Broken rotor bars in induction motors can be dependably detected by analyzing the current signatures under sufficient motor load conditions. Detection becomes less dependable under light motor load conditions. There are also cases in which tolerable motor operating conditions generate current signatures similar to those of motors with broken rotor bars. These cases may present security concerns when the detection element is set to trip the motor and to send alarms. In this paper, we aim to achieve the following: show how broken rotor bars cause characteristic current signatures; show how to detect broken rotor bars with a zero-setting protection element, which uses the current signature method; use cases with different motor operating and fault conditions to analyze the performance of the zero-setting broken bar protection element; identify cases when the current signature method is dependable and cases when security is a concern, and present solutions to address security concerns.

Heating and losses of induction motor (IM) grow with the supply voltage unbalance and harmonic distortion. To avoid thermal overload, the IEEE 141-1993, the IEEE 3004.8, NEMA MG1-2014, and the IEC 60034-26 standards establish derating factors for IMs operating under those conditions. In this article, we show that while the derating factors proposed by these standards adequately protect standard-efficiency IMs, they are only marginally adequate to protect modern higher-efficiency IMs. To this end, we compare the derating factors provided by the standards with the derating factors required to maintain the losses at rated values in a standard-efficiency IM, in a premium-efficiency IM, and in a super-premium-efficiency IM. To extrapolate the results from these IM, we compared the nameplate data of 548 IMs of different efficiency classes and found that higher efficiency classes correlate to higher IM starting currents and lower impedances to the negative sequence and harmonic voltages. These lower impedances in turn may lead to higher losses for unbalanced and harmonic voltages conditions.

Effects of voltage unbalance on the induction motors power, torque and vibrations are analyzed in this paper. Voltage unbalance produces power oscillations at twice the supply frequency, which introduces, in turn, torque oscillations and vibrations at this frequency. The dependence of these power oscillations on the unbalance level and motor load is analyzed by simulation. The effects of voltage unbalance on the oscillation of motor power, torque and vibrations are extensively analyzed through experimental results. These experiments are performed for induction motors with different rotor characteristics, showing that the motor behavior under voltage unbalance significantly changes according to the rotor construction.

Small efficiency improvement in induction motors (IMs) often lead to large monetary savings over time. These monetary savings justify the current movement toward the use of higher efficiency IMs. In many cases, replacing a standard efficiency IM with a higher efficiency one is transparent to the user. In other cases, however, there are side effects which need to be considered. This paper presents data showing that vibrations due to voltage unbalance are significantly more pronounced on higher efficiency IMs than in standard efficiency ones. This paper presents the mathematical expressions for active power for an IM under voltage unbalance. Then the paper describes a dynamic simulation framework to validate the paper's hypothesis over a wide range of IMs and a laboratory the test setup used to validate the mathematical expressions with experimental results of standard (IE1), premium (IE3) and super premium (IE4) IMs. Finally, the paper presents vibration measurements due voltage unbalance, and compares the results with the measured vibration levels with the ISO 20816-1 standard vibration severity guidelines