This invention relates to an improvement of a device adapted to ground an elevator winder.
FIG. 1 shows an elevator winder driven by an electronic motor controlled by a thyristor Leonard system. In FIG. 1, reference characters R, S and T designate a three-phase AC supply and 2a, the armature of a DC motor in which the applied voltage is controlled by a thyristor converter 1 (the field not shown). The shaft 3 of the armature 2a, and a machine supporting stand 4 are set through vibration-proof rubber members 5 in a machine room. A grounding line 6 is used to ground the machine supporting stand 4. Bearings supportings 7 and 8 of the shaft 3 are disposed on the stand 4.
A traction sheave 9 is fixedly mounted on the shaft 3. A secondary sheave 10 is mounted on a shaft 11. Bearings 12 and 13 support the shaft 11 on the machine supporting stand 4. Element 14 is a main rope wound on both of the traction sheave 9 and the secondary sheave 10. A cage 15, a balance weight 16 complete the elevator structure. The capacitance 17 between the armature 2a and the shaft 3 thereof and stray capacitances 18, 19 and 20 between the supply lines and the ground complete the electrical circuit.
The machine supporting stand 4 is grounded through the grounding line 6 in accordance with Electrical Equipment Installation Standards (Japan). Accordingly, the shaft 3, and the bearings 7, 8, 12 and 13 are also grounded.
It is well known in the art that, when an electric motor is controlled by the use of a high-speed switching element such as a thyristor, then shaft current flows in the elecric motor. More specifically, when the thyristor converter 1 is operated, a pulsive current flows through a circuit (R--1--2a--17--3--7--4--6--19--S) as indicated by the arrow A, or a pulsive current flows through a different circuit (R--1-- 2a--17--3--9--14--10--11--13--4--6--19--S). In general, the bearings 7, 8, 12 and 13 are ball-and-roller bearings. Therefore, when current flows in these bearings, the coating surfaces of the balls or the rollers are electrically peeled off, and the balls or the rollers are quickly damaged.
In order to prevent the flow of the shaft current through the bearings 7, 8, 12 and 13, a grounding brush 22 as shown in FIG. 2, is provided in contact with the shaft 3 of the electric motor 2 with the grounding brush 22 being grounded through a grounding line 23. However, this method is disadvantageous in that it is necessary to extend the shaft 3 outwardly, that is, the overall dimension is increased. Also, the current flowing in the bearings 7, 8, 12 and 13 is not always zeroed depending on the contact resistance of the grounding brush 22, and the maintenance of the brush 22 is troublesome.