Electrical Drives I. Week 4-5-6: Solid state dc drives- closed loop control of phase controlled DC drives

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Jade Lester
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1 Electrical Drives I Week 4-5-6: Solid state dc drives- closed loop control of phase controlled DC drives

2 DC Drives control- DC motor without control Speed Control Strategy: below base speed: V t control (terminal voltage control) above base speed: flux control via V f control (field weakening) For the separately excited dc motor, assuming that the field excitation is held constant, the transfer characteristic between the shaft speed and the applied voltage to the armature can be expressed as: The feedback is a naturally existed loop which is physically present in the motor itself. Normally, speed is the subject and is the variable to be controlled, thus the input signal is NOT voltage. V a E a - 1 Ra L a s I a K T K E Td Tl - 1 Js F If the load torque is neglected, the transfer characteristics between the motor speed ω and the applied terminal voltage V t ω V t = sl a + R a K T Js + F + K E K T

3 DC Drives control- DC motor without control The characteristic roots can of the pervious equation can be determined as: s + 1 τ a s + F J + 1 τ a τ m Where: τ a = electrical time constant = L a R a τ m = mechanical time constant = R a J K E K T The un-damped natural frequency and damping ratio can be given as: ω m = 1 τ a 1 τ m + F J σ = ζω m = τ a + F J The speed response over the whole operating range upon the application of a load torque can be determined by: Δω ΔT L = 1 + sτ a 1 + sτ a Js + F + K EK T R a

4 Closed Loop Control of DC Drives Closed loop control is when the firing angle is varied automatically by a controller to achieve a reference speed or torque This requires the use of sensors to feed back the actual motor speed and torque to be compared with the reference values DC motor control normally employ a two loop control technique. This has an inner feedback loop to control the current (and hence torque) and an outer loop to control speed. When position control is called for, a further outer position loop is added. Reference signal + Controller Plant Output signal Sensor Additional Control variables: Protection Enhancement of response fast response with small overshoot Improve steady-state accuracy Main Control variables: Speed Torque Position

DC Drives control Normally, a typical drive system would look like the following assuming the following scenario: If the motor running light at a set speed and the speed reference signal is suddenly

5 DC Drives control Normally, a typical drive system would look like the following assuming the following scenario: If the motor running light at a set speed and the speed reference signal is suddenly increased, the reference speed is now greater than the actual speed and there will be a speed error signal, represented by the output of the left-hand summing point. A speed error indicates that acceleration is required, which in turn means torque, i.e. more current. The speed error is amplified by the speed controller and the output serves as the reference or input signal to the inner control system. The inner feedback loop is a current-control loop, so when the current reference increases, so does the motor armature current, thereby providing extra torque and initiating acceleration. As the speed rises the speed error reduces, and the current and torque therefore reduce to obtain a smooth approach to the target speed. Schematic diagram of analogue controlledspeed drive with current and speed feedback control loops

6 Closed Loop Control of DC Drives Cascade control structure Control variable of inner loop (eg: speed, torque) can be limited by limiting its reference value Torque loop is fastest, speed loop slower and position loop - slowest

Closed Loop Control of DC Drives-Torque loop Inner Torque (Current) Control Loop: Current control loop is used to control torque via armature current (i a ) and maintains current within a safe limit

7 Closed Loop Control of DC Drives-Torque loop Inner Torque (Current) Control Loop: Current control loop is used to control torque via armature current (i a ) and maintains current within a safe limit Amplifies the difference (or current error), and using the resulting amplified current error signal (an analogue voltage) to control the firing angle α and hence the output voltage of the converter Accelerates and decelerates the drive at maximum permissible current and torque during transient operations Torque (Current) Control Loop. Could be in terms of torque or current Proportional plus integral controller (PI) are usually employed for zero steady state error operation Feedback current signal is obtained either from a d.c. current transformer, or from a.c current transformer/rectifiers in the mains supply lines

8 Closed Loop Control of DC Drives-Torque (current) loop The inner current loop is usually employed for current limitation. As long as the current control loop functions properly, the motor current can never exceed the reference value. Hence by limiting the magnitude of the current reference signal (by means of a clamping circuit), the motor current can never exceed the specified value. For small errors in speed, the current reference increases in proportion to the speed, thereby ensuring linear system behavior with a smooth approach to the target speed. If the speed error exceeds a limit, the output of the speed-error amplifier saturates and there is thus no further increase in the current reference. By arranging for this maximum current reference to correspond to the Electronic current limitation! full (rated) current of the system there is no possibility of the current in the motor and converter exceeding its rated value, no matter how large the speed error becomes.

9 Closed Loop Control of DC Drives- Speed Loop Cascade control structure Speed Control Loop: Ensures that the actual speed is always equal to reference speed * Provides response to changes in *, T L and supply voltage without exceeding motor and converter capability Proportional plus integral controller (PI) is usually employed Speed Control Loop Speed feedback is provided by a d.c. tachogenerator. Actual and reference speeds are fed into the speederror amplifier

10 Closed Loop Control of DC Drives- Speed Loop With the motor at rest (and unloaded for the sake of simplicity), we suddenly increase the speed reference from zero to full value. The speed error will be 100%, so the output I ref from the speed error amplifier will immediately saturate at its maximum value, which has been deliberately clamped so as to correspond to a demand for the maximum (rated) current in the motor. The motor current will therefore be at rated value, and the motor will accelerate at full torque. Speed and E a will therefore rise at a constant rate, the applied voltage V t increasing steadily so that the difference (V t E a ) is sufficient to drive rated current through the armature resistance. The output of the speed amplifier will remain saturated until the actual speed is quite close to the target speed, and for all this time the motor current will therefore be held at full value. Only when the speed is within a few percent of target will the speed-error amplifier come out of saturation. Thereafter, as the speed continues to rise, and the speed error falls, the output of the speed-error amplifier falls below the clamped level. Speed control then enters a linear regime, in which the correcting current (and hence the torque) is proportional to speed error, thus giving a smooth approach to final speed.

11 Closed Loop Control with Controlled Rectifiers Two-quadrant Two-quadrant Three-phase Controlled Rectifier DC Motor Drives Field is separately excited and field supply is kept constant or regulated, depending on the need for field weakening Most important part is the design of the speed and current controllers time constants. Current Control Loop System without motor model Speed Control Loop

Closed Loop Control with Controlled Rectifiers Two-quadrant Actual motor speed m measured using the tachogenerator (Tach) is filtered to produce feedback signal mr To do list The reference speed r *

12 Closed Loop Control with Controlled Rectifiers Two-quadrant Actual motor speed m measured using the tachogenerator (Tach) is filtered to produce feedback signal mr To do list The reference speed r * is compared to mr to obtain a speed error signal The speed (PI) controller processes the speed error and produces the torque command T e * T e * is limited by the limiter to keep within the safe current limits and the armature current command i a * is produced i a * is compared to actual current i a to obtain a current error signal The current (PI) controller processes the error to alter the control signal v c v c modifies the firing angle to be sent to the converter to obtained the motor armature voltage for the desired motor operation speed Design of speed and current controller (gain and time constants) is crucial in meeting the dynamic specifications of the drive system Controller design procedure: 1. Obtain the transfer function of all drive subsystems a) DC Motor & Load b) Current feedback loop sensor c) Speed feedback loop sensor 2. Design current (torque) control loop first 3. Then design the speed control loop

Transfer Function of Subsystems DC Motor and Load Assume load is proportional to speed T L B L m DC motor has inner loop due to E a magnetic coupling, which is not physically seen.

13 Transfer Function of Subsystems DC Motor and Load Assume load is proportional to speed T L B L m DC motor has inner loop due to E a magnetic coupling, which is not physically seen. This creates complexity in current control loop design due to the cross coupling. These two loops need to be decoupled by reconfiguring the block diagram System without motor block diagram System with motor block diagram System without control

14 Closed Loop Control with Field Weakening Two-quadrant Motor operation above base speed requires field weakening Field weakening obtained by varying field winding voltage using controlled rectifier in: single-phase or three-phase Field current has no ripple due to large L f Consists of two additional control loops on field circuit: Field current control loop (inner) Induced emf control loop (outer): Induced emf loop is estimated and is sensitive to parameter variation thus needs to be adaptive and also depends on the type of motor

15 Closed Loop Control with Field Weakening Two-quadrant Field weakening

16 Closed Loop Control with Field Weakening Two-quadrant Field weakening Field current controller (PI-type) Estimated machine - induced emf e V a R a i a L a dia dt Induced emf reference Induced emf controller (PI-type with limiter) Field current reference

17 Closed Loop Control with Field Weakening Two-quadrant The reference e a is compared to e a to obtain the induced emf error signal (for speed above base speed, e a kept constant at rated emf value so that φ 1 ω) The e a (PI) controller processes the error and produces the field current reference i f i f is limited by the current limiter to keep within the safe field current limits i f is compared to actual field current i f to obtain a current error signal The field current (PI) controller processes the error to alter the control signal v cf (similar to armature current i a control loop) v cf modifies the firing angle f to be sent to the converter to obtained the motor field voltage for the desired motor field flux

18 Closed Loop Control Four-quadrant Four-quadrant Three-phase Controlled Rectifier DC Motor Drives Control operation of either converter 1 or converter 2

19 Closed Loop Control Four-quadrant Control very similar to the two-quadrant dc motor drive. Each converter must be energized depending on quadrant of operation: Converter 1 for forward direction / rotation Converter 2 for reverse direction / rotation Changeover between Converters 1 & 2 handled by monitoring Speed Current-command Zero-crossing current signals (to transfer control from one converter to another Speed and current loops shared by both converters

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