This control program starts a three-phase induction motor in star, removes the star contactor after 5 s, waits 300 ms, and then energizes the delta contactor. The evidence identifies a Mitsubishi PLC but does not specify the PLC family, timer time base, stop-button input, or complete I/O allocation. Confirm those items in the actual hardware configuration before commissioning. The described design explicitly excludes overload protection.
I/O Assignment and Wiring Constraints
| PLC point | Field device or function | Purpose |
|---|---|---|
| X1 | SB2 start button, normally open contact | Requests a start |
| X2 | KM1 normally open auxiliary contact | Confirms operation of the main contactor |
| X3 | KM2 normally open auxiliary contact | Monitors the delta contactor and blocks a direct delta start |
| Y0 | KM1 coil | Controls the main contactor |
| Y1 | KM2 coil | Controls the delta contactor |
| Y2 | KM3 coil | Controls the star contactor |
| T0 | Star-running timer | Times 5 s from the confirmed start |
| T1 | Transition timer | Provides a 300 ms open interval before delta operation |
When KM1 and KM3 close, the motor is connected in star. When KM1 and KM2 close, it is connected in delta. Retain the hardwired electrical interlock between the KM2 and KM3 coils and implement a separate software interlock. The source permits the PLC-provided 24 VDC supply or a separately sized supply for the inputs; it also notes that relay-input test hardware may use 220 VAC. Apply only the voltage supported by the installed input module and its wiring arrangement.
Star-to-Delta Operating Sequence
- Close QS and press SB2. Permit Y0 only when X3 indicates that KM2 is not closed; this prevents a direct delta, full-voltage start.
- Energize Y0. When KM1 operates, its auxiliary contact closes X2. Use X2 with the Y0 seal-in condition to maintain Y0 and start T0.
- While T0 is timing and the delta output is off, energize Y2. KM1 and KM3 then place the motor in star.
- After 5 s, remove Y2 to end star operation and start T1.
- After the 300 ms transition delay, energize Y1 only while the star output is off. KM1 and KM2 then place the motor in delta.
- Seal in Y1 through the stated logic because energizing Y1 causes T0, and consequently T1, to reset. Pressing the stop button must remove Y0 so that either Y1 or Y2 also drops out.
Interlocks and Fault Response
Place the normally closed logic state of X3 in series with the X1 start request. If KM2 has welded main contacts or a mechanism that cannot release, its normally open auxiliary contact remains closed, X3 turns on, and the start path to Y0 opens. The PLC therefore refuses to energize KM1 instead of initiating a delta-connected start.
Use a normally closed Y1 condition in the Y2 rung and a normally closed Y2 condition in the Y1 rung. These software interlocks prevent simultaneous star and delta commands, while the retained contactor-coil hard interlock provides an independent electrical barrier. Auxiliary-contact feedback monitors actual contactor action; output-bit interlocking alone does not confirm mechanical release.
Commissioning and Verification
Enter and download the ladder program, then test the sequence while monitoring X2, X3, Y0, Y1, Y2, T0, and T1. Verify that Y0 and Y2 energize for star starting, Y2 drops after 5 s, neither Y1 nor Y2 is energized during the following 300 ms, and Y1 then energizes for delta operation. Confirm that Y1 and Y2 never turn on together.
Test the protective paths: simulate X3 active before a start and verify that Y0 remains off; press stop during both star and delta operation and verify that Y0 and the active connection contactor drop out. Record the observed sequence and results. Because overload protection is outside the described design, do not interpret this program as providing motor overload protection.
FAQ
How long should this Mitsubishi PLC keep the motor in star?
T0 holds the star-start sequence for 5 s. At timeout, the program removes Y2 and de-energizes KM3.
Why is there a 300 ms delay between star and delta?
T1 creates a 300 ms interval after KM3 is switched off and before KM2 is energized. This interval is specified to reduce the risk of an arc-related short circuit during transition.
How does the PLC prevent a direct delta start?
KM2 auxiliary feedback is wired to X3, and the normally closed logic state of X3 is placed in series with start input X1. If KM2 remains closed, X3 blocks Y0 and prevents KM1 from starting the motor.