A large starting motor is provided, especially for reversible, pumped storage water wheel generator-motor units, having a rotor member which comprises a relatively thin ferromagnetic rim and a closed container of cooling fluid disposed adjacent and radially inward thereof. Heat developed due to circulation of starting currents within the rim is conducted to the cooling fluid through the inside diameter surface of the rotor rim. Provision is made for venting and replenishing the cooling fluid which heats up and in extreme cases vaporizes in response to the thermal energy transferred from the rim.
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