Single Phase Synchronous Motor

The constant speed motor operated from a single phase ac supply is simply called as the single phase synchronous motor.They are suitable for applications such as clocks,timers,etc.

There are two types of single phase synchronons motor.They are described below in brief.

Reluctance motor:

A reluctance motor is a motor which depends on reluctance torque for its operation. Reluctance torque is the torque induced in an iron object (such as a pin) in the presence of an external magnetic field, which causes the object to line up with the external magnetic field. This torque occurs because the external field induces an internal magnetic field in the iron of the object, and a torque appears between the two fields, twisting the object around to line up with the external field. In order for a reluctance torque to be produced in an object, it must be elongated along axes at angles corresponding to the angles between adjacent poles of the external magnetic field.
A simple schematic of a reluctance motor is shown in Figure 1

    fig:1

It can be shown that the torque applied to the rotor of this motor is proportional to sin 2δ, where δ is the electrical angle between the rotor and the stator magnetic fields. Therefore, the reluctance torque of a motor is maximum when the angle between the rotor and the stator magnetic fields is 45°. Like a normal synchronous motor, it has no starting torque and will not start by itself.A self-starting reluctance motor that will operate at synchronous speed until its maximum reluctance torque is exceeded can be built by modifying the rotor of an induction motor. In this figure, the rotor has salient poles for steady-state operation as a reluctance motor and also has cage windings for starting. Although no torque is developed due to induction motor action, however torque due to salient pole structure of rotor structure of rotor will not be zero.The power developed by the rotor of 3-phase synchronous motor with salient pole structure is given by,

The second part of the above power is due to the difference between the value of Xd and Xq which is due to the saliency of the motor and is called reluctance power that varies with sin2δ.Because of this torque, the rotor continues to rotate, even the emf and current in the rotor conductor is zero and thus it rotates at synchronous speed.

Hystersis motor:

Hysteresis motor employs the phenomenon of hysteresis to produce a mechanical torque. The rotor of a hysteresis motor is a smooth cylinder of magnetic material with no teeth or windings. The stator of the motor can be either single- or three-phase; but if it is single-phase, a permanent capacitor should be used with an auxiliary winding to provide as smooth a magnetic field as possible, since this greatly reduces the losses of the motor. Figure 2 shows the basic operation of a hysteresis motor.

fig:2

 When a three phase (or single-phase with auxiliary winding) current is applied to the stator of the motor, a rotating magnetic field appears within the machine. This rotating magnetic field magnetizes the metal of the rotor and induces poles within it. When the motor is operating below synchronous speed, there are two sources of torque within it. Most of the torque is produced by hysteresis. When the magnetic field of the stator sweeps around the surface of the rotor, the rotor flux cannot follow it exactly, because the metal of the rotor has a large hysteresis loss. The greater the intrinsic hysteresis loss of the rotor material, the greater will be the angle by which the rotor magnetic field lags the stator magnetic field. Since the rotor and stator magnetic fields are at different angles, a finite torque will be produced in the motor. In addition, the stator magnetic field will produce eddy currents in the rotor, and these eddy currents produce a magnetic field of their own, further increasing the torque on the rotor. The greater the relative motion between the rotor and the stator magnetic field, the greater the eddy currents and eddy-current
torques. When the motor reaches synchronous speed, the stator flux ceases to sweep across the rotor, and the rotor acts like a permanent magnet. The induced torque in the motor is then proportional to the angle between the rotor and the stator magnetic field, up to a maximum angle set by the hysteresis in the rotor. Since the amount of hysteresis within a particular rotor is a function of only
the stator flux density and the material from which it is made, the hysteresis torque of the motor is approximately constant for any speed from zero to synchronous speed. The Ieddy-current torque is roughly proportional to the slip of the motor. These two facts taken together account for the shape of the hysteresis motor's torque-speed characteristics.

It is commonly used as the driving mechanism in electric clocks. An electric clock is therefore synchronized to the line frequency of the power system, and the resulting clock is just as accurate (or as inaccurate) as the frequency of the power system to which it is tied.

                                          

Split phase Induction Motor

It is known that the single phase induction motor is not self starting and therefore it is necessary to employ some means to make it self starting.For this purpose,an additional winding is provided on stator,which is called starting or auxiliary winding as shown in fig 3.

                             

                                                                    fig.3  

The two windings which are spaced 90° electrical apart on the stator of a motor are excited by the alternating emfs that are 90° displaced in time phase.Then,a rotating magnetic field is produced.If two windings so placed  are connected in parallel to a single phase source,the field produced will be alternate but will not revolve since two windings are equivalent to one phase winding.If an impedance is connected in series with one of these winding,the current may be made to differ in time phase.By proper selection of such impedance,the current may be made to differ in time phase as much as 90°,thereby producing a rotating field like a field of a two phase motor.This is the principle of phase splitting and single phase inductor motor employing this principle for starting is split phase induction motor.

Split phase induction motor are of following types:

Ø Resistant Start Motor

Ø Capacitor Start Motor

Ø Capacitors Start and Run Motor

Ø Permanent Capacitor Motor

Ø Shaded Pole Motor

Resistant Starting Motor:

                         

                                                                fig: 3.1

In this motor,resistor of certain resistance is used for starting of single phase inuction motor as shown in fig 3.1.Since the starting winding is made up of comparatively thin wire with respect to main winding,its resistance will be more with compare to the resistance of the main winding and its inductance will be less with compare to induction of main winding.Since the resistance of the starting winding is high and its inductance is low,the current through  the starting winding(I_s) lags the applied voltage by a small angle.Whereas,the current through the main winding(I_m) lags the applied voltage by a larger angle resulting a phase difference of α between I_m and I_s.Because of this phase difference,the motors act as a two phase motor and it will produce a rotating magnetic field.Hence,the motor rotates in a particular direction.When the speed reaches about 70-80% of its normal rated speed,the centrifugal switch gets open and the motor acts as a single phase induction motor.At this speed,the forward torque will be more than backward torque.Hence the motor continue to rotate in forward direction.

The starting torque of such motor is proportional to phase angle α.Since the value of phase angle is small in this motor,this type of motor produces low starting torque.It has a starting torque of 150-200% of full load torque and starting current of 6-8 times of full load current.It operates at the speed of 1440 rpm and its size can vary from 1/20 KW to 1/4 KW.

Such motors find wide applications for low inertia loads,continuous operating loads and applications requiring moderate starting torque such as for driving washing machines,fans,blowers,centrifugal pumps,domestic refrigerators,duplicating machines,wood working tools,grinders,oil burners,etc.

Capacitor Start Motor:

                                 

                          

                                                                  fig: 3.2

This is an improved form of resistant start motor,with the addition of capacitor in the starting winding as shown in fig 3.2. If high starting torque is required,then a capacitor of proper value can be connected in series with the starting winding so that the phase angle is nearly 90° resulting high starting torque.Such motor is known as single phase capacitor start induction run motor.It can be used for the applications requiring high starting torque without excessive high starting 

current.After the machine attains normal speed,the starting winding is cut out and its operating characteristics are identical to those of resistant start motor.

Capacitor Start – Capacitor Run Motor:

In this type of motor,two capacitors C₁ and C₂ are used and connected in parallel.The parallel combination is connected in series with starting winding and C₂ has centrifugal switch in series with it as shown in fig 3.3.

                          

                                                                   fig:3.3

Both the capacitors C₁ and C₂  are in circuit during starting.After the motor speed has reached about 70-80 % of normal rated speed,the centrifugal switch opens and C₂  gets disconnected.The starting winding and C₁ remains in the circuit during the running condition too.Thus,the motor is actually a two phase motor even the supply voltage is single phase.The use of C₁ during running condition improves the power factor of the motor and improves the efficiency.The starting winding and C₁ have to be designed for continuous duty rating.Hence,this type of motor is more expensive than the capacitor start induction run motor.The value of C₂ has to be chosen higher value than that of C₁ in order to produce high starting torque.The typical torque-speed characteristics is shown in fig 3.3. Both the starting currents and running currents are low,i.e. full load current may be 3/4^th of that for an equivalent capacitor start motor.

In spite of its high cost,it is used in splendid machines where load requirements are severe as in case of compressors,refrigerators,fire strokers,pumps,conveyors,hospital,studies and so forth.The direction of rotation of motor may be reversed by interchanging the connections to the supply of either the main or auxiliary winding.

Permanent Capacitor Motor:

Permanent Capacitor single phase induction motor has two stator winding placed mutually 90 electrical degree apart.The main or running winding is connected directly across the single phase ac supply.A capacitor in series with with the auxiliary winding is also connected across the supply lines.There is no centrifugal switch,since the auxiliary winding is energized at all the time when the motor is in operation.The schematic diagram of permanent capacitor motor and its corresponding torque-speed characteristics is shown in fig 3.4.

                                

                                                                          fig:3.4

The auxiliary winding is always in motor circuit, and therefore the operation of motor when loaded resembles more closely to that of two phase motor.

It is used in the applications,where the motors starts with light load or no load on its shaft,i.e. high torque is not required.Hence,It is possible to avoid the cost of centrifugal switch and starting capacitor by removing them and keeping only starting winding and C₁ permanently connected as shown in the circuit in the figure.

Shaded Pole Motor:

Small motors requiring small starting torque,can be made self starting by using shaded band in the pole as shown fig 3.5. and this motor is called shaded pole motor.

                                    

                                                                             

                                                                              fig: 3.5

The stator of this type of motor has shaded ppole structure.The low resistance copper band C and D are placed so as to surround approximately half of the pole face.When stator winding is supplied by single phase ac voltage source,emf will induce in the shaded band and induced current will glow in these shaded band.These induced current opposes the change of flux which which will cause to lag flux φ_s with respect to flux φ_m  (according to the Lenz law) as shown in the phasor diagram.Due to phase angle α between φ_m and φ_s ,a rotating magnetic field will produce ehich rotates in the direction from the unshaded portion to the shaded portion. Starting torque of such motor will be very less. Therefore, shaded pole motors are only suitable for small capacity motor requiring low starting torque.Because of their low starting torque,they are best suited for driving fans or other loads that are easily started.They may have multiple taps near one electrical end of the winding,which provides variable speed and power via selection of one tap at a time,as in ceiling fans.Morever,they are compatible with TRIAC-based variable speed controls,which often are used with fans.They are built in power sizes up to about ¼ horsepower output.Above 1/3 horsepower, they are not common, and for larger motors,other designs offer better characteristics.

Double Field Revolving Theory

Double field revolving theory states that the pulsating magnetic field produced by the single phase winding is equivalent to the phasor sum of two oppositely rotating magnetic fields each having a magnitude of 0.5φ_m with a synchronous speed of N_s=120f/P.

fig:2

Fig 2 shows two rotating magnetic fields,namely OA and OB,each having a magnitude of 0.5φ_m.OA is rotating in forward(clockwise) direction and OB is rotating in backward (anticlockwise) direction with the synchronous speed.

When the two rotating phasors reach the positions such as shown in fig 2,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=0 of the pulsating field shown in fig 1.Let us assume this position as reference point.

After 30° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.5φ_m  and the direction is upward. This condition is equivalent to ωt=30° of the pulsating field shown in fig 1.

After 60° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.866φ_m  and the direction is upward. This condition is equivalent to ωt=60° of the pulsating field shown in fig 1.

After 90° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to φ_m  and the direction is upward. This condition is equivalent to ωt=90° of the pulsating field shown in fig 1.

After 120° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to 0.866φ_m  and the direction is upward. This condition is equivalent to ωt=120° of the pulsating field shown in fig 1.

After 180° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to zero.This condition is equivalent to ωt=180° of the pulsating field shown in fig 1.

After 210° rotation from the reference position,the positions of two phasors will be as shown in fig 2. Here,the net resultant of two rotating phasors is equal to -0.5φ_m  and the direction is downward. This condition is equivalent to ωt=210° of the pulsating field shown in fig 1 and so on.

Based on double field revolving theory,the torque-speed characteristics of single phase induction motor can be drawn as shown in fig 2.a,where torque-speed characteristics are shown,one due to forward rotating magnetic field and other due to backward rotating magnetic field.

fig:2.a

Here, OA=Forward starting torque

          OB=Backward starting torque

These two torques are equal and opposite. Hence,the net starting torque of single phase induction motor is zero.so,the single phase induction motor is not self starting.

The equivalent circuit of single phase induction motor is shown in the figure below (fig 2.b) :

fig:2.b

x

Single Phase Induction Motors: Construction and Characteristics

Single phase induction motor or fractional kilowatt motor is a motor built in a frame having a continuous rating of smaller than 1KW.It is constructionally more or less similar to a three phase induction motor with the difference that its stator is provided with single phase winding.When single phase AC voltage is supplied to the single phase stator winding,it will not produce a rotating magnetic field like in three phase induction motor,rather it will produce an alternating magnetic field which magnitude varies with respect to time,and its direction changes by 180° at a particular instant at a particular time interval.

With the connection of single phase AC supply to the single phase induction motor,alternating current will flow in its stator winding and the polarity of stator pole would alternatively be N and S.The field so produced will be pulsating i.e. polarities will be alternating with flux rising and falling in strength.The current induced in the rotor will tend to rotate in both directions alternatively and  thus the rotor will be at standstill due to inertia.If a rotor is given a push by hand or by another means in any direction,it will pick up speed and continue to rotate in same direction developing operating torque,Thus,the single phase induction motor is not self starting and requires special starting means.

                                                                      fig: 1.a

                                                                     
                                                                 fig: 1.b

Fig 1.a shows the basic construction of single phase induction motor and Fig 1.b shows the nature of air gap flux produced by single phase winding.