A permanent magnet rotor, comprising: a rotor core having a generally cylindrical shape with an outer circumferential surface and a rotational axis; a plurality of magnet insertion holes formed in the rotor core and arranged circumferentially at a preset angular interval about the rotational axis, each hole extending in a direction parallel to the rotational axis and having a cross section of a generally elongated slit; at least one of the elongated slits is curved concave relative to the circumferential surface and having a radially inward side, a radially outward side, and two ends that are respectively spaced apart from the outer circumferential surface by two bridges thereby to form a rotor center core section located circumferentially between the bridges and located radially between the radially outward side of the slit and the outer circumferential surface of the rotor core; and more than one permanent magnet inserted in each of the holes; wherein the magnets in each hole are arranged such that each of the magnets is in direct contact with an adjacent magnet so that centrifugal forces imparted on a center portion of the magnets is transferred through outer portions of the magnets to the bridges, characterized in that each of the bridges have a thicker portion and a thinner portion that are respectively positioned near the radially inward and outward sides of the hole, and extend between the end of the hole and the outer circumferential surface of the rotor core, and the bridges have recesses that are open and not filled with the magnets thereby forming air gaps between the permanent magnets and the thinner portions, such that the thicker portions are in direct contact with the magnets and the gaps increases continuously from the one end of the thinner portion, and cause the centrifugal forces to be transferred to the thicker portion of the bridge and relieve stress in the thinner portion of the bridge. a rotor core having a generally cylindrical shape with an outer circumferential surface and a rotational axis; a plurality of magnet insertion holes formed in the rotor core and arranged circumferentially at a preset angular interval about the rotational axis, each hole extending in a direction parallel to the rotational axis and having a cross section of a generally elongated slit; at least one of the elongated slits is curved concave relative to the circumferential surface and having a radially inward side, a radially outward side, and two ends that are respectively spaced apart from the outer circumferential surface by two bridges thereby to form a rotor center core section located circumferentially between the bridges and located radially between the radially outward side of the slit and the outer circumferential surface of the rotor core; and more than one permanent magnet inserted in each of the holes; wherein the magnets in each hole are arranged such that each of the magnets is in direct contact with an adjacent magnet so that centrifugal forces imparted on a center portion of the magnets is transferred through outer portions of the magnets to the bridges, characterized in that each of the bridges have a thicker portion and a thinner portion that are respectively positioned near the radially inward and outward sides of the hole, and extend between the end of the hole and the outer circumferential surface of the rotor core, and the bridges have recesses that are open and not filled with the magnets thereby forming air gaps between the permanent magnets and the thinner portions, such that the thicker portions are in direct contact with the magnets and the gaps increases continuously from the one end of the thinner portion, and cause the centrifugal forces to be transferred to the thicker portion of the bridge and relieve stress in the thinner portion of the bridge.