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Types of Single-Phase Induction Motors

By: Tirffneh Y.(M.Tech)May 2014

Mekelle University- MIT

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Introduction

• Each single-phase induction motor derives its name from the method used to make it self-starting

• Some are split-phase motor capacitor-start motor capacitor-start capacitor-run motor permanent split-capacitor motor the shaded-pole motor

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• For an IM to be self-starting, it must have at least two phase windings in space quadrature and must be excited by a 2-Ph or 3-ph source

• The currents in the 2-ph windings are 900 electrical out of phase with each other

• The placement of the two phase windings in space quadrature in a 1-ph motor is no problem

• However, the artificial creation of a second phase requires some basic understanding of resistive, inductive, and capacitive networks

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Split-Phase Motor• employs 2 separate windings that are placed in

space quadrature and are connected in parallel to a 1-ph source

• One winding, known as the main winding, has a low R and high L. This winding carries current and establishes the needed flux at the rated speed

• The second winding, called the auxiliary winding, has a high R and low L. This winding is disconnected from the supply when the motor attains a N of nearly 75% of its Ns

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• The disconnection is necessary to avoid the excessive power loss in the auxiliary winding at full load

• A centrifugal switch is commonly used to disconnect the auxiliary winding from the source at a predetermined speed

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• At the time of starting, the two windings draw currents from the supply

• The main-winding current lags the applied voltage by almost 900

• The auxiliary-winding current is approximately in phase with the applied voltage

• In practice a well-designed split-phase motor, the phase difference between the two currents may be as high as 600

• It is from this phase-splitting action that the split-phase motor derives its name

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• Since the two phase-windings are wound in space quadrature and carry out of-phase currents, they set up an unbalanced revolving field

• It is this revolving field, albeit unbalanced, that enables the motor to start

• The starting torque developed by a split-phase motor is typically 150% to 200% of the full-load torque

• The starting current is about 6 to 8 times the full-load current

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• speed-torque characteristic of split-phase motor:Note the drop in torque at the time the auxiliary

winding is disconnected from the supply

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Capacitor-start Motor

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• In a capacitor-start motor a capacitor is included in series with the auxiliary winding

• If the capacitor value is properly chosen, it is possible to design a capacitor-start motor such that the main-winding current lags the auxiliary-winding current by exactly 900

• Therefore, the starting torque developed by a capacitor motor can be as good as that of any poly-phase motor

• The need for an external capacitor makes the capacitor-start motor somewhat more expensive than a split-phase motor

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• However, a capacitor-start motor is used when the starting torque requirements are 4 to 5 times the rated torque. Such a high starting torque is not within the realm of a split-phase motor

• Since the capacitor is used only during starting, its duty cycle is very intermittent. Thus, an inexpensive and relatively small ac electrolytic-type capacitor can be used for all capacitor-start motors

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Capacitor-Start Capacitor-Run Motor

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• Although the split-phase and capacitor-start motors are designed to satisfy the rated load requirements, they have low pf at the rated speed

• The lower the power factor, the higher the power input for the same power output

• Thus, the efficiency of a single-phase motor is lower than that of a poly-phase induction motor of the same size

• Since this motor requires two capacitors, it is also known as the two-value capacitor motor

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• The efficiency of a single-phase induction motor can be improved by employing another capacitor when the motor runs at the rated speed

• This led to the development of a capacitor-start capacitor-run (CSCR) motor

• One capacitor is selected on the basis of starting torque requirements (the start capacitor), whereas the other capacitor is picked for the running performance (the run capacitor)

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• The start capacitor is of the ac electrolytic type, whereas the run capacitor is of an ac oil type rated for continuous operation

• Since both windings are active at the rated speed, the run capacitor can be selected to make the winding currents truly in quadrature with each other

• Thus, a CSCR motor acts like a two-phase motor both at the time of starting and at its rated speed

• Although the CSCR motor is more expensive because it uses two different capacitors, it has relatively high efficiency at full load compared with a split-phase or capacitor-start motor

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Permanent Split-Capacitor Motor(PSC)

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• Is a less expensive version of a CSCR motor• A PSC motor uses the same capacitor for both

starting and full load operation• Since the auxiliary winding and the capacitor stay

in the circuit as long as the motor operates, there is no need for a centrifugal switch

• For this reason, the motor length is smaller than for the other types discussed above

• The capacitor is usually selected to obtain high efficiency at the rated load

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• Since the capacitor is not properly matched to develop optimal starting torque, the starting torque of a PSC motor is lower than that of a CSCR motor

• PSC motors are, therefore, suitable for blower applications with minimal starting torque requirements

• These motors are also good candidates for applications that require frequent starts

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• Other types of motors discussed above tend to overheat when started frequently, and this may badly affect the reliability of the entire system

• With fewer rotating parts, a PSC motor is usually quieter and has a high efficiency at full load

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Shaded-Pole Motor• When the auxiliary winding of a single-phase

induction motor is in the form of a copper ring, it is called the shaded-pole motor

• The pole is physically divided into two sections• A heavy, short-circuited copper ring, called the

shading coil, is placed around the smaller section

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• shaded-pole motor is very simple in construction and is the least expensive for fractional horsepower applications

• Since it does not require a centrifugal switch, it is not only rugged but also very reliable in its operation

• Has low efficiency and low starting torque

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Principle of Operation• consider changes in the flux produced by the

main winding at three time intervalsa) When the flux is increasing from zero to maximumb) When the flux is almost maximumc) When the flux is decreasing from maximum to zero• Any change in the flux in each pole of the motor

is responsible for an induced emf in the shading coil in accordance with Faraday's law of induction

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• Since the shading coil forms a closed loop having a very small resistance, a large current is induced in the shading coil

• The direction of the current is such that it always creates a magnetic field that opposes the change in the flux in the shaded region of the pole

• With this understanding, let us now analyze the effect of the shading coils during the time intervals mentioned above

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Interval a: • During this time interval the flux in the pole is

increasing and so is the current induced in the shading coil

• The shading coil produces a flux that opposes the increase in the flux linking the coil

• As a result, most of the flux flows through the un-shaded part of the pole

• The magnetic axis of the flux is then the center of the un-shaded section of the pole

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Interval b:• During this time interval the magnetic flux in the

pole is near its maximum value, therefore, the rate of change of flux is almost zero

• Hence, the induced emf and the current in the shading coil are zero, so, the flux distributes itself uniformly through the entire pole

• The magnetic axis, therefore, moves to the center of the pole

• This shift in magnetic axis has the same effect as the physical motion (rotation) of the pole

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Interval c:• During this time interval the magnetic flux produced

by the main winding begins to decrease, therefore, the current induced in the shading coil reverses its direction in order to oppose the decrease in the flux

• In other words, the shading coil produces the flux that tends to prevent a decrease in the flux produced by the main winding

• As a result, most of the flux is confined in the shaded region of the pole

• The magnetic axis of the flux has now moved to the center of the shaded region

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shading-pole action during the positive half cycle of a flux waveform

a) wt < π/2: Almost all the flux passing through unshaded region; b) wt = π /2: No shading action, flux is uniformly distributed over

the entire pole; c) wt > π /2: Most of the flux is passing through the shaded region.

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• Note that without the shading coil, the center of the magnetic axis would always be at the center of the pole

• The presence of the shading coil forces the flux to shift its magnetic axis from the unshaded region to the shaded region

• The shift is gradual and has the effect of revolving magnetic poles

• In other words, the magnetic field revolves from the unshaded part toward the shaded part of the motor

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• The revolving field, however, is neither continuous nor uniform

• Consequently, the torque developed by the motor is not uniform but varies from instant to instant

• Since the rotor follows the revolving field, the direction of rotation of a shaded-pole motor cannot be reversed once the motor is built

• To have a reversible motor, we must place two shading coils on both sides of the pole and selectively short one of them

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• To increase the starting torque, the leading edge of the shaded-pole motor may have a wider air-gap than the rest of the pole

• It has been found that if a part of the pole face has a wider gap than the remainder of the pole, the motor develops some starting torque without the auxiliary winding

• Such a motor is called a reluctance start motor• This feature is commonly employed in the design of a

shaded-pole motor to increase its starting torque

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Speed-torque characteristic of a shaded-pole motor• To cancel some of the third-harmonic effect, we

can use a relatively high-resistance rotor• However, any increase in the rotor resistance is

accompanied not only by a decrease in the operating speed of the motor but also by a drop of motor efficiency

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Universal Motor• A universal motor is defined as a motor which

may be operated either on DC or 1-ph a.c. supply at approximately the same speed and output

• A universal motor is wound and connected just like a dc series motor, i.e., the field winding is connected in series with the armature winding with some modifications

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Principle of Operation

• When a series motor is operated from a dc source, the current is unidirectional in both the field and the armature windings

• Therefore, the flux produced by each pole and the direction of the current in the armature conductors under that pole remain in the same direction at all times

• Hence, the torque developed by the motor is constant

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When a series motor is connected to an ac source,

• Current and flux directions in a universal motor during (a) the positive and (b) the negative half cycles

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• During the positive half cycle the flux produced by the field winding is from right to left

• For the marked direction of the current in the armature conductors, the motor develops a torque in the counterclockwise direction

• During the negative half cycle, the applied voltage has reversed its polarity, consequently, the current has reversed its direction

• As a result, the flux produced by the poles is now directed from left to right

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• Since the reversal in the current in the armature conductors is also accompanied by reversal in the direction of flux in the motor, the direction of the torque developed by the motor remains unchanged

• Hence, the motor continues its rotation in the counterclockwise direction

• If Ka is the machine constant, ia is the current through the field and the armature windings at any instant, and ɸp is the flux per pole at that instant, the instantaneous torque developed by the motor is Kaiaɸp

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• Thus, the instantaneous torque developed by the motor is proportional to the square of the armature current

• In other words, the average value of the torque developed is proportional to the root-mean-square (rms) value of the current

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• It is obvious from the waveform above that the torque developed by the universal motor varies with twice the frequency of the ac source

• Such pulsations in torque cause vibrations and make the motor noisy

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The equivalent circuit, the phasor diagram, and the speed-torque characteristics of a universal motor

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• The back emf Ea, the winding current Ia, and the flux per pole ɸp are in phase with each other as shown

• Rs and Xs are the resistance and the reactance of the series field winding. Ra and Xa are the resistance and the reactance of the armature winding

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Design Considerations

1. When a series motor is to be designed as a universal motor, its poles and yoke must be laminated in order to minimize the core loss produced in them by the alternating flux

• If a series motor with an unlaminated stator is connected to an ac supply, it quickly overheats owing to excessive core loss

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2. Under steady-state operation of a dc series motor, the inductances of the series field and armature windings have little effect on its performance

• However, the motor exhibits reactive voltage drops across these inductances when connected to an ac source, which have a two-fold effect:

(a) reducing the current in the circuit for the same applied voltage, and

(b) lowering the power factor of the motor. The reactive voltage drop across the series field winding is made small by using fewer series field turns

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3. The decrease in the number of turns in the series field winding reduces the flux in the motor. This loss in flux is compensated by an increase in the number of armature conductors

4. Under ac operation, an emf is induced by transformer action in the coils undergoing commutation. This induced emf (a) causes extra sparking at the brushes, (b) reduces brush life, and (c) results in more wear and tear of the commutator. To reduce these harmful effects, the number of commutator segments is increased and high-resistance brushes are used in universal motors

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5. The increase in the armature conductors results in an increase in the armature reaction. The armature reaction can, however, be reduced by adding compensating windings in the motor as:

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With all these drawbacks, why do we use a universal motor?

1. A universal motor is needed when it is required to operate with complete satisfaction on dc and ac supply.

2. The universal motor satisfies the requirements when we need a motor to operate on ac supply at a speed in excess of 3600 rpm (2-pole induction motor operating at 60 Hz). Since the power developed is proportional to the motor speed, a high-speed motor develops more power for the same size than a low speed motor

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3. When we need a motor that automatically adjusts its speed under load, the universal motor is suitable for that purpose. Its speed is high when the load is light and low when the load is heavy.

Application• Some applications that require variation in

speed with load are saws and routers, sewing machines, portable machine tools, and vacuum cleaners

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• Example: A 120-V, 60-Hz, 2-pole, universal motor operates at a speed of 8000 rpm on full load and draws a current of 17.58 A at a lagging power factor of 0.912. The impedance of the series field winding is 0.65 + j1.2 Ω. The impedance of the armature winding is 1.36 + j1.6 Ω. Determine (a) the induced emf in the armature, (b) the power output, (c) the shaft torque, and (d) the efficiency if the rotational loss is 80 W.

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Solution (a) From the equivalent circuit of the motor we have

As expected, the induced emf is in phase with the armature current

(b) The power developed by the motor is

The power output:

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Questions???