0.5% to 5.0% for standard Design B torque curve induction motors. The induction motor's essential character is that it is created solely by induction instead of being separately excited as in synchronous or DC machines or being self-magnetized as in.

For rotor currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( n s {\displaystyle n_{s}} types and applications of single phase induction motor aria-hidden="true" alt="n_{s}" />); otherwise the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field induced in the rotor and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as "asynchronous motors".

An induction motor can be used as an, or it can be unrolled to form a which can directly generate linear motion.

### Synchronous speed[]

An AC motor's synchronous speed, n s {\displaystyle n_{s}} , is the rotation rate of the stator's magnetic field,

n s = 2 f p {\displaystyle n_{s}={2f \over {p}}} ,

where f {\displaystyle f} is the motor supply's frequency, where p {\displaystyle p} is the number of magnetic poles and where n s {\displaystyle n_{s}} and f {\displaystyle f} have identical units. For f {\displaystyle f} in unit and n s {\displaystyle n_{s}} in, the formula becomes

n s = 2 f p ⋅ ( 60   s m i n ) = 120 f p ⋅ ( s m i n ) {\displaystyle n_{s}={2f \over {p}}\cdot \left({\frac {60\ \mathrm {s} }{\mathrm {min} }}\right)={120f \over {p}}\cdot \left({\frac {\mathrm {s} }{\mathrm {min} }}\right)} .

For example, for a four-pole three-phase motor, p {\displaystyle p} = 4 and n s = 120 f 4 {\displaystyle n_{s}={120f \over {4}}} = 1,500  and 1,800 , RPM synchronous speed, respectively, for 50 Hz and 60 Hz supply systems.

The two figures at right and left above each illustrate a 2-pole 3-phase machine consisting of three pole-pairs with each pole set 60º apart.

### Slip[]

Typical torque curve as a function of slip, represented as "g" here

Slip, s {\displaystyle s} , is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed. Thus

s = n s − n r n s {\displaystyle s={\frac {n_{s}-n_{r}}{n_{s}}}\,}

where n s {\displaystyle n_{s}} is stator electrical speed, n r {\displaystyle n_{r}} is rotor mechanical speed. Slip, which varies from zero at synchronous speed and 1 when the rotor is at rest, determines the motor's torque. Since the short-circuited rotor windings have small resistance, even a small slip induces a large current in the rotor and produces significant torque. At full rated load, slip varies from more than 5% for small or special purpose motors to less than 1% for large motors. These speed variations can cause load-sharing problems when differently sized motors are mechanically connected. Various methods are available to reduce slip, VFDs often offering the best solution.

### Torque[]

#### Standard torque[]

Speed-torque curves for four induction motor types: A) Single-phase, B) Polyphase cage, C) Polyphase cage deep bar, D) Polyphase double cage
Typical speed-torque curve for NEMA Design B Motor

The typical speed-torque relationship of a standard NEMA Design B polyphase induction motor is as shown in the curve at right. Suitable for most low performance loads such as centrifugal pumps and fans, Design B motors are constrained by the following typical torque ranges:

• Breakdown torque (peak torque), 175-300% of rated torque
• Locked-rotor torque (torque at 100% slip), 75-275% of rated torque
• Pull-up torque, 65-190% of rated torque.

Over a motor's normal load range, the torque's slope is approximately linear or proportional to slip because the value of rotor resistance divided by slip, R r ′ / s {\displaystyle R_{r}^{'}/s} , dominates torque in linear manner. As load increases above rated load, stator and rotor leakage reactance factors gradually become more significant in relation to R r ′ / s {\displaystyle R_{r}^{'}/s} such that torque gradually curves towards breakdown torque. As the load torque increases beyond breakdown torque the motor stalls.

#### Starting[]

There are three basic types of competing small induction motors: single-phase, split-phase and shaded-pole types and small polyphase motors.

In two-pole single-phase motors, the torque goes to zero at 100% slip (zero speed), so these require alterations to the stator such as to provide starting torque. A single phase induction motor requires separate starting circuitry to provide a rotating field to the motor. The normal running windings within such a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.

In certain smaller single-phase motors, starting is done by means of a shaded pole with a copper wire turn around part of the pole. The current induced in this turn lags behind the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational field energy to start the motor. These motors are typically used in applications such as desk fans and record players, as the required starting torque is low, and the low efficiency is tolerable relative to the reduced cost of the motor and starting method compared to other AC motor designs.

Larger single phase motors are and have a second stator winding fed with out-of-phase current; such currents may be created by feeding the winding through a capacitor or having it receive different values of inductance and resistance from the main winding. In capacitor-start designs, partnersuche kostenlos grafschaft bentheim the second winding is disconnected once the motor is up to speed, usually either by a centrifugal switch acting on weights on the motor shaft or a which heats up and increases its resistance, reducing the current through the second winding to an insignificant level. The capacitor-run designs keep the second winding on when running, improving torque. A resistance start design uses a starter inserted in series with the startup winding, creating reactance.

Self-starting polyphase induction motors produce torque even at standstill. Available squirrel cage induction motor starting methods include direct-on-line starting, reduced-voltage reactor or auto-transformer starting, star-delta starting or, increasingly, new solid-state soft assemblies and, of course, VFDs.

Polyphase motors have rotor bars shaped to give different speed-torque characteristics. The current distribution within the rotor bars varies depending on the frequency of the induced current. At standstill, the rotor current is the same frequency as the stator current, and tends to travel at the outermost parts of the cage rotor bars (by ). The different bar shapes can give usefully different speed-torque characteristics as well as some control over the inrush current at startup.

Although polyphase motors are inherently self-starting, their starting and pull-up torque design limits must be high enough to overcome actual load conditions.

In wound rotor motors, rotor circuit connection through slip rings to external resistances allows change of speed-torque characteristics for acceleration control and speed control purposes.

#### Speed control[]

Resistance
Typical speed-torque curves for different motor input frequencies as for example used with

Before the development of semiconductor, it was difficult to vary the frequency, and cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used DC drives or wound rotor motors (WRIM) with for rotor circuit connection to variable external resistance allowing considerable range of speed control. However, resistor losses associated with low speed operation of WRIMs is a major cost disadvantage, especially for constant loads. Large slip ring motor drives, termed slip energy recovery systems, some still in use, recover energy from the rotor circuit, rectify it, and return it to the power system using a VFD.

The speed of a pair of slip-ring motors can be controlled by a cascade connection, or concatenation. The rotor of one motor is connected to the stator of the other. If the two motors are also mechanically connected, they will run at half speed. This system was once widely used in three-phase AC railway locomotives, such as.

Variable-frequency drive

Main article:

In many industrial variable-speed applications, DC and WRIM drives are being displaced by VFD-fed cage induction motors. The most common efficient way to control asynchronous motor speed of many loads is with VFDs. Barriers to adoption of VFDs due to cost and reliability considerations have been reduced considerably over the past three decades such that it is estimated that drive technology is adopted in as many as 30-40% of all newly installed motors.

## Construction[]

Typical winding pattern for a three-phase (U, V, W), four-pole motor. Note the interleaving of the pole windings and the resulting.

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field, windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90° from the single-phase supply and feeds it to the second motor winding. Single-phase motors require some mechanism to produce a rotating field on startup. Cage induction motor rotor's conductor bars are typically skewed to avoid magnetic locking.

Standardized NEMA & IEC motor frame sizes throughout the industry result in interchangeable dimensions for shaft, foot mounting, general aspects as well as certain motor flange aspect. Since an open, drip proof (ODP) motor design allows a free air exchange from outside to the inner stator windings, this style of motor tends to be slightly more efficient because the windings are cooler. At a given power rating, lower speed requires a larger frame.

## Rotation reversal[]

The method of changing the direction of rotation of an induction motor depends on whether it is a three-phase or single-phase machine. In the case of three-phase, reversal is straightforwardly implemented by swapping connection of any two phase conductors.

In a single-phase split-phase motor, reversal is achieved by changing the connection between the primary winding and the start circuit. Some single-phase split-phase motors that are designed for specific applications may have the connection between the primary winding and the start circuit connected internally so that the rotation cannot be changed. Also, single-phase shaded-pole motors have a fixed rotation, and the direction cannot be changed except by disassembly of the motor and reversing the stator to face opposite relative to the original rotor direction.

## Power factor[]

The of induction motors varies with load, typically from around 0.85 or 0.90 at full load to as low as about 0.20 at no-load, due to stator and rotor leakage and magnetizing reactances. Power factor can be improved by connecting capacitors either on an individual motor basis or, by preference, on a common bus covering several motors. For economic and other considerations, power systems are rarely power factor corrected to unity power factor. Power capacitor application with harmonic currents requires power system analysis to avoid harmonic resonance between capacitors and transformer and circuit reactances. Common bus power factor correction is recommended to minimize resonant risk and to simplify power system analysis.

## Efficiency[]

• Friction and, 5–15%
• Iron or, 15–25%
• Stator losses, 25–40%
• Rotor losses, 15–25%

Various regulatory authorities in many countries have introduced and implemented legislation to encourage the manufacture and use of higher efficiency electric motors. There is existing and forthcoming legislation regarding the future mandatory use of premium-efficiency induction-type motors in defined equipment. For more information, see:.

## Steinmetz equivalent circuit[]

Many useful motor relationships between time, current, voltage, speed, power factor, and torque can be obtained from analysis of the Steinmetz (also termed T-equivalent circuit or IEEE recommended equivalent circuit), a mathematical model used to describe how an induction motor's electrical input is transformed into useful mechanical energy output. The equivalent circuit is a single-phase representation of a multiphase induction motor that is valid in steady-state balanced-load conditions.

The Steinmetz equivalent circuit is expressed simply in terms of the following components:

• and ( R s {\displaystyle R_{s}} , X s {\displaystyle X_{s}} ).
• resistance, leakage reactance, and slip ( R r {\displaystyle R_{r}} , X r {\displaystyle X_{r}} or R r ′ {\displaystyle R_{r}^{'}} , X r ′ {\displaystyle X_{r}^{'}} , and s {\displaystyle s} ).
• ( X m {\displaystyle X_{m}} ).

Paraphrasing from Alger in Knowlton, an induction motor is simply an electrical transformer the magnetic circuit of which is separated by an air gap between the stator winding and the moving rotor winding. The equivalent circuit can accordingly be shown either with equivalent circuit components of respective windings separated by an ideal transformer or with rotor components referred to the stator side as shown in the following circuit and associated equation and parameter definition tables.

Steinmetz equivalent circuit

The following rule-of-thumb approximations apply to the circuit:

• Maximum current happens under locked rotor current (LRC) conditions and is somewhat less than V s / X {\displaystyle {V_{s}}/X} , with LRC typically ranging between 6 and 7 times rated current for standard Design B motors.
• Breakdown torque T m a x {\displaystyle T_{max}} happens when s ≈ R r ′ / X {\displaystyle s\approx {R_{r}^{'}/X}} and I s ≈ 0.7 L R C {\displaystyle I_{s}\approx {0.7}LRC} such that T m a x ≈ K ∗ V s 2 / ( 2 X ) {\displaystyle T_{max}\approx {K*V_{s}^{2}}/(2X)} and thus, with constant voltage input, a low-slip induction motor's percent-rated maximum torque is about half its percent-rated LRC.
• The relative stator to rotor leakage reactance of standard Design B cage induction motors is
X s X r ′ ≈ 0.4 0.6 {\displaystyle {\frac {X_{s}}{X_{r}^{'}}}\approx {\frac {0.4}{0.6}}} .
• Neglecting stator resistance, an induction motor's torque curve reduces to the Kloss equation
T e m ≈ 2 T m a x s s m a x + s m a x s {\displaystyle T_{em}\approx {\frac {2T_{max}}{{\frac {s}{s_{max}}}+{\frac {s_{max}}{s}}}}} , where s m a x {\displaystyle s_{max}} is slip at T m a x {\displaystyle T_{max}} .

## Linear induction motor[]

Main article:

Linear induction motors, which work on same general principles as rotary induction motors (frequently three-phase), are designed to produce straight line motion. Uses include, linear propulsion,, and pumping.

1. That is, electrical connections requiring, separate-excitation or self-excitation for all or part of the energy transferred from stator to rotor as are found in, and motors.
2. NEMA MG-1 defines a) breakdown torque as the maximum torque developed by the motor with rated voltage applied at rated frequency without an abrupt drop in speed, b) locked-rotor torque as the minimum torque developed by the motor at rest with rated voltage applied at rated frequency, and c) pull-up torque as the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs.

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## Classical sources[]

One of the most commonly used is induction motor. We also call this motor as because it runs at speed less than its synchronous speed.

### Synchronous Speed

Here we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine, and it depends upon the frequency and number poles of the machine.

An induction motor always runs at speed less than synchronous speed. Because the rotating magnetic field produced in the stator will create flux in the rotor and hence will make the rotor to rotate. Due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach it's rotating magnetic field speed, i.e. the synchronous speed. There are basically two types of induction motors. The types of induction motors depend upon the input supply. The and. Single phase induction motor is not a self-starting motor, and three phase induction motor is a self-starting motor.

### Working Principle of Induction Motor

We need to give double excitation to make a to rotate. In a, we give one supply to the stator and another to the rotor through brush arrangement. But in induction motor, we give only one supply, so it is really interesting to know how an induction motor works. It is very simple, from the name itself we can understand that here, induction process is involved. When we give the supply to the stator winding, a magnetic flux is produced in the stator due to the flow of current in the coil. The rotor winding is arranged in such a way that each coil becomes short-circuited in the rotor itself.

The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited, according to, current will start flowing through the coil of the rotor. When the current through the rotor coils flows, another flux gets generated in the rotor. Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic field. This is the working principle of an induction motor of either type – single and three phase.

#### Single Phase Induction Motor

1. Split Phase Induction Motor
2. Capacitor Start Induction Motor
3. Capacitor Start and Capacitor Run Induction Motor

#### Three Phase Induction Motor

1. Squirrel Cage Induction Motor
2. Slip Ring Induction Motor
We have already mentioned above that single phase induction motor is not a self-starting and three phase induction motor is self-starting. So what is self-starting? When the machine starts running automatically without any external force to the machine, then it is called as self-starting. For example, we see that when we put on the switch the fan starts to rotate automatically, so it is self-starting. Point to be noted that fan used in home appliances is single phase induction motor which is inherently not self-starting. How? A question arises how it works? We will discuss it now.

### Why is Three Phase Induction Motor Self Starting?

In, there are three single phase lines with 120° phase difference. So the rotating magnetic field has the same phase difference which will make the rotor to move. If we consider three phases a, b, and c when phase a is magnetised, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetised and it will attract the rotor and then phase c. So the rotor will continue to rotate.

### Why Single Phase Induction Motor is not Self Starting?

It has only one phase still it makes the rotor to rotate, so it is quite interesting. Before that, we need to know why single phase induction motor is not a self-starting motor and how we overcome the problem. We know that the AC supply is a sinusoidal wave and it produces a pulsating magnetic field in the uniformly distributed stator winding. Since we can assume the pulsating magnetic field as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting, and hence the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by an external force, then the motor will start to run. We can solve this problem by making the stator winding into two winding, one is main winding, and another is auxiliary winding. We connect one capacitor in series with the auxiliary winding. The capacitor will make a phase difference when current flows through both coils. When there is phase difference, the rotor will generate a starting torque, and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor gets disconnected from the motor, but if we rotate with the hand, it will start rotating. That is why we use a capacitor in the single phase induction motor.
There are several advantages of induction motor which make this motor to have wider application. It has good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is a disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the phase sequence of three-phase supply, i.e. if RYB is in a forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor, but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.
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