Wound-rotor motors are invariably started on full voltage but with external resistance in the secondary circuit. Ordinarily sufficient resistance is provided to give 100% torque at standstill, which means that 100% current will be drawn from the line. If a higher torque is required to start the load, less external resistance must be used, and the current drawn is proportionately higher.

As the motor accelerates, the external secondary resistance is short-circuited in one or more steps. The locked-rotor values in Table 20-8 are generally recognized as the minimum needed by motor designers to obtain the required torque characteristics for general-purpose motors.

Squirrel-cage motors with these values are usually acceptable for full-voltage starting on power lines and also on combined light and power secondaries of 208 or 230 V, if manually controlled (infrequently started). In the case of automatically controlled (frequently started) equipment, with 208- or 230-V motors supplied from combined light and power secondaries, current-reducing starters to reduce the current to about 65% of these values may be required, unless consultation with the power company indicates that the available system capacity will permit use of full-voltage starting. In any case, consultations with the power company for motor applications above 25 hp are advisable.

Autotransformer starters (compensators) are the most popular of any reduced-voltage type. They have the advantage that the ratio of torque developed by the motor to the current drawn from the line remains substantially the same as for full-voltage starting.

The motor torque and the current drawn from the line (neglecting the magnetizing current of the autotransformer) are both reduced in proportion to the square of the voltage impressed on the motor. The magnetizing current of the autotransformer generally does not exceed 25% of motor full-load current. Normally, the motor accelerates nearly to full speed on the reduced-voltage connection and is then transferred to full voltage.

Since the circuit to the motor is opened and then immediately reclosed, a transient inrush of current occurs which may be of much greater magnitude than the current normally drawn by the motor at the speed at which the transfer is made. This transient inrush, however, is of such extremely short duration that it does not produce an objectionable voltage disturbance on the average power system.

Standard autotransformer starters are provided with 65% and 80% voltage taps in sizes up to 50 hp and with 50%, 65%, and 80% voltage taps in the larger sizes. “Part-winding” starting is being more widely used for reducing starting current. This involves arranging the stator winding so that, by use of adequate control devices, one part of the stator winding is first energized and subsequently the remainder of the winding is energized in one or more steps.

The purpose is to reduce the initial values of the starting current drawn and/or the starting torque developed by the motor. The usual arrangement involves energizing one-half the stator winding on the first step, resulting in approximately 50% of normal locked-rotor torque and approximately 60% of normal locked-rotor current. While this torque may be insufficient to start the motor in some applications, it permits drawing full-winding starting current from the system in two increments.

Another method is to connect two-thirds of the winding on the first step, by using a 4-pole contactor, in which case the motor should accelerate promptly to full speed. The remaining third of the winding is then connected by closing a second contactor with only two poles.

Resistor-type reduced-voltage starters are sometimes used. They have the disadvantage that the current drawn from the line is reduced in direct ratio to the impressed voltage, while the torque developed by the motor is reduced as the square of this voltage.

The resistor is short-circuited, either all at once or in steps, when the motor comes up to speed. The circuit for the motor is not broken in transferring to full voltage, as is the case with the autotransformer starter. These features make the resistor-type starter adapted for use where “increment-type” starting current restrictions exist.

With the resistor-type starter, the contactors, which short-circuit the resistors as well as the line contactors, must carry the full current of the motor, whereas in part-winding starting, the contactors for the two parts of the winding each carry only half the total current.

Reactor-type reduced-voltage starters are sometimes used on larger motors, most frequently on high voltage motors (2300 V or above), where oil circuit breakers are necessary to provide sufficient current-interrupting capacity. In such cases, the reactor and starting circuit breaker are placed in the neutral of the motor. The breaker can then be of low-interrupting capacity, since the fault current at this point is limited by the reactance of the motor windings.

Wye-delta starting, though quite common abroad, is used in the United States primarily for refrigeration compressors. This starter consists of a switching arrangement that transfers the motor winding from Y for starting to delta for running.

The current drawn and the torque developed by the motor are thus reduced to only one-third their full voltage values. This very low torque, the extra contactors required, and the current inrush when the circuit is reclosed on delta make this scheme less attractive than others.

Motors are frequently supplied from power systems consisting of complex networks for which calculation of the voltage drop would be difficult. The voltage drop may be estimated, however, if the short-circuit kVA is known at the point of power delivery.

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