Inside the Delta vs. Wye-Delta

Inside the Delta vs. Wye-Delta

Wye, Delta, Wye-Delta, three, six, nine or 12 lead motors.

There are many different ways a motor can be connected and to match, and there is a myriad of starting methods.  

Some motors are more versatile than others for starting options depending upon access to the motor leads themselves. 

One common question of confusion is around updating a starter for a wye-delta six or 12 lead motor. Are there other options out there instead of replacing the electro-mechanical starter?  

The short answer is yes!

Electro-mechanical starters have successfully started wye-delta starters for decades. Little has changed over time with this method. Simply put, a traditional wye-delta starter starts in the wye configuration and transitions to a delta run, thus allowing reduced voltage and current when starting. 

The starting voltage, current and torque are reduced in traditional wye-delta starting. The percent voltage applied during the start in the wye is 58% of full voltage, resulting in a percent of full load starting torque of 33%. This is a great way to start a delta motor designed for a wye-delta start. You achieve a consistency of torque output that is built to last. 

Going further, there are two starting methods with the electro-mechanical, open and closed transition wye-delta.  

Open transition:

  • Utilizes three contactors, overload relay, timer and an interlock typically. When transitioning from wye to delta, this method could produce a large current spike because of the physical disconnect between the wye and delta configuration, thereby the term, open transition. The magnitude of the current is determined by the timer and the motor load.

Closed transition:

  • In addition to the components utilized with the open transition, an additional contactor and power resistors are used to reduce the current spike of the transition. 

What are the disadvantages of either method?

  • Real estate. Electro-mechanicals take up some space in a cabinet in addition to consuming time devoted to wiring and connecting the pieces together.
  • Overtime, as the motor gets older, adjustments to the amount of torque maybe needed. 

An easier way to utilize a wye-delta designed motor, or for that matter, a delta motor with six accessible leads, is to use a soft starter enabling zero transition to start the motor. A soft starter can produce the same amount of torque as an electro-mechanical starter. Torque is needed to start the motor. This can be done in two ways, one is connecting the motor in line configuration, the second is inside-the-delta.

The line connection is accomplished by bringing back three (or pairs of) leads to the soft starter from the motor. To accomplish this, leads may need to be paired up.

Inside the delta is even easier than that. You would just bring back the same six leads connected to the electro-mechanical wye-delta starter and connect them to the soft starter. Inside the delta is a configuration where the SCRs of the soft start are “inside the delta.”

Figure A illustrates how the SCRs per phase are wired inside the delta. Figure B is the connection for line connected.

Not only can the soft starter provide similar amount of torque as an electro-mechanical, it can provide higher or lower torque if needed just by adjusting the starting current limit or initial torque for soft starting. 

In addition, the form factor is typically much smaller with a soft starter than that of an electro-mechanical. 

Rockwell Automation provides soft starters capable of direct wiring up to wye-delta motors. View Wye-Delta and Solid State Starter Application Guide for more information and detail on wye-delta starting.

Bill Bernhardt
Posted September 19, 2016 By Bill Bernhardt, Senior Commercial Engineer, Rockwell Automation
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