What would be the type of Reduced voltage magnetic starter is best suited to use in a 300-hp 440v 3phase motor

Question

I just want to know the appropriate reduced voltage magnetic starter type of motor controller, to use in controlling a 300-hp or up to 1000-hp, 440v, 3-phase motor; Does Star-Delta magnetic starter appropriate? How about an Auto-transformer reduced voltage?

Answer 1

The appropriate reduced voltage starter is not determined by the motor except that star-delta starting is only possible if the motor has terminals brought out for star-delta and the available supply voltage corresponds to the motor's delta voltage. Part winding starting also requires a motor with appropriate connections available.

The various starting methods have advantages and disadvantages in terms of performance, physical size, cost and complexity. The electromechanical methods include direct on line (DOL), auto transformer, series resistor or reactor, part winding and star-delta. There are also electronic reduced voltage starters. A variable frequency drive (VFD) can be used as a starter that provides regulated current up to 150% of rated torque or perhaps a little higher with percentage current close to the same as percentage torque.

Among the electromechanical methods, direct on line is the lowest cost, followed by part winding, star-delta, series resistor/reactor and autotransformer in approximate order of increasing cost and complexity. A solid state starter might not cost more than star-delta. A except in very small sizes  VFDs are more expensive than any other alternative and are suitable only for extremely high inertia loads that take a very long time to accelerate.

Compared to DOL, star-delta provides 33% torque for 33% current, and part winding provides 48% torque for 65% current. With typical component selection, series resistor or reactor starting provides about 49% torque for 70% current. Auto transformer starting typically provides three voltage tap alternatives for 25%, 42% and 64% torque with 25%, 42% and 64% current. Note that the above percentage torque and current values are percentages of the DOL starting torque and current values. DOL starting provides something like 150% torque at stall and 225% torque as the motor approaches full speed with a maximum current that is something like 500% to 700% of the rated full load running current.

Electronic reduced voltage starters provide adjustable current limiting with a controlled rate of acceleration and current increase. If the current is allowed to rise to a level equal to the current require for DOL starting, available torque is comparable to DOL starting. With current limited to a level comparable to series resistor/reactor starting, the torque is comparable to that starting method.

There are other performance issues not covered here including duty cycle limits, current balance and suitability for long acceleration times.