Electrical Machines Exam - Theory PDF

ELECTRICAL MACHINES EXAM – THEORY 1. Four basic principles describe how magnetic fields are used in these devices A current-carrying wire produces a magnetic field in the area around it. A time-changing magnetic fi eld induces a voltage in a coil of wire ifit passes through...

ELECTRICAL MACHINES EXAM – THEORY 1. Four basic principles describe how magnetic fields are used in these devices A current-carrying wire produces a magnetic field in the area around it. A time-changing magnetic fi eld induces a voltage in a coil of wire ifit passes through that coil. (This is the basis of transformer action.) A current-carrying wire in the presence of a magnetic field has a force induced on it. (This is the basis of motor action.) A moving wire in the presence of a magnetic field has a voltage induced in it. (This is the basis of generator action.) 2. Force induced in a conductor A conductor though which the i current flows situated in a magnetic field of B density will develop a force F, given by the relation below (l being the total length of the conductor situated in the magnetic field): 3. Faraday's Motional emf Expression The magnitude of the electromagnetic induction is directly proportional to the flux density B, the number of loops giving a total length of the conductor I in meters and the rate or velocity, v at which the magnetic field changes within the conductor in meters/ second or m/ s, giving by the motional emf expression : 4. Transformer load operation 5. Secondary parameters referred to primary It is done in order to eliminate the transformation ratio from the equations. The secondary winding is replaced by a fictional one having NI turns and all parameters recalculated in order not to influence the operating mode. It is assumed that the fluxes, powers and voltage drops of the secondary winding remain the same after the secondary parameters are reffered to the primary. To reffer the secondary e.m.f. to the primary, from the condition that the maximum useful flux value to be constant, it results: 6. Transformer equivalent diagrams T-shape equivalent diagram 7. Parallel operation of transformer - It is necessary for ensuring the continuity in energy supply for the consumers when one of the transformers is out of operation because of a fault or due to repairs or revisions. - Moreover, when the supplied load varies throughout the day, more transformers operating in parallel ensure a better overall efficiency. - Two or more transformers operate in parallel when their primary windings are supplied from the same source and their secondary windings are connected to the same busbar form where the consumers are supplied. - In order not to have circulating currents between transformers and to have a load distribution (between parallel operating transformers) proportional to their nominal power, several conditions should be accomplished: - To have the same nominal voltages in primary and secondary - To have the same vector group and to be connected to the same terminals - To have the same short-circuit voltages. - Their nominal power ration not bigger than 3. 8. Induction Machine operating principle 9. Referring the motor quantities to the stator in case of the induction machine 10. Induction machine equivalent diagrams (+ power transfer) 11.Induction machine power losses + torque - When loaded, the 3— induction motor draws from the mains active 𝑃 12.Induction Machines Operations Mode 13.Induction Machine Starting and Speed control The process involves using a 3-phase rheostat in the rotor circuit of a wound rotor motor to decrease starting current and increase starting torque. During startup, the rheostat is set to maximum resistance, gradually reducing it as the rotor speed increases until it is short-circuited, allowing the motor to enter normal operation. 14.Induction Machines Generator and Brake Mode Braker Mode: In brake mode, the machine converts electrical and mechanical power to heat in the rotor circuit when the torque opposes rotation. Regenerative Braking: As the machine switches from motor to generator mode, it absorbs excess mechanical energy at the shaft, feeding it back into the grid. Plugging or reverse voltage braking: Quickly stops the mechanism in electric drive systems by altering phase interconnection and adding resistors to the rotor to limit current. Dynamic Braking: The stator, now powered by a DC source, generates a stationary magnetic field at its inner periphery. To the still-spinning rotor, this field appears as a revolving one. 15.Synchronous Machines Synchronous generators use rotors with either salient or non-salient poles. Salient poles protrude, while non-salient poles are flush. Non-salient poles suit two- and four-pole rotors, while salient poles are for four or more poles, common in low-speed hydro turbines. To reduce eddy current losses, rotor construction involves thin laminations. Salient pole rotors in low-speed hydro turbines may have multiple pole pairs for the required power frequency, featuring large diameters and short lengths. Synchronous machines (SM) and induction machines (IM) are both AC machines. IM rotors turn slightly slower than synchronous speed, making them asynchronous. SM rotors spin at synchronous speed. Both machine types create a revolving magnetic field through AC currents in the stator's three-phase windings.  Stator magnetic circuits and windings in both types are similar, generating a rotating magnetomotive force and field with a three-phase system of stator currents. Synchronous machine rotors may have excitation windings or permanent magnets. Excitation windings are supplied with DC currents to create the rotor magnetomotive force and flux. Alternatively, rotors may have built-in permanent magnets without windings. Rotor position defines the flux position. 17. Synchronous motors: Torque equation, torque-speed characteristic, v curves The Synchronous Motor Torque-Speed Characteristic Synchronous motors power constant-speed devices connected to larger power systems, making the terminal voltage and frequency constant regardless of load. This results in a steady-state speed that remains constant from no load to maximum torque, with a speed regulation of 0%. Pull-out torque, occurring at 90°, is typically three times the full-load torque of a synchronous motor. When torque exceeds this limit, known as slipping pole, the rotor slips behind the stator's magnetic fields, causing significant torque surges and severe vibrations. The Synchronous Motor V-curves: Minimum armature current on the curve occurs at unity power factor, with only real power supplied to the motor. At other points, reactive power is supplied or consumed. With field currents below the minimum, armature current lags, consuming reactive power. Above the minimum, armature current leads, supplying reactive power. Controlling field current allows control of reactive power in the power system. 18.DC machines: Commutation + formula for internal generated voltage Commutation is the process of converting the AC voltages and currents in the rotor of a dc machine to DC voltages and currents at its terminals. LAB 8 – PROBLEME