ENB205 Electrical and Computer Engineering Week 3.dotx.docx

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Negareh Ghasemi

ENB205 Lecture Electrical and Computer Engineering Permanent Magnet DC Motors Can only generate small amount of torque because the field relies upon ‘residual’ magnetic flux in the permanent magnets. Current in the machine tends to cause a reaction in the field, demagnetising the field poles. Only good for small applications Armature Reaction Current in the rotor works to increase or decrease flux density at trailing/leading edges of the magnet. This reaction works to weaken the magnets in a PMDC motor. Air-Gaps By increasing the air-gap between rotor and poles at the edges of the pole faces, the magnetic field lines are less inclined to ‘bunch up’ reducing the armature reaction. 1 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering B-H Curve De-Magnetisation Ferromagnetic B-H curves are not the same going up, as down. PM Material needs to have a high residual flux density and a high magnetic intensity to demagnetise. PM Material Choice Neodymium-iron-boron (rare earth magnets) have both a very high H, and high maximum residual flux density. NdFeB good choice for PM motors. Speed-Torque (PMDC) Straight line –ignoring armature reaction Armature reaction works to weaken flux, lower flux means lower induced EMF and hence the motor can spin faster for the same terminal voltage. Speed Control (PMDC) Since terminal Voltage is proportional to speed of the motor, assuming the same torque is applied:  Decrease in terminal voltage, decrease in speed  Increase in terminal voltage, increase in speed. Wound Field DC Machine Wound Field Poles A much higher flux density can be achieved in the same material, if magnetising force is applied. Wound field poles have the capacity to generate higher flux density and higher torque. Cost is they require constant current to maintain magnetic field intensity, losses in field windings. Advantage is they have a controllable field density, will not demagnetise and can be wound to negative the effect of armature reaction. Shunt Field Winding Shunt machine: Field winding is placed across same Separately Excited: Field winding is placed across a different supply as the rotor (parallel). supply and controlled independently. Note that the winding has an inductance, and resistance. Remember an inductor is simply a coil of wire, and that inductors in DC act as a dead short. If the voltage for the two windings is identical, there is no difference between Shunt and Separately Excited machines. 2 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering Speed-Torque (Shunt) Straight line, still suffers from armature reaction, however can be countered by compensating windings. Speed does not decrease significantly with increased load, and hence is said to have good ‘speed regulation’ Small difference in speed, good speed regulation. Speed Regulation Speed regulation is the difference between the ‘no-load’ speed and the ‘full load’ speed, compared to the full load speed. No-load speed is the theoretical maximum speed. Full-load speed is the speed when the armature current is at its rated value. Speed Control (Shunt) Controlling field current Increase Radjwill:  Decreases field current  Decreases flux per pole  Decreases e  Allows more current to flow in armature, allowing the motor to accelerate to a new equilibrium Increasing the field resistance has an effect on the speed-torque curve, making it steeper. At very high loads the decrease in flux will not cause sufficiently high enough increase in current and the torque output will actually decrease, slowing the motor. 3 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering Magnetising Curves Always provided at ‘rated’ or ‘full-load’ speed Changing the field current, does not translate to a linear change in speed for the Shunt DC Motor. Due to saturation of the magnetic material used to create the field poles, the B-H curve is not linear, and hence the back-EMF generated due to the field current is not linear. A magnetisation curve is often used to predict
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