Starting current

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. 

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

Consider a 750 kW, 4-pole motor, having a rated starting torque as 125% of the rated and starting current k as 600% of the rated. At 80% of the rated voltage the starting torque Ts, will reduce to... 

In the same context, the starting current will also decrease linearly according to this factor and not according to the supply voltage. In the above example, the starting current /s, at 80% of the rated voltage will therefore reduce to... 

NEMA recommendations on starting currents 2.5 Load torqUO Of Opposing torqUO... 

Generalizing the above equation, autotransformer tapping for a particular starting current, /at >s... 

From this equation, the desired tapping of the auto-transformer, to limit the starting current to a desired value can be determined. 

A squirrel cage motor has its 4, on DOL as six times its rated current. Find the required tapping on an autotransformer to limit the starting current to 1.5 times. 

For a 3.3 kV 450 kW motor, with a full-load current of 100 A and starting current on DOL as six times the rated current, the kVA rating of the transformer for 50% tapping will be... 

Soft starting minimizes the starting mechanical and thermal stresses/shocks on the machine and the motor. It results in reduced maintenance cost, fewer breakdowns and hence longer operating life for both. Reduced starting current is an added advantage. 

The external resistance adds up to the total impedance of the motor windings and limits the starting current. It also improves the starting power factor. 

Restriction in starting current and a requirement for high starting torque to accelerate heavy rotating masses sometimes limit the use of a squirrel cage motor. For such applications a slip-ring motor provides a better alternative. 

The starting current and the peak current during each cycle... 

Voltage can be adjusted to maintain the starting current constant... 

Starting current (/, ) Can be controlled to any desired level subject to the minimum required Cannot be controlled. Although duration of the starting inrush is very short because of no-load start Can be controlled to any desired level. Normally up to 1.5 times the rated current... 

This is the time taken by the stator or the rotor, whichever is less, to reach the limiting temperature rise, as specified in Table 7.5, when the starting current /s, is pas.sed through the stator windings after the motor has reached thermal equilibrium, underrated conditions. For increa.sed safety motors, this time should not be less than 5 seconds (preferably 10 seconds or more). 

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