PURPOSE: To alleviate amount of iron core and realize light weight by arranging two sheets of support plates at the upper and lower areas between the magnetic pole and rear yoke and keeping the size between poles to the specified value by extending them with a jack.

CONSTITUTION: The support plates 7 which bridges the yoke 1a and pole 1b are arranged at the upper and lower areas at the part covering he excitation coil 3 of iron core 1. A jack 8 is provided between such support plates 7 and these plates 7 are adjusted to extend. A stepped part 9 is formed at the iron core in the rear side in the side of poles 1b and yoke 1a in order to make constant the position of support plates 7. This jack 8 can be comprised easily by lifting or pushing down the support plates 7. The internal side of coil 3 can be supported by inserting a rubber sheet 10 between the support plates 7 and coil 3.