1.  Field of the Invention
The present invention relates to a method of starting a rotating machine, and more particularly, to a method of starting a rotating machine, such as a wind turbine generator, disposed in a cold environment. 
2.  Description of the Related Art
Wind turbine generators are increasingly constructed across the globe in favor of promotion of use of renewable energy.  It is therefore demanded to construct wind turbine generators even in a cold environment. 
One of issues necessary to be considered in constructing a wind turbine generator in a cold environment is that lubricant of a lubrication system is cooled when the wind turbine generator stops operating.  When the lubricant is cooled after the wind turbine generator stops operating, the viscosity of the lubricant is increased, causing deterioration of the circularity of the lubricant.  When the viscosity of the lubricant is excessively increased, load is imposed on equipment (such as a lubricant pump) constituting the lubrication system and pipes, potentially causing equipment malfunction or leakage of the lubricant.  This problem is serious particularly when the wind turbine generator in an extremely cold environment is to be started. 
To solve such a problem, a heater that heats lubricant is generally provided in a wind turbine generator disposed in a cold environment.  At the starting of the wind turbine generator is started, a lubricant pump is actuated to start the lubrication system, after the lubricant is heated by the heater.  For example, US Patent Application Publication No.  US2009/0191060A1 discloses a technique for providing a heater in a drain pipe discharging lubricant from a speed-up gear to a lubricant pump, thereby avoiding damages of the lubricant pump. 
According to study of the inventor of the present invention, however, the technique for supplying the heated lubricant is not always appropriate for a component, such as a main bearing, having a large heat capacity and a large contact area with the lubricant.  The main bearing is cooled after the wind turbine generator stops operating.  When the heated lubricant is supplied to the cooled main bearing, then the lubricant is instantly cooled to increase the viscosity of the lubricant, causing deterioration of the lubricant discharge efficiency.  When lubricant of an amount exceeding the amount of lubricant dischargeable from the main bearing is supplied to the main bearing, leakage of the lubricant may occur. 
To provide a heater heating the main bearing may be an approach for avoiding the above-stated problem; however, the approach of providing the heater heating the main bearing suffers from problems of a larger heat capacity of the main bearing of the wind turbine generator, and inevitable heat transmission to the nacelle base, which has a large heat capacity.  To heat the main bearing to a necessary temperature using the heater, it is necessary to provide a heater having a considerably large heating capacity and a considerably long heating time is required.  In these circumstances, it may be impractical to heat the main bearing using a heater.  The above-described problem also applies to other rotating machines including a bearing with a large heat capacity (for example, shield machines, printing machines and industrial machines such as power generator turbines).