The present invention relates to overtemperature protection for electric motors, and more particularly, to protection against excessive temperature of the rotor winding bars of squirrel-cage induction motors.
Squirrel-cage induction motors in the smaller sizes normally have cast squirrel-cage rotor windings of aluminum alloy, and overtemperature of the rotor winding is not usually a problem in motors of this size. In larger induction motors, however, squirrel-cage windings consisting of copper or copper-alloy bars brazed or otherwise joined to copper-alloy end rings are commonly used. Under locked rotor conditions, or under high slip conditions, such as at starting, very high currents occur in the rotor winding bars of these larger motors, while the cooling airflow is at a minimum when the motor is at standstill or running at low speed. Very high bar temperatures can, therefore, occur but as the motor accelerates, the rotor currents decrease and the airflow increases so that the bars do not reach dangerous temperatures if the motor accelerates normally. Under some conditions, however, the rotor bar temperature rise may become excessive. Thus, if the motor stalls, or starts slowly and does not accelerate normally, very high temperatures can result. This may occur when driving high inertia loads, for example, or if the line voltage is abnormally low, so that the motor starts very slowly. Repeated starting of the motor with insufficient cooling time between starts can also cause excessive temperature. Under any of these conditions, the temperatures may become high enough to soften or melt the brazed connections of the bars to the end rings or cause other damage resulting in mechanical or electrical failure.
Protection against overtemperature of the rotor bars has presented a difficult problem because of the difficulty of getting temperature information from the rotor while it is rotating. Slip rings have been tried for this purpose but are not satisfactory because of the space required and because they are not sufficiently reliable. Transmission of temperature signals from the rotor by means of radio transmitters, or by the use of rotary transformers having one winding on the rotor and another winding on the stator, have also been proposed as shown, for example, in Smith U.S. Pat. No. 3,824,857. Such arrangements are undesirable, however, since rotary transformers are relatively bulky devices requiring substantial extra space for the windings on both rotor and stator, and a second rotary transformer is also required to provide power to the signal transmitting transformer. If a radio transmitter is used on the rotor, a power source must also be provided for the transmitter, which may be a rotary transformer with the disadvantages just mentioned, or a battery which is necessarily of substantial size and weight and involves mechanical mounting and balancing problems as well as possible reliability problems. This type of solution to the problem of overtemperature protection, therefore, has not heretofore been found to be satisfactory. It has also been attempted to avoid the problem entirely by imposing restrictions on the type of load with which the motor may be used, or by limiting the number of starts that can be made in a given time. This approach, however, is undesirable because users and operators of these motors have a very strong tendency to disregard such instructions.