Figure 1- Motor Overview

Alternating current AC induction motors (ACIM) are popular in industry and consumer electronics for a number of reasons, see Figure 1. Their construction is extremely optimized, since they have been produced for years. They are very simple and manufacturing costs are favorable. They have no brushes and require minimum maintenance. The robustness of the motor is another strong advantage. Traditionally, these motors have been run with invariable speed control, and are started and stopped frequently in order to achieve the desired result. About 50% of the electricity used during such a process is wasted. Many new methods of reducing electricity are being considered, including new electric motor efficiency technologies. System costs and power consumption can be drastically reduced utilizing digital control of an analog motor circuit. Presented solution of a 3-phase AC Induction motor vector drive based on Freescale's DSC MC56F8013 / 23 microprocessor takes advantage of a cost-efficient solution for consumer and industrial motor drives.

Three-Phase AC Induction Motor

The ACIM is a rotating electric machine designed to operate from a 3-phase source of alternating voltage. Slots in the inner periphery of the stator accommodate 3-phase winding a,b,c. The turns in each winding are distributed so that a current in a stator winding produces an approximately sinusoidally-distributed flux density around the periphery of the air gap. When three currents that are sinusoidally varying in time, but displaced in phase by 120° from each other, flow through the three symmetrically-placed windings, a radially-directed air gap flux density is produced that is also sinusoidally distributed around the gap and rotates at an angular velocity equal to the angular frequency of the stator currents.

The most common type of induction motor has a squirrel cage rotor in which aluminum conductors or bars are cast into slots in the outer periphery of the rotor. These conductors or bars are shorted together at both ends of the rotor by cast aluminum end rings, which also can be shaped to act as fans.

As the sinusoidally-distributed flux density wave produced by the stator magnetizing currents sweeps past the rotor conductors, it generates a voltage in them. The result is a sinusoidally-distributed set of currents in the short-circuited rotor bars. Because of the low resistance of these shorted bars, only a small relative angular velocity between the angular velocity of the flux wave and the mechanical angular velocity of the two-pole rotor is required to produce the necessary rotor current. The relative angular velocity is called the slip velocity. The interaction of the sinusoidally-distributed air gap flux density and induced rotor currents produces a torque on the rotor.

Vector Control of AC Induction Motor

In order to achieve variable speed operations in a three-phase AC induction motor, a variable voltage and variable frequency need to be supplied to the motor. Modern three-phase variable speed drives (VSD) are supplied with digitally controlled switching inverters which can considerably reduce overall system power consumption.

Using a variable speed drive based on induction motor can provide up to 60% electric energy economy, 3-4 times increase of resources and functional possibilities unattainable before. Power range of this drive makes up 0.2-0.4 kW - fridge compressors, 0.8-l kW - washing machines, 2-3 kW- conditioners, 3-100 kW - electric drives for housing and communal services (pumps for cold and hot water in many-storied houses, trunk cold water pipelines etc.).

The control algorithms can be sorted into two general groups. The first group is referred to scalar control. The constant Volt per Hertz control is a very popular technique representing scalar control.

The other group is called vector or field oriented control (FOC). The vector oriented techniques bring overall improvements in drive performance over scalar control. Advantages of FOC: higher efficiency, full torque control, decoupled control of flux and torque, improved dynamics, etc.

The basic idea of the FOC algorithm is to decompose a stator current into flux and torque producing components. Both components can be controlled separately after decomposition. The structure of the motor controller is then as simple as that for a separately excited DC motor.

Figure 2 shows the basic structure of the vector control algorithm for the AC induction motor. To perform vector control, it is necessary to follow these steps:

• Measure the motor quantities (phase voltages and currents)

• Transform them into the 2-phase system (?,?) using a Clarke transformation

• Calculate the rotor flux space-vector magnitude and position angle

• Transform stator currents into the d-q reference frame using a Park transformation

• The stator current torque (isq) and flux (isd) producing components are separately controlled

• The output stator voltage space vector is calculated using the decoupling block

• The stator voltage space vector is transformed by an inverse Park transformation back from the d-q reference frame into the 2-phase system fixed with the stator

• Using space vector modulation (SVM), the output 3-phase voltage is generated

Figure 2 " Vector Control Transformations

Description of Vector Control Algorithm

The overview block diagram of the implemented control algorithm is illustrated in Figure 3. Similarly, as with other vector control oriented techniques, it is able to control the excitation and torque of the induction motor separately. The aim of control is to regulate the motor speed. The speed command value is set by high level control. The algorithm is executed in two control loops. The fast inner control loop is executed with a 125?s period. The slow outer control loop is executed with a period of one millisecond.

Figure 3 - Vector Control Algorithm Overview

To achieve the goal of the induction motor control, the algorithm utilizes a set of feedback signals. The essential feedback signals are as follows: DC-bus voltage, three-phase stator current reconstructed from the DC-bus current, and motor speed. For correct operation, the presented control structure requires a speed sensor on the motor shaft. In the case of the presented algorithm, an incremental encoder is used.

The fast control loop executes two independent current control loops. They are the direct and quadrature-axis current (isd,isq) PI controllers. The direct-axis current (isd) is used to control rotor magnetizing flux. The quadrature-axis current (isq) corresponds to the motor torque. The current PI controllers' outputs are summed with the corresponding d and q axis components of the decoupling stator voltage. Thus we obtain the desired space-vector for the stator voltage, which is applied to the motor. The fast control loop executes all the necessary tasks to be able to achieve an independent control of the stator current components. This includes:

• Three-Phase Current Reconstruction

• Forward Clark Transformation

• Forward and Backward Park Transformations

• Rotor Magnetizing Flux Position Evaluation

• DC-Bus Voltage Ripple Elimination

• Space Vector Modulation (SVM) The slow control loop executes speed and field-weakening controllers and lower priority control tasks. The PI speed controller output sets a reference for the torque producing quadrature axis component of the stator current (isq). The reference for flux producing direct axis component of the stator current (isd) is set by the Field-Weakening controller. The Adaptive Circuit performs correction on the rotor time constant to minimize the error of the rotor flux position estimation.

System Concept

The Freescale MC56F80xx family is well suited to digital motor control, combining the DSP's calculation capability with the MCU's controller features on a single chip.

The MC56F80xx family members provide these peripheral blocks:

• One PWM module with PWM outputs, fault inputs, fault-tolerant design with dead-time insertion, supporting both centre-aligned and edge-aligned modes

• 12-bit ADCs, supporting two simultaneous conversions; ADC and PWM modules can be synchronized

• One dedicated 16-bit general-purpose quad timer module

• One serial peripheral interface (SPI)

• One serial communications interface (SCI) with LIN slave functions

• One inter-integrated circuit (I2C) port

• On-board 3.3 V to 2.5 V voltage regulator for powering internal logic and memories

• Integrated power-on reset and low-voltage interrupt module

• All signal pins multiplexed with general-purpose input/output (GPIO) pins

• Computer operating properly (COP) watchdog timer

• External reset input pin for hardware reset (may also be assigned as GPIO)

• JTAG/On-Chip Emulation (OnCE) module for unobtrusive, processor-speed-independent debugging

• Phase-locked loop (PLL) based frequency synthesizer for the hybrid controller core clock, with on-chip relaxation oscillator

Figure 4 - System Concept

The three-phase ACIM vector control a with single shunt sensor benefits greatly from the flexible PWM module, fast ADC, and quad timer module. The PWM offers flexibility in its configuration, enabling efficient three-phase motor control. The PWM module is capable of generating asymmetric PWM duty cycles in centre-aligned configuration. We can benefit from this feature to achieve a reconstruction of three-phase currents in critical switching patterns. The PWM reload SYNC signal is generated to provide synchronization with other modules (Quadtimers, ADC). The application uses the ADC block in simultaneous mode scan. It is synchronized to the PWM pulses. This configuration allows the simultaneous conversion of the required analogue values for the DC-bus current and voltage within the required time. The ADC conversions are triggered directly by the PWM without need for the DSC core to relay the event, resulting in predictable and constant relative timing.

The quad timer is an extremely flexible module, providing all required services relating to time events. The application uses four channels of the quad timer for:

• One channel for PWM-to-ADC synchronization

• Two channels for reading quadrature encoder signals (one channel in case the tachogenerator is used instead of a quadrature encoder)

• One channel for system base of slow control loop (1ms period)

An adaptive closed loop rotor flux estimator enhances control performance and increases the overall robustness of the system. Sensitivity of the parameter drift can be considerably minimized in this way. Minimizing system cost the algorithm implements a single shunt current sensing reducing three current sensors to one. The high range of motor operating speeds up to 20,000RPM is another advantage of the presented design.

Such high speed is required e.g. by washer producers. The ratio between motor and drum is about ten in horizontal washers. Thus, for achieving drum speed 2,000RPM, motor has to run in 20,000RPM. Three-phase induction motors for washers are designed for nominal speed much lower than 20,000RPM (mostly about 6,000RPM). Higher speed is achieved utilizing so-called field-weakening algorithm which allows exceeding the nominal motor speed while the motor magnetizing flux maintaining at a level to prevent it from exceeding the nominal motor voltage. This feature introduces next cost and energy saving by using motor designed for lower nominal speed but that is possible to run up to 20,000RPM with a field-weakening algorithm.

Three-Phase Current Reconstruction

The vector control algorithm requires the sensing of the three motor phase currents. A standard approach is to sense the phase currents directly through current transformers, or Hall Effect sensors, directly coupled to the motor phase lines that carry the current between the switches and the motor. To reduce the number of current sensors and overall cost of the design, the three-phase stator currents are measured by means of a single DC-Link current shunt sensor; see Figure 5.

Figure 5 - DC-Link Current Sensor

The DC-Link current pulses are sampled at exactly timed intervals. A voltage drop on the shunt resistor is amplified by an operational amplifier inside the 3-phase driver and shifted up by 1.65V. The resultant voltage is converted by the ADC.

Based on the actual combination of switches, the three-phase currents of the stator are reconstructed. The AD converter measures the DC-link current during the active vectors of the PWM cycle. When the voltage vector V1 is applied, current flows from the positive rail into the phase A winding and returns to the negative rail through the B and C phase windings. When the voltage vector V2 is applied, the DC link current returning to the negative rail equals the T phase current. Therefore, in each sector, two phase current measurements are available. The calculation of the third phase current value is possible because the three winding currents sum is zero. The voltage vector combination and corresponding reconstructed motor phase currents are shown in Table 1.

Table 1 - Measured Current

Estimating Fan Energy Cost Example

Pumps and fan systems account for almost 40% of all motor applications in industry. The input power of a fan is proportional to cube of the flow. For instance For example, if 100% flow requires full power, 75% flow theoretically requires (0.75)3 = 42% of full power. Although this is the theoretical saving under zero static head conditions, even in practical applications, a substantial energy saving can be achieved. Most energy savings from the application of VSD's are available in the use of centrifugal fans or pumps. When estimating the saving from such applications, it is useful to use the Fan Laws, which relate air (fluid) flow, input power and motor speed.

An example of how the Fan Laws may be applied in practice is a centralized plant supplying a chilled water system with two flow requirements. The system is controlled by an electronic actuator that throttles the water flow based on the system requirements.

With the throttle fully open, the flow rate is 80 l/s while power consumption has been measured at 20 kW. When the flow rate reduced to 65 l/s using the throttling valve the power consumption is measured at 18 kW. The system operates for 8,760 hours per year divided into 60% and 40% for the throttled and un-throttled conditions, respectively. By installing a VSD, savings can be calculated as follows.

This shows that for the 60% of the time that the system is throttled, the savings would be in the order of:

When VSD should be used:

• Production volume fluctuates

• Currently are used multi-speed motors

• Dampers, control valves or recycle loops are used to control flow

• Required very accurate control of speed and torque

• Having speed control systems which do not perform satisfactorily

• Systems operate more than 80 hours per week

Conclusion

The solution presented based on of Freescale's Digital Signal Controller MC56F80xx shows cost-effective design for wide range of both industrial and consumer motor control application. Digital controllers completely control the AC induction motor so it always functions at optimum efficiency. While the capacity of the motor corresponds to the needs of the process, variable speed drives can give major savings, compared to the wasteful practice of running the motor at full speed. This extends motor life and reduces stress and strain on the motor, both electrically and mechanically. With energy prices on the rise, Freescale DSC's introduces a smart option for energy savings on electric motors.