Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-08-06
2001-10-30
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S220000, C310S221000, C310S045000
Reexamination Certificate
active
06310418
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to rotating electrical machines.
More particularly the invention relates to rotating electrical machines rated at powers in excess of 10 kilowatts, of the kind having a rotor comprising conductor bars of copper or similar conducting material located in slots in a rotor core of magnetic material and connected together to form a winding. These will be referred to below as electrical machines of the kind specified. Examples of such machines are induction motors wherein the rotor conductor bars are connected at their ends to form a cage winding and synchronous motors wherein the rotor conductors are connected to form a damper cage winding.
2. Description of the Related Art
During manufacture of the rotor, it is customary to insert the copper or aluminum conductor bars by force-fitting them into the slots, thereby slightly distorting the bars and/or the slots. This procedure is intended to ensure a very tight fit of the bars into the slots, thereby obviating any relative movement between them and so enabling the bars to remain fixed securely in the slots throughout the service life of the machine.
In such machines the rotor conductor bars are usually uninsulated. However, in recent years it has become apparent that in some circumstances sparking can occur between the uninsulated conductor bars and the rotor core. Surprisingly, such sparking can occur even when uninsulated bars have previously been tightly located in their slots in the rotor core. Such sparking is unacceptable in machines required to operate in explosive atmospheres.
It has been discovered that such sparking can be prevented by ensuring that the conductor bars are continually in perfect electrical contact with the rotor core. However, while such contact can perhaps be achieved in a new machine, it is impossible to guarantee that such contact is maintained throughout the service life of the machine, due for example to in-service cyclic stresses and to the different thermal expansions of the materials of the conductor bars and the rotor core.
An alternative solution to the problem of sparking is to insulate the conductor bars electrically from the rotor core. However, the selection of a satisfactory method of insulation has proved difficult. For example, wrapping the conductors in insulating tape or sheet has proved unsatisfactory because it is not possible to wrap the tape or sheet sufficiently tightly to achieve adequate tightness of the wrapped conductors in the rotor core slots to avoid fatigue failure of the bars.
Two other methods of insulation that have been proposed are disclosed in, for example, Japanese Patent No. 63-253 842 and U.S. Pat. No. 3,213,302.
Japanese Patent No. 63-253842 is concerned with the provision of insulation between a field coil and an iron core and between the turns of the field coil and proposes the use for the insulating material between the turns, of an inorganic insulating material, resin and a curing agent applied to a glass cloth. U.S. Pat. No. 3,213,302 on the other hand, discloses the use of sodium phosphate insulation with steel laminations and conductor bars as used in magnetic cores of electric power equipment.
The possibility of applying a silicon dioxide coating to copper rods in a squirrel cage motor is mentioned in U.K. Pat. No. 824 861 but such a coating is not designed to prevent sparking and would not be utilizable in the large machines with which the present invention is concerned.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rotating electric machine of the kind specified, wherein the problem of sparking is alleviated so as to ensure long term safe operation of the machines in potentially explosive atmospheres.
According to the present invention there is provided a rotating electrical machine rated at a power in excess of 10 kilowatts having a rotor comprising: a rotor core of magnetic material and having slots therein. Conductor bars are located in the slots in the rotor core and electrically connected together to form a winding. A coating on the conductor bars of a heat treated ceramic-based material has a firing temperature. The coating is applied to the conductor bars and the ceramic-based material undergoes a heat treatment before the conductor bars are located in the slots. The heat treatment is at or above the firing temperature of the ceramic-based material, and the coating is adapted to prevent sparking between the conductor bars and the rotor core.
The coating suitably consists of a ceramic-based material, preferably a micro-porous alumina ceramic-based material, such a coating being suitably applied to the conductor bars using a plasma spraying process. Alternatively, the coating may be a glass ceramic, such as a lithia/zinc oxide/silicate (LiO
2
/ZnO/SiO
2
) glass ceramic.
The coating's thickness should be less than 500 micrometers, but greater than about 15 micrometers, preferably 20 to 250 micrometers, with glass ceramic coatings being at the bottom end of the latter range.
When the coating is of micro-porous material, it is suitably impregnated with a synthetic resin material before insertion of the conductor bars into the slots.
In a further aspect, the invention provides a rotor for a rotating electrical machine, the rotor comprising conductor bars and a magnetic iron rotor core having slots therein for the reception of the conductor bars. The conductor bars are force-fitted into the slots and consist of softer metal than the iron rotor core. To prevent sparking from occurring between the conductor bars and the rotor core, and to inhibit loosening of the conductor bars in the slots during the service life of the machine due to unwanted abrasion of the conductor bars during and after the force-fitting process, the conductor bars are electrically insulated from the rotor core by a coating on the conductor bars of a ceramic-based material. The coating is applied prior to force-fitting of the conductor bars into the slots, whereby said unwanted abrasion is transferred from the relatively soft conductor bars to the harder iron core and thereby reduced.
In yet another aspect, the invention provides a rotor for a rotating electrical machine, comprising: a rotor core of magnetic material, the rotor core having slots therein. Conductor bars are located in the slots in the rotor core and are electrically connected together to form a winding. A plasma-sprayed ceramic-based powder coating is provided on the conductor bars. The coating has a microporous structure comprising the sprayed powder with voids therebetween. The coating is applied to the conductor bars before the conductor bars are located in the slots and the coating is adapted to prevent sparking between the conductor bars and the rotor core. The coating is impregnated with a synthetic resin material such that the voids are filled with the synthetic resin material, whereby the insulation, resilience and heat transfer characteristics of the coating are improved.
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Abstract of the contents of p. 86 to 132 ofDie Isolierstoffe der Elektrotechnik(the Insulating materials used in Electrical Engineering), pp. 1-6 and full text.
Clark Paul Eaton
Regan Roger Henry
Alstom UK Limited
Kirschstein et al.
Lam Thanh
Ramirez Nestor
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