Motor Neutral Point Research Articles

Online temperature monitoring of a grid connected induction motor

This paper proposes a novel method for continuous temperature measurement of the stator winding of an induction machine. The method is based on measuring stator winding resistance by injecting a DC current test signal into the motor neutral point without interrupting its normal operation. The entire stator winding is used as a sensor without additional thermal sensors, while DC voltage and current need to be measured solely. Virtually equal currents are injected in each phase thereby minimizing the influence on the motor torque inherent to contemporary signal injection-based methods. The measurement results obtained using the proposed method are compared to results obtained using the standard resistance-based method and direct measurements obtained from thermocouples embedded in the windings. An excellent match is achieved under various loading levels and variable cooling conditions, thus proving the accuracy and robustness of the proposed method.

A Passive EMI Filter for Preventing High-Frequency Leakage Current From Flowing Through the Grounded Inverter Heat Sink of an Adjustable-Speed Motor Drive System

This paper focuses on the high-frequency leakage current flowing through the grounded heat sink of a voltage-source PWM inverter in an adjustable-speed motor drive system. A passive EMI filter is proposed, designed and tested for a 200-V 3.7-kW adjustable-speed motor drive system. This filter is unique in the sense that it requires access to the ungrounded motor neutral point. An equivalent common-mode circuit for the motor drive system equipped with the EMI filter is derived from experimental waveforms along with theoretical discussions. Experimental results verify that the passive EMI filter is viable and effective in eliminating both heat-sink and motor-frame leakage currents from the motor drive system.

An Approach to Suppressing Both Shaft Voltage and Leakage Current in an AC Motor Driven by a Voltage-Source PWM Inverter-A Passive EMI Filter Making Use of the Motor Neutral Line-

This paper proposes a practical approach to suppressing both shaft voltage and leakage current in an ac motor driven by a voltage-source PWM inverter. This approach is characterized by using a neutral line of the ac motor. A common-mode inductor is connected between the inverter and the motor. Moreover, a resistor and a capacitor are connected in series between the motor neutral point and the inverter negative dc bus. This unique circuit configuration makes the common-mode inductor effective in reducing the common-mode voltage appearing at the motor terminals. As a result, both shaft voltage and ground current are significantly suppressed with low cost. Over-voltages at the end of a cable can be suppressed by a normal-mode inductor and a resistor which are connected in parallel. The validity and effectiveness of the new approach are verified by experimental results from a 5-kVA laboratory system.

Development of a Robust Random Switching Hysteresis PWM Inverter for Linear Positioning Control

Robust random switching control for a three-phase hysteresis current-controlled PWM inverter to promote its ability in servo driving control is studied in this paper. After observing the effect of varying band on the output harmonic distribution characteristic of a hysteresis current-controlled PWM inverter, a random switching hysteresis PWM scheme is developed. A robust spectrum shaping technique is devised such that the randomized inverter output harmonic spectrum is further shaped to the desired pattern. In the resulted frequency spectrum, the magnitudes of lower-frequency harmonics are reduced significantly. This makes the hysteresis PWM inverter suitable to serve as a driving source for position servo motor drives. The effect of AC motor neutral point isolation on the random switching performance is observed. The developed random switching inverter is used to power a linear permanent magnet synchronous motor-driven magnetic suspended positioning stage, and the resulted position control performance is demonstrated by some simulated and measured results.

A High Performance AC Servo System with Permanent Magnet Synchronous Motors

A high performance ac servo system with permanent magnet synchronous motor, including a field-weakening control and a high-speed current response control of PWM inverter is proposed. For the field weakening, the d-axis armature current is adequately controlled through a microprocessor. The novel current control scheme of PWM inverter includes the motor neutral point potential stabilization and the switching carrier frequency equalization.