80 Questions
What is the benefit of semiclosed slots in terms of air gap characteristics?
Better air gap characteristics
What is the effect of increasing the number of stator slots?
Increased cost and weight
What is the formula to calculate the stator slot pitch at the air gap surface?
tss = pD/Sss
What is the purpose of the cooling fan located at the non-driving end of an induction motor?
To provide forced cooling for the stator and rotor
What is the effect of higher current density in stator windings?
Reduction in cross-section and weight
What is the typical range of current density values assumed for stator windings?
3-5 amps/mm²
What is the unit of synchronous speed (ns) in the output equation?
RPM
What percentage of the slot area is occupied by insulation?
Around 25%
What is the output coefficient (Co) equal to?
(11 Bav q Kw cos x 10-3)
What is the formula to calculate the turns per phase (Tph) of an induction motor?
Tph = Eph/4.44fFkw
Why are open slots rarely used in 3F induction motors?
Because they lead to higher air gap contraction factor and hence poor power factor
What is the purpose of the terminal box?
To receive the external electrical connections
What is the condition for selecting the number of stator slots for fractional slot windings?
Number of slots/pole/phase is 3.5 or more
What is the relation between the pole pitch and core length in order to obtain the best power factor?
Pole pitch/Core length = 0.18
What is the maximum peripheral speed for a normally designed induction motor?
30 m/s
What is the purpose of the two end-flanges in an induction motor?
To support the two bearings
What is the typical range of slot space factor?
0.25 to 0.4
Why should stator slots not be too wide?
It leads to thin tooth width and weak mechanical strength
What is the primary function of induction motors in industries?
To act as prime movers
What is the thickness of the slot liner?
0.5 mm to 0.7 mm
What is the empirical formula to calculate the length of the mean turn?
lmt = 2L + 2.3 tp + 0.24
What is the main difference between DC machines and induction motors?
Presence of commutator and laminated stator
What is the formula to calculate the resistance of the stator winding per phase?
R = (0.021 x lmt x Tph) / as
What is the purpose of laminating the magnetic path in the stator?
To reduce eddy currents and losses
What is the maximum flux density allowed in the stator tooth?
1.8 Tesla
What is the outer cylindrical frame of the motor made of?
Welded sheet steel or cast aluminum alloy
What is the ratio of slot depth to slot width?
3 to 5
What is the function of the stator core?
To carry magnetic flux
What is the thickness of the coil separator for low voltage machines?
0.5 mm to 0.7 mm
How many sets of windings are required for a 3-phase motor?
3
What is the purpose of the air gap in an induction motor?
To provide a uniform and small air gap
What is the significance of the rotor in an induction motor?
It is the rotating part of the motor
What is the value of specific magnetic loading (Bav) in the given design?
0.48 Tesla
What is the value of specific electric loading (q) in the given design?
26000 ac/m
What is the full load efficiency (?) of the induction motor?
0.88
What is the value of winding factor (Kw) in the given design?
0.955
What is the calculated value of D2L in the given design?
0.018 m3
What is the calculated value of stator diameter (D) in the given design?
0.33 m
What is the calculated value of stator length (L) in the given design?
0.17 m
What is the revised number of turns per phase (Tph) in the given design?
162
What is the primary function of induction motors in industries?
To act as prime movers
What is the purpose of laminating the magnetic path in the stator?
To reduce eddy currents, reducing losses and heating
How many sets of windings are required for a 3-phase motor?
3
What is the significance of the rotor in an induction motor?
It is the rotating part of the motor
What is the function of the stator core?
To carry magnetic flux
What is the main difference between DC machines and induction motors?
Presence of commutator
What is the outer cylindrical frame of the motor made of?
Welded sheet steel, cast iron or cast aluminum alloy
What is the purpose of the air gap in an induction motor?
To practically maintain a uniform and small air gap
What is the typical range of slot space factor?
0.25 to 0.4
What is the formula to calculate the length of the mean turn?
lmt = 2L + 2.3 tp + 0.24
What is the maximum flux density allowed in the stator tooth?
1.8 Tesla
What is the ratio of slot depth to slot width?
3 to 5
What is the thickness of the coil separator for low voltage machines?
0.5 mm to 0.7 mm
What is the formula to calculate the resistance of the stator winding per phase?
R = (0.021 x lmt x Tph) / as
What is the purpose of the wedge in the stator slot?
To hold the coils in position
Why should stator slots not be too wide?
To prevent thin tooth width and high flux density
What is the value of output coefficient (Co) in the given design?
99.2
What is the calculated value of stator diameter (D) in the given design?
0.33 m
What is the calculated value of stator length (L) in the given design?
0.17 m
What is the number of stator slots in the given design?
54
What is the number of conductors per slot in the given design?
5.5
What is the revised number of turns per phase (Tph) in the given design?
162
What is the value of specific electric loading (q) in the given design?
26000
What is the full load efficiency (?) of the induction motor?
0.88
What is the main advantage of using a cooling fan located at the non-driving end of an induction motor?
To provide forced cooling for the stator and rotor
What is the unit of the output equation Q in the given design?
Kilowatts
What is the purpose of assuming a relation between the gross length and pole pitch of the stator?
To separate the diameter and length of the stator from the D2L product
What is the significance of the peripheral speed in the design of an induction motor?
It determines the maximum safe operating speed of the motor
What is the effect of increasing the number of conductors per phase (Zph) on the output equation?
It increases the output power of the motor
What is the significance of the specific electric loading (q) in the design of an induction motor?
It determines the stator design and the magnetic flux
What is the purpose of using a steel shaft in an induction motor?
To transmit the mechanical power to the load
What is the effect of the air gap flux per pole () on the output equation?
It increases the output power of the motor
What is the primary reason for selecting a higher number of stator slots?
To reduce the leakage reactance and tooth pulsation losses
What is the effect of using tapered slots on the assembly of windings?
It makes the assembly of windings more difficult and time-consuming
What is the purpose of assuming a suitable value of current density for the stator windings?
To determine the sectional area of the stator conductors
What is the typical range of values assumed for the winding factor (kw) in full-pitch distributed windings?
0.955 unless otherwise specified
What is the relationship between the number of stator slots (Sss) and the stator slot pitch (tss) at the air gap surface?
tss is inversely proportional to Sss
What is the effect of increasing the number of stator slots on the iron losses?
It increases the iron losses
What is the purpose of selecting a suitable shape and size of the conductor based on the sectional area?
To determine the type of conductor to be used
What is the effect of selecting a higher number of stator slots on the magnetizing current?
It increases the magnetizing current
Study Notes
Induction Motors
- Induction motors are widely used in industrial applications, from small workshops to large industries.
- They are employed in applications such as centrifugal pumps, conveyors, compressors, crushers, and drilling machines.
Constructional Details
- An induction motor consists of a stationary member called the stator and a rotating member called the rotor.
- The stator and rotor are each made up of:
- An electric circuit (insulated copper or aluminum winding) to carry current.
- A magnetic circuit (laminated silicon steel) to carry magnetic flux.
- The stator:
- Consists of an outer cylindrical frame, a magnetic path (laminated steel laminations), and insulated electrical windings.
- Windings are placed inside the slots of the laminated stator core.
- Cross-sectional area of the windings must be large enough for the power rating of the motor.
Output Equation
- The output equation gives the relation between the various physical and electrical parameters of the electrical machine.
- Output Q (in kW) = Input x Efficiency.
- Input to motor = 3Vph Iph cos x 10^-3 kW (for a 3-phase machine).
- Output equation can be derived and simplified to Q = Co D^2L/ns kW.
Design of Stator
- Stator core and stator slots are the two main components of the stator.
- Stator slots:
- Two types: open slots and semiclosed slots.
- Semiclosed slots are commonly used in 3-phase induction motors.
- Selection of number of stator slots:
- Affects the weight, cost, and operating characteristics of the motor.
- Higher number of slots reduces leakage reactance, tooth pulsation losses, and increases overload capacity, but increases cost, weight, and magnetizing current.
Turns per Phase
- EMF equation of an induction motor: Eph = 4.44fFTphkw.
- Turns per phase (Tph) can be obtained from the EMF equation.
- Tph = Eph / 4.44fFkw.
Conductor Cross-Section
- Sectional area of the stator conductor can be estimated from the stator current per phase and a suitably assumed value of current density for the stator windings.
- A suitable value of current density has to be assumed considering the advantages and disadvantages.
Length of the Mean Turn
- Calculated using an empirical formula: lmt = 2L + 2.3tp + 0.24.
Resistance of Stator Winding
- Calculated using the formula: rs = (0.021 x lmt x Tph) / as.
- Total copper losses in stator winding can be calculated using the calculated resistance.
Flux Density in Stator Tooth
- Calculated considering the dimensions of the stator slot pitch, width of the slot, and width of the stator tooth.
- Limited to 1.8 Tesla.
Depth of Stator Core
- Calculated assuming a suitable value for the flux density in the stator core.
- Generally, the flux density in the stator core varies between 1.2 to 1.4 Tesla.
Induction Motors
- Induction motors are widely used in industrial applications, from small workshops to large industries.
- They are employed in applications such as centrifugal pumps, conveyors, compressors, crushers, and drilling machines.
Constructional Details
- An induction motor consists of a stationary member called the stator and a rotating member called the rotor.
- The stator and rotor are each made up of:
- An electric circuit (insulated copper or aluminum winding) to carry current.
- A magnetic circuit (laminated silicon steel) to carry magnetic flux.
- The stator:
- Consists of an outer cylindrical frame, a magnetic path (laminated steel laminations), and insulated electrical windings.
- Windings are placed inside the slots of the laminated stator core.
- Cross-sectional area of the windings must be large enough for the power rating of the motor.
Output Equation
- The output equation gives the relation between the various physical and electrical parameters of the electrical machine.
- Output Q (in kW) = Input x Efficiency.
- Input to motor = 3Vph Iph cos x 10^-3 kW (for a 3-phase machine).
- Output equation can be derived and simplified to Q = Co D^2L/ns kW.
Design of Stator
- Stator core and stator slots are the two main components of the stator.
- Stator slots:
- Two types: open slots and semiclosed slots.
- Semiclosed slots are commonly used in 3-phase induction motors.
- Selection of number of stator slots:
- Affects the weight, cost, and operating characteristics of the motor.
- Higher number of slots reduces leakage reactance, tooth pulsation losses, and increases overload capacity, but increases cost, weight, and magnetizing current.
Turns per Phase
- EMF equation of an induction motor: Eph = 4.44fFTphkw.
- Turns per phase (Tph) can be obtained from the EMF equation.
- Tph = Eph / 4.44fFkw.
Conductor Cross-Section
- Sectional area of the stator conductor can be estimated from the stator current per phase and a suitably assumed value of current density for the stator windings.
- A suitable value of current density has to be assumed considering the advantages and disadvantages.
Length of the Mean Turn
- Calculated using an empirical formula: lmt = 2L + 2.3tp + 0.24.
Resistance of Stator Winding
- Calculated using the formula: rs = (0.021 x lmt x Tph) / as.
- Total copper losses in stator winding can be calculated using the calculated resistance.
Flux Density in Stator Tooth
- Calculated considering the dimensions of the stator slot pitch, width of the slot, and width of the stator tooth.
- Limited to 1.8 Tesla.
Depth of Stator Core
- Calculated assuming a suitable value for the flux density in the stator core.
- Generally, the flux density in the stator core varies between 1.2 to 1.4 Tesla.
Learn about the construction and applications of induction motors, widely used in industrial settings. Understand the stator and rotor components and their roles.
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