From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical letter explaining the operation and function of a thermostat unit for a generator.
| Identifier | ExFiles\Box 48\3\ Scan003 | |
| Date | 15th April 1921 guessed | |
| X. 4200 Rolls-Royce of America, Inc. Springfield, Mass. Attention: Mr. M. {Mr Moon / Mr Moore} Olley Gentlemen: I am mailing to you under separate cover one of our thermostat units. You will note that it is a simple resistance which is short circuited by a pair of points, one of them being mounted on a bi-metal member. When the generator becomes hot, the points are open and the resistance is thrown into the field circuit. This device is made to mount under the commutator cover band and is attached to the machine body proper. Therefore, it is dependent for its temperature on the conduction of heat from the metal to which it is attached. We use this in conjunction with the third brush machine. We set the output cold at 20 to 21 amperes. In case it is desirable, we can set the output as high as 25 or 26. This high amperage, of course, will heat the machine up quickly. The thermostat is normally adjusted to open at 160° Fahrenheit. In special cases it may be made to open at as low a temperature as 120 or as high as 200. When the generator body heats up, causing the thermostat points to open, the resistance unit is put into the field circuit, thereby reducing the output. Since our machines will operate continually at outputs as great as 18 amperes, it makes it practical for us to fix the low output of the machine at any amount from 5 to 15 amperes. Normally we have found that an output of 8 to 10 amperes, when the thermostat points are open, is best suited to motor vehicle demands. On motor trucks, we fix the low output as low as 5 amperes. Since the temperature of the generator is greatly affected by the atmosphere passing over it, the rate at which the battery is charged is much higher in cold weather than it is in hot weather - for instance, at zero weather, the generator will not heat up sufficiently to open the thermostat, unless the car is driven continually for at least 35 to 45 minutes, thereby giving a high battery charging rate, in the winter time when most current is demanded from it. However, if a long hard drive is made in the winter, the thermostat will open and protect the battery from a too long continued high charge rate. | ||