Resistance Start/Induction Run (RSIR)
This motor type is also referred to as Split-phase motors. They are usually cheaper than other single-phase motor types used by industries, but also have some limitations in terms of performance.
The starting device of RSIR motors comprises two separate windings in the stator. One of these is used exclusively for starting and is wound with a smaller wire size with higher electrical resistance than the main windings. This causes a time delay for the rotating field, which in turn makes the motor start. A centrifugal
or electronic switch disconnects the starting winding when the motor reaches approximately 75% of the rated speed. The motor will then continue running according to standard induction motor principles.
As mentioned above, certain limitations apply to RSIR motors. Their starting torques are low, often between 50 to 150 per cent of rated load. The motor also develops high starting currents, approximately 700 to 1000 % of rated nominal current. As a result, overly long starting times will cause the starting winding to
overheat and fail. Of course, all this means that this motor type should not be used in situations where high starting torques are required.
RSIR motors are designed for single voltage, which naturally limits the range of potential applications. Their maximum running torques range from 100 to 250% of normal. It should also be mentioned that thermal protection is difficult because it is hard to find
a protector with trip-off time fast enough to prevent the starting winding from burning out.
RSIR motors are well suited for small grinders, fans, and other applications with low starting torque and power needs from 0.06 kW to 0.25 kW. They are not suitable for applications which require high torques or high cycle rates.
RSIR motor. Schematic diagram and torque/speed curve
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