Transcript Document
Fundamentals of Electric Drives: DC Drives Separately excited DC drive Shunt DC drive Power sources are variable DC voltage sources: 3phase or 1phase controlled rectifier (converter) or DC-DC converter. This allows torque and speed control within an broad range, by • varying terminal/armature voltage •Varying field current/voltage •Reversing direction of armature or field voltage or current. A DC Drive •Friction is negligible DC Drive control Generation (forward) Speed w No load speed Va/(KIf) Braking Forward motoring Torque Reverse motoring No load speed with reversed armature voltage or field current Braking Generation (Reverse) Modes of drive operation • Motoring (forward or reversed) – source supplies energy to the armature; • Dynamic braking (or dynamic plugging) – source is disconnected and energy in the winding dissipates across an Rbrake; • Regenerative braking (forward and reverse) – an excess energy in the armature winding is used to charge the power source. Is used to quickly stop the motor by reversing polarity of Vf and Va and operating the source as an inverter – Charging current creates a large braking torque. • These modes are used to control DC drives, e.g, to reduce speed: The motor normally operates in reverse or forward motoring mode, but can be switched to plugging or regenerative braking mode when necessary. Four Quadrants of Control “Reverse generation” Speed control using 3phase controlled converters: Forward Motoring (1st Quadrant) • • At full speed, the firing angle is usually at least 15-20 deg, The converter supplies Va equal to (these are equations for 3 phase full converter) • • • • • 3 3 Vm,0 pk cos a 3 6 V phase , RMS cos a 3 2 Vline, RMS cos a 1.35Vline, RMS cos a Va>Eg by an amount of equal to Ra*Ia drop. To reduce speed, the firing angle must be increased, this will immediately reduce Va. Interruptions in the instantaneous source current through the thyristors increase, and the motor “coasts” through the “no current” intervals to a lower speed. Average current Ia remains positive at all times. As speed reduces, Eg gradually reduces, and eventually it becomes lower than the new value of Va. This process of voltage Eg reduction continues until the developed torque restores to the original value (or until the output power reaches the power demand at a lower speed). The developed torque builds up and the average current Ia settles to the new value at a lower speed. Va Simulations using Matlab/Simulink 3-phase full converters angles: A=60 deg, F=90 deg. If the input power reduces or increases, Va needs to be controlled to restore speed. Note Ia>0, and Va>Eg at all times (forward motoring). dcmotor_full_3phase_forward_motoring.mdl, Torque 2 1200, A=60 and 62 deg. Forward Regeneration (4th Quadrant) • When torque reduces by a large amount, reduction of Va may cause the drive “enter” 4th quadrant, i.e, Ia may become negative for a short period of time while w>0. • E.g., in the previous example, this occurs when load torque drops from 1500Nm to 500Nm, and to restore original speed, A must change from 60 deg to 66 deg; I.e., Va experiences a significant deep. Simulations using Matlab/Simulink: Forward Generation (4th Quadrant) The load torque reduces significantly, from 1500Nm to 500Nm. Va needs to be controlled to restore speed. Increase Armature converter angle: A from 60 deg to 66 deg. Note Ia<0, for a short time, then it restores to a positive value, and drives returns to forward motoring. Two quadrant speed control (1st & 2nd Quadrants) • A quicker speed reduction may be required when load torque changes sign (e.g., a locomotive goes over the hill, a pump suction needs to be reversed). • This can be achieved using dynamic plugging (i.e, letting the armature winding discharge via an insert rersistor) or using reverse regenerative braking (preferred for large motors, as this allows kinetic energy be converted to electric energy.) • To take advantage of reverse regenerative braking, the converter must be operated as inverter, to allow current flow from the armature back to the 3 phase network – Polarity of Va must be reversed; – Polarity of Eg must also be reversed; this achieved by reversing polarity of the filed current, or changing commutation of the armature winding. – Finaly |Va| must be made smaller than |Eg|, to allow for regeneration. • These are conditions for the 2nd quadrant. Simulations using Matlab/Simulink: Regenerative Braking (2nd Quadrant) After torque is reversed, and the drive speeds up, Va and Eg are reversed (by reversing If), Va with A=120 deg. Next, the armature converter angle A was reduced to 102 deg to reduce |Va| below |Eg| while keeping polarity of |Va| reversed. Note Ia>0, 0>Va>Eg, and |Va|<|Eg|. That is the armature supplies power, while running at original speed. dcmotor_full_3phase_forward_mototing.mdl Reverse motoring • Required when load torque changes direction (eg, fans, pumps). Then speed must also change to supply power. • Va must be controlled to provide required speed and direction of rotation. • Reverse motoring can be achieved by delaying thyristors for more than 90 deg. Simulations using Matlab/Simulink: Reverse Motoring (3rd Quadrant) After torque reverses, increase armature converter angle: A to 120 deg (i.e, beyond 90 deg), to reverse the polarity of Va. Note Ia<0, 0>Eg>Va, and |Va|>|Eg|, that is the armature converter supplies power, but speed is reversed. dcmotor_full_3phase_forward_mototing.mdl Regenerative braking with positive torque • Is required when torque does not change, but direction of rotation needs to be reversed (hoists, elevators, etc.) • To reverse speed and keep it constant but negative, the polarity of Va is reversed by delaying thyristors beyond 90 deg. • The armature develops a braking torque which eventually reverses rotation. At steady state Ia>0, 0>Va>Eg, and |Va|<|Eg|. That is, the armature charges the source. • These are conditions for regenerative braking, but without reversing field. Regenerative Braking (2nd Quadrant), with positive torque The speed needs to be reversed, but torque remains positive (“lower the hoist by reversing the motor”). Va must be controlled to reverse speed and keep it const. Change armature converter angle A to 102 deg to reverse |Va|, and keep the polarity of Va reversed. Note Ia>0, 0>Va>Eg, and |Va|<|Eg|. That is the armature supplies power, but speed is reversed. DC-DC converters DC-DC Converter classification • First quadrant converter • Second quadrant converter • 1st and 2nd quadrant converter • 3rd and 4th quadrant converter • Four quadrant converter 2 1 3 4 1-2 and 3-4 quadrant converters 1st quad: S1, D4 2nd quad: S4, D1 3rd quad: S3, D2 4th quad: S2, D3 Polarity of the load EMF is reversed.