FOR M 2
The Patents Act, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A method of determining the speed of rotation of a three phase induction motor
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Mylavarapu Ramamoorty and Samsul Ekram, both Indian nationals and both of Crompton Greaves Ltd, Advanced Motor Design and Technology Centre (AMDTC), CG Global R and D Centre,. Kanjurmarg (E), Mumbai -400042, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF INVENTION
This invention relates to a method of determining the speed of rotation of a three phase induction motor.
This invention also relates to a three phase induction motor.
PRIOR ART DESCRIPTION
A three phase induction motor comprises a control unit for controlling the operation thereof. The controller unit comprises a speed controller electrically connected to a speed sensor such as Hall Sensor located in the proximity of the motor. It also comprises current sensors such as current transformers connected to the three phases and to the speed controller. It further comprises a reference rotor speed voltage generator connected to the speed controller. During operation of the motor, the speed controller continuously senses the current following in the phases and also the speed of rotation of the rotor. Further it compares the sensed rotor speed with the input from the reference speed voltage generator, compensates for any error in the sensed speed and drives the motor at the designed or required speed. Such a motor configuration requires the speed sensor and further requires it to be physically connected to the speed controller which is located in the controlled room, whereas the motor is generally located away or far away from the speed controller. Therefore, electrical connection of the sensor to the speed controller is often cumbersome and difficult and also works out to be expensive. In the case of retrofit applications it may at times be very difficult or practically impossible to mount the speed sensor on the motor shaft.


OBJECTS OF THE INVENTION
An object of the invention is to provide a method of determining the speed of rotation of three phase induction motor which method eliminates the speed sensor and associated electrical connection of the speed sensor to the speed controller and resultant inconveniences and expenses.
Another object of the invention is to provide a method of determining the speed of rotation of three phase induction motor which method is simple and easy to carry out.
Another object of the invention is to provide a three phase induction motor which eliminates the speed sensor and associated electrical connection of the speed sensor to the speed controller and resultant inconveniences and expenses.
Another object of the invention is to provide a three phase induction motor which is simple in construction and operation.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a method of determining the speed of
rotation of a three phase induction motor comprising the following steps:
i) sensing the voltages and currents Va and Ia, V2 and Ib and Vc and Ic, in the three
phases of the motor, respectively;


ii) converting the sensed three phase voltages and currents into coordinate voltages and currents using the model matrix as follows :

wherein the model V0 and I0 are zero sequence voltage and current, respectively;
iii) calculating the magnitude and angle of the rotating flux in the stator of the motor by the following equations:

wherein Rs is the stator resistance;
iv) converting the coordinate voltage and current and flux values into direct axis (d-axis) and quadrature axis ( q axis) using the Park's transformation equation as follows:

v) choosing the direct axis for coordinate transformation along the computed flux axis such that the flux component along the quadrature axis is zero without controlling the stator current and according to the decoupling theory flux in the q- axis is zero and y/, lies
in the d-axis, that is to say y/^= y/j and y/qs = 0; therefore, calculating the rotor current in the d- axis and q- axis (which is a function of the stator current and flux) by the following equations :

c/-axis component of the rotor current,

^r-axis component of the rotor current

wherein Lis is the leakage inductance and Lm is the mutual inductance.
vi) calculating the rotor speed using the following equations :
as the rotor of the motor is short circuited
and - therefore the d-axis and g-axis components of the


0000000
vii) filtering the calculated rotor speed to remove high frequency noise;
viii) comparing the calculated rotor speed with a reference rotor speed voltage; and

ix) compensating for any speed error and generating a command signal matching with the reference rotor speed voltage and outputting the command signal to the speed controller of the control unit of the motor to drive the motor at the required or designed speed.
Preferably, the calculated rotor speed is filtered in step vii in a low past filter to remove high frequency noise.
According to the invention there is also provided a three phase induction motor comprising a control unit comprising voltage sensor and a current sensor connected to each of the three phases of the motor and a microcontroller connected to the voltage sensors and current sensors and to a speed controller, the micro controller being further connected to a reference rotor speed voltage generator, the microcontroller being programmed to sense the voltages and currents in the three phases, process the sensed voltages and currents to calculate a corresponding rotor speed value, compare the calculated rotor speed value with a reference rotor speed voltage, compensate for any sensed speed error and output a command signal to the speed controller to drive the motor at the required or designed speed.
The following is a detailed description of the invention with reference to the accompanying drawings, in which the sole Fig 1 is a block diagram of the control unit of a three phase induction motor according to an embodiment of the invention. The control unit comprises a speed controller 1 connected to a three phase supply 2 and to a three phase induction motor 3. 4 is a micro controller connected to the speed controller and to


voltage sensors 5a, 5b and 5c and current sensor 6a, 6b and 6c in each of the three phases of the motor, respectively. The microcontroller is also connected to a reference speed voltage generator 7. The voltage sensors are preferably potential transformers and the current sensors are preferably current transformers. Preferably, the micro controller is a microprocessor or digital signal processor. The microcontroller is programmed based on the method as described herein. During operation of the induction motor the micro controller senses the voltages and currents in the three phases of the motor through the respective voltage sensors and current sensors and processes the sensed voltages and currents to calculate a corresponding rotor speed value. The calculated rotor speed value is compared with a reference rotor speed voltage generated by the generator 7. The microcontroller compensate for any speed error and outputs a command signal to the speed controller to drive the motor at the required or designated speed.
The invention eliminates the speed sensor for sensing the motor speed and associated electrical connection of the speed sensor to the speed controller and resultant inconveniences and expenses. The method of the invention is simple and easy to carry out. The motor of the invention is simple in construction and operation.


We claim:
1. A method of determining the speed of rotation of a three phase induction motor
comprising the following steps:
i) sensing the voltages and currents Va and Ia, V1 and lb and Vc and Ic, in the three
phases of the motor, respectively;
ii) converting the sensed three phase voltages and currents into coordinate voltages and currents using the model matrix as follows :

wherein the model V0 and I0 are zero sequence voltage and current, respectively;
iii) calculating the magnitude and angle of the rotating flux in the stator of the motor by the following equations:

wherein R, is the stator resistance;

iv) converting the coordinate voltage and current and flux values into direct axis (d-
axis) and quadrature axis (q axis) using the Park's transformation equation as follows:

v) choosing the direct axis for coordinate transformation along the computed flux axis such that the flux component along the quadrature axis is zero without controlling the stator current and according to the decoupling theory flux in the q- axis is zero and y/, lies
in the d-axis, that is to say y/ds = Wi and y/qs - 0; therefore, calculating the rotor current in the d- axis and q- axis (which is a function of the stator current and flux) by the following equations :

wherein Lis is the leakage inductance and Lm is the mutual inductance.
vi) calculating the rotor speed using the following equations :
as the rotor of the motor is short circuited
and therefore, the d-axis and g-axis components of the

rotor speed are , respectively and the average rotor speed,

,, , wherein cos = d0/dt is obtained from two consecutive
samples of q during operation of the motor and dt is the sampling interval and Rr is the rotor resistance;
vii) filtering the calculated rotor speed to remove high frequency noise;
viii) comparing the calculated rotor speed with a reference rotor speed voltage; and
ix) compensating for any speed error and generating a command signal matching with the reference rotor speed voltage and outputting the command signal to the speed controller of the control unit of the motor to drive the motor at the required or designed speed.
2. The method as claimed in claim 1, wherein the calculated rotor speed is filtered in step vii in a low pass filter.
3. A three phase induction motor comprising a control unit comprising voltage sensor and a current sensor connected to each of the three phases of the motor and a microcontroller connected to the voltage sensors and current sensors and to a speed controller, the micro controller being further connected to a reference rotor speed voltage generator, the microcontroller being programmed to sense the voltages and currents in the three phases, process the sensed voltages and currents to calculate a corresponding rotor speed value, compare the calculated rotor speed value with a reference rotor speed voltage, compensate for any sensed speed error and output a command signal to the speed controller to drive the motor at the required or designed speed.


4. The three phase motor as claimed in claim 3, wherein the voltage sensors are potential transformers and the current sensors are current transformers.
5. The three phase motor as claimed in claim 3 or 4, wherein the microcontroller is a microprocessor or digital signal processor.
.



Abstract

A method of determining the speed of rotation of a three phase induction motor comprising the following steps:
i) sensing the voltages and currents Va and Ia, lb and lb and Vc and Ic, in the three
phases of the motor, respectively;
ii) converting the sensed three phase voltages and currents into coordinate voltages and currents using the model matrix as follows :

wherein the model V0 and I0 are zero sequence voltage and current, respectively;
iii) calculating the magnitude and angle of the rotating flux in the stator of the motor by the following equations:


wherein Rs is the stator resistance;
iv) converting the coordinate voltage and current and flux values into direct axis (d-
axis) and quadrature axis ( q axis) using the Park's transformation equation as follows:

v) choosing the direct axis for coordinate transformation along the computed flux
axis such that the flux component along the quadrature axis is zero without controlling the stator current and according to the decoupling theory flux in the q- axis is zero and y/, lies in the (/-axis, that is to say = 0; therefore, calculating the rotor
current in the d- axis and q- axis (which is a function of the stator current and flux) by the following equations :
d-axis component of the rotor current,

q-axis component of the rotor current

wherein Ils is the leakage inductance and Lm is the mutual inductance.