Commonly used starting methods for squirrel cage motors are
- direct-on-line starters
- star-delta starters
- frequency drives
- soft starters
The simplest and most common starting device is the direct-on-line starter where the equipment consists of a main contactor and a thermal or electronic overload relay.
The disadvantage of the direct-on-line method is very high starting current (6 to 10 times the rated motor currents) and high starting torque, causing
- slipping belts, heavy wear on bearings and gear boxes
- damaged products in the process
- water hammers in piping systems
A star-delta starting device consists normally of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).
The starting current is about 30 % of the direct-on-line starting device. The starting torque is about 25 % of the direct-on-line starting torque.
The stress on an application is reduced compared to the direct-on-line starting method.
With a variable frequency drive the electrical frequency to the motor is modulated between typical 0-250 Hz.
- the rated motor torque is available at lower speed
- the starting current is low - ranging 0.5 - 1 times the rated motor current
The frequency drive can also be used for soft stops.
With soft starters thyristors are used to reduce starting voltage. With lower motor voltage
- the starting current and starting moment can be very low compared to other methods
Stress on the application can be close to minimum compared to other methods.
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