Electric Motor Electrical Fault of Electric Vehicles
Faults in PMSM motor drives are mainly divided into three groups: electrical faults, mechanical faults and sensor faults. These errors can occur in the motor or inverter parts.
As mentioned before, the main electrical faults are turn short circuit faults (ITSF), open or short phase and demagnetization faults related to the motor. In addition, switch open or short circuit, intermediate circuit capacitor failures are also inverter related.
1) Short circuit faults
Faults in the stator windings and inter-turn insulation degradation in PMSMs are usually caused by voltage surges, moisture or mechanical, electrical or thermal stresses that cause winding short circuits. This failure is called an Intermittent Short-Circuit Failure (ITSF) and has the highest failure rate of all engine failures.
The shorted winding creates an additional current loop that is connected to the flux linkage generated by other motor windings and rotor magnets. The low impedance and high combined flux linkage voltages cause large fault currents in the ITSF winding, leading to stator overcurrents and overheating.
In the early stages of ITSF, the motor can continue to operate at reduced power even if only a few percent of the windings fail. However, the heat generated by the overcurrent can damage the insulation of nearby windings and quickly spread to the entire phase, causing phase-to-phase or phase-to-earth short circuits, causing catastrophic motor failure in a short time and expensive to repair. Expensive.
In addition, high residual currents in the additional current paths can permanently demagnetize the rotor’s permanent magnets. Therefore, detecting incipient faults is important for ITSF. Typically, the ratio of shorted turns to coil turns is considered the severity of ITSF. As the severity increases, the induced back-EMF voltage of the shorted winding increases, which results in a rapid increase in short circuit current and a large system imbalance.
2) Demagnetization Error
Physical damage, high temperature operation, aging, or reverse magnetic field can all cause demagnetization, which reduces the strength of the permanent magnets (PMs) in IPMSMs.
Reversible demagnetization and irreversible demagnetization are two forms of demagnetization. The former is caused by field weakening control, while the latter suffers from permanent demagnetization. The combined effects of temperature and permeance curve shifts lead to an improper operating point of IPMSM, which is the main cause of irreversible demagnetization. When demagnetization occurs, the PM linkage flux decreases, and therefore the torque of PMSM decreases.
This has a negative impact on engine characteristics and efficiency. To compensate for the weakened PM and generate the same torque as in the normal state, the current in the demagnetized PMSM must be increased. However, this means higher copper losses and higher temperatures. Meanwhile, higher temperatures can lead to even more severe irreversible demagnetization.
As a result, the reliability and security of the system are compromised. To avoid such consequences, the use of error detection and diagnostic technologies is essential. Demagnetization errors can cause additional frequency components in the stator current and vibration, resulting in torque and speed pulsations.
3) Switches in the inverter are open or shorted
Inverters are used as the core components of the drive systems of electric motors. Due to high frequency operation, high power loads, aging, and other conditions, switching devices are the components most likely to fail during operation (about 38% of driver failures), and usually manifest as short or open circuit failures. Such an error does not stop the operation of the drive system.
The system operates in phase-locked mode, since an open-circuit fault stops the stimulation of the defective phase winding in the switching device. This causes the drive system to become unbalanced, leading to unbalanced forces on the rotor, which in turn can lead to a significant decrease in system performance, noticeable vibrations, and ultimately to motor failure due to the lack of FDD. Short-circuit faults usually occur as a result of overvoltage, overheating, failure of protective components, or incorrect gate signals.
As a result, the faulty phase generates a large reverse braking torque during the demagnetization phase, which significantly impairs the stability of the drive system and leads to a subsequent failure of the entire system.
In this case, an immediate overcurrent occurs, which triggers the protection circuit and shuts down the inverter. Accurate and fast identification and isolation of power transistor faults and their location are therefore crucial for the safe operation of PMSM drives.