STEADY STATE OPERATION OF DC MACHINES

Electric DC machines, as indeed any other type of electric machine, can be used to either produce electric energy from the input mechanical energy, or to convert electric energy into output mechanical energy. These two possible operating regimes are called generation and motoring.
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STEADY STATE OPERATION OF DC MACHINESENGNG2024 Electrical Engineering STEADY STATE OPERATION OF DC MACHINES1. INTRODUCTION Electric DC machines, as indeed any other type of electric machine, can be used toeither produce electric energy from the input mechanical energy, or to convert electric energyinto output mechanical energy. These two possible operating regimes are called generation andmotoring. As already mentioned, DC machines used to be in the past the major source of DCpower. In order to produce electric power DC machines were operated as generators.Nowadays, however, use of DC generators is becoming more and more rare. DC power isobtained instead by means of power electronic converters. The remaining applications of DCmachines are today restricted to motoring. In this operating regime a DC machine is operatedas a DC motor: it consumes DC power, while delivering at its shaft mechanical power. Theshaft drives a certain load, that is characterised with the load torque. A DC machine consists of stationary part, called stator, and rotating part, called rotor.Both stator and rotor are equipped with one winding. Stator winding is supplied from a DCvoltage source and the role of this winding is to produce magnetic flux in the air gap of themachine. This flux is stationary in space. Rotor winding is again supplied from a DC voltagesource (for motoring): DC supply is connected to the rotor winding through a specialassembly, that is composed of brushes and commutator. Brushes are stationary, whilecommutator is fixed to the rotor and hence rotates together with the rotor. This assemblyenables supply of electric power from stationary power supply to rotating winding on rotor.Principle of operation of this assembly is illustrated by means of Fig. 1. Rotor winding is shownin a very simplified manner, as consisting of just one coil, connected to two segments of thecommutator. Motoring action is assumed and the current is therefore delivered to the rotorwinding through the stationary brushes and rotating commutator. Two positions of the rotorwinding are shown in Fig. 1. Terminal current (current brought to the brushes) and the windingcurrent are illustrated in Fig. 2. As can be seen from these two figures, current inside the rotorwinding is reversed (commutated) after each half-revolution of the rotor. Current inside therotor is therefore AC, while the terminal current is DC. Frequency of the current inside therotor winding equals frequency of rotation. θ θ A B ia = Ia ia ia = Ia ia i=Ia i=−Ia B A Fig. 1 - Current reversal (commutation) in rotor coil by means of the commutator. E Levi, 2001 1ENGNG2024 Electrical Engineering ia Ia i Ia 0 θ = ωt 0 π 2π θ=ωt − Ia Fig. 2 - Terminal current and current through rotor coil. Note that such a situation regarding frequencies in the two windings is the only possibleone that satisfies the condition of average torque existence. Since the stator winding is suppliedwith pure DC current of zero frequency, the machine can develop an average torque if andonly if the rotor winding frequency equals the frequency of rotation. This means that it is notpossible to realise an electric machine with DC currents flowing in both stator and rotorwindings. Such a situation would result in the possibility of developing an average torque atzero speed only. At zero speed however converted power equals zero and therefore such amachine could not do the process of electromechanical energy conversion. Let the stator winding, which is called excitation or field winding as well, be suppliedwith constant DC voltage equal to Vf . Current that flows through this winding is in steady-state operation determined withIf = Vf /Rf (1)Flux produced in the air gap of the machine is, neglecting saturation of the magnetic circuit,proportional to this current. HenceΦf = c1 If (2)It has to be emphasised that the excitation winding can be replaced with permanent magnets.Many of the modern DC motors rely on permanent magnet excitation and in s ...