Solid-state soft starter combination-controller with a high efficiency motor.

The squirrel cage ac induction motor is widely used and delivers more power to industry than any other electric device. There is a catch however. These motors are sometimes hard to start because of the large inrush current (locked-rotor current), which ranges from about seven times full-load current for a standard motor to twenty times full-load current for some high-efficiency motors.

Inrush current causes dynamic stresses on electrical distribution systems. Starting large motors may cause a transient-line undervoltage condition. A large starting current causes a considerable voltage drop on the line due to line impedance. This is true especially on older power distribution systems that often have over-loaded conductors and undersized substation transformers with poor voltage regulations.

Line undervoltage, although temporary, may cause nuisance conditions, such as lights dimming, to more serious ones such as contactors or relays dropping out. There are known cases of generator start or automatic transfer to an alternate power source as a consequence of an undervoltage condition due to starting large motors across the line.

Locked-rotor current in mechanical terms translates as locked rotor torque. This high starting torque causes mechanical stresses on the drive components or equipment, leading to excessive wear. It may also result in fatigue failures of couplings, bearings and gearboxes.

Reduced-voltage starters are the most popular method used to prevent locked rotor current. These devices control voltage from the instant it is applied to the instant it reaches the nominal line level and the motor is energized.

When selecting a reduced-voltage starter, engineers should consider several factors:
• Initial voltage needed to break loose the load.
• Necessary or allowed acceleration time.
• The shape of the voltage ramp curve, which is how the voltage changes and how it increases during the acceleration time.
• Motor demand for current and driven load demand for torque.
• Limits on the inrush current and starting torque due to driven loads.
• Starter efficiency and its effect on power demand.
• Electrical codes for installation and operation.
• Environmental conditions of the installation site.
• Utility penalties for exceeding peak demand and undervoltage conditions for transmission lines.

Starter types

The various types of reduced-voltage devices are:

The autotransformer starter uses three contactors and an autotransformer with selectable taps. It enables:
• Two-step voltage starting with smooth acceleration and current-limiting of the transformer inductance.
• Flexibility of application choices.
• Acceleration times up to 30 sec.

This graph shows the starting currents of the different reduce-dvoltage starters.
Select figure to enlarge.

The initial voltage is usually 65% of the line voltage. It can be set as low as 50% to reduce the starting torque. Or it can be set as high as 80% for higher starting torque when necessary, however, the resultant transformer losses may reduce efficiency.

Constraints are its high price and its large size, when compared to other types.

A primary resistor starter uses contactors and resistors. The number of contactors determines the voltage steps.

Low circuit inductance makes voltage steps abrupt. The large voltage drop on the resistor bank and heat losses lower this starter’s efficiency. Heating usually limits the acceleration to 5 sec.

This starter offers good starting torque and relatively low cost. But it requires space for the resistor bank and contactors.

A primary reactor starter is similar to the primary resistor type and offers smooth acceleration to 15 sec. Efficiency is poor due to additional inductance. Its poor power factor negatively affects motor flux and torque-producing current components. Applications are limited to medium and high voltage large horsepower motors.

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