12-10-2012, 01:24 PM
Induction Motors Behavior Under Non Sinusoidal Voltage Supply Condition and Assessment of its Damages in Iran Network
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Abstract
An induction motor behaviors in low power quality condition is presented in this paper. Simulations are done for both ideal supply and non proper supply conditions. In this paper, we use two case study including one 3hp (small range) motor and one 50hp (medium range) motor and effects of non ideal supply voltage on motors characteristics are studied. Efficiency reduction, increase in motors temperature and oscillations in steady state torque are some of most important issues of this phenomena. Considering the traditional derating on induction motors in such situations and motors efficiency reduction and other effects of non sinusoidal supply on induction motors, the harmfulness of this type of supply on Iran network (considering abundant usage of induction motors) has been calculated and the importance of preventing non sinusoidal supply is illustrated.
INTRODUCTION
Induction motors are commonly used in industry and sensitive to voltages harmonics and their operation and efficiency can be affected intensely their supplied power quality. Some oscillations in three phase induction motors torque appear on launch time which these oscillations damped in very short time. Although the torque amount can be negative in that time but average value remain positive because of motors stable operation. If we use from speed-torque curve for depict of motor behavior in steady state. In very applications these oscillations will not be appreciable and can be negligible. In non sinusoidal supply conditions the oscillations existing in transient state is not damped and survive to steady state (with small value). This paper is research about induction motors transient behavior on non ideal supply condition.
HARMONIC EFFECTS ON INDUCTION MOTORS
Voltage harmonic because of heat and oscillations produce in rotor cause most important damage to induction motor. Rotors oscillations are because of torque ripples and these ripples emerge from positive and negative ordinary harmonics. Rotors oscillations can increase friction losses of bearings. Since motors temperature in present of any harmonic would be higher than normal state and this will damage to bearings and stator coil and consequently motors life will be increased. Amount of this decrease is dependent to motors class. Fig.1 shows relation between motors life and its temperature in E,B and F classes. It is seen from fig.1 that 8 degree increase iin motors temperature in class E and 10 degree in class B and 12 degree in class F lead to motors life reduction to half. Preventing this affair is required derating on motors. This prohibit from motors temperature increase in non sinusoidal supply condition. Equation(2) determine amount of decrease in motors nominal values for prevent from temperature increase.
THREE PHASE INDUCTION MOTOR
Induction motors consist of one multi phase coil which distributed in stator evenly and other coil which has been short circuit in induction motors alternative current (AC) is applied to stator and the current is induced in rotor.[2]
We accept in this paper assumptions listed in below.
• Air gap in motor is even.
• Magnetic circuit is linear.
• Rotors and stators resistance are fix.
• Stator coil is distributed so generate sinusoidal MMF
• Some phenomena such as core losses and saturation and harmonics of MMF and resistors changes with temperature are neglected.
CONCLUSION
In this paper the harmonic generator factors and their effects on induction motors performance was studied. These effects include efficiency reduction which was shown for VDF=10%, generation of torque oscillations in steady state. Also these oscillations cause mutual effect of stator and rotor and lead to motors temperature increase. For preventing of excessive rise of motors temperature, motors should be derated. This derating leads to increase of energy consumption and cause more damages to consumers. With consideration to abundant consumption of this type of motors in Iran the amount of damages emerge from this phenomena was calculated and importance of preventing from non proper supply in induction motors was proved.