Originally Posted by
Mike Henderson
Just a correction - Induction motors are not synchronous motors. They always have some "slip" (the difference between the hertz and the RPM). As the motor is loaded (any loading, not just overloaded) the slip increases, which means the RPM decreases. The specification of the RPM of a two pole 60hz motor is usually 3450RPM which means that at full load, the motor will run at (approximately) that RPM. When loaded lightly, it will run at a higher RPM. When loaded beyond the specification of the motor it will run at an RPM less than 3450. The change in RPM is smooth (up to a point), meaning that if you add a small increment of load, you get a small decrease in RPM.
If you load it enough the RPMs "crash" towards zero (the motor stalls). When you overload a motor, the motor is trying to create more HP than it was designed for. HP is torque times RPM (times a constant) so as you ask it to create more and more HP, the RPMs go down. If the motor cannot generate more torque to counter the slowdown in the RPMs the HP goes down. Once you reach that point, the motor stalls. Most motors can actually produce a bit more HP than the nameplate but they will be drawing excess current, which leads to excess heat. If continued, the motor fails from the heat.
Even when completely unloaded, an induction motor will not run at 3600RPM. An induction motor requires some slip to operate.
There are synchronous motors and those have an energised rotor - meaning that there are brushes and current is provided to the rotor. In an induction motor there are no brushes and the current in the rotor is "induced" by the difference between the hertz and the RPM.
Mike