Three Phase Induction Motor as Traction Motor

Last updated on April 5th, 2015 at 06:14 am

Working Principle of Three Phase Induction Motor

AC induction motor is equivalent to Transformer where secondary is short circuited and free to rotate. Three phase primary winding is mounted on Stator distributed spatially at 120 degree. Resultant Field flux produced in the air gap, due to time and space varying each phase flux, is of constant magnitude but rotating at a synchronous speed of Ns. The value of Ns is given by 120(f/P ) where f is frequency and P is number of poles. This flux induces emf and current in the rotor and causes it to rotate in the direction of stator flux and tries to catch it up. The induction of voltage and current in the rotor circuit will depend upon the relative motion between field and rotor.  The induced emf, rotor reactance and frequency is having value of sE, sX and sf where s is called slip frequency given by (Ns-Nr)*100/Ns

Equivalent Circuit of Induction Motor

Equivalent circuit is same as that of transformer except that all values in the secondary side is at slip frequency because of induction of voltage and current in the rotor will be based on the relative motion of synchronous speed and rotor speed.

1. Air gap Flux is produced by the magnetising current I_m flowing through inductive winding of the stator L_m and Φ_ag = (L_m I_m)/Ns

• From Faraday’s Law we know that;          e_ag=N_s(d Φ_ag/dt)

Air gap flux at any instant is given by       Φ_ag(t)= Φ_agsinωt

From this we get                             e_ag=N_sΦ_agωcosωt or N_sΦ_ag(2πf)cosωt

Rms value                                                 ∝Φ_agf means proportional to air gap flux and frequency

Ratio of applied voltage and frequency shall be maintained constant for air gap flux to be constant.

2. Air gap flux produced is rotating at a synchronous speed of ω_s and induces voltage in the rotor circuit. The frequency of induced voltage depends on the relative motion between synchronous speed and the rotor speed at any instant of time. It will be f at locked rotor and will keep on reducing to very low value with increase in speed.

Slip                                                                         s(per unit)=(ω_s-ω_r)/ω_s

Slip Speed( ω_sl)                                                 sω_s

Slip frequency(f_sl)                                            sf∝ sω_s

Rotor Voltage E_r                                               ∝f_slΦag=R_rI_r +j2πf_slL_rI_r

Power crossing the airgap                            E_agI_r

Rotor Power Loss                                             3R_rI_r²                               

                        Now multiplying rotor voltage equation by f/f_sl   then we get

E_ag = (f/f_sl ) E_r =R_rI_r(f/f_sl ) +j2πf_slL_rI_r(f/f_sl)

Multiplying with I_r

P_ag = E_agI_r= R_rI_r²(f/f_sl ) +j2πfL_rI_r² = (Active power per phase+ Reactive Power per phase)

Total active power 3P_ag=3 R_rI_r²(f/f_sl )

                        Electromagnetic power of rotor = Air gap power – rotor losses

P_em=3 R_rI_r²(f/f_sl )- 3R_rI_r²   = 3 R_rI_r²[(f/f_sl)-1] where f/f_sl = 1/s

T_em=P_em/ω_r; ω_r=ω_s(1-s)

P_ag/P_em={3 R_rI_r²(f/f_sl)]/ 3 R_rI_r²[(f/f_sl)-1]= (f/f_sl)/ [(f/f_sl)-1]=1/(1-s)=ω_s/ω_r

           T_em=3( Rr/ω_r)Ir²[(f/f_sl)-1] =3[ Rr/ ω_s(1-s)]Ir²[(1-s)/s]=3(1/ω_s) I_r² (R_r/s)

I_r=sE_ag/Z_r= sE_ag/√R_r²+s²X_r²;

T_em=[3(1/ω_s)( E_ag²)( R_r/s)]/[( R_r/s)²+X²]

Now there are two options

• When s is low-R_s/s is low as compared to X, then  T_em= [3(1/ω_s)( E_ag²)( s/R_r)]; T_em∝s
• When s is high-R_s/s is low as compared to X, then   T_em=[3(1/ω_s)( E_ag²)( R_r/X²)](1/s); T_em∝1/s

Figure 2

Stability of the torque from  zero slip to pull out can be confirmed like that if there is sudden demand of torque, speed reduces and if load speed increases, torque demand reduces. This is not so in zone of pull out torque to pull in torque where if speed increases  demand of load also  increases. Induction motors are preferred to work much below pull out torque as shown T_rated to deliver full power at rated speed.  T_max  is short time torque to utilize short term thermal rating of the motor.

Speed Control of Traction Motor

Speed of an Induction motor depends only on number of poles and frequency of applied power supply from the formula ω_s=120f/p or to some extent by reducing the supplied voltage which results in reduction of generated torque resulting in reduction of speed. These speed control method could not find any universal application for any purposeful gain. There is one example of using pole change from 4 to six pole for twin speed vacuum exhauster used  on WAG1 class of locomotive for quick creation of vacuum after application of brakes.

With the development of advance power devices like thyrister, GTO and IGBT, frequency control became feasible.

Speed-torque characteristics of Induction motor derives the  Tractive effort vs speed characteristics of Three phase Induction Motor.

Constant Torque Zone

Now I_r∝f_slΦ_ag and T_em∝Φ_agI_r we get T_em∝(Φ_ag)²f_sl; from this equation it may be derived that if we keep the air gap flux at the maximum and constant along with f_sl also constant, motor will deliver maximum torque and at constant level. Torque speed curve at different frequencies is shown below. T_max is the maximum torque motor can deliver for a short period without increasing the maximum permissible temperature.

Constant Air gap Flux and Constant f_sl = sf        results constant torque up to rated frequency

Constant Power Zone

Φ_ag can be maintained constant when voltage is increased in proportion to frequency. This is possible up to rated voltage thereafter, air gap flux reduces as indicated below

Φ_ag=E_ag/f; T_em∝(Φ_ag)²f_sl or T_em∝(E_ag/f )²f_sl or ∝E_ag²f_sl/f ² or ∝ f_sl/f ² or ∝ sf/f ² or ∝ s/f

It means torque varies inversely proportion to frequency by keeping slip constant.

Reduced Power Zone

Torque reduces with reduction of slip and therefore slip can only be reduced to the limit of minimum torque which is essentially required by load. Thereafter slip reduces in proportion to frequency, keeping sf constant and in this situation T_em is in inverse proportional to  square of frequency.

Voltage Boost at Low Voltage

Air gap flux is   required to be  maintained constant to derive constant tractive effort up to constant tractive effort zone of the drive. Voltage applied provides air gap flux as well as drop across stator resistance therefore voltage boost is required at low voltages to maintain constant air gap.

V_s = kf+ I₁R₁

The percentage I₁R₁ drop is significant at lower frequency operation therefore voltage boost is required to compensate for the voltage drop as shown below

Regenerative Braking

This is the most important feature of traction motor provides opportunity for saving on energy but also smoothness in braking, saving on wear and tear in mechanical braking system. An Induction motor works as a generator if mechanical speed is more than the synchronous speed. When called upon to brake, the frequency applied to the stator is lowered below the mechanical speed, and what happens is explained as follows:

A Traction motor is operated at frequency f₀,Slip s₀ and delivering torque τ₀ shown on curve below. Now the frequency of the stater is reduced to f₁, the demand of torque slip characteristics now shifts to curve B requiring point of operation shift.  to curve B where torque and slip becomes negative and shifts to point B on the curve B with regeneration to start. Maximum value o regenerative tractive effort is limited to 25T and so is negative torque of the traction motor. The rate of change of frequency decides the maximum setting; voltage is simultaneously reduced along with frequency.