Voltage reduced

Reduced-voltage starting. A reactor, resistor, or transformer is temporarily connected ahead of the motor during start to reduce the current inrush and limit voltage dip. This is accompanied by reduced starting torque. For reactor or resistor start, the torque decreases as the square of current for transformer start, the torque decreases directly with line current. The reactor, resistor, or transformer can be adjusted to give a proper balance between torque and current. 

Reduced-Voltage Starting Reduced-voltage starting is used to reduce system voltage dip. Voltage dips must be limited otherwise, they may drop other motors off the line, cause synchronous motors on the system to pull out of step, or cause objectionable lamp flicker. 

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. 

Sometimes the load itself may call for a soft start and a smoother acceleration and a reduced voltage switching may become essential. 

A few common methods to achieve a reduced voltage start are described below. 

To determine the starting current, when the test is conducted at a reduced voltage (l/ /3Vr). allowance must be made for the saturation effect while estimating the value of /s, at the rated voltage. See also Section 1.6.2(A) also (the important note on starting torque). 

By circulating roughly 50-60% of the rated current under a locked rotor condition, at a reduced voltage. To facilitate easy escape of the moisture to the atmosphere, the bearing covers may be removed. 

During the test the main busbars are fed at the rated current, for which the switchgear assembly is designed. They are heated naturally and therefore no resistance of the main bus need be measured. The busbars are shorted at one end and the current is fed from the other through a variable-current injection set at a reduced voltage of 3-10 V, or enough to achieve the rated current. The arrangement saves on power requirement and consum-... 

When the required test voltage is not available at site, the reduced voltage power frequency withstand test may be carried out at still lower test voltages, depending upon the voltage availability at site. Then the duration of the test must be increased as shown in Table 14.8, and IS 10118-3 and BS 159. 

It is advisable to choose the CTs with low secondary current, say, at 1 A, to permit a lower relay setting lor the voltage and the current trip coils. The reduced voltage acr(vss the relay will also improve the stability level of the proieetion scheme. 

Direct on-line starting (DOL) Reduced voltage starting... 

The voltage drop during starting should be calculated using the speed current data, locked rotor power factor, and the distribution system con stants. Speed-torque values at this reduced voltage can be calculated, assuming that the torque varies as the voltage squared. These values are compared to driven equipment curves. 

An early appraisal of motor and driven equipment speed-torque characteristics, particularly at the reduced voltage occurring during starting, is necessary. The following data should be obtained ... 

Full- or reduced-voltage or part-winding starting(ac). 

Design A produces exceptionally high breakdown torques but at the expense of high locket rotor currents that normally require provision for starting with reduced voltage. This motor is suitable for machines in which the friction and inertia loads are small. 

Typical torque-speed characteristics of wound-rotor motors are shown in the curves in Figure 3-11 for full voltage and for reduced voltage that are obtained with different values of secondary resistance. 

Start/delta, auto transformer, primary resistance Reduced voltage is applied to the motor ... 

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