FORM 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 making a compact AC induction motor having a rated power
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Lokesh Jayal of Crompton Greaves Ltd, M5 Division, Ahmadnagar, Maharashtra, India, an Indian National
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 THE INVENTION
The present invention relates to making a compact AC Induction motor having a rated power. More specifically, the invention relates to a method of designing a compact AC induction motor for a given rated power.
BACKGROUND OF THE INVENTION
An induction motor is a type of alternating current motor which converts electrical energy to mechanical energy. An AC induction motor consists of two main parts, stator and rotor, where the rotor is rotating part and stator is stationery part. In a typical induction motor, the electrical power is converted to mechanical power in the rotor where the power is supplied to the rotor by means of electromagnetic induction.
Induction motors find applications in centrifugal pumps, where the centrifugal pumps are commonly used to move fluids through a piping system. However, in the existing systems, the induction motors are huge in size and are therefore costly and cumbersome. The power of an induction motor is directly proportional to magnetic flux density, diameter and length of the stator. If the diameter of the stator is reduced, then in order to maintain the rated power and magnetic flux density, the length has to be increased. As a result, there is no significant reduction in the size of the motor at a given power and magnetic flux density.
Therefore, there is a need for a method of making a compact AC induction motor with reduced dimensional parameters at a rated power.
OBJECTS OF THE INVENTION
An object of the invention is to make a compact AC induction motor with reduced dimensional parameters at a rated power
Another object of the present invention is to decrease the diameter of stator of an induction motor without increasing its length at a rated power.

DETAILED DESCRIPTION OF THE INVENTION
According to the invention, there is provided a method of making a compact AC induction motor having a rated power Q governed by the equation Q α Bav1D1 L1, in which, D1 and L1 are the diameter and length of the stator of the motor respectively, and BaV1 is the average magnetic flux density in the stator core. The method comprising reducing the diameter of the stator, without varying the length, to a value at which the average magnetic flux density increases to a maximum threshold value, the threshold value being determined based on the maximum acceptable limit of current induced by the average magnetic flux density in the stator windings of the motor, such that the rated power of the induction motor is governed by the equation Q α Bav2E>22 L1, wherein D2 is the reduced diameter of the stator, L1 is the length of the stator, and BaV2 is the increased average magnetic flux density in the stator windings, wherein an increment in the current density in the stator windings due to increased average magnetic flux density is compensated by increasing the cross-sectional area of the conductor of the stator windings.
Preferably, at the rated power 5 HP, the reduced diameter of the stator is 82.55 mm, the length of the stator is 100mm and the increased average magnetic flux density in the stator core is 0.65 webers per square meter.
These and other aspects, features and advantages of the invention will be better understood with reference to the following detailed description, accompanying drawings and appended claims, in which,
Fig 1 is a flowchart illustrating a method of making a compact AC induction motor having a rated power.
With reference to figure 1, a method of making a compact AC induction motor at a rated power is described. An induction motor comprises a stator and a rotor, where the rotor is rotating part and stator is stationary part. The stator of the motor has windings around it which carry a supply current to induce an alternating magnetic field that surrounds the rotor. The alternating magnetic field created by the stator induces an

alternating current in the rotor conductors. The induced alternating current further interacts with the alternating magnetic field and to cause a rotational motion on the rotor.
For an AC induction motor, the output power is governed by the equation
Q=C0D2Lns, (1)
where Q is kVA rating of the motor, C0is output coefficient, D is diameter of the stator bore, L is length of stator core and ns is rated speed of the motor. The output coefficient is further governed by the equation
C0=llKwBav-ac*10-3 (2)
where Bav is average magnetic flux density in the stator core, Kw is the winding factor, and 'ac' is ampere conductors. The 'ampere conductors'' is the product of the number of conductors in the stator windings and the current circulating in these conductors. The 'ampere conductors' represent the electrical loading of the motor which in turn determines the temperature rise, insulation and current density in the motor.
Combining equations (1) and (2),
Q=llKwBavD2Lns-ac*10-3 (3)
When the value of ampere conductors', winding factor and rated speed of a motor is kept constant, then the output power of the motor depends on average magnetic flux density, diameter and length of the stator,
i.e. QaBavD2L (4)
In an embodiment of the present invention, for a given rated power, an AC induction motor is made which is compact with respect to existing motor designs. An existing motor design at a rated power may have values of average magnetic flux density,

length and diameter as BaV1, D1 and L1 respectively, wherein the rated power is governed by the equation Q a BaV1D12L1
In step 101, the diameter of the stator is reduced, without varying its length. In an embodiment of the present invention, the diameter of the stator is reduced from an initial value D1 at a constant length L1. It may be noted that when the diameter of the stator is reduced, the average magnetic flux density in the stator core is increased. The average magnetic flux density increases due to decrease in surface area of the stator. Further, the magnetic flux density in the stator induces a magnetic current in the stator windings. Therefore, when the average magnetic flux density in the stator core increases, the magnetic current induced in the stator windings increases, and so the heat dissipated due to magnetic current. As a result, the reduction in diameter of the stator has a direct effect on amount of heat dissipation in the stator core.
In step 102, it is checked whether the average magnetic flux density Bav1 in the stator core increased due to reduction in diameter has reached a maximum threshold value BaV2- The maximum threshold value BaV2 is determined based on the maximum acceptable limit of magnetic current which would cause an allowable amount of heat dissipation in the stator windings.
If the average magnetic flux density has reached the maximum threshold value BaV2, then the corresponding diameter of the stator is set as the reduced diameter D2. If the magnetic flux density has not reached a maximum threshold value, then the diameter of the stator is further reduced and step 102 is repeated.
At reduced diameter D2, increased magnetic flux density B2, and constant length L1, the rated power of the induction motor is governed by the equation Q α BaV2D22 L1.
The increment in magnetic current due to increased average magnetic flux density leads to increase in overall current in the stator windings, which leads to increased current density thereof. The increment in the current density in the stator windings is compensated by increasing the cross-sectional area of the conductor of the stator windings.

In an embodiment of the present invention, the increment in cost of the conductor of the stator windings due to increased cross-sectional area is compensated by reduction in overall length of conductors required for stator windings due to decreased diameter of the stator.
Example:
At rated power 5hp, an existing design is taken which has a stator bore diameter as 95
mm, length as 100 mm (112F stator frame) and average magnetic flux density around the
stator periphery at the air gap is 0,51 webers per square meter. By decreasing the diameter
to 82.55mm (100F stator frame) at constant length, the average magnetic flux density is
increased from 0.51 to 0.65. At a rated power of 5 hp, the parameters
of existing and proposed designs are summarized in the below table:

Parameter Existing Design Proposed Design
Stator bore diameter (mm) D1 =95 D2 = 82.55
Length of stator (mm) L,=100 L1 - 100
Average Magnetic flux density (webers per square meter) Bav1 = 0.51 Bav2 = 0.65
Using above-mentioned values, the output power Q1 and Q2 of existing and proposed designs are compared using equation (4):

The output power of existing and proposed designs are approximately same. Therefore, it can be inferred, at a rated power, the proposed design has a reduced diameter and is therefore compact with respect to an existing design. The invented design has a smaller size stator frame and can be applicable in high voltage band application in centrifugal pumps leading to cost reduction and improved output.

Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the invention as defined in the appended claims.

We claim:
1. A method of making a compact AC induction motor having a rated power Q
governed by the equation Q a Bav1D12L1, in which D1 and L1 are the diameter and length
of the stator of the motor respectively, and Bav1 is the magnetic flux density in the stator
core, the method comprising:
reducing the diameter of the stator, without varying the length, to a value at which the magnetic flux density increases to a maximum threshold value, the maximum threshold value being determined based on the maximum acceptable limit of current induced by the magnetic flux density in the stator windings of the motor, such that the rated power of the induction motor is governed by the equation Q a Bav3D22 L1, wherein D2 is the reduced diameter of the stator, L1 is the length of the stator, and Bav2 is the increased magnetic flux density in the stator windings, wherein an increment in the current density in the stator windings due to increased magnetic flux density is compensated by increasing the cross-sectional area of the conductor of the stator windings.
2. The method as claimed in claim 1, wherein at the rated power 5 HP, the reduced
diameter of the stator is 82.55 mm, the length of the stator is 100mm and the increased
magnetic flux density in the stator core is 0.65 webers per square meter.