A 3-phase induction motor uses current delivered in three phases in a sequence into the coils of a stator to create a rotating magnetic field. This induces an electric field in a coil or squirrel cage to drive a rotor. The difference in speed between rotor, the synchronous speed and the rotating magnetic field is called the slip.
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Working principles of a 3-phase induction motor
In a 3-phase AC induction motor, there are three stator windings, each usually in two halves, with the rotor winding short-circuited by end rings. As the current passes through the coils on opposite sides of the stator, a two-pole electromagnet is established, creating a two-pole motor. Applying a phase to each of the electromagnets in turn creates the rotating magnetic field that is strong enough to start moving the rotor.
More winding can create more poles in the motor, with more complex control required but more accuracy in positioning the rotor. A four-pole motor is regarded as optimum for the torque and responsiveness needed to for the motor drives of electric cars, for example. But higher pole counts are only possible with more sophisticated control schemes.
The typical drive has three half-bridges, each delivering a sine-wave voltage to the stator. This uses power MOSFETs or IGBTs with high-voltage gate drivers, or power modules that combine the three half-bridges and related gate drives. These can use scalar algorithms that vary the voltage to determine the frequency of the phases, or volts/hertz. More sophisticated algorithms such as vector control or Field-Oriented Control (FOC) are used to control the frequency of multiple phases in high-end motors are now increasingly popular across the range of three-phase induction motors.
Polyphase motors generally cover three-phase motors using multiple poles.
Self-starting and soft-start controllers
A soft-start controller is used in three-phase AC induction motors to reduce the load on the self-starting motor and the current surge of the motor during start-up. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.
Induction motors can have inrush currents seven to ten times that of the operational current. Starting torques can be 3 times higher to overcome the starting conditions, causing mechanical stress on the components in the motor. So electronic soft starters use a control system to reduce the torque by temporarily reducing the voltage or current input until the induction motor reaches its synchronous speed.
A digital soft-starter controller continuously monitors the voltage during start-up, adjusting to the load of the motor to provide a smooth acceleration and the speed control. This is often done with connected silicon-controlled rectifiers (thyristors) controlling each phase separately to give the optimum control.
Direct torque control
The torque generated in the rotor of a 3-phase induction motor is proportional to the flux generated by each stator pole, the rotor current and the power factor of the rotor. Direct torque control (DTC) is a technique used in variable frequency drives. It comes from estimating the magnetic flux from the voltage and current of the motor. This is compared to a reference value to control the torque.
This allows the flux and the torque to be changed quickly by changing the references, making the motor more efficient and reducing power losses as only the exact current is used. This also avoids the rotor overshooting, allowing more accurate control over the motor.
Three-phase induction motors are a key part of almost every industrial process. So there are many methods for fault detection and diagnosis to make sure that the motors keep the production lines running.
However, despite the high level of reliability of these motors, most of the methods require a good deal of expertise to apply successfully, looking at the voltage, current, vibration or the thermal profile. Simpler approaches are needed so that any line operators can make reliable decisions. And motor makers want to reduce the number of sensors in the motor as they can fail and cause reliability problems.
Rotor faults may occur during production as small faults, or may result from production faults or mechanical, environmental, electromagnetic or thermal pressure on the rotor when the motor runs. Even if these faults are small at first, the faults grow over time, and a broken or cracked rotor can cause neighboring components to fail from increased currents and thermal activity.
Machine learning is increasingly being used to monitor the performance of motors, comparing the patterns of different types of data used in the control systems to predict any potential failure.