Real-time loss minimization control in induction machines based on DSP TMS 320 LF 2812

This paper presents a DSP based implementation of simple and very useful control algorithm for the real-time efficiency optimization of the indirect vectorcontrolled induction motor drives. Conventional field-oriented induction motor drives operate at rated flux even at low load. To improve the efficiency of the existing induction motors, it is important to regulate the magnetization flux of the motor in the desired operating range. This paper presents techniques for minimizing power loss (the copper and core losses) of induction motor based on determination of an optimum flux level for the efficiency optimization of the vector-controlled induction motor drive. An induction motor (IM) model in d-q coordinates is referenced to the rotor magnetizing current. Thus the decomposition into d-q components in the steady-state motor model can be utilized in deriving the motor loss model. The algorithm offers a fast convergence. The complete closed loop vector control of the proposed LMC-based IM drive is successfully implemented in real-time using digital signal processor DSP TMS320LF2812 for 1HP motor induction motor. The close agreement between the simulation by Matlab/Simulink and the experimental results confirms the validity and usefulness of the proposed techniques. The proposed LMC in a comparison with conventional FOC can reduce total losses from 5% to 67.2% for all load ranges.


ABSTRACT:
This paper presents a DSP based implementation of simple and very useful control algorithm for the real-time efficiency optimization of the indirect vectorcontrolled induction motor drives.Conventional field-oriented induction motor drives operate at rated flux even at low load.To improve the efficiency of the existing induction motors, it is important to regulate the magnetization flux of the motor in the desired operating range.This paper presents techniques for minimizing power loss (the copper and core losses) of induction motor based on determination of an optimum flux level for the efficiency optimization of the vector-controlled induction motor drive.An induction motor (IM) model in d-q coordinates is referenced to the rotor magnetizing current.Thus the decomposition into d-q components in the steady-state motor model can be utilized in deriving the motor loss model.The algorithm offers a fast convergence.The complete closed loop vector control of the proposed LMC-based IM drive is successfully implemented in real-time using digital signal processor DSP TMS320LF2812 for 1HP motor induction motor.The close agreement between the simulation by Matlab/Simulink and the experimental results confirms the validity and usefulness of the proposed techniques.The proposed LMC in a comparison with conventional FOC can reduce total losses from 5% to 67.2% for all load ranges.

INTRODUCTION
Produced worldwide is used by motors, mainly induction motors, which constitute around Environmental concerns, increasing energy demand, and limited resources have driven efficiency improvement in all aspects of electrical engineering.Over 50% of the electrical energy 60% of the industrial electrical load.The importance of the efficiency maximization for an induction motor (IM) drives may be realized from different perspectives [1], [2], [3], [4].
Induction motors are normally dimensioned for constant voltage and frequency, in such a way that they have the optimal efficiency near 75% load and almost optimal efficiency at nominal load.So, operation of an IM is very efficient at or near rated load with rated flux.However, operation with rated flux causes low efficiency at light load [2].Thus, in cases where a motor drive has to operate in a wider load range, the minimization of losses has great significance.Efficiency optimization controls for such machines will not only earn a recurring benefit in economy but also have a very good impact on the global environment [4].[5], [7].
Electrical motor drive losses consist of grid loss, converter loss, motor loss and transmission loss.In an effort to improve efficiency, there have been improvements in the materials, design and construction techniques.However, converter loss and motor loss are still greatly dependent on control strategies, especially when the motor operates at light load.In transitional motor control the core losses and magnetizing-currentinduced stator copper loss are almost constant, so, at light load, the motor efficiency decreases drastically.However, at light loads, rated flux operation causes excessive core loss, thus impairing the efficiency of the drive.Since drives operate at light load most of the time, optimum efficiency can be obtained by programming the flux [2].

This
paper presents loss-model-based controller (LMC) for minimizing power loss of induction motor and an approach of simultaneously optimizing the efficiency of induction-motor drives and selecting flux reference through the minimization of the copper and core losses while ensuring high dynamic performance.The loss-model-based controller based induction motor (IM) drive is implemented in real time for 1HP induction motor using digital signal processor DSP TMS320LF2812.

LOSS MODEL OF INDUCTION MOTOR
Losses in an IM constitute copper loss and core loss in stator and rotor, mechanical loss, and stray load loss.Core loss and copper loss depend on the magnetic and electric loading of the machine and, therefore, are controllable.The stray load loss depends mainly on the construction of the motor (type of stator and rotor slots, length of overhang, etc.) and also on the harmonics in the supply voltage [2].
Usually, for a given motor and specified load, the sum of stray load loss and the mechanical loss do not exceed 25-30% of the total losses and may be assumed to remain constant [2].The electric loss is dominated by the copper and core losses, and when minimized, it improves the overall system efficiency.Thus, the motivation of loss minimization is to look for an optimum balance of the variable losses to make the total loss minimum.Copper loss reduces with decreasing stator and rotor currents while the core loss essentially increases with increasing airgap flux density.And a given load torque, there is an air-gap flux density at which the total loss is minimized.
Hence, an electrical loss minimization process ultimately comes down to the selection of the appropriate air-gap flux density of operation.Since the air-gap flux density must be variable when the load is changing, control schemes in which the (rotor, air gap) flux linkage is constant will yield a suboptimal efficiency operation especially when the load is light.[3].
In this paper focuses in minimization stator copper, rotor copper and iron losses by determining an optimal air-gap flux density when the load is changing.An equivalent circuit for IM can be varied by the different choices of flux linkage constants [6].In this paper, we utilize an equivalent circuit referenced to the rotor magnetizing current by defining the rotor flux as we :Electrical rotor speed.
wr :Rotor magnetizing current.is: Stator voltages complex space vector.us: Stator current complex space vector.
The induction motor losses that can be minimized by excitation adjustment are the following.The total losses of an induction motor consist of stator and rotor copper losses Pcu, core losses Pfe and mechanical losses Pm.In the steady state, the stator and rotor copper losses are given by cus Fe cur Copper losses: These are due to flow of the electric current through the stator and rotor windings and are given by Iron losses: These are the losses due to eddy currents and hysteresis, given by '2 () From Figure 1, the rotor current can be expressed as Substituting from ( 7) into ( 6 To minimize a power loss with a constant torque, the differentiation of the loss expression (8) has been done.
Putting from ( 9), (10) isqTe into (11) leads to Note, in the steady state imr=isdandan optimum magnetizing current level for minimum loss is given by ' 3.

PROPOSED REAL-TIME LOSS MINIMIZATION CONTROL SCHEME
The proposed efficiency optimization control system, which based on Field Oriented Controlis presented in Figure 2. Our target is to consider an application that requires both high dynamic performance and maximum efficiency.Therefore, an indirect vector-controlled IM drive is considered where an additional outer loop is placed for the efficiency optimization.The vector control not only has the advantage of excellent dynamic performance, but, also, due to the inherent decoupling of the -axis (flux-producing) and -axis (torque-producing) currents in the steady state, the flux control and torque control may be thought of separately.The vector control part involves speed and current controllers in the synchronously rotating reference frame, vector rotators, and reference transformations.Speed is measured by using an encoder.The reference optimal rotor magnetizing current is defined by loss-model-based controller, which has been explained in Section 2 by equation ( 13).

SIMULATION OF THE PROPOSED CONTROL SCHEME
In order to verify the effectiveness of the proposed loss minimization scheme, a simulation model is developed in Matlab/Simulink software.Simulation has been performed 1HP induction motor parameters, which are presented in Table 1.The simulation is done for conventional FOC scheme and proposed loss minimization control (LMC) in three cases study to compare characteristics of these control systems) No-load starting an induction motor; b) Simulation is done at various speed levels with load torque of 5 Nm the motor speed is constant but load torque is changed and c) Load torque is constant but motor speed is changed.   1 and the Appendix.The reference speed is ramped from 0 to 1000 r/min while seeking to minimize the total electrical loss.The simulation results show that the rotor flux linkage quickly builds up and maintains an almost constant rated value and becomes small at steady state, its near 0.2Wb.The total loss reduces as the developed torque becomes very small.But, in the conventional FOC scheme, the total loss is more and reaches about 80W and the rotor flux linkage is rated.When the proposed LMC is utilized, a slight reduction in total losses is observed while there was no difference in the dynamic performance of the controller.Thus, the improvement of the proposed method is verified.It is observed that the proposed scheme has smaller loss factor than the traditional one in all the speed ranges, which indicate reduced loss for the proposed LMC.

5.HARDWARE IMPLEMENTATION AND EXPERIMENTAL RESULTS
In order to implement the proposed LMC in real time the DSP TMS320LF2812is used.To simplify code development and shorten debugging time, a Code Composer Study tool is provided.In addition, an onboard JTAG connector provides interface to emulators.The DSP is supplemented by a set of on-board peripherals used in digital control systems including analog to digital (A/D), digital to analog (D/A) converters, digital I/O, serial interface and incremental encoder interfaces.The block diagram of experimental system is shown in Fig. 8.
The PC-based controller produces numerical switching commands sent to DSP board and outputs of the DSP board are sent to the base drive circuit to drive the inverter.The actual motor currents are measured by the Current sensors HX-05-P and fed back to the DSP board through the A/D channel system.
Rotor position is sensed by an optical incremental encoder of 2000-line resolution and is fed back to the DSP board through the encoder interface.The IM is coupled to a dc machine.The dc machine is operated as a generator in order to adjust load to the IM.

EXPERIMENTAL RESULTS.
In order to further investigate the effectiveness of the proposed LMC, real-time experiments are carried out on the available 1 HP motor.The parameters of this motor are obtained through no load test and locked rotor test, and shown in Table 1.Figures 9-10 show experimental results of conventional FOC and the proposed LMC, is consists of the developed torque, speed, magnetizing current command, and total.The motor is operated at 1000 rpm and torque conditions is changing by steps: (10%;20%;30%; 40%) rated load torque by every 20s.One can see that the speed and developed torque remains same regardless while the total loss is reduced.To investigate the effects of proposed LMC at different speeds, the motor is operated at various speeds under the same 10% of rated load torque.Operation of an IM is very efficient at near or rated conditions.In this case, the proper selection of the flux level in the motor can achieve the energy saving.For this reason, the experiment was carried out at light load (10% of the rated torque) rather than the rated torque in order to clearly demonstrate the effectiveness of the proposed loss minimization scheme.The total power loss is calculated using (6) and fed for ease of getting stable measurement in real-time.

CONCLUSIONS
A new online loss-minimization-based controller for IM drive has been developed and presented in this paper.The complete closed loop vector control of the proposed LMC-based IM drive is successfully implemented in real-time using digital signal processor DSP TMS320LF2812 for 1HP motor induction motor.The performance of the proposed controller has been tested in both simulation and experiment at different operating conditions.It is found from the results that the performance of the drive with the proposed LMC has been improved in terms of power saving as compared to a conventional FOC.The performance of the drive is tested only in steady state, as any industrial drive runs in steady state for a long time and hence the steadystate loss is the main concern for drives.A comparison of the proposed LMC with the conventional FOC showed that the proposed LMC can results in higher loss reduction in all speed ranges as compared to the conventional one.The proposed LMC in a comparison with conventional FOC can reduce total losses from 5% to 67.2% in depending on load, and the total loss can be reduced significant, specially, and can reach to 72% when load is about 10% of rated load.

Figure. 1 .
Figure. 1. Steady-state IM equivalent circuit in a field-oriented frame.(a) d-axis equivalent circuit.(b) q-axis equivalent circuit.From figure1, rotor magnetizing current and current through the iron loss resistor are defined by

Fig. 2 .
Fig. 2. The proposed control scheme based Indirect Field Oriented Controlled induction motor

Figures 3 (
Figures 3(a), (b) present the no-load starting transient of an experimental 1-hp induction machine whose parameters are shown in Figure1and the Appendix.The reference speed is ramped from 0 to 1000 r/min while seeking to minimize the total electrical loss.The simulation results show that the rotor flux linkage quickly builds up

Figure 3 .Figure 4 .Figure 5 .
Figure 3. Motor starting from zero speed to 1000 r/min.for FOC (a) and proposed LMC (b): developed torque (Nm), actual rotor speed (rpm), rotor flux linkage (Wb), total electric loss.Figures 4(a),(b)show the sample comparisons of total losses between the traditional FOC and the proposed LMC while the motor was running at the following constant speed 1000 rpm and torque conditions is changing by steps: (10%;20%;30%;40%;100%) of rated load.As expected, the rotor flux responds to the changing

Figure 6 .
Figure 6.Comparison of total loss with and without the proposed LMC

Figure 7 .
Figure 7.Comparison of total loss with and without the proposed LMC

Figure. 8 .
Figure. 8. Block diagram of experimental The control algorithms are implemented through developing a real-time Simulink model.Then the model is downloaded to the DSP board utilizing Control Desk software and real-time workshop (RTW).The Graph Tool of Code Composer Studio software is provided to observe experimental results online in PC by JTAG interface between DSP2812 and PC.

Table 1 .
Induction motor data for simulation and experiment

Table 2 .
The simulation result: A comparison total loss for two control schemes with various loads, a constant speed equal 1000 rpm