The Fleming s Left Hand Rule shows what happens when electrons in a current enter a magnetic field.

Transcription

1 M4: Electrical Actuators M4.1 Fleming s Left Hand Rule The Fleming s Left Hand Rule shows what happens when electrons in a current enter a magnetic field. According to this rule if the index finger is pointed in the direction of a magnetic field, and middle finger, is at a 90 degree angle to your index and points in the direction of the current, then your extended thumb points in the direction of the exerted force. M4.2 Right-hand grip rule for magnetic field

2 The right-hand rule for determining the direction of the magnetic field states that if you wrap your right hand around a wire with the thumb pointing in the direction of the electrical current, the fingers would be in the direction of the magnetic field. M4.3 Determine the direction of rotation of the given motor N S M4.4 DC Motors A DC motor runs on direct current (DC) electricity. It consists of two main parts. These are, 1) Stator 2) Rotor. The stator consists of the housing, permanent magnets, and brushes. The rotor consists of the output shaft, windings and commutator. The image below shows a sectional view of a dc motor by General Electric Co. Cutaway view of typical dc motor (General Electric Co.)

3 A simplified version of a DC motor is shown below. S N There are two main types of DC motors. Brushed DC motor The brushed DC electric motor generates torque directly from DC power supplied to the motor by using commutation, stationary magnets (permanent or electromagnets), and rotating electrical magnets. Brushless DC motors In brushless DC motors permanent magnets are placed on the rotor and the electromagnets are moved to the stator. A controller board (with high-power transistors) is used to charge up the electromagnets as the shaft turns. The brushless motor does not have a commutator. M4.5 DC motor control To control a DC motor a current and a direction has to be given as inputs to a motor. A current is applied to the motor circuit by using a voltage. The amount of current input to the motor controls the output power. It controls the torque and speed of the motor. In order to control the current you can apply a voltage or a specific PWM signal. In order to control the direction we can use an H bridge. Controlling the speed or torque There are two main methods used in controlling the voltage. 1. By applying a direct voltage Under this method a direct voltage between 0 V and maximum voltage (eg. 24 V etc.) is applied to the motor. 2. By applying a voltage as a pulse width modulation (PWM) signal Under this method voltage is applied as a waveform. The PWM method is very efficient for controlling dc motors.

4 Controlling the direction An H bridge is used to control the direction of a DC motor. H-bridge is named so as it has four switching elements at the "corners" of the H and the motor forms the cross bar. An H bridge is built with four switches. By choosing the appropriate switches to close current could be channeled to obtain the movement of the rotor in the desired direction. A simplified H bridge circuit L298 H-Bridge Motor Driver circuit

5 H-Bridge Control of a DC motor in clockwise direction H-Bridge Control of a DC motor in counter clockwise direction