1. | Reduce driving power until the load has been transferred to the other alternators, adjusting field rheostat to obtain minimum current; |
| 2. | Open main switch; |
| 3. | Open field switch. |
Ques. What precaution should be taken?
Ans. Never open field switch before main switch.
Fig. 2,852.—General Electric 500 kw., horizontal mixed pressure Curtis turbine connected to a 500 kw. dynamo. In a Curtis turbine it is not necessary to use the whole periphery of the first stage for low pressure steam nozzles. A section can be partitioned off and equipped with special expanding nozzles to receive steam at high pressure direct from the boilers. Such nozzles deliver their steam against the same wheel as do the low pressure nozzles, but occupy only a small portion of its periphery. The steam is expanded in these nozzles from high pressure all the way down to the normal pressure of the first stage, and in such expansion acquires a high velocity and consequently contains a great deal of energy—much more than does an equal quantity of low pressure steam. In consequence of this, high pressure steam is used with a far lower water rate than is obtained with low pressure steam, or with high pressure steam reduced to low pressure in a reducing valve. This construction is called "mixed pressure." Its function is the same as that of the reducing valve, that is, it makes up for a deficiency of low pressure steam by drawing direct on the boilers. With this construction, the full power of the turbine can be developed with: All low-pressure steam, all high pressure steam, or, any necessary proportion of steam of each pressure. Furthermore, the transition from all low pressure to all high pressure, through all the conditions intermediate between these extremes, is provided for automatically by the turbine governor; a deficiency of low pressure steam causes the high pressure nozzles to open automatically.
Ques. What is the ordinary method of cutting out an alternator?
Ans. The main switch is usually opened without any preliminaries.
Ques. What is the objection to this procedure?
Ans. It suddenly throws all the load on the other alternators, and causes "hunting."