Lab 15: Star-Delta (Y-Δ) Starter

Application Scenario

A water utility runs a 75 hp pump motor. When a large induction motor is switched straight onto the line (across-the-line starting), it draws 6 to 8 times its full-load current for several seconds. That current surge dims the lights in the plant, can trip the supply breaker, and puts a mechanical shock on the pump and piping.

A star-delta (wye-delta) starter reduces the starting current to about one third:

1. The motor starts with its windings connected in star (Y). Each winding gets only about 58% of the line voltage, so the current and torque are much lower.
2. After the motor has accelerated (a few seconds), the star contactor opens.
3. After a short transition gap, the delta (Δ) contactor closes, and the motor runs at full voltage.

The star and delta contactors must never be closed at the same time: that would short-circuit the supply through the motor leads. The control circuit must provide the timing and the interlocks.

In this lab, the relays play the parts of the three contactors of a real starter:

Learning Outcomes

After this lab you will be able to:

• Explain why large motors use reduced-voltage starting, and how star-delta starting reduces current.
• Build a timed transition from one contactor to another with a gap between them.
• Combine a timer changeover contact with electrical interlocks (Lab 06) in one circuit.
• Read a sequence from a timing diagram and verify it with a stopwatch.

Background

Star vs. delta. A star-delta motor has six leads (both ends of all three windings). In star, one end of every winding is tied together, and each winding is connected between a line and that common point: it sees the phase voltage (line voltage ÷ √3 ≈ 58%). In delta, each winding is connected directly between two lines: it sees the full line voltage. Because power depends on voltage squared, starting in star gives about one third of the delta starting current and torque.

The sequence in this circuit.

1. START: MAIN (CR1) seals in. TR1 starts timing. STAR (CR2) is ON through TR1A N.C.T.O. (1–4) and the delta interlock TR4B N.C.T.O. (11–8). R1 lights.
2. Star time over (TR1 times out): TR1A opens, and STAR drops (R1 OFF). TR1B N.O.T.C. (8–6) closes.
3. Transition gap: the DELTA timer TR4 is fed through TR1B and the star interlock CR2B N.C. (10–2). It can only start once STAR has really dropped. TR4 counts the gap.
4. DELTA: TR4 times out; TR4A N.O.T.C. (1–3) turns ON the DELTA lamp G1, and TR4B opens to lock STAR out.
5. STOP: MAIN drops, and every other rung, fed from N3, loses power.

Timing Diagram

Functional Requirements

1. Pressing PB1 energizes MAIN (CR1) and STAR (CR2) together. R1 (STAR) lights.
2. After 5 s, STAR drops (R1 OFF). MAIN stays ON.
3. After a 1 s gap, DELTA (TR4) comes ON. G1 (DELTA) lights and stays ON.
4. STAR and DELTA are never ON at the same time.
5. Pressing PB2 at any time stops everything. The next START begins again in STAR.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. Use pencil. This design uses 7 grid lines. Rungs 3–5 all take their power from N3.

Step 1 — Rungs 1–2: MAIN contactor

1. Grid line 1: PB2 (N.C.), PB1 (N.O.), CR1 coil (2–7). Mark N2 and N3.
2. Grid line 2: CR1A N.O. seal-in (1–3) from N2 to N3.

Step 2 — Rung 3: STAR timer

1. From N3, draw a line down (this vertical line feeds Rungs 3, 4, and 5). On grid line 3, draw the TR1 coil (2–7).

Step 3 — Rung 4: STAR contactor

1. From N3: TR1A N.C.T.O. (1–4), then TR4B N.C.T.O. (11–8), then the CR2 coil (14–13).
2. Mark N4 (between TR1A and TR4B) and N5 (between TR4B and the CR2 coil).

Step 4 — Rung 5: DELTA contactor (with gap)

1. From N3: TR1B N.O.T.C. (8–6), then CR2B N.C. (10–2), then the TR4 coil (2–10).
2. Mark N6 (between TR1B and CR2B) and N7 (between CR2B and the TR4 coil).

Step 5 — Rungs 6–7: lamps

1. Rung 6: from L1, CR2C N.O. (11–7), then R1 (STAR). Node N8.
2. Rung 7: from L1, TR4A N.O.T.C. (1–3), then G1 (DELTA). Node N9.

Step 6 — Trace your drawing with a pencil

1. t = 1 s after START: which of CR1, TR1, CR2, TR4 are energized? Which lamps are ON?
2. t = 5.5 s: which are energized now? Why is neither lamp ON?
3. t = 7 s: which are energized? Could STAR come back ON at this point? Which two contacts prevent it?
4. Suppose CR2B were missing from Rung 5, and CR2's contacts were slow to open (for example, welded). What could happen at t = 5 s + 1 s?
5. PB2 is pressed at t = 3 s (still in STAR). What happens? What happens at the next START?

Step 7 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them, and fill in the wiring table on the worksheet. The N7 row is done for you.

Get instructor approval before wiring.

Key Wiring Points

1. One timer changeover ends STAR and starts the DELTA sequence: TR1A (N.C.T.O.) in the star rung, TR1B (N.O.T.C.) in the delta rung.
2. Interlock both ways, as in Lab 06: a DELTA N.C. contact in the STAR rung, and a STAR N.C. contact in the DELTA rung.
3. The gap comes from a timer that starts only after STAR has really dropped (because CR2B is in its rung). A gap based only on time settings would not detect a stuck star contactor.
4. Feed every sequence rung from N3, so STOP resets the whole sequence.
5. Mind the pin families: CR1/TR1 (1-3-4, 8-6-5, coil 2–7), CR2 (9/10/11 commons, coil 14–13), TR4 (1-3-4, 11-9-8, coil 2–10).

Wiring Procedure

1. Power OFF. Set TR1 to 5 s. Set TR4 to on-delay mode, 1 s.
2. Wire the circuit from your approved wiring table, one node at a time.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:2 to TR1:1 and CR1:2 to TR1:8 should read near 0 Ω (N3).
   • TR1:1 to CR2:14 should read near 0 Ω (TR1A and TR4B both N.C.).
   • TR1:8 to CR2:10 should read high (TR1B N.O.T.C. open).
   • CR2:10 to TR4:2 should read near 0 Ω (CR2B N.C. closed).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Explain why starting in star reduces the starting current to about one third.
2. Why must there be a gap between STAR opening and DELTA closing? What could happen at the contacts if there were no gap?
3. The delta timer TR4 can only start after CR2B closes. Why is this safer than simply setting TR1 to 5 s and another timer to 6 s, both starting at START?
4. What happens if the star time is set too short (the motor has not accelerated)? What if it is set too long?
5. List two other methods of reduced-voltage starting and one advantage of each compared with star-delta.

Applying This Circuit Pattern

The break-before-make transfer with a timed gap pattern: one output turns OFF, an interlock proves it is OFF, a timer waits, and only then the second output turns ON.

Design question for the future: Reduced-voltage starting is also done today with soft starters and variable-frequency drives (VFDs). If you were replacing this star-delta starter with a soft starter, which parts of this control circuit would you still need?

Extension

During the 1 s gap, both lamps are OFF, although the MAIN contactor is still ON. Add lamp G2 as a MAIN indicator using CR1's free contact set (CR1B). Connect G2:2 to L2. Which pins do you use?