Single Phase Motor : Concepts - 5

HYSTERESIS MOTOR:

These motors consist of a chrome-steel cylinder of high retentivity so that the hysteresis loss is high. It has no winding. Once the magnetic polarities are induced in the rotor, it revolves synchronously with the revolving magnetic field. Since the rotor has no slots or winding, the motor is noiseless free from vibrations. Such a motor is ideal for sound equipment.

SUMMARY OF CHARACTERISTICS AND APPLICATIONS OF SINGLE PHASE MOTOR:

Motor type

Main characteristics

Applications

Split phase:
                                      Poor starting torque.                                        Non-reversing drives with
                                      Low power factor and  efficiency.                      light loads on starting.
                                      Shunt speed characteristics.
Capacitor motor:
                                      Moderately good starting torque.                    Suitable for reversing as well as
                                Higher power factor and efficiency                  non reversing drives without heavy starting 
                               than split phase type. Quiet operation             loads Used in passenger lifts, domestic                                      shunt speed characteristics.                                   refrigerators,   fans, etc.
                                    
Repulsion:
                               Good starting torque. Shunt speed                         Suitable for heavy starting
                              characteristics. . 

                               Additional winding needed for reversing.

     

Universal (Series):
                              Good starting torque. High power factor.           Vacuum cleaners,  motorized hand tools.
                                
 Small Synchronous motor:
                            Constant speed operation.                                 clocks, timing mechanisms, 
                            Poor starting                                                    picture and sound reproduction.
                            Low power factor. 
    

Mathematical analysis of single phase motor:

According to rotating field theory:

They are 2 rotating fields, Ff forward rotating field and Fb - Backward rotating field. Slip of rotor w.r.t forward rotating field is

Ns = (120 F) / P R.P.M.

Slip of rotor w.r.t backward rotating field :

2-s ωs= (2 π Ns)/60 rad/sec.

Rotating field Equivalent Single φ motor under running condition

Vf and Vb are components of stator voltage Vm.

Main winding current.

Im = Vm/ |Ztotal| = Vm/ |Zf/2 + Zb/ 2|

At x =m, magnetizing current is neglected.

The Circuit Model of Single-phase, single-winding motor is shown in the figure below :

Air gap power for forward field, pgf = ½ I 2m Rf

Air gap power for backward, pgb = ½ I 2m Rb

Rf and Rb are real parts of Zf’ and Zb’

Torques produced by 2nd fields

Tf = 1/ωs Pgf and Tb = 1/ωs Pgb

ωs = synch. Speed in rad/sec

Total torque. T= Tf – Tb = 1/ ωs (Pgb - Pgb) = I2m/2 ωs (Rf - Rb)

Total rotor Cu loss: Rotor Cu loss corresponding to forward field + rotor Cu loss corresponding to backward field

S. Pgb + (2-s) Pgb

Total electric power. connected to gross mech-form

Pm = (1-s) ωs T = (1-s) (Pgf - Pgb) = (1-s) Pgf + [1-(2-s)]Pgb

Total electric power input to motor.

P Elect = Pgf + Pgb

Simplified formula

Emf/Emb = (r2/s + jX2 )/ (r2/2-s + jX2) at X = infinity

Impedence offered to Vf component, Zf= ½ [{r1m + r2/(2-s}}]+ j (X1 + X2)

Impedence offered to Vb component, Zb= ½ [{r1m + r2/(2-s}}]+ j (X1 + X2)

Z total = Zf + Zb Vf/Vb= Zf/Zb

Tf/Tb= Pgb/Pgb = (2-s)/s, Tf=1/ I2m r2/2s

Tb=1/ ωs I2 m r2/(2(2-s))

Ttotal = Tf - Tb = (I2mr2)/2 ωs [1/s – 1/ (2-s)]Nω-m

Tf/Ttotal = [1/s]/[1/s – 1/(2-s]],

Tb/Ttotal = [1/(2-s)) ]/ [1/s – 1/(2-s)]

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Single Phase Motor : Concepts - 4

3. Reluctance motor:

Reluctance motor operates on the principle that when a magnetic material is placed in the magnetic field, a force of magnetic pull exerts on the magnetic material tending to bring the piece of magnetic material in the dynest portion of the magnetic field. Thus magnetic material align itself at the place where the reluctance is minimum.

STEPPER MOTOR:

It is an electromechanical device which actuates a train of step angular (or linear) movements in response to a train of input pulses on one to one basis i.e. one step actuation for each pulse input.

Depending upon the construction, there are 2 types of stepper motor.

(i) Variable reluctance Motor:

A 3 φ motor with 6 stator teeth and 4 rotor tooth are shown here. The 3 phases are excited in succession.

At the time when first phase is excited, the magnetic flux axis is shown. When 2nd phase is excited, the magnetic flux axis shifts and rotor rotates. Hence by changing the magnetic flux axis, rotor rotates in steps.

(ii) Permanent magnet stepper motor:

A four phase permanent magnet motor is shown here. The rotor is a cylindrical permanent magnet and coils are wound on stator tooth

When phases are excited in sequence, rotor rotates clockwise. Since it is a 4-phase motor, the step angle is 45°. For small step angles, no. of magnetic poles and stator tooth are increased.

Universal Motor:

They can be operated either on ac or dc supply at approximately the same speed and output. The armature is of the same type as ordinary series motor. These are of 2 types :

1. Concentrated-pole. non-compensated type (Low power rating).

2. Distributed field. compensated type (high power rating).

The power factor is improved by reducing the inductance of field winding and hence no. of turns. It decreases the magnetic native force in air-flux gap for a given current. The armature turns are increased to get the same torque but the reactance of motor increases. It is neutralized by using compensating winding in which current is proportion to armature current but 180° out of phase.

The voltage induced by transformer action in the coils during its commutation does not produce serious commutation problem High resistance brushes are used to aid the commutation.

These motors are used in sewing machines, vacuum cleaners, kitchen appliances, drills hair dryers etc.

The speed torque characteristics for dc and ac input supply is shown below :

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Single Phase Motor : Concepts - 3

Capacitor run motor:

There is no switch in the starting winding and capacitor is permanently connected.

Starting torque is lower about 50 to 100% of full load torque.

 

Capacitor start and capacitor run Motor:

Two capacitors are used. One for starting and the other for running purpose. These motors give best performance in terms of efficiency and power factor.

2. Shaded pole motor:

Each shaded pole has its own exciting coil. About 1/3 rd portion of each pole core is surrounded, by a strap of copper forming a shading coil. When the flux is increasing in the pole, a portion of the flux attempts to pass through the shaded tip or shaded portion of the pole. This flux induces voltage and hence current in the copper ring, and by Lenz's law the direction of current is such that it opposes the flux entering the coil. Hence in the beginning, the greater portion of the flux passes through un shaded side of each pole. When the flux reaches its maximum value, its rate of change is zero thereby the e.m.f. and hence current in the shading coil becomes zero. Large amount of main flux then links with the coil. After the main flux tends to decrease, the current induced in the shading coil now tends to prevent the flux linking with shading coil from decreasing. The shading coils thus causes the flux in shaded portion as shown to lag the flux in shaded portion of the pole.

Speed torque characteristics is shown here starting torque is very small about 50% of full load torque.

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Single Phase Motor : Concepts - 2

1. Split phase motor:

The main winding has low resistance but high reactance and starting winding has high resistance but low reactance.

Current in the auxiliary winding called starting winding lags the supply voltage by lesser angle. The current in the main winding being highly inductive lags the supply voltage with greater angle. Now there are two currents which are not in phase with each other. As shown in the figure Im is the current in the main winding lagging the voltage by angle φm and Is, the current in the starting winding lagging the voltage by angle φs. The angle between the two currents is θ as shown in the figure. The torque produced is such that it gives circular movement to the rotor and is proportional to the sine of angle θ. A centrifugal switch which is normally closed is incorporated in series with the starting winding. When the motor comes up to speed about 75% of synchronous speed it opens automatically with the help of centrifugal force and puts the starting winding out of circuit.

Typical torque speed characteristics is shown here.

Capacitor Start Motor:

The starting winding has a capacitor in series with it. The motor gives high starting torque due to large value of C.

Torque speed characteristics is shown below.

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Synchronous Motor : Concepts - 3

Since the speed of the motor is always constant, above expression also expresses the torque developed by the motor.

The condition for the maximum power is Θ = α

Hence the value of maximum power

Pmax = (EbV)/ Zs – E2b / Zs cos (θ)

When, θ=90o , cos (θ) =0 so that,

Pmax = (Eb . V)/ Zs

'Load angle of Power angle:

It is the space angle between the axis of stator revolving magnetic field and the rotor pole axis, both locked and running at synchronous speed.

Hunting:

Due to sudden change of load, the angle changes causing oscillations of rotor about the mean position due to inertia of the rotor. Thus hunting is usually of low frequency oscillations superimposed on normal synchronous speed.

Synchronous reactance:

A synchronous machine can be represented as an emf source in series with internal impedance of the machine. This internal impedance, Zs is called synchronous impedance. The synchronous reactance takes into account the effect of armature reactions and the flux produced by the armature current. The emf takes into account the flux produced by field excitation.

Effect of change of Excitation in synchronous motor:

The real power is P = VI cosφ .The value of excitation for which back emf, Eb is equal to applied voltage V, is known as 100% excitation. An over excited motor (more than 100% excitation) takes leading power factor current. An under excited motor takes lagging power factor current.

Change in excitation currents does not change the load or power output of the synchronous motor. To change the power output, the mechanical load should change.

SYNCHRONOUS MOTOR V-CURVES:

The power factor of a synchronous motor is controlled by the variation of the filed current. Increasing the field current from small value, the line current decreases until a minimum line current occurs showing that the motor is operating at unity power factor. If we go on increasing the field current, the line current increases again operating the motor at leading power factor.

(a)V curves of a synchronous motor

(b) Power Factor versus field current at different loads

Open circuit characteristics of Synchronous Motor:

The open circuit characteristics of a synchronous machine is a curve of the armature terminal voltage on open circuit as a function of the field excitation, when the machine is running at synchronous speed.

Short circuit characteristics of Synchronous Motor:

If the armature terminals of a synchronous machines (driven as a generator at synchronous speed) are short-circuited through suitable, ammeter, and the field current is gradually increased until the armature current has reached a maximum safe value (about 1.5 times rated current), data can be obtained from which short circuit armature current can be plotted against the field current. Such a characteristic is known as short-circuit characteristic.

SYNCHRONOUS CONDENSERS:

It is a synchronous motor used for power factor correction which is operated at no load but at greatly over excited field. Such conditions cause the motor to take a current that leads the voltage by 90° which is equivalent to a capacitor.

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Synchronous Motor : Concepts - 2

DIFFERENCE BETWEEN INDUCTION MOTOR AND SYNCHRONOUS MOTOR:

	Induction Motors	Synchronous Motor
1.	These motors have wound rotor with slip rings or a squirrel cage rotor.	These motors have dc poles on rotor energized by excitation system.
2.	Rotor current is ac and is induced by magnetic induction.	The field current (excitation current) can be changed to vary the power factors.
3.	These motors run at less than synchronous speed. Full load slip is about 4%.	These motors always run at synchronous speed without slip.
4.	These motors take lagging power factor current.	These motors take different p.f. currents depending upon the excitation level.
5.	These motors have inherent starting torque.	These motors do not have any, inherent starting torque.
6.	These motors start unaided.	These motors have to be started by suitable means and brought to synchronous speed and then synchronized.
7.	These motors are used for variable speed and variable load drives.	These motors are used for constant speed and constant load drives.
8.	For the same size, these motors are cheaper.	These motors are costly due to additional cost of excitation system.

 

METHODS OF STARTING Synchronous Motor

(1) Using a starting motor:

This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor.

(2) Starting as an induction motor:

This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism. Change of direction of rotation. For this a running motor may be stopped. By interchanging any two phase sequences of stator terminals, the motor will run in reverse direction.

Synchronous Motor: TORQUE DEFINITIONS

(A) Synchronous torque:

It is the steady state torque required to drive the motor and the load at the synchronous speed.

(B) Pull-in torque:

It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied.

(C) Nominal pull in torque:

It is the value of pull in torque at 95% of the synchronous speed with the rated voltage and frequency applied to the stator when the motor is running with the winding current.

(D) Pull out torque:

It is the maximum sustained torque which the motor will develop at synchronous speed for 1 minute with rated frequency and with rated field current.

(E) Pull up torque:

It is the minimum torque developed between standstill and pull in point. This torque must exceed the load torque by sufficient margin to ensure satisfactory acceleration of the load during starting.

(F) Reluctance torque:

It is fraction of the total torque with the motor operating synchronously. It results from saliency of the poles. It is approximately 30% of the pull-out torque.

(G) Locked rotor torque:

It is the maximum torque which a synchronous motor will develop at rest for any angular positions of the rotor at the rated voltage and frequency.

Losses. Various losses occurring in the motor are :

1. Armature Copper Loss Ia2Ra

2. Iron and friction losses.

Torque. In a synchronous motor, the power per phase

Pm =( Eb.V)/ Zs cos (θ - α) – Eb2 / Zs cos (θ)

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