Types of motors and their mode of operation
Mode of operation of DC motors
The basic components of a DC motor are as follows
- The field magnet, which is also called the stator
- The rotatable armature, which is also referred to as a rotor or runner
- The collector and the carbon brushes that the Supply armature winding with electrical energy
There are different ways of explaining the functional principle of a DC motor.
The rotor is located in the magnetic field of the stator, i.e. the field magnet (permanent or electromagnet). The anchor consists of several current-carrying conductors. If current-carrying conductors are in a magnetic field, forces act. Since the rotor is rotatably mounted in the magnetic field, the forces acting outside the axis of rotation cause a torque. This torque then causes the rotor to rotate.
Another approach would be that a magnetic field is built up by the field magnet, i.e. the stator. In the field, the rotor is therefore the rotatably mounted electromagnet. The armature is connected to a voltage source via sliding contacts called carbon brushes. The current flowing in the armature ensures that magnetic poles are formed in the armature. A torque is generated by the magnetic field in the stator and the magnetic field in the armature. Unequal magnetic poles attract and identical magnetic poles repel each other. The resulting attractive and repulsive forces between the magnetic poles ensure that a rotary movement occurs.
Three-phase asynchronous motor
The most frequently installed electric motor worldwide is the three-phase asynchronous motor. Its greatest advantage compared to other electric motors is that it does not have sliding contacts. This makes it extremely robust in terms of its construction, very low maintenance and operationally reliable. Asynchronous motors are built in the most varied of power sizes, so the smallest three-phase asynchronous motors have a few watts up to the large drives with tens of megawatts. To prevent eddy currents and the associated losses due to the rotating magnetic field, both main parts, i.e. the stator and the rotor, are laminated. This means that instead of a single iron core, it is built up from several individual sheets of metal that are isolated from one another. These laminated iron cores contain grooves that are evenly distributed over the circumference and into which the respective windings are introduced (stator and rotor winding). The stator and the rotor are separated by a small air gap.
In most cases the stator winding is designed as a symmetrical three-phase winding. A three-phase winding consists of three individual winding strands which, in a motor with a number of pole pairs of p = 1, are spatially offset from one another by 120 °. & Nbsp; The number of pole pairs determines the speed of the motor. With a number of pole pairs of p = 1 and a frequency of 50 Hz, the rotating field has a speed of 3000 rpm (synchronous).
The speed can be calculated as follows:
nd = f * 60 / p
nd = rotating field speed
f = frequency
p = number of pole pairs
The three individual strings are connected either in a star or in a delta, depending on the voltage available. & nbsp;
Let us now consider the runner. There are 2 different versions. On the one hand the three-phase asynchronous motor with slip-ring rotor and the three-phase asynchronous motor with squirrel-cage rotor. The slip ring rotor also has a three-phase winding in the rotor, which is connected in star or delta. The switching ends of the rotor winding are made accessible via three sliding contacts, i.e. one sliding contact for each winding phase.
The 50 Hz rotating magnetic field in the stator generates a magnetic field in the armature (principle of induction). The ball-bearing armature is now set in rotation by the magnetic field. The speed of the armature is always below the rotating field speed at nominal load. This speed difference is referred to as slip.
