WYE – DELTA MOTOR STARTER WIRING BASIC TUTORIALS

A common misapplication that is encountered is the improper sizing of the six motor leads between the still very popular wye-delta reduced voltage motor starter and the motor. This is best demonstrated by an example.

Assume that you have a 500-ton electrical centrifugal chiller operating at 460 V, three-phase, 60 Hz, with a nameplate rating of 588 full load amps (FLA).You would normally apply the correct factor of 125 percent required by NEC Article 440, to arrive at the required conductor ampacity: 588 × 1.25 = 735 ampacity for each of the three conductors.

Because there will be six conductors between the load side of the starter and the compressor motor terminals, the 735 ampacity is divided by two; you would select six conductors, each having an ampacity of not less than 368 A. Referring to NEC Article 310, Table 310-16 for insulated copper conductors at 75°C would result in the selection of 500-kcmil conductors.

This wire size is incorrect when used between the wye-delta starter and motor terminals. The problem is caused by a common failure to recognize that the motor may consist of a series of single-phase windings.

To permit the transition from wye-start to delta-run configuration, the motor is wound without internal connections. Each end of the three internal motor windings is brought out to a terminal, as shown in Figure 8.5.

FIGURE 8.5 Wye-to-delta internal motor windings brought out to terminals.

The motor windings are configured as required for either starting or running at the starter as shown in Figure 8.6, panels a and b, respectively. In the running-delta configuration, the field wiring from the load side of the starter to the compressor motor terminals consists of six conductors, electrically balancing the phases to each of the internal motor windings as described below in Figure 8.7.

FIGURE 8.6 Wye-start, delta-run motor winding configuration.

FIGURE 8.7 Field wiring between starter and motor in wye-start, delta-run configuration.

Note, for example, that motor winding T1 − T4 is connected to the line voltage across phase L1 − L2. It should be apparent that the windings within the motor are single phase- connected to the load side of the starter. Thus, the interconnecting field wiring between the starter and motor must be sized as though the motor were single-phase.

Electrical terminology simply describes this motor as being phase-connected, and the current carried by the interconnecting conductors as phase amps.

To correctly size the conductors between the motor starter and the motor, therefore, it is necessary to calculate the ampacity with the 125 percent feeder-sizing factor required by the NEC on a single-phase basis as follows:

Ampacity per terminal conductor = three-phase FLA × 1.25/1.73

For the example given:

Ampacity per terminal conductor = 588 × 1.25/1.73 = 424

Thus, it is clear that the current in the conductors between the starter and the motor on a single-phase basis is 58 percent of the three-phase value, not 50 percent as originally assumed, because the current in one phase of a three-phase system in the delta-connected winding is one divided by the square root of three due to the vector relationship.

In the original example, the conductors were sized for a minimum ampacity of 368 A.From the NEC, 500-kcmil copper conductors at 75°C have a maximum allowable ampacity of 380.The preceding calculation discloses that the conductors should be selected for not less than 424 ampacity.

Referring to the NEC again, 600-kcmil conductors have a maximum allowable ampacity of 420. In many cases, depending upon the interpretation of the local electrical inspector, 600 kcmil would be acceptable (usually within 3 percent is acceptable).

Five-hundred– kilocircular mil wire would not be. Almost needless to say, the conductors supplying the line side of the wye-delta starter are sized as conventional three-phase motor conductors.

WOUND MOTORS STARTING TYPES BASIC INFORMATION

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.