Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The most common method of start­ing squirrel cage three-phase motors is across the line (direct-on-line). Some applications require limiting the mo­tor starting current and/or torque to reduce the stress on the electrical and mechanical systems. 

Although there are other meth­ods such as autotransformer, reactor and using a variable frequency drive (VFD), the focus in this article will be on the reasons behind the selection of the three most common methods of achieving these objectives. Specifically, these methods are part winding, wye-delta, and electronic soft-starting. The windings in all three of these methods usually have 6 leads.

Part winding start
Part winding start (PWS) is a re­duced starting current method primar­ily intended to reduce the momentary inrush current to a motor so that the utility automatic voltage recovery system can respond to avoid an unac­ceptable voltage dip. Among the issues that could result from an excessive voltage dip are light flicker and contac­tors going to their “off” state due to the holding coil not being able to maintain the “on” status.  

As its name implies, only part of the winding is energized for the initial step (see Figure 1), thus the heating effect is more severe than if the entire winding was energized with the reduced current.  In fact, the starting current on the energized part of the winding may be proportion­ally higher than when the entire winding is energized across the line. For example, the starting current is typically 70% of rated starting current, and being ap­plied to only 50% of the winding, the energized winding is subjected to 140% (70%/50%) of rated current.  

Fortunately, the motor winding is typically energized for no more than about 2 to 3 seconds during the initial step of the part winding start, and then the entire winding is energized for the run mode of operation. Since the intended purpose is only momentary, a longer start time period is not neces­sary and could rapidly overheat the energized portion of the motor wind­ings. Further, the motor rotor may not reach operating speed and may have a significant amount of electrical noise during the initial step.  

Note: A variation of the PWS meth­od utilizes a series delta initial step fol­lowed by a parallel delta run. With that method, the starting current is about 25% of rated starting current, and the current applied to the entire winding during starting is therefore about 50% of its normal starting current. A motor connected in this configuration could, therefore, operate for more than 2 to 3 seconds in the start mode.

The PWS winding has 6 leads, labeled 1, 2, 3, 7, 8 and 9.  Leads 1, 2 and 3 are energized for the start mode, and leads 1&7, 2&8 and 3&9 are ener­gized in the run mode. Note that the numbers on the PWS motor are not the same as for the wye-delta start motor described below. If the winding is supposed to be wye-delta and is con­nected for PWS in error, it will have to be internally reconnected. Thus, if this error is detected after a motor is installed, it will need to be either partially dismantled at the installa­tion or returned to the service center. A practical method for determining if the motor starter is PWS versus wye-delta is to count the number of contactors. The PWS starter uses 2 contactors and the wye-delta has 3; the additional contactor is used to create the wye during starting.

Wye-delta start
The wye-delta start method ener­gizes the entire winding in a wye con­figuration at the start and then switches to the delta configuration for normal “run” operation (see Figure 2). Initial operation in the wye mode results in a significant reduction in starting current and torque. Theoretically, the power capability in wye is about 1/3 that of the delta connection, and the starting current is also about 1/3 of rated starting current. Because the entire winding is energized at this reduced current, it can remain in the wye “start” mode for a considerable period of time. In some applications, such as a centrifuge, the start period may be up to 30 minutes.  

Although the motor torque is very low in the wye mode, that is often an advantage for loads that require “soft” starting, such as some air compressors. If the motor for that type of air com­pressor started across the line it could damage the relatively slender (small diameter) shaft, or gears if connected to the motor through a gearbox.

Like the PWS motor, the wye-delta winding has 6 leads. In this case they are labeled 1, 2, 3, 4, 5 and 6. Leads 1, 2 and 3 are energized for the start mode, with leads 4, 5 and 6 connected together by the wye contactor; and leads 1&6, 2&4 and 3&5 are energized in the run mode. Again, note that the numbers on the wye-delta motor are not the same as for the PWS start mo­tor described above. If the winding is supposed to be PWS and is connected for wye-delta in error, it will have to be internally reconnected. To determine if the starter is PWS or wye-delta, count the number of contactors as described at the end of the discussion above for the PWS starting method.

Note:  A motor with a 6 lead, wye-delta connection may be connected delta if it is to be operated from an across the line (direct online) starter. If you are uncertain of the 6-lead arrange­ment of a motor connection, see the May 2008 Currents article titled “Identifying Unmarked Leads Of 6-Lead Motors With 1 or 2 Windings.”

Electronic soft-start
With an electronic soft-starter, the motor is connected for the across-the-line arrangement. There are two variations of the electronic soft-starter connection. One version inserts silicon controlled rectifiers (SCRs) in series with the line leads of a 3-lead motor (see Figure 3). The other variation inserts the SCRs “in­side the delta” of a 6-lead delta (actu­ally wye-delta) connected motor (see Figure 4). Both methods achieve the same purpose: Variable reduced the voltage at the motor termi­nals at starting. The same electronic soft-starter may be connectable for either configuration. The voltage is raised from a minimum until the motor accelerates to rated speed, or near rated speed, after which the soft-starter typically goes into bypass mode and the motor is then connected directly across the line.

Note that with the “inside-the-delta” configuration the line leads connect directly to 3 of the 6 motor leads. There should be a line contactor to electrically isolate the motor with this arrangement. The advantage of the “inside-the-delta” arrangement compared to the series arrangement is that the starter SCRs only see phase current, thus the capacity of the starter is theoretically increased by a 1.73 multiplier. However, due to increased SCR heating, a more practical multi­plier is 1.50.

The advantage of the electronic soft-start compared to the wye-delta method is that the starting current, torque, time and acceleration can be controlled. Bear in mind that with any reduced voltage or torque start­ing method, the load inertia will affect the start time. Thus, although a high inertia load might be able to be started with a wye-delta starter, the electronic soft-starter could overheat while attempting to have the motor accelerate the same high inertia load.  

Note: A variation of the 3-lead mo­tor electronic soft starter (see Figure 3 on Page 4) is a variable frequency drive (VFD). Although less common than the conventional electronic soft starters, the VFD has the additional advantages of providing speed and current control, and sometimes torque control, during starting.

Categories: AC motors, Stators/windings, Winding connections, Design/theory/application

Tags: Winding connections Motor startup

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