Understanding Electric Motors: Components, Operation, and Control
Electric Motor Components
Electric Motors: Key components include the rotor and stator.
- Rotor: The rotating part housed inside the magnetic circuit of the stator.
- Stator: The fixed part formed by the magnetic plate with slotted windings.
External Engine Parts
External Engine Parts: These include the terminal box, type plate, axle casing, fan cap, and base fixation.
Motor Phases
Motor Phase: Single-phase motors have two windings inside: one working winding and one starting winding, along with two neutral terminals. They have a single direction of rotation. The starting winding is connected in series with a capacitor (either external or using a centrifugal switch).
Three-Phase Squirrel Cage Rotor Motor
Three-Phase Squirrel Cage Rotor Motor: These motors have three windings, one for each phase. Each winding has two terminals: a beginning and an end (U1, W2, V1, U2, W1, V2). Three-phase motors have a terminal box with 6 terminals. Delta connection: 230V, Star connection: 400V.
Starting a Three-Phase Motor Directly
Starting a Three-Phase Motor Directly: Low-power motors (less than 0.75kW) can be started directly using a three-pole switch or by closing phases for a short time.
Phase Failure
Phase Failure: This causes an overcurrent, potentially damaging the motor windings. If the motor hums and fails to start, or continues to run with unusual noise, it may be at risk of burning out.
Reversing Direction
Reversing Direction: Change the phase sequence of the power supply.
Induction Motor Starting
Induction Motor Starting: The starting torque is critical, as the motor must overcome the load to reach its normal operating speed. Speed curves are important.
Star-Delta Starting
Star-Delta Starting: This is a common method to reduce overcurrent during startup. The motor is initially connected in a star configuration, consuming three times less current than in normal operation (delta). Once the motor reaches a certain speed, it switches to the delta configuration.
Wound Rotor Induction Motor
Wound Rotor Induction Motor: Used for applications requiring high torque. The terminal box has 6 normal terminals and 3 additional terminals (KLM). Designed to operate briefly in a shorted condition to protect the switchgear and drivers.
Rotor Resistance Starting
Rotor Resistance Starting: Designed to operate with the rotor winding shorted. Starting with this connection results in very high current consumption. Starting in stages reduces the current. These motors have 9 terminals: 6 for the stator winding and 3 for the rotor winding.
Asynchronous Motor Starters by Progressive Starters
Asynchronous Motor Starters by Progressive Starters: These are power electronics devices that allow induction motors to start gradually, limiting current spikes during startup. They have a power block and a control block.
Induction Motor Braking
Induction Motor Braking: In some applications, it’s necessary to stop the motor abruptly. This can be achieved through:
- Current Injection Braking: Applying a temporary, small DC voltage to the stator generates a fixed magnetic field that brakes the rotor.
- Electromechanical System: An electromechanical system applies friction to the motor shaft to stop it.
- Plugging: Feeding the motor with two inverted phases creates a torque opposite to the normal operation. The normal engine operation is done by connecting KM1. When this is disabled, the motor disconnected from the mains supply and automatically enters KM2, in this situation the engine is fed THROUGH resistance, braking immediately.
Motor Speed Regulation
Motor Speed Regulation of AC Motors: N = 60XF / P, meaning that changing the number of pole pairs (P) or the frequency (F) changes the speed (N).
Changing the Number of Poles
Exchange Rate Variation of Number of Poles: If a motor has different speeds, the winding is designed to allow selection of the desired speed. These types of motors have 2 terminal boxes, one for each winding. Two common methods are:
- Motor with 2 separate windings
- Motor with shared windings (Dahlander motor)