ElectromechanicsLearningYear 2

Electromechanics: Rotating Machines

electromechanics4

An example of a rotating machine is depicted in Figure 1. It consists of a stator, rotor and the air gap between them. Also you can see the stator and rotor windings  in Figure 1. The rotor is installed into the stem, and the stem connects to the motor and any other loads. The windings are there to carry the electrical current that generates magnetic fields for the electrical load. The closed loops for voltages can be generated there.

Figure 1. The simplified scheme of a rotating electrical machine

A distinction can be made by the types of windings. The current can operate in the rotating machine for magnetic field generation – this current is called magnetising. This tipe of winding is called field winding.

The current is low power DC, and the windings can also carry the load current, and will be called Armature. In DC and AC machines, windings that carry magnetising current and load current are different. However, in some machines the same windings can carry the load and magnetising currents – this happens in induction motors. This winding is called the primary winding. The output winding is the secondary winding.

When talking about energy conversion, electric machines can be classified in the following way:

  • Generator – a machine that creates the electrical energy from mechanical;
  • Motor – machine that converts the electrical energy to the mechanical.

The classification of a rotating machine is the following:

  • DC machines: direct-current machines;
  • Synchronous machines: here the DC current goes through one winding, and AC current through the other winding;
  • Induction machine: here AC current goes through both

In order for the rotating machine to turn and generate electrical currents, the permanent magnets are used, as is the AC and DC input currents. The magnetic field creates the torque in the electric motor and the electromagnetic laws help the generator to create electric current in the magnetic field.

Let’s consider physics that occur in electrical machines. In all electric machines the force on the wire is f=iw[I,B], where iw – is the current through the wire, and B is the magnetic field. The torque at a minimum on the coil is T=KBiwsinα, here the K is the coefficient, depending on the geometry of the coils, α is the angle between the magnetic field B and the current. There are two fields that are generated – in the stator and rotor. The stator magnetic field generates the magnetic field that is described with the following formula: B=μNi2πR.

Rotating machines are energy conversion machines that are characterised with efficiency and energy losses. The generator and rotor may be characterised with different types of losses when the direct current passes through. The losses in rotating machines are:

  • Electrical losses
  • Core losses
  • Mechanical lossesGenerator and motor losses can be classified the following way:
    • Rotational losses
      • Non-load rotational losses
      • Stay load losses
    • Armature -circuit copper losses
      • Armature losses
      • Brush losses
    • Field copper losses
      • Series-field losses
      • Shunt-field losses

However, they have different distribution and value for generator and motor loss structure. Electrical losses usually occur because of the DC resistance. Mechanical losses usually occur because of the friction or windage. The losses may also occur in rotation machines for cooling purposes. Open-circuit core losses consist of hysteresis and eddy current losses.

The efficiency of a rotation machine is usually expressed with an efficiency map. The efficiency map consists of torque-speed characteristics. The torque-speed characteristics vary for different rotation machines, and depend on the rotation speed of the machine. The torque-speed characteristics resemble with the voltage-current characteristics of the power source of the circuit.  

The torque-speed characteristics determine the actual motor speed, connected to the load. An important fact is that the motor can produce a non-zero torque-speed. This is because the motor is connected to the electrical source. And this electrical source can be the reason behind some of the motor torque. This is called starting torque.

It is importnat to care about the typical working parameters that are indicated on the motor. Mentioned on the nameplate is type of device, the manufacturer, rated voltage and frequency, rated current and volt-amperes, rated speed and horsepower. In this case the rated voltage tells us the voltage value needed to produce the required magnetic flux. High frequency operation will increase the magnetic core losses.

The rated current and rated volt-amperes are the currents and power required for stable work and for the motor to not overheat during the operation. Peak power operation may exceed the motor power, torque, and other characteristics, but that will ultimately lead to motor overheating and operation failure.

Another important factor to consider is the voltage and speed regulation of electric machines. The regulation creates the possibility to keep voltage or speed still during operation with a changing load for the motor. How to calculate the important ratings for the rotating machines are:

(SR)  speed regulation = S0Speed at zero loadSLSpeed at certain loadSLSpeed at certain load  (VR)voltage regulation= V0Voltage at zero loadVLVoltage at certain loadVLVoltage at certain load

Resuming the opeartion of rotating machines, we can see that magnetic attraction and repulsion helps to generate the mechanical torque of a rotation machine. The magnetic field can also generate the voltage and curent in the windings of a rotating maching.

In the case of our rotating machine, when current flows through the conductors in the magnetic field, it produces certain torque and the rotating structure rotates with a certain velocity. In this case the wires that go to the rotating machine rotates too, producing the oppositional electromotive force. If the rotating machine is connected to some mechanical action source, rotation produces the electromotive force that moves in the magnetic field, which generate electrical currents through the conductors.

The rotating machines have magnetic poles. And the torque is generated by a set of magnetic forces of attraction and repulsion between magnetic poles of the stator and rotor. (Figure 2).

Figure 2. The scheme of the rotating machines, which shows the magnets of rotor and stator. The stator magnet is static, the rotor magnet is moving.

 

Leave a Reply