ENERGY CONVERSION ONE (Course 25741) CHAPTER SEVEN INDUCTION MOTORS … (Induced Torque…)

…INDUCTION MOTOR INDUCED-TORQUE EQUATION ZTH=RTH+jXTH = jXM(R1+jX1)/[R1+j(X1+XM)] Since XM>>X1 and XM+X1>>R1 , Thevenin resistance & reactance can be approximated as: RTH ≈ R1 ( XM/ [X1+XM] ) ^2 XTH ≈ X1 resulting equivalent circuit:

…INDUCTION MOTOR INDUCED-TORQUE EQUATION The current I2 obtained as follows: I2= VTH / (ZTH + Z2)=VTH/[RTH +R2/s + j XTH +j X2] Magnitude of current: I2= VTH /√(RTH+R2/s)^2+(XTH+X2)^2 Air gap power PAG = 3 I2^2 R2/s= 3 VTH^2 R2/s / [(RTH+R2/s)^2+(XTH+X2)^2] and rotor induced torque is: Tind=PAG/ ωsync Tind=3VTH^2 R2/s / {ωsync [(RTH+R2/s)^2+(XTH+X2)^2]} a plot of torque as function of speed (& slip) shown in next figure:

…INDUCTION MOTOR INDUCED-TORQUE EQUATION A typical induction motor torque-speed characteristic curve

…INDUCTION MOTOR INDUCED-TORQUE EQUATION A plot of speed above & below normal range shown below:

INDUCTION MOTOR TORQUE-SPEED CURVE Torque-speed characteristics curve provides several important information 1- induced torque of motor is zero at syn. Speed 2- torque-speed curve is nearly linear between no load and full load. In this range rotor resistance is much larger than its reactance so rotor current, rotor magnetic field & induced torque increase linearly with increasing slip 3- there is a maximum possible torque that cannot be exceeded (pullout torque) is 2 to 3 times rated full-load torque of motor (calculated in next section) 4-starting torque on motor is slightly larger than its full-load torque, so this motor will start carrying any load that it can supply at full power 5- Note: that torque on motor for a given slip varies as square of applied voltage. This is useful in one form of induction motor speed control that will be described

INDUCTION MOTOR TORQUE-SPEED CURVE 6- if rotor of induction motor driven faster than sync. Speed, direction of Tind reverses & machine become Gen. converting Pmech to Pelec (discussed later) 7- if motor turning backward relative to direction of magnetic fields, induced torque will stop machine very rapidly & will try to rotate it in other direction since reversing direction of magnetic field rotation is simply a matter of switching any two stator phases, this fact can be used as a way to very rapidly stop an induction motor act of switching two phases in order to stop motor very rapidly is called plugging

INDUCTION MOTOR TORQUE-SPEED CURVE Power converted to mechanical in an induction motor: Pconv=Tindωm Note: peak power supplied by induction motor occurs at a different speed than maximum torque; and of course no power is converted to mechanical form when rotor is at zero speed

INDUCTION MOTOR TORQUE-SPEED CURVE Induced Torque & Power Converted versus motor Speed in r/min (4 pole induction motor)

INDUCTION MOTOR TORQUE-SPEED CURVE Maximum (Pullout) Torque in induction motor Tind=PAG/ωsync maximum possible torque occurs when air gap power is maximum Since air-gap power = power consumed in R2/s maximum induced torque will occur when power consumed by this resistor is maximum If angle of load impedance is fixed, maximum power transfer theorem states : maximum power transfer occur when magnitude of that impedance = source impedance magnitude Equivalent source impedance: Zsource=RTH+jXTH+jX2 So maximum power transfer occurs when: R2/s=√RTH^2 + (XTH+X2)^2