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with a prime mover. Actually, during a system short circuit the synchronous motor acts like a generator and delivers shortcircuit current to the system instead of drawing load cur- rent from it (Fig. 1.4). As soon as a short circuit is established, the voltage on the system is reduced to a very low value. Consequently, the motor stops delivering energy to the mechanical load and starts slowing down. However, the inertia of the load and motor rotor tends to prevent the motor from slow- ing down. In other words, the rotating energy of the load and rotor drives the synchronous motor just as the prime mover drives a generator. 6 SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES The synchronous motor then becomes a generator and delivers short- circuit current for many cycles after the short circuit occurs on the system. Figure 1.5 shows an oscillogram of the current delivered by a synchronous motor during a system short circuit. The amount of current depends upon the horsepower, voltage rating, and reactance of the synchronous motor and the reactance of the system to the point of short circuit. LOAD CURRENT SYNCHRONOUS MOTOR -€t FIG. 1.4 Normally motors draw load current from the source or utility system but produce rhort- circuit current when a short cir- wit occurs in the d a d . U l I L I T Y SYSTEM ,- \ SHORT CIRCUIT CURRENT FROM MOTOR . . -. . . SYSTEM SYNCMOYOUS ' FIG 1 5 IBmlowl l roce of 0s- Yoroll . . - . . ._ ,. _ _ .. ,. . . .. . . .. cillogrclm of short-circuit current produced by a synchronous motor SHORT ' . - I CIRCUIT SHORT CIRCUIT CURRENT DELIVERED BY A SYNCHRONOUS MOTOR. SHORT.CIRCUIT-CURRENT CALCULATING PROCEDURES 7 HOW INDUCTION MOTORS PRODUCE SHORT-CIRCUIT CURRENT The inertia of the load and rotor of an induction motor has exactly the same effect on an induction motor as on a synchronous motor; i.e., it drives the motor after the system short circuit occurs. There is one major difference. The induction motor has no d-c field winding, but there is a flux in the induction motor during normal operation. This flux acts like flux produced by the d-c field winding in the synchronous motor. The field of the induction motor is produced by induction from the stator rather than from the d-c winding. The rotor flux remains normal as long as voltage is applied to the stator from an external source. How- ever, if the external source of voltage is removed suddenly, as it is when a short circuit occurs on the system, the flux in the rotor cannot change instantly. Since the rotor flux cannot decay instantly and the inertia drives the induction motor, a voltage is generated in the stator winding causing a short-circuit current to flow to the short circuit until the rotor flux decays to zero. To illustrate the short-circuit current from an induction motor in a practical case, oscillograms were taken on a wound- rotor induction motor rated 150 hp, 440 volts, 60 cycles, three phase, ten poles, 720 rpm. The external rotor resistance was short-circuited in each case, in order that the effect might he similar to that which would he obtained with a low-resistance squirrel-cage induction motor. Figure 1.6 shows the primary current when the machine is initially running light and a solid three-phase short circuit is applied a t a point in the circuit close to its input (stator) terminals a t time TI. The current shown is measured on the motor side of the short circuit; so the short- circuit current contribution from the source of power does not appear, but only that contributed by the motor. Similar tests made with the machine initially running a t full load show that the short-circuit current produced T. FIG. 1.6 , Tracer of oxillograms of short-circuit currents produced by an induction motor running at light load. 8 SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES by the motor when short-circuited is substantially the same, regardless of initial loading on the motor. Note that the maximum current occurs in the lowest trace on the oscillogram and is about ten times rated full-load current. The current vanishes almost completely in four cycles, since there is no sustained field current in the rotor to provide flux, as in the case of a synchronous machine. The flux does last long enough to prodnce enough short-circuit current to affect the momentary duty on circuit breakers and the interrupting duty on devices which open within one or two cycles after a short circuit. Hence, the short-circuit current produced by induction motors must he considered in certain calculations. The magnitude of short-circuit cur- rent produced by the induction motor depends upon the horsepower, voltage rating, reactance of the motor, and the reactance of the system to the point of short c. "cuit. The machine impedance, effective a t the time of short circuit, cmesponds closely with the impedance a t standstill. Consequently, the i iitial symmetrical value of Short-circuit current is approximately equnl to the full-voltage starting current of the motor. TRANSFORMERS Transformers are often spoken of as a source of short-circuit current. Strictly speaking, this is not correct, for the transformer merely delivers the short-circuit current generated by generators or motors ahead of the transformer. Transformers merely change the system voltage and mag; nitude of current but generate neither. The short-circuit current deliv- ered by a transformer is determined by its secondary voltage rating and reactance, the reactance of the generators and system to the terminals of the transformer, and the reactance of the circuit from the transformer to the short circuit. ROTATING-MACHINE REACTANCE The reactance of a rotating machine is not one simple value as it is for a transformer or a piece of cable, but is complex and variable with time. For example, if a short circuit is applied to the terminals of a generator, the short-circuit current behaves as shown i n Fig. 1.7. The current starts out a t a high value and decays to a steady state after some time has elapsed from the inception of the short cirroit. Since the field excitation voltage and speed have remained snbstantially constant within the short interval of time considered, a change of apparent react,ance of the machine may he assumed, to explain the change in the magnitude of short-circuit current with time. The expression of such variable reactance at any instant after the SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES 9 occurrence of any short circuit requires a complicated formula involving time as one of the variables. For the sake of simplification in short-cir- cuit calculating procedures for circuit-breaker and relay applications, three values of reactance are assigned to generators and motors, viz., subtransient reactance, transient reactance, and synrhronous reactance. The three reactances can be briefly described as follows: 1. Subtransient reactance X y is the apparent reactance of the stator winding at the instant short circuit occurs, and it determines the current Row during the first few cycles of a short circuit. 2. Transient reactance X i is the apparent initial reactance of the stator winding, if the effect of all amortisseur windings is ignored and only the field winding considered. This reactance determines the cur- rent following the period when subtransient reactance is the controlling value. Transient reactance is effective up to 45 see or longer, depending upon the design of the machine. 3. Synchronous reactance X d is the apparent reactance that deter- mines the current flow when a steady-state condition is reached. It is not effective until several seconds after the short circuit occurs; consequently,