WO2012131709A1

WO2012131709A1 - Adaptive speed control of compressor motor

Info

Publication number: WO2012131709A1
Application number: PCT/IN2012/000200
Authority: WO
Inventors: Praveen Shankar JAMBHOLKAR
Original assignee: Praveen Shankar JAMBHOLKAR
Priority date: 2011-03-28
Filing date: 2012-03-26
Publication date: 2012-10-04

Abstract

A method and system for adaptive speed control of a compressor motor is disclosed. The method includes steps of measuring predetermined electrical parameters of an input power supply to the compressor motor, measuring pressure and temperature at the compressor output, comparing the measured electrical parameters with standard voltage versus frequency characteristics, tracking variations in the measured electrical parameters, calculating rate of change of speed of the compressor motor, calculating efficiency of the compressor and controlling the electrical parameters of the input power supply based on the calculated efficiency and rate of change of speed to vary the speed of the compressor motor. The system includes the first measuring means to measure predetermined electrical parameters of the input power supply to the compressor motor, the second measuring means to measure pressure and temperature at the compressor output and processing means to control speed of the compressor motor.

Description

TITLE

ADAPTIVE SPEED CONTROL OF COMPRESSOR MOTOR

FIELD OF DISCLOSURE

The present disclosure relates to the field of compressor motors.

BACKGROUND

Air conditioning is the removal of heat from indoor air for thermal comfort. In other words, the term refers to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air. An air conditioner (often referred to as an AC or Air Con) is an appliance, system, or machine designed to stabilize the air temperature and humidity within an area (used for cooling as well as heating depending on the properties of air at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

A basic cooling cycle of an air conditioning system comprises the following main components:

Compressor: A compressor is used to compress a refrigerant to a higher pressure. The refrigerant entering the compressor is usually in vapor phase. The temperature of the refrigerant also increases as the refrigerant is compressed to a high pressure. Condenser: A condenser is used to condense the refrigerant in vapor phase to liquid phase. In the condenser the heat energy of the liquid is lowered as it changes its state. The output is high temperature, high pressure liquid.

Thermostatic Expansion Valve: Also known as TXV, the function of this component is to reduce the pressure of the liquid refrigerant and thereby reduce its temperature.

Evaporator: The liquid refrigerant at low temperature and low pressure enters the evaporator coil, absorbs the heat from the air blown over the evaporator coil and changes back into vapor state. The air which passes across the evaporator coil loses heat and gets cooled. This air is used to cool the ambient atmosphere.

Conventional air conditioners typically employ fixed single speed compressors. These type of compressors either run or stop_; The main drawback of such compressors is that they consume more power. Typically, an air conditioner works for a few minutes, stops for a while and again starts after a few minutes. A user typically sets a target temperature. Till this target temperature is reached, the compressor runs at a rated speed. When the ambient temperature cools down to the target temperature after reaching the desired cooling level, the air conditioner has to stop further cooling so that the temperature is not reduced further. Hence the compressor stops functioning, thereby maintaining the temperature for a while. As the room temperature increases again and when the room temperature exceeds the target temperature, the air conditioner starts working, i.e. the compressor starts functioning and this cycle repeats. Hysteresis is done to prevent frequent starts and stops. This wastes further energy in extra cooling required due to hysteresis.

Air conditioners known in the art consume high energy. A major drawback with the above operation is that the power consumed by the compressor during restarts is very high. Every time the compressor starts/restarts, due to the high starting current, the power consumed is four to five times its operational power. Many times other appliances connected to the same power source are cut-off due to the operation. As a result extra protection may be required for other appliances connected to the same source of power.

Also, it is difficult to run air conditioners known in the art on home inverters. Further, a lot of noise is generated every time a conventional air conditioner starts. To overcome some of these issues, an inverter based air conditioner was introduced but it proved to be an expensive" alternative. What is needed is an economically viable air conditioner that overcomes the drawbacks of the air conditioners known in art.

Hence, there is a need for a compressor that consumes less power and is cost effective. Further, there is a need for controlling the motor of the compressor so that it consumes less power. Particularly there is a need for controlling the speed of the motor of the compressor to achieve energy efficiency at varying load conditions. OBJECTS

Some of the objects of the present disclosure are listed herein below.

An object of the disclosure is to provide a compressor that is cost effective.

An object of the disclosure is to provide efficient control of the motor of the compressor so that is consumes less power.

Another object of the disclosure is to provide efficient control of the speed of the motor of the compressor to achieve energy efficiency at varying load conditions.

Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure

SUMMARY

A method for adaptive speed control of a compressor motor, the method comprising the following steps, measuring p^redetermined electrical parameters of the input power supply to the compressor motor, measuring pressure and temperature at the output of the compressor, comparing the measured electrical parameters with standard voltage versus frequency characteristics, tracking variations in the measured electrical parameters,_* calculating rate of change of speed of the compressor motor, calculating efficiency of the compressor motor based on the tracked variations in the measured electrical parameters, controlling the electrical parameters of the input power supply based on the calculated efficiency and rate of change of speed to vary the speed of the compressor motor in a closed loop.manner.

Additionally, the step of calculating efficiency comprises the step of comparing the measured pressure and temperature with standard temperature versus entropy characteristics.

Typically, the step of controlling is a variable frequency control technique.

A system adapted to provide adaptive speed control of a compressor motor, the system comprising, first measuring means adapted to measure predetermined electrical parameters of the input power supply to the compressor motor, second measuring means adapted to measure pressure and temperature at the output of the compressor, and processing means adapted to control speed of the compressor motor in a closed loop manner, the processing means comprising comparing means adapted to compare the measured electrical parameters with standard voltage versus frequency characteristics, tracking means adapted to track variations in the measured electrical parameters, first calculating means adapted to calculate rate of change of speed of the compressor motor, second calculating me^'ans adapted to calculate efficiency of the compressor motor, and controlling means adapted to control the electrical parameters of the input power supply based on the calculated efficiency and rate of change of speed. Typically, the electrical parameters are selected from the group consisting of current, voltage and frequency.

Preferably, the processing means is a microcontroller.

Typically, the processing means is a variable frequency drive.

Additionally, the input power supply is provided by at least one solar photovoltaic panel.

Typically, the compressor is an air conditioning system compressor. Typically, the compressor is a refrigeration system compressor.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

The disclosure will now be described with the help of accompanying drawings, in which:

FIGURE 1 illustrates a cooling cycle of an air conditioner as known in the art;

FIGURE 2 illustrates a graphical representation of energy consumption and temperature hysteresis versus time for a fixed speed compressor 's known in the art;

FIGURE 3 illustrates a schematic representation of a sensor less field oriented control of an induction motor as known in the art.; FIGURE 4 illustrates a graphical representation of the temperature versus entropy chart of the air conditioning system;

FIGURE 5 illustrates a graphical representation of the pressure versus enthalpy chart of the air conditioning system;

FIGURE 6 illustrates a graphical representation of the voltage versus frequency characteristics of a motor; and

FIGURE 7 illustrates a graphical representation of temperature and energy versus time for a variable speed compressor in accordance with the present disclosure.

DETAILED DESCRIPTION OF ACCOMPANYING DRAWINGS

The disclosure will now be described with reference to the embodiments shown in the accompanying drawings. The embodiments do not limit the scope and ambit of the disclosure. The description relates purely to the exemplary preferred embodiments of the disclosed structure and its suggested applications.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The description herein after, of the specific embodiments will so. fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Referring to Figure 1-, a basic cycle of operation used in conventional air conditioners known as the Vapor Compression Cycle (100), is illustrated. The Vapor Compression Cycle (100) operates by continuously changing the physical properties of fluid through a cycle. The processes constituting the standard Vapor Compression Cycle (100) typically are:

• 1-2 illustrates compression of a refrigerant to a high pressure in the compressor (101). The refrigerant entering the compressor (101) is in vapor phase. The refrigerant is warm vapour at low pressure (1 1). The temperature of the refrigerant also increases as the refrigerant is compressed to a high pressure. The refrigerant at the output of the compressor is high temperature, high pressure vapour (22).

• 2-3 illustrates dissipation of heat from the refrigerant at a constant pressure and the condensation of the refrigerant in the condenser (102). The condenser (102) is used to condense the refrigerant in vapor phase to liquid phase. The refrigerant in the vapour phase enters the condenser at a high pressure and high temperature. A fan (1 12) is used to pass external air over the condenser while the refrigerant passes through the condenser (102) coils. This causes condensation thereby changing the state of the refrigerant from vapour phase to liquid phase. The heat energy of the liquid is lowered in the condenser (102) as it changes its state. The output is a high temperature, high pressure liquid (33). In accordance with one embodiment, the temperature of the liquid refrigerant is 43.3 °C and the pressure of the liquid refrigerant is 15.5 bar.

• 3-4 illustrates an expansion of the refrigerant to a low pressure and a low temperature in a Thermostatic Expansion Valve (TXV) (103). The function of TXV (103) is to reduce the pressure of the liquid refrigerant and thereby reduce its temperature. The output is very low temperature, low pressure liquid (44). In accordance with one embodiment, the temperature of the liquid refrigerant is 4.4 °C and the pressure of the liquid refrigerant is 4.8 bar.

• 4-1 illustrates absorption of heat from the air and evaporation of the refrigerant in an evaporator (104). The liquid refrigerant at low temperature and low pressure enters the evaporator coils. A blower (114) is used to blow air over the evaporator (104) while the liquid refrigerant passes through the evaporator (104) coils. The liquid refrigerant absorbs heat from the air blown over the evaporator (104) coils and changes back into vapor state. The air which passes across the evaporator. (104) coils loses heat and gets cooled. This air is used to cool the ambient atmosphere.

The refrigerant at the output of the evaporator (104) is high temperature, low pressure vapour. This refrigerant is then supplied tc the compressor and the cycle repeats.

Referring to Figure 2, a graphical representation of energy consumption (E) and temperature (T) hysteresis (H) versus time (t) for a fixed speed compressor as known in the art is illustrated. The energy (E) consumption during the starting/restarting of the compressor is very high as can be seen from the peaks in the graphical representation depicting the energy (E) consumption. Hysteresis (H) is done to prevent frequent starts and stops which causes further waste of energy in extra cooling required due to hysteresis (H).

The systems known in the art are plagued by drawbacks including high energy consumption, low efficiency and running cost. To improve the efficiency/Coefficient of Performance (COP) of conventional air conditioners, Variable Speed Compressors which function at varying speeds, are employed.

A Variable Speed Thrive (VSD) Air Compressor is an air compressor that takes advantage of variable-speed drive technology. The drive used in this type of compressor controls the speed (RPM) of the compressor, which in turn saves energy as compared to a fixed speed equivalent. The most common form of VSD technology in the air compressor industry is a variable frequency drive, which converts the incoming AC power to DC power and then converts the DC power back to a quasi-sinusoidal AC power using an inverter switching circuit. A variable-frequency drive (VFD) is a system for controlling the rotational speed of an r.lternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

VFD operation is as explained herein below.

When an induction motor is connected to a full voltage supply, it draws current of the order that is several times (up to about six times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.

In contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus, starting the motor at such a low frequency avoids the high inrush current that occurs when the motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop about 150% of its rated torque while the VFD draws less than 50% of its rated current from the mains in a low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. Power electronics are used to control the drive and ensure proper functioning of the air conditioner. In air conditioning systems employing a variable speed drive compressor, the speed of the compressor is controlled based on the difference in a target temperature set by a user and an ambient temperature of the room. Power is saved since the compressor speed is reduced based on the cooling requirement. Power is further saved since the target temperature is steadily maintained without resorting to hysteresis and energy efficiency is achieved. Referring to Figure . 3, a schematic representation of a sensor less field oriented control of an induction motor as known in the art is illustrated. This type of sensor-less field oriented control which typically includes a Field Oriented Control algorithm improves the energy efficiency of the air conditioning system, but is inherently difficult to implement. This is due to the fact that, in this method of sensor less field oriented control, the position and velocity of the rotor are not measured directly, but are estimated using currents and voltage measurements. This makes compressors using this technique expensive.

Referring to Figures 4 and 5, a graphical representation of the temperature (T) versus entropy (S) chart of the air-conditioning system and a graphical representation of the pressure (P) versus enthalpy (H) chart of the air conditioning system is illustrated. A horizontal line within the two phase regions, the liquid phase (L) and the vapour phase (V), indicates that pressure (P) and temperature (T) both remain constant during any given isobaric phase change. However, entropy (S) and enthalpy (H) increase significantly during vaporization.

When liquid boils at a given value of applied pressure, it extracts heat from the surroundings or other available medium. If vapour produced by boiling the liquid is transported to a new location and compressed to a higher pressure, it can be condensed at a correspondingly higher temperature, yielding its heat of condensation to new surroundings or any cooling medium that may be supplied.

Reducing the speed of the compressor of the air-conditioning system leads to reduction in the output pressure and the output temperature, thereby reducing entropy. This results in an exponential decrease in total power required versus the speed of the compressor.

In accordance with the present disclosure, the method for adaptive speed control of a compressor motor is a combination of adaptive control and power electronics which enables sensor-less speed control of the_^compressor motor. In particular, this is a combination of adaptive control of electrical parameters using a hopping technique and the temperature versus entropy chart for the air conditioning cycle so that discharge flow, temperature and pressure are minimized. This adaptive control is a closed loop control and changes according to particular thermal load requirements of the air conditioner making it efficient and easy to implement. The adaptive control is typically achieved using a microcontroller.

Referring to Figures 6 and 7, a graphical representation of the .voltage (V) versus frequency (F) characteristics of a motor and a graphical representation of temperature (T) and energy (E) versus time (t) for a variable speed drive compressor in accordance with the present disclosure is illustrated. Any deviation from the voltage (V) versus frequency (F) characteristics for a particular motor results in improper movement and more significantly, poor efficiency. In accordance with the present disclosure, the method for adaptive speed control uses standard voltage versus frequency characteristics' look-up table and performs a perturb and observe technique. While deviating in a direction, the efficiency and rate of change of speed of the motor is tracked and calculated. If the voltage versus frequency characteristics of the motor are perturbed in a particular direction causes the efficiency of the motor to increase, the voltage versus frequency characteristics are further perturbed in the same direction. Alternately, if the efficiency of the motor decreases, direction of perturbation is reversed.

The temperature versus entropy chart indicates temperature and pressure at the output of the compressor. In accordance with an embodiment of the present disclosure, both temperature and pressure at the output of the compressor are minimized, thereby reducing the power required to drive the motor of the compressor at a given speed resulting in optimized power usage.

With this criteria and the basic look-up table, voltage and frequency supplied to the motor are varied thereby varying the speed of the motor to provide adaptive speed control of the motor of the compressor resulting in less power consumption and achieving energy efficiency at varying load conditions.

In accordance with an embodiment, a system is adapted to provide adaptive speed control of a compressor motor which is an air-conditioning system compressor. The air-conditioning system typically comprises a chassis, a blower, a compressor with a three phase induction motor, a condenser connected to the compressor, means for expansion of a refrigerant, an evaporator connected through a copper pipe, and a microcontroller using an adaptive control method to achieve sensor-less speed control of the compressor motor. The microcontroller uses structured hopping of electrical parameters in accordance with the method disclosed herein above to run the compressor smoothly with minimum power consumption, and also takes advantage of decreasing the back pressure of the compressor motor, thereby minimizing the motor torque along with its speed.

In accordance with an embodiment, the system is adapted- to provide adaptive speed control of a compressor motor which is a refrigeration system compressor.

In accordance with another embodiment, the system is adapted to provide adaptive speed control of a compressor motor that is typically powered by mains power (AC voltage, single phase).

In accordance with yet another embodiment, the system is adapted to provide adaptive speed control of a compressor motor that is powered by Solar PV panels.

In accordance with still another embodiment, the system is adapted to provide adaptive speed control of a compressor motor that is provided with an adaptive variable frequency drive. The speed control of the compressor motor as described herein above results in optimizing the energy efficiency of the air conditioning / refrigeration system with respect to ambient conditions.

TECHNICAL ADVANCEMENTS AND ECONOMIC SIGNIFICANCE

The technical advancements offered by the present disclosure' include the realization of:

• low power consumption;

• lower variation in temperature;

• use of solar power;

• continuous operation of a variable speed drive compressor without being cut-off from the power supply;

• prevention of frequent starting and stopping of the compressor at periodic intervals thereby reducing electrical surges, ensuring smooth operation and also prevention of any damage to other appliances connected to the same source of power; and

• energy saving in the range of 30% to 50% is achieved under various test conditions of operation of the air conditioning / refrigeration system in accordance with the present disclosure.

Throughout this specification the word "comprise", or Variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements, as the use may be in one of the embodiments to achieve one or more of the desired objects or results.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

While considerable, emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims

1. A method for adaptive speed control of a compressor motor, said method comprising the following steps:

• measuring predetermined electrical parameters of the input power supply to the compressor motor;

• measuring pressure and temperature at the output of the

compressor;

• comparing the measured electrical parameters with standard

voltage versus frequency characteristics;

• tracking variations in the measured electrical parameters;

• calculating rate of change of speed of the compressor motor;

• calculating efficiency of the compressor motor based on the

tracked variations in the measured electrical parameters;

• controlling the electrical parameters of the input power supply based on the calculated efficiency and rate of change of speed to vary the speed of the compressor motor in a closed loop manner.

2. The method as claimed in claim 1, wherein the step of calculating efficiency comprises the step of comparing the measured pressure and temperature with standard temperature versus entropy characteristics.

3. The method as claimed in claim 1, wherein the step of controlling is a variable frequency control technique.

A system adapted to provide adaptive speed control of a compressor motor, said system comprising:

• first measuring means adapted to measure predetermined

electrical parameters of the input power supply to the

compressor motor;

• second measuring means adapted to measure pressure and

temperature at the output of the compressor; and

• processing means adapted to control speed of the compressor motor in a closed loop manner,

said processing means comprising:

^■ comparing means adapted to compare the measured

electrical parameters with standard voltage versus frequency characteristics;

^■ tracking means adapted to track variations in the measured electrical parameters;

^■ first calculating means adapted to calculate rate of change of speed of the compressor motor;

^■ second calculating means adapted to calculate efficiency of the compressor motor; and

^■ controlling means adapted to control the electrical

parameters of the input power supply based on the calculated efficiency and rate of change of speed.

The system as claimed in claim 4, wherein said electrical parameters are selected from the group consisting of current, voltage and frequency.

6. The system as claimed in claim 4, wherein said processing means is a microcontroller.

7. The system as claimed in claim 4, wherein said processing means is a variable frequency drive.

8. The system as claimed in claim 4, wherein said input power supply is provided by at least one solar photovoltaic panel.

9. The system as claimed in claim 4, wherein said compressor is an air conditioning system compressor.

10. The system as claimed in claim 4, wherein said compressor is a

refrigeration system compressor.