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How this is calculated#

You give the motor's shaft power, line voltage, power factor and efficiency. The tool works the full load current and full load torque the same way the motor tool does. Then you give the locked-rotor current and torque from the motor data, and the torque the load needs to break away from rest.

A cage induction motor at standstill behaves like a transformer with its secondary shorted. Its current follows the voltage at its terminals, and its torque follows the square of that voltage. Every starting method is a way of lowering the voltage at the motor, or of controlling the current directly. So the tool scales the locked-rotor current and torque for each method:

• direct on line takes the full locked-rotor current and torque;
• star-delta puts the line voltage divided by the square root of 3 across each winding, so the line current and the torque both fall to a third;
• an autotransformer at a tap gives the motor the tap voltage; the motor current falls with the tap, the line current and the torque with the tap squared;
• primary resistance leaves part of the voltage at the motor; the current falls with it, the torque with its square;
• a soft starter holds the current at its limit, and the torque falls with the square of that limit over the locked-rotor current;
• a drive starts at rated flux, so its torque follows its current limit.

A method starts the load where its starting torque is above the breakaway torque. With a fault level at the motor terminals, the tool estimates the dip as the starting current over the fault current.

Worked examples#

• An 11 kW centrifugal pump. Every method starts it. Direct on line draws 124.5 A and star-delta 41.5 A. The lowest start that still turns the pump is the variable speed drive, at 31.13 A.
• A 22 kW loaded conveyor. Star-delta gives 73.3 % torque against a 120 % load, so it cannot start the belt. The lowest start that does is direct on line, at 290.57 A.
• A 37 kW fan on a board with 6 kA. The dip is 8.1 % direct on line and 2.7 % in star-delta. The lowest start is the variable speed drive, at 104.72 A and a 1.7 % dip.
• A 5.5 kW bore pump at the end of a rural line. Direct on line dips the supply 9.1 %, a soft starter at 3.5 times full load current 4.5 %. The lowest start is the variable speed drive, at 15.57 A.

Limits#

The comparison is at standstill. It does not follow the torque through the run-up, so a star-delta start that breaks the load away can still stall before the change to delta. The drive figures assume rated flux; the supply side of a drive draws less than the motor side at low speed. The dip is an estimate for comparing methods, not a flicker study. Choose the circuit-breaker so its magnetic trip clears the starting current of the method used, and set the overload to the full load current.

See also: Motor starting methods: current and torque, Starting a loaded conveyor and Motor.