Q: What does it mean to say that a reversible motor is rated for 30 minutes?
A: Reversible motors require a larger input power than induction motors to increase the starting torque and improve the instant reversing characteristics. This means that loss is high and the temperature rises more during continuous operation. If operated continuously, the motor will burn out. It is thus designed to provide maximum performance if operated for no more than 30 minutes continuously.
When using a reversible motor intermittently for a short period of time, a large current flows, which causes the generation of a large amount of heat when starting or reversing. However, as the natural cooling effect of the motor is high when the motor is left stopped for a longer period of time, you can curb rises in temperature.
The motor case temperature equals the rise in motor temperature plus the ambient temperature, Generally, if the case temperature of the motor is 90degC (194F) or less, continuous motor operation is possible with the same operating conditions, considering the insulation class of motor winding. However, the lower the motor temperature is, the longer the bearing grease life is.
The motor temperature varies according to conditions such as the load, the operating cycle, the mounting method of the motor and the ambient temperature.
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