Motor starters and a two-pump station

Objectives

1. Translate the direct-on-line and star–delta starter schematics of Chapter 2 into PLC programs, with overload and interlock functions.
2. Prove by fault injection that the star and delta contactors can never be closed together.
3. Automate a pumping station with level hysteresis, lead–lag alternation of two pumps and dry-run protection.

Background

The starters are those of Chapter 2 and TD Exercise 2.2: a main contactor KM1 sealed in by its own auxiliary contact and, for star–delta, a star contactor KM2 and a delta contactor KM3 switched by an on-delay timer and interlocked against each other. In a PLC the contactors become outputs, their auxiliary contacts become contacts of those outputs, and the timer becomes a PLC timer. The pumping station applies the lead–lag principle of Chapter 2 to two pumps filling an elevated reservoir.

Bench set-up, Parts A and B

With SW5 and SW6 ON, the relays RQ0.0–RQ0.5 click like contactors. You may also switch SW3 ON to repeat Q0.0–Q0.5 on the traffic-light lamps.

Preparation

1. Redraw the DOL control circuit of TD Exercise 2.2 as a ladder network, and mark which element of the hard-wired circuit each contact replaces.
2. Choose the timers: the star period is 5 s, and the open-transition gap between KM2 opening and KM3 closing is 60 ms. For each, give a timer number of the right resolution and its preset.
3. For the pumping station of Part C, draw the GRAFCET (or state table) of the pump selection logic.

Timer choice

The S7-200 timer number fixes its type and resolution. Enter a duration to see the preset of every group.

Duration (s)

Type	Resolution	Timer numbers	Preset
TON/TOF	1 ms	T32, T96	+1000	exact
TON/TOF	10 ms	T33–T36, T97–T100	+100	exact
TON/TOF	100 ms	T37–T63, T101–T255	+10	exact
TONR	1 ms	T0, T64	+1000	exact
TONR	10 ms	T1–T4, T65–T68	+100	exact
TONR	100 ms	T5–T31, T69–T95	+10	exact

Part A: direct-on-line starter

1. Program the starter: START sets KM1, KM1 seals itself in, STOP or an overload releases it. An overload trip latches FAULT, which only RESET clears, and the motor must not restart after a reset without a new START.

Part B: open-transition star–delta starter

1. Extend the program: on START, KM1 and KM2 close (star). After 5 s KM2 opens, and 60 ms later KM3 closes (delta). STOP or an overload opens every contactor.
2. Add the interlocks in software: KM2 may not close while KM3 is closed, and conversely.
3. Follow the sequence in the status chart, and time the transition with a 1 ms timer that runs while neither KM2 nor KM3 is closed.

Phase	KM1	KM2	KM3	Duration measured
Star
Transition (open)
Delta
After STOP

Part C: two-pump station

Switch SW3 and SW6 OFF. Wire the buzzer BZ+ to relay output RQ0.2 and BZ− to 24 V−, with the relay common RC2 to 24 V+; enable SW6 for this relay only when you test the alarm.

Tank 1 is the sump, fed from the network; tank 2 is the elevated reservoir that supplies the users. Pumps P1 and P2 transfer water from the sump to the reservoir. The levels are not measured: the program simulates them, as in manual Exercise 8-1.

Level simulation. Levels are V-words from 0 (empty) to 1000 (full), updated every 100 ms. The sump rises by 4 while SUPPLY is open. Each running pump moves 3 from the sump to the reservoir, but only while both isolating valves are open. The reservoir falls by 5 while DEMAND is on. Levels stay within 0–1000.

Control specification.

1. The lead pump starts when the reservoir falls to 300 and stops at 900 (hysteresis).
2. If the reservoir still falls below 150, the lag pump starts as well; it stops with the lead pump.
3. The lead and lag roles are exchanged at every new start, so that the two pumps share the running hours.
4. Dry-run protection. No pump may run with the sump below 50. When this happens the pumps stop and the alarm sounds until the sump is back above 150.
5. No-flow protection. If a pump has run for 10 s without the reservoir level rising, it is stopped and the alarm sounds; the fault stays latched until a reset.
6. The four lamps show full (≥ 950) and empty (≤ 50) for each tank.

Work. Program the level simulation, then the control. Over a test of several minutes with changing demand, record each pump's starts and cumulated running time.

Fault-injection tests

Questions

1. In industry the star–delta interlock exists twice: once as a hard-wired auxiliary contact and once in the PLC program. Why is the software interlock alone not considered sufficient?
2. What is the purpose of the 60 ms gap? With reference to the simulated starting transients of Chapter 2, what happens in the motor when the delta contactor closes?
3. Why does lead–lag rotation extend the life of the pumps, and why does it not reduce the energy consumed?
4. The no-flow protection infers a fault from the absence of an expected change. Give two other examples of this technique in the mechanisms of the course.