T-Phi Network (Star-Delta) Using Relays and Timer

Fajar Purnama
5 min readMar 10, 2020

1. Introduction

In electric circuit star and delta circuit consist of 3 loads which in star circuit they are connected in form of a star while in delta circuit are connected in delta form as in Figure 1.

The application is usually used in (Alternate Current) AC motor where it has to start slowly. Starting the motor through a high current immediately could blow the motor as the starting current equals to the normal current multiplied by v3 (=v3 x I). It’ll be fine if the current is 5A (ampere) (I_start=v3 x 5 A=8.66 A), what happens if it’s 50A it’ll be (I_start=v3 x 50 A=86.6 A). The difference between 50A and 86.6A is quite significant. Therefore a switch to initially start a motor through star condition (low current) is needed, then after a while controlled by a time change the circuit to delta (high current). Full diagram can be seen in Figure 2.

Figure 2. star Delta Motor Diagram

2. Simulation Design

For the experiment here we have limited resource, we don’t have the star-delta motor which made me think whether there’s an alternative to do the star-delta motor simulation. I got an idea of using a low voltage lamp or LED as an indicator by using the brightness of the lamp. Instead I’d use 3 resistors (1 K? for this experiment) to form both star and delta circuit. We’ll see the brightness of the lamp instar and delta circuit, and for this we’ll use Direct Current (DC). Since there’s only positive (+) and negative (-) for the source we need to figure out an alternate circuit (AC have 3 sources R-S-T). For star circuit is T circuit and for delta circuit is Pi circuit in DC as shown in Figure 3 (If I have time I’ll include the calculation). Figure 3 shows that the current passed through T circuit is smaller which should be used as initial circuit, and Pi circuit passed larger current which should be switched later afterwards.

To realize this experiment we used:

  • 24V DC adapter
  • 3 1K resistor
  • LEDs



Fajar Purnama

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