The Vector Control Principle Engineering Essay

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This chapter focuses on implementation of the method of vector control. The objectives of this chapter shall become familiar with the implementation of this method, to better understand some of the data recorded, to examine to what extent such control keeps the rotor flux constant during changes in load torque and speed, and to observe the improvement of the dynamic response to the scalar control method by simulation. Due to strong industry demand for machine tools world-wide there must be a high standard for quality control of electrical drives with high reliability and maintainability.

The above requirements are complete drive systems for AC machines and frequency converters with vector control system. Vector control is the analogy between AC machines and DC. The price of the solution is complicated by the coordinate transformation, the phases of conversion, and processing difficult transformation of the feedback signal. Practical realization of this system is complex and needs feedback from micro-computers and VLSI technology. In the vector control engineering world of AC motors has gained popularity and can be taken as a standard method the method of orientation in the field is done in different version practice. Vector control is applied to an induction machine fed by an inverter power source, voltage source or current operations when the control is achieved. Thus, the vector a control machine carries out the operation of four quadrants and achieves a high dynamic response. The induction motor is powered by a pulse width modulated (PWM) inverter thyristor power supply, the switching frequency is very low and always in the range of about 100 Hz - 1 kHz. Converters use ratings up to about 100kW, so that most of the more important applications are high dynamic performance servo controlled.

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Progress in the fields of power electronics and microelectronics has made it possible to realize high-performance control-techniques for a.c. machines. Vector control is such a technique with which it is possible to operate a.c. machines with dynamic performance comparable to that of DC. Machines. Among various vector controlled a.c. machines, squirrel-cage induction machine is of particular interest because of its ruggedness and low cost. In this chapter, transient and steady state performance of vector-controlled induction motor drive is investigated by Matlab simulation.

7.1 Vector Control Principle

In this section, Figure 7-1 is used to illustrate the principle of vector control. In this frame of reference control method, a dq coordinate fixed to the space vector of rotor flux is used to achieve decoupling between the motor flux and torque. Stator direct axis current and quadrature axis current can be controlled separately, respectively, as in a DC motor.

Figure (7.1): Field-oriented variable-frequency induction motor drive

As shown in Figure ( ), the induction motor is powered by a current-controlled PWM converter, which operates as a three-phase sinusoidal current source. The motor speed w is compared to the w* reference and the error is generated by the speed control

producing a torque command Te *. As shown below in Figure (7.2), the rotor flux and torque can be controlled separately by the stator direct axis current ids and quadrature- axis current iqs, respectively.

Figure (7.2) field-oriented control principle

The quadrature of the axis stator current reference iqs* is calculated from torque reference Te * as

(7.1)

Where

Lr is the rotor inductance

Lm is the mutual inductance

|r|est is the estimated rotor flux linkage given by

(7.2)

Where

r = Lr / Rr is the rotor time constant.

The stator direct-axis current reference ids* is obtained from rotor flux reference input

|r|*

(7.3)

The position ofe rotor flux required for the transformation of coordinates is generated from the rotor speed m and slip frequency SI:

(7.4)

"The slip frequency is calculated from the stator reference current iqs* and the motor parameters".

(7.5)

The iqs* and ids* current references are converted into phase current references ia*, ib*, ic* for the current regulators. The regulators procedure the measured and reference currents are produced inverter gating signals. The function of the speed controller has to maintain a motor speed equal to the speed reference input in steady state and to provide excellent dynamic during transients. This can be proportional-integral.

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7.2 Simulink Vector Controller Structure

This figure shows the induction motor drive in which blocks from the power system and Simulink is used to model the induction motor drive in greater detail.

The induction motor fed by a current controlled PWM inverter which was built using universal bridge blocks...

The motor used in this case study is a 50HP , 460 V , Four pole , 60 Hz and the motor has the following parameters : Rs = 0.087 Ω L1s = 0.8mh Rr = 0.078 Ω , L1r = 0.8mH , Lm = 34.7mH .

Figure (7.3) Simulink Model for Vector Controller

Above figure is shown Vector control of a variable -frequency induction motor drive. The IGBT inverter is modeled by a universal bridge block in the power electronic device. The input voltage of the DC link is 780 V dc voltage supply.

Built from Simulink blocks current regulator which consists of three hysteresis controller. Thus, the motor currents are provided by the measuring machine output of the induction machine block.

Figure (7.5) shows conversation between abc and dq reference frames are implemented by the abc to dq0 transformation and also dq0 to abc transformation blocks in variable speed field oriented induction motor drive.

Figure (7.6) is shown how to calculate rotor flux buy the flux.

"The rotor flux position (Θe) is calculated by the Teta calculation block of vector control of a variable-frequency induction motor drive". As shown Figure (7.7)

"The stator reference quadrature axis current (iqs *) is calculated by the (iqs*) block Calculation shows variable speed field-oriented drive induction motor"as shown figure (7.8).

"The stator direct-axis current reference (ids*) is calculated by the id*_ calculation block shown in vector control of a variable-frequency induction motor drive" as shown figure (7.9).

The simulation Parameters dialog was set as follows:

Power ,voltage and frequency [pn(VA) Vn (Vrms),fn(Hz)]

[ 50*746, 460,60 ]

Stator resistance and inductance [ Rs (ohm) L1s(H) ]:

[ 0.087 0.8e-3 ]

Rotor resistance and inductance [Rr,(ohm) Llr,(H) ]:

[ 0.228 0.8e-3 ]

Mutual inductance Lm (H):

[ 34.7e-3]:

Inertia, friction factor and pole pairs [ J(Kg.m 2 ) F(N.m.s) p0]:

[ 1.662 0.12]

Initial conditions

[ 1,0,0,0,0,0,0,0 ]

Table (7.1) Squirrel-cage motor parameters case study

Start time

0

Integrator type

0de23tb

Stop time

4s

Relative tolerance

1e-3

Absolute tolerance

auto

Maximum step size

auto

Initial step size

auto

Table (7.2) the simulation parameters in the simulation parameters case study.

7.2.1 Simulation Results:

To investigate drive dynamic performance in the two following cases (speed regulation performance versus reference and load torque changes) two changing operating conditions to the drive have been applied: a step change in speed reference and a step change in load torque, the final state vector achieved from the previous simulation was employed. As the initial condition, so that the simulation should be started from steady state.

Simulating the vector controller with a 780 volt DC supply provided a number of worthwhile results. The control algorithm was evaluated under different operating condition; the start-up characteristics with no load torque, with 11.9 Nm, with step speed and step torque. Figures (7.10) and (7.12) show the starting characteristics of the motor without load and with a load torque of 11.9 Nm. It can be observed low the graph of motor speed is slightly overshot to set the point of 120 radians per second, and then it quickly returns to the minimum value of the steady state error. Vector control is perfectly acceptable in some applications; for example, in electric vehicle application.

The drive was started from standstill by specifying initial condition for all state variable in the power and also specifying [ 1,0,0,0,0,0,0,0]as the initial condition for the induction machine block in this case study, the speed reference was stepped from 0 to 120 rad/s at the torque equal 0. t = 0.Transient response from the induction motor was as shown below when the motor started, current and torque were very high, at the point 0.75s the response time of torque and current were at 0.7 s after they dropped to achieve steady state, so the speed was reduced to 2s then reached steady state before any errors in speed could be seen. We see all graphs have the same characteristics, because the motor is without load so that case is called steady state.

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Figure (7.10) Startup characteristics with no load torque

Figure (7.11) Startup characteristics with no load torque

Figure (7.12) Startup characteristics with 11.9Nm load torque

7.2.1.1Steady state voltage, current and torque waveforms:

The steady state being reached, the simulation was halted and the current, voltage and torque were plotted by the scope. Thus, the following figure shows the motor current, voltage and torque achieved when the motor runs without load (Torque = o Nm) at the speed 120rad/s.

Figure (7.13) Steady state motor current, voltage and torque

This two below cases the speed and torque were 120 rad/s and 50Nm.

In the next simulation, as shown in Figure (7.14), the controller's step response to a change in the loading torque was investigated. The motor was started with an initial torque value of 50 Nm then this value was increased to 100 Nm after two seconds causing a drop in motor speed. The controller took less than a second to respond to this increase in torque and the motor returned to the speed set point.

Figure (7.14) Torque step response

Figure (7.15) Torque step response

In the analysis of the motor's speed step response performance, as shown in Figure (7.16), the initial reference speed of 120 radians per second was increased to 175 after 1.5 seconds. The controller algorithm was able to respond quickly to a change in speed .As seen at the speed step set point (1.5s), the torque increased from 11.9 Nm (steady state) to about 300 Nm, as it was at the starting point, before quickly returning to steady state. Also at that point (1.5s) there was an increase in the current (is), but not the same as the starting current. In general, the control performance of the transient and steady state was very impressive.

Figure (7.16) Speed step response

Figure (7.17) Speed step response

In the next simulation, the controller's step response to a change in the speed and in the loading torque was investigated. The motor was started with an initial speed value of 120 rad/s and initial torque of 50 Nm, then these values were increased to 175-rad/s and 100Nm.The simulation results showed that after one second there was an increased torque where the step speed was set. The controller took less than a second to respond to this increase and return the motor to the initial torque value set point. If one looks at where the torque increased, there was a slight drop in the speed for a short period of time, then it went back to the steady state condition, the torque also returning to the set point over all, using speed step and torque step change in the same simulation showed that the transient and steady state performance of the vector controller was very impressive, even though by stepping both the speed and the torque.

Figure (7.18) Speed step response and torque step response

7.2.1.2 Speed regulation dynamic performance: (Creates X Initial)

To study drive dynamic performance (speed regulation performance versus reference and load torque changes), two changes in the operating conditions were applied to the drive: a step change in speed reference (from 120 rad/s at t = 0.5 s) and a step change in load torque (11.9 Nm at t = 2 s.). The final state vector obtained with the previous simulation was used as the (X Initial state) and the response time of torque and speed at the same point (0.5).On the other hand, the torque was starting from 0-300 Nm, then it decreased to reach steady state in two seconds; the torque was applied and increased to 100Nm and also the current later reached the same situation but the speed was different. It started from 120 rad / s, increased to 175rad / s after 1.5 seconds, then slightly decreased to 2 seconds, then dropped more, because at this point, the motor loading. The response of the induction motor drive to successive changes in speed reference and load torque is shown in Figure (7.19)

Figure (7.19) Speed step response and torque step response

Figure (7.20) Speed step response and torque step response

7. 3 Vector control dissection

As regards methods of scalar control, accuracy and dynamic characteristics were significantly improved.

Aim to maneuver the magnitude and space phase of the internal motor fields for improved performance.

Probable to control, directly and separately, the torque and flux of AC machines.

Provides accurate management of transient and steady state. In addition to this advantage, vector control of induction motors solves problems inherent to mechanical commutation with DC machines.

Static error obtained from steady state vector control offers good performance and is smaller than with scalar control

A huge technological step in the field of IM drive.

Vector control is a controlled high torque and fast transient response to load.