Single Phase Motor : Concepts -- 2 ( sai saikumar jn)

Single Phase Motor : Concepts

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 :

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. Low power factor and efficiency. Shunt speed characteristics.	Non-reversing drives with light loads on starting.
Capacitor motor	Moderately good starting torque. Higher power factor and efficiency than split phase type. Quiet operation shunt speed characteristics.	Suitable for reversing as well as non reversing drives without heavy starting loads. Used in passenger lifts, domestic refrigerators, fans, etc.
Repulsion	Good starting torque. Shunt speed characteristics.	Suitable for heavy starting. Additional winding needed for reversing.
Universal (Series)	Good starting torque. High power factor.	Vacuum cleaners, motorized hand tools.
Small Synchronous motor	Constant speed operation. Poor starting torque. Low power factor.	Non-reversing drives requiring absolutely constant speed, with light starting. Used in clocks, timing mechanisms, picture and sound reproduction.

Mathematical analysis of single phase motor:

According to rotating field theory:

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

N_s = (120 F) / P R.P.M.

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

2-s ω_s= (2 π N_s)/60 rad/sec.

Rotating field Equivalent Single φ motor under running condition
V_f and V_b are components of stator voltage V_m.

Main winding current.

I_m = V_m/ |Z_total| = V_m/ |Z_f/2 + Z_b/ 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, p_gf = ½ I ²_m R_f

Air gap power for backward, p_gb = ½ I ²_m R_b

R_f and R_b are real parts of Z_f’ and Z_b’

Torques produced by 2^nd fields

T_f = 1/ω_s P_gf and T_b = 1/ω_s P_gb

ω_s = synch. Speed in rad/sec

Total torque. T= T_f – T_b = 1/ ω_s (P_{gb -} P_gb) = I²_m/2 ω_s (R_f - R_b)

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

S. P_gb + (2-s) P_gb

Total electric power. connected to gross mech-form
P_m = (1-s) ω_s T = (1-s) (P_{gf -} P_gb) = (1-s) P_gf + [1-(2-s)]P_gb

Total electric power input to motor.
P _Elect = P_gf + P_gb

Simplified formula

E_mf/E_mb = (r₂/s + jX₂ )/ (r₂/2-s + jX₂) at X = infinity

Impedence offered to V_f component, Z_f= ½ [{r_1m + r₂/(2-s}}]+ j (X₁ + X₂)

Impedence offered to V_b component, Z_b= ½ [{r_1m + r₂/(2-s}}]+ j (X₁ + X₂)

Z _total = Z_f + Z_b V_f/V_b= Z_f/Z_b

T_f/T_b= P_gb/P_gb = (2-s)/s, T_f=1/ I²_m r₂/2s

T_b=1/ ω_s I² _m r₂/(2(2-s))

T_total = T_f - T_b = (I²_mr₂)/2 ω_s [1/s – 1/ (2-s)]Nω-m


T_f/T_total = [1/s]/[1/s – 1/(2-s]],

T_b/T_total = [1/(2-s)) ]/ [1/s – 1/(2-s)]



from ur's -- Bellapuri saikumar
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