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Title:
DESIGN OF STATOR AND ROTORS OF INTERNAL ROTOR AC SQUIRREL CAGE ELECTRIC MOTORS AND WINDING
Document Type and Number:
WIPO Patent Application WO/2019/026083
Kind Code:
A1
Abstract:
The present invention relates to a squirrel cage motor, comprising:a stator;a rotor core rotatably disposed axially in the stator; the rotor core having a first end face and a second end face; and a plurality of longitudinal slots radially disposed along a circumference of the rotor core at an interval and a plurality of conductor bars housed in the longitudinal slots; and a pair of coaxial end rings abutting each of the first end face and the second end face of said rotor such that the cross-sectional area of coaxial ringdistalto the axis has a cross section smaller than the coaxial ring proximal from the axis.

Inventors:
SINDHI SUBHASH C (IN)
Application Number:
PCT/IN2018/000042
Publication Date:
February 07, 2019
Filing Date:
August 02, 2018
Export Citation:
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Assignee:
SINDHI SUBHASH C (IN)
International Classes:
H02P1/00; H02K15/00
Foreign References:
CN204316291U2015-05-06
US20060273683A12006-12-07
US1694061A1928-12-04
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Claims:
I/We Claim:

1. A squirrel cage motor, comprising:

a stator;

a rotor core rotatably disposed axially in the stator; the rotor core having a first end face and a second end face; and

a plurality of longitudinal slots radially disposed along a circumference of the rotor core at an interval and a plurality of conductor bars housed in the longitudinal slots, characterized in that

a pair of coaxial end rings abut each of the first end face and the second end face of said rotor,

each pair of coaxial end rings has a first coaxial ring and a second coaxial ring coaxially disposed and radially spaced apart by an annular groove, and

the first coaxial ring is proximal to the axis and the second coaxial ring is distal from the axis, the second coaxial ring having a cross sectional area smaller than a cross- sectional area of the first coaxial ring.

2. The squirrel cage motor as claimed in claim 1 , wherein width of the groove is up to 1.5 mm.

3. The squirrel cage motor as claimed in claim 1 , wherein width of the second coaxial ring (w2) and first coaxial ring (wl) is in the ratio of 1 : 10, preferably in the ratio of 1:5 and more preferably in the ratio of 2.5:4.5.

4. The squirrel cage motor as claimed in any one of the claims 1-3, wherein height of the second coaxial ring (h2) is smaller than height of the first coaxial ring (hi).

5. The squirrel cage motor as claimed in any one of the claims 1 -3, wherein the height of the second coaxial ring (h2) is equal to the height of the first coaxial ring (hi).

Description:
TECHNICAL FIELD

The present disclosure is related to anAC squirrel cage motor, more particularly relates to an AC squirrel cage rotor having an improved coaxial end rings that improves the starting characteristics of the motor without compromising with the full load performance.

BACKGROUND OF THE DISCLOSURE

Induction motors are commonly widely used as drive power sources for various machines and tools because of the merits that they are simple in construction, strong and durable, easily inspected for maintenance during operation, and reliable. Rotors of induction motors of small capacity are mostly designed to include squirrel cage windings, and such rotors are manufactured by the die casting of a metal material such as aluminum from the viewpoint of high productivity.

Squirrel cage induction motor consists of a stator and a rotor. The stator of asquirrel cage induction motor is made up with number of stampings, and these stampings are slotted to receive the stator winding. The stator may be wound with a single phase fed by a single phase current or a 3 phase winding which is fed from a 3 phase supply. It is wound for a defined number of poles, and the number of poles is determined from the required speed.

The speed and torque of three phase induction motor is given by: * ~ P

Where, f = frequency and P is the number of poles

When the motor is running at full speed, the speeds of the stator and rotor fields differ by a small amount called slip.

The speed of induction motor is given by,

N = :V, ( l - . )

Where,

N is the speed of the rotor of an induction motor,

Ns is the synchronous speed,

S is the slip.

The slip is usually a few percent of the stator frequency, and this difference is the rotor frequency. At start-up, however, the rotor is at rest, and the difference is the full amount of the stator frequency. Thus at startup, stator and rotor current can reach up to 5 to 6 times their normal operating value. The power applied to the stator circulates in the stator windings, while resulting rotor current circulate in the rotor bars and end rings placed in the rotor.

Motor may take the advantage of the skin effect of electricity by increasing rotor resistance during startup or reducing rotor resistance after startup. According to the skin effect, electricity of high frequency flows only in an outer skin of a conductor, while electricity of low frequency can penetrate conductors and more fully utilize the conductors available. At start up, rotor frequency is high, and the skin effect limits the penetration of rotor currents into the rotor bars and end rings.

The torque produced by three phase induction motor is given by,

Where,

E2 is the rotor emf

Ns is the synchronous speed

R2 is the rotor resistance

X2 is the rotor inductive reactance

Thus, the problem of high starting current with comparatively low starting torque can be solved by increasing the rotor resistance at starting. The rotor resistance at starting can be made high by using the skin effect. Previous attempts to use the skin effect to reduce start-up current and increase locked rotor torque have mainly involved the details of rotor slots. Other non-electromagnetic methods for lowering startup current and increasing locked rotor torque are mechanically involved and add parts and complexity to the motor.

Rotor of a squirrel cage motor, as shown in US Pat no. 4064410 consist of a cylindrical laminated core, having parallel slots on it. These parallel slots carry rotor conductors. In this type of rotor, heavy bars of copper, aluminum or alloys are used as rotor conductors instead of wires. The ends of rotor bars are brazed or fusionwelded toend rings at both ends. The end rings are dimensioned such that the outer diameter of the end rings is slightly smaller than the rotor diameter and the inner diameter of the ring is slightly lager that shaft of the motor. The rotor bars are thus permanently short circuited. One of the limitation associated with such an induction motor is that it is not possible to add any external resistance to armature circuit.

Double squirrel cage rotor and a deep slot squirrel cage rotor are known as squirrel cage rotor capable of improving the starting torque, as shown in US Pub. No. 2012/0187796. However, due to the fact that the depth of the slots formed in the rotor core of each of these rotor is increased and the distance between the bottom of the slots and the surface of the shaft, or the so called back height, becomes considerably small, the magnetic flux density become high in the magnetic path of the rotor core, and the magnetic saturation of the rotor core will be caused and the exciting current will be increased, which causes problems such as lowering the motor efficiency and overheating the winding.

Another limitation in the double squirrel cage rotor or the deep slot squirrel cage rotor, the length of the tooth portion between adjacent slots increases in the radial direction, and hence a greater loss occur in the magnetomotive force. Further, limitation associated with it is extra labor, expenses and inconvenience of two different types of rotor bar material.

US patent no. 6246141 provides a high torque and reducing the current electric motor by using a motor with squirrel cage rotor and end rings with reduced cross section over the outer portion of the rotor and increased cross section over the inner portion of the rotor. The end rings have an inner hole of diameter slightly greater than that of shaft of the motor. Although the solution provides advantages by way ofmcrease in resistance at time of starting, leading to small increase of up to 10% in starting torque and decrease in the resistance after start up, the poor efficiency and lack of sufficient increase in the starting torque is still a problem. Therefore, the present disclosure is directed to overcome one or more of the problems as set forth above.

SUMMARY OF THE DISCLOSURE

The present disclosure provides for a squirrel cage motor, comprising: a stator; a rotor core rotatably disposed axially in the stator; the rotor core having a first end face and a second end face; and a plurality of longitudinal slots radially disposed along a circumference of the rotor core at an interval and a plurality of conductor bars housed in the longitudinal slots, characterized in that a pair of coaxial end rings abut each of the first end face and the second end face of said rotor, each pair of coaxial end rings has a first coaxial ring and a second coaxial ring coaxially disposed and radially spaced apart by an annular groove, and the first coaxial ring is proximal to the axis and the second coaxial ring is distal from the axis, the second coaxial ring having a cross sectional area smaller than a cross-sectional area of the first coaxial ring.

In an embodiment of the present disclosure, width of the groove is up to 1.5 mm.

In an embodiment of the present disclosure, thewidth of the second coaxial ring (w2) and first coaxial ring (wl) is in the ratio of 1 : 10, preferably in the ratio of 1 :5 and more preferably in the ratio of 2.5:4.5.

In yet another embodiment of the present disclosure, height of the second coaxial ring (h2) is smaller than the height of the first coaxial ring (hi).

Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

FIG.lshows a sectional view of the rotor end rings,according to an embodiment of the present disclosure;

FIG.2showsa perspective view of a rotor assembly, according to an embodiment of the present disclosure;

DETAILED DESCRIPTION OF THE DISCLOSURE

Provided below is a non-limiting exemplary embodiment of the present disclosure and a reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Wherever possible, corresponding or similar reference numbers will be used throughout the drawings to refer to the same or corresponding parts. Moreover, references to various elements described herein, are made collectively or individually when there may be more than one element of the same type. However, such references are merely exemplary in nature. It may be noted that any reference to elements in the singular may also be construed to relate to the plural and vice-versa without limiting the scope of the disclosure to the exact number or type of such elements unless set forth explicitly in the appended claim.

The two main components of squirrel cage motorare the stator (stationary element) and the rotor (rotating element).The stator is a fixed part of the stationary motor. An iron core is in the stator housing made from thin iron sheets. These iron sheets have punched-out sections for single or three-phase windings. The windings and the stator core generate the magnetic field. The number of pairs of poles (or poles) determines the speed at which the magnetic field rotates. The rotor is mounted on a motor shaft. The rotor comprising a shaft to be disposed on an axis of the stator; a cylindrical rotor core fixedly mounted on the shaft; and a pair of end rings disposed adjacent to the opposite end faces of the rotor core.

When a motor is started, electric currents are induced in the rotor. In order to obtain a significant improvement in the starting torque, it is necessary to reduce the rotor currents through increase in resistance during start-up. Once the motor is running at normal speed, lower resistance is needed for economical operation of the motor. This result is achieved by providing the squirrel cage motor in combination with the end rings such that the cross- sectional area of first coaxial ring has a cross section smaller than the secondcoaxial ring.

FIG. 1& FIG. illustrates the sectional view of the rotor end rings and a perspective view of a rotor assembly respectively. A pair of coaxial end rings abut each of the first end face and the second end face of the rotor. Each pair of coaxial end rings has a first coaxial ring and a second coaxial ring coaxially disposed and radially spaced apart by an annular groove, and the first coaxial ringis proximal to the axis and the second coaxial ring is distal from the axis, the second coaxial ring having a cross sectional area smaller than a cross-sectional area of the first coaxial ring.

For a motor of up to 1 kW, the width of the groove can be up to or lower than 1.5 mm depending upon a manufacturing process used. It was found that decreasing the size of the groove will not have significant impact on the starting torque or full load torque of the motor. For example, for a motor of up to 5 kW, the width of the groove can be up to 3mm.

The width of the first coaxial ring (wl) and second coaxial ring (w2) is in the ratio of 1 : 10, preferably in the ratio of 1 :5, and more preferably in the ratio of 2.5:4.5.

In the preferred embodiment of the present invention, the height of the second coaxial ring (h2) is smaller than the height of the first coaxial ring (hi).

With the above configuration, the cross -section of the proximal portion is not reduced so that the rotor current will have little resistance during the normal operation.Upon start up, the skin effect limits the rotor current to the distal portion, thus assuring high resistance and thereby high starting torque for the motor. Once the motor started up, the rotor current were able to penetrate into the rotor and use the full proximal portion for attaining lower resistance. Thus, the motor runs with lower resistance after startup. It can thus be seen that the present invention provides a squirrel cage motor with significant increase in the starting torque of about 40 % without compromising with the full load performance. Thus, present invention provides a highly efficient, robust in construction and economical motor.

Example 1:

The motor was made in 108* 108 square stacks with 32 mm stack height, meant for semiautomatic washing machine. The outer diameter of rotor is 68 mm and shaft is of 12mm. The rotor is provided with pair of coaxial end rings includes two rings coaxially disposed radially spaced apart by an annular groove of width 1.4 mm such that the OD/ID of the first portion being distal to the axis is 66/61 mm and the OD/ID of the second portion being proximal from the axis is 59/50 mm. The width of the second portion is (66-61 )/2 i.e. 2.5 mm and width of the first portion is (59-50)/2 i.e. 4.5 mm.

Results: It was found that the starting torque before providing a groove is 0.075 kg-m and the starting torque after providing the groove of width 1.4mm is 0.105 kg-m. Thus, with this configuration, significant improvement of about 40 % greater starting torque is achieved thereby providing advancement over the state of art with no compromise with the full load torque thereby, providing a highly efficient motor.

Example 2: Similar motor was made, but this time each pair of coaxial end rings of the rotor includes two rings coaxially disposed radially spaced apart by an annular groove of width 2 mm.

Results: It was found that with this configuration, significant improvement of starting torque is achieved over the state of art but the full load torque was compromised.

Advantages

In an embodiment, the disclosed squirrel cage motor with end rings divided into two coaxial rings by a groove, in such a manner that the coaxial ring distal to the axis has high electrical resistance than the coaxial ring proximal to the axis.

In an embodiment, the disclosed squirrel cage motor with end rings divided into two coaxial rings by a groove,significant improvement of about 40 % greater starting torque is achieved over the state of art. In an embodiment, the disclosed squirrel cage motor with end rings divided into two coaxial rings by a groove, significant improvement of starting torque is achieved with no compromise with the full load torque.

In an embodiment, the disclosed squirrel cage motor with end divided into two coaxial rings by a groove, provides a highly efficient motor, robust in construction and economical.

While aspects of the present invention have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by modification of the disclosed device without departing from the scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present invention as determined based upon claims and any equivalents thereof.