Worksheet: motor control circuits
Material: the free motor control circuit simulator at examrobotic.com (examrobotic.com/en/tools/motor-control-circuit-simulator), on a computer or phone. Time: about 40 minutes.
Aim: understand the seal-in contact, why the stop is a normally closed contact and what the interlocks of a reversing starter are for, and learn to find faults by following the voltage on the diagram.
1. Start-stop with seal-in
- Choose «Start-stop with seal-in». Press Start and let go. Which contact keeps the KM1 coil energised once you are no longer pressing S1? Write its name and terminals.
- Hold Stop and press Start at the same time. What happens? Why is it wired that way?
- With the motor running, which contact lights pilot lamp H1? Why is that better than connecting H1 across the coil?
- If the power fails while the motor is running and comes back later, does it restart by itself? Explain why.
2. Forward-reverse
- Choose «Forward-reverse». Look at the power diagram: which two phases does KM2 swap?
- With the motor running forward, press Reverse without pressing Stop. Which contact prevents it?
- Tick the push-button interlock and reverse on the fly. Write down the current peak and compare it with a start from
standstill:
Peak when reversing: ________ A Peak when starting: ________ A - Untick the electrical interlock, keep the push-button one and reverse on the fly. What happens, and why are the buttons not enough?
- Reverse several times in a row without stopping. Which protection eventually acts? Which of its contacts opens the control circuit?
3. Find the fault
Press «Surprise fault» three times (a different fault each time). Fill in the table before choosing your answer.
| No. | What the machine does | How far the voltage gets | Fault |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
Answers (for the teacher)
- KM1's normally open auxiliary contact 13-14, in parallel with S1.
- Stop wins: S0 is in series with everything. It is a safety rule: given conflicting commands, the machine stops.
- KM1's auxiliary 23-24. That way the lamp shows the contactor has really closed, not just that its coil has voltage.
- No. When the power fails, KM1 drops out and its 13-14 opens; when power returns nothing feeds the coil until someone presses start.
- L1 and L3 (L2 goes straight through).
- KM1's normally closed 61-62 contact, in series with the KM2 coil (electrical interlock).
- When reversing, the motor plug-brakes and the peak is slightly higher than a direct-on-line start (in the simulator, around 6.5 times In against just over 6).
- Q1 trips on a short circuit. KM1 takes a few milliseconds to open its contacts and the arc carries on conducting for a few more; KM2 closes during that time. KM1's 61-62 only lets KM2 in once KM1 has really dropped out.
- Overload relay F1: its 95-96 contact opens the control circuit and 97-98 signals it.
Possible faults in the simulator: KM1 seal-in not making (only runs while pressed), loose wire on S0 (nothing starts; the voltage stops at S0), KM1 or KM2 coil open (the voltage reaches the coil but the contactor does not close), S1 wired to its NC contact (starts by itself), KM2 not swapping phases (turns the same way both ways), KM1 61-62 not closing (reverse does not pull in; the voltage stops at that contact) and KM2 seal-in not making.