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An induction motor draws very high starting current during its acceleration to full rated speed, typically 6 to 10 times the full load current. Reduced starting current is desirable where the electrical grid is not of sufficient capacity, or where the driven load cannot withstand high starting torque.
Self-starting polyphase induction motors produce torque even at standstill. Available squirrel-cage induction motor starting methods include direct-on-line starting, reduced-voltage reactor or auto-transformer starting, star-delta starting or, increasingly, new solid-state soft assemblies and, of course, variable frequency drives (VFDs). [39]
AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed.
By adjusting the shape of the bars in the rotor, the speed-torque characteristics of the motor can be changed, to minimize starting current or to maximize low-speed torque, for example. Squirrel-cage induction motors are very prevalent in industry, in sizes from below 1 kilowatt (1.3 hp) up to tens of megawatts (tens-of-thousand horsepower).
Nevertheless, across-the line starting of induction motors is accompanied by inrush currents up to 7-10 times higher than running current, and higher efficiency motors can experience inrush currents 10-15 times running current. In addition, starting torque can be up to 3 times higher than running torque. The starting torque transient can create ...
In the past, repulsion-start wound-rotor motors provided high starting torque, but with added complexity. Their rotors were similar to those of universal motors, but their brushes were connected only to each other. Transformer action induced current into the rotor.
Public Domain. In the 1930's, Nikola Tesla, who is known for creating the induction motor and refining AC currents, imagined a machine that would allow you to project a mental image in real life ...
A resistance start motor is a split-phase induction motor with a starter inserted in series with the startup winding, creating reactance. This added starter provides assistance in the starting and initial direction of rotation. The start winding is made mainly of thin wire with fewer turns to make it high resistive and less inductive.
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