Fast operating protection of low-voltage electric motors on reed switches sensitive for stator winding short-circuits

• The relationships between current phase angles and shorted turns have been identified. • Monitoring the angles between the currents allows detecting 5–10% of shorted turns. • The response time for detecting shorted turns does not exceed 5–10 ms. • A new protection device for a low-voltage electric motor (EM) is presented. This article is devoted to the development of a new device to protect asynchronous electric motors (EM) rated below 1 kV (low-voltage) from inter-turn and phase-to-phase short circuits (SC). The protection is based on monitoring the angles between the stator phase currents. Experiments were conducted with various stator winding faults: phase-to-phase SCs, inter-turn SCs, and an open-phase condition. It was experimentally proven that the principle of monitoring the angles between stator phase currents is highly sensitive for detecting inter-turn SCs, whereas traditional protections based on monitoring current magnitude or stator winding temperature are not as effective. It is shown that this principle allows detection of inter-turn SCs involving a small number of shorted turns in one phase within a very short time. A new reed-switch-based protection device for low-voltage EM was developed, which detects the aforementioned SCs faster than existing solutions while not using bulky current transformers. The use of reed switches in the device design also reduces weight and size, which for the first time enabled a mounting structure for attaching the device to a motor’s terminal box – significantly simplifying installation and expanding the device’s applicability. The developed reed-switch protection device is suitable for protecting EM stator windings and preventing emergency situations in applications requiring high-speed fault detection.