Induction Motor Construction Overview

Induction Motor Construction

• Induction motors are commonly used in various industries, from small workshops to large industrial facilities.
• They power diverse equipment like pumps, conveyors, compressors, crushers, and drilling machines.
• Similar to DC motors, but with key differences:
  • Laminated stator core for reducing eddy currents and minimizing losses
  • Absence of a commutator, simplifying construction and increasing durability
  • Uniform and small air gap for efficient magnetic flux transfer
  • Nearly constant speed operation
• The motor consists of a stationary stator and a rotating rotor.
  • Stator:
    • Outer frame made of steel, cast iron, or aluminum alloy
    • Laminated stator core inside the frame to reduce eddy currents
    • Insulated electrical windings placed within the slots of the laminated stator - three sets for three-phase motors, connected in star or delta configuration.
  • Rotor:
    • Contains a set of conductors embedded in the rotor core
    • This set of conductors forms a closed circuit similar to a squirrel cage.

Output Equation

• The output equation relates the motor's physical and electrical parameters.
• For an 'm' phase motor:
  • Output Q (kW) = Input x efficiency
  • Input = mVph Iph cos x 10-3 kW
  • For a 3-phase motor: Input = 3Vph Iph cos x 10-3 kW
• Assuming Vph = Eph = 4.44 f TphKw, where:
  • Vph = phase voltage
  • Eph = phase emf
  • f = frequency
  • Tph = number of turns per phase
  • Kw = winding factor
• Output equation simplifies to:
  • Q = (11 Bav q Kw cos x 10-3) D2L ns kW
  • Where:
    • Co = (11 Bav q Kw cos x 10-3) = output coefficient
    • D = Diameter of stator
    • L = Gross core length
    • ns = Synchronous speed in rpm
    • Bav = Average flux density
    • q = Specific electric loading (air gap flux per pole)

Main Dimensions (D & L)

• The output equation relates D2L to the motor's output.
• Design considerations for achieving optimal ratios between length (L) and pole pitch (pD/p):
  • Minimum cost: 1.5 to 2.0
  • Good efficiency: 1.4 to 1.6
  • Good overall design: 1.0 to 1.1
  • Good power factor: 1.0 to 1.3
• To optimize power factor, the following relation is usually assumed for separating D and L:
  • Pole pitch/Core length = 0.18/pole pitch
  • D = 0.135PvL (where D and L are in meters)

Peripheral Speed

• For typical induction motors, the calculated diameter should ensure a peripheral speed below 30 m/s.
• Specially designed rotors can achieve peripheral speeds up to 60 m/s.

Stator Design

• Stator core and slots:
  • Stator slots are typically either open or semi-closed.
• Open slots:
  • Slot opening equal to the slot width.
  • Easier winding assembly and repairs.
  • Higher air gap contraction factor, resulting in poorer power factor.
  • Rarely used in three-phase induction motors.
• Semi-closed slots:
  • Slot opening smaller than slot width.
  • More difficult and time-consuming winding assembly, leading to higher costs.
  • Better air gap characteristics compared to open slots.
• Tapered slots:
  • Slot opening smaller than slot width.
  • Slot width varies from top to bottom, with minimum width at the bottom.
  • Offer advantages similar to semi-closed slots.
• Selection of Stator Slots:
  • The number of slots significantly affects the motor's weight, cost, and operating characteristics.
  • Advantages of higher number of slots:
    • Reduced leakage reactance.
    • Reduced tooth pulsation losses.
    • Higher overload capacity.
  • Disadvantages of higher number of slots:
    • Increased cost.
    • Increased weight.
    • Increased magnetizing current.
    • Increased iron losses.
    • Poorer cooling.
    • Increased temperature rise.
    • Reduction in efficiency.
  • General guidelines:
    • Three or more slots per pole per phase for integral slot windings.
    • 3.5 slots per pole per phase for fractional slot windings.
    • Stator slot pitch at the air gap surface (tss) should be between 1.5 to 2.5 cm.
    • tss = pD/Sss (where Sss is the number of stator slots)
• Turns per phase:
  • From the EMF equation (Eph = 4.44fFTphkw):
    • Tph = Eph/ 4.44fFkw
  • Induced emf (Eph) is typically assumed equal to the applied voltage per phase.
  • Flux per pole = Bav x pDL/P
  • Winding factor (Kw) can be assumed as 0.955 for full pitch distributed winding unless otherwise specified.
• Number of conductors per phase (Zph):
  • Zph = 2 x Tph
  • Total number of stator conductors (Z) = 6 Tph
  • Conductors per slot (Zs) = Z/Ss (integer for single layer winding and even number for double layer winding)
• Conductor cross-section:
  • Area of cross-section of stator conductor (as) estimated from stator current per phase (Is) and current density (ds).
  • as = Is / ds
  • Stator current per phase: Is = Q / (3Vph cos ?)
  • Advantages of higher current density:
    • Smaller cross-section.
    • Reduced weight.
    • Lower cost.
  • Disadvantages of higher current density:
    • Increased resistance.
    • Higher copper losses.
    • Increased temperature rise.
    • Lower efficiency.
  • Typically, 3 to 5 amps current density for stator windings.
  • Circular conductors for sectional areas below 5 mm2.
  • Rectangular conductors for areas above 5 mm2, with a width to thickness ratio of 2.5 to 3.5.
• Stator slot area:
  • Occupied by conductors and insulation (over 25% for insulation).
  • Slot space factor = Copper area in the slot / Area of each slot (between 0.25 and 0.4)
  • Detailed dimension of the slot can be estimated.
• Slot design:
  • Full-pitched double layer windings typically employed for stator windings with even number of conductors per slot.
  • Slot width should be balanced to avoid mechanically weak teeth and exceeding permissible flux density (1.6 to 1.8 Tesla).
  • Slot depth should not be excessive to avoid increasing leakage reactance, with a depth to width ratio of 3 to 5 generally recommended.
• Slot insulation:
  • Slot liner with thickness 0.5 mm to 0.7 mm.
  • Coil separator with thickness 0.5 to 0.7 mm (low voltage) or 0.8 to 1.2 mm (high voltage)
  • Wedge (3.5 to 5 mm) to hold coils in position.
  • Slot lip (1.0 to 2.0 mm).

Length of Mean Turn (lmt)

• lmt = 2L + 2.3 tp + 0.24 (where L is gross length of stator and tp is pole pitch in meters)

Resistance of Stator Winding (rs)

• rs = (0.021 x lmt x Tph)/ as (where lmt is in meters and as is in mm2)

Copper Losses in Stator Winding

• Total copper losses = 3 (Is)2 rs

Flux Density in Stator Tooth

• Calculated at 1/3rd height from the narrow end of the tooth.
• Diameter at 1/3rd height (D'): D' = D + 1/3 x hts x 2 (hts = height of stator slot)
• Slot pitch at 1/3rd height (t's): t's = pxD'/Ss
• Tooth width at this section (b't): b't = t's - bs (bs = slot width)
• Area of one stator tooth (a't): a't = b't x li (li = length of tooth)
• Area of all stator teeth per pole (A't): A't = b't x li x number of teeth per pole
• Mean flux density in stator teeth (B't): B't = F/A't (F = flux)
• Maximum flux density in stator teeth should be less than 1.5 times B't.

Depth of Stator Core

• The solid portion below the slots.
• Flux density in stator core (Bc) is typically 1.2 to 1.4 Tesla.
• Flux in stator core section (Fc): Fc = ½ F
• Area of stator core (Ac): Ac = F/2Bc
• Area of stator core (Ac): Ac = Li x dcs
• Depth of the core: dcs = Ac / Li (dcs = depth of stator core)

Outer Diameter of Stator Core

• Do = D + 2hss (where hss = height of stator slot)

Example Design (30 kW Induction Motor)

• Given: 30 kW, 440 V, 3-phase, 6 pole, 50 Hz, delta connected squirrel cage induction motor.
• Assumed values:
  • Specific magnetic loading (Bav) = 0.48 Tesla
  • Specific electric loading (q) = 26000 Ac/m
  • Full load efficiency (?) = 0.88
  • Full load power factor (cosF) = 0.86
  • Winding factor (Kw) = 0.955
• Calculated values:
  • Main dimensions (D & L):
    • D2L = Q/ (Co ns) m
    • Co = 11 Bav q Kw ?cosF x 10-3 = 99.2
    • ns = 16.67 rps
    • D2L = 30/(99.2 x 16.67) = 0.0182 m3
    • D = 0.33 m and L = 0.17 m.
  • No. of stator turns (Tph):
    • F = (pDL/p) Bav = 0.0141 wb
    • Assuming Eph = Vph = 440 volts
    • Tph = Eph / 4.44fFkw = 148.
  • No. of stator slots (Sss):
    • Assuming 3 slots/pole/phase: Sss = 3 x 3 x 6 = 54
  • No. of conductors per slot (Zs):
    • Total no of conductors = 148 x 2 = 296
    • No.of conductors /slot = 296/54 = 5.5
    • Revised no.of turns/phase = 162, with 76 conductors/slot.