Problem 14 control — AND/OR inputs and timed stopping
Try the contact-logic exercise on this page, no sign-in ↓
Problem 14 compares two different start conditions using the same two sensors. PB1 needs both LS inputs; PB2 needs at least one. This lesson uses the Q-Net 2025-08-04 edition and the site's problem-14 example. The earlier automatic forward/reverse explanation was incorrect for this problem.
Read the power circuit and self-hold lesson first. M1 and M2 are separate loads. Changing an LS state is not an automatic command to start the other motor.
Read the sensors first
LS1 drives the X1 relay state and LS2 drives X2. In the first operating path, their NO contacts are in series; in the second, they are in parallel. The exam notice treats LS devices as external switches connected through a terminal block. Distinguish that supplied-material scope from the clickable LS symbols in the simulator.
| LS1 | LS2 | PB1 can start MC1 | PB2 can start MC2 |
|---|---|---|---|
| Released | Released | No | No |
| Detected | Released | No | Yes |
| Released | Detected | No | Yes |
| Detected | Detected | Yes | Yes |
This assumes the protection/stop path and other starting conditions are satisfied. The last row does not establish mutual exclusion between the motors. Do not add a reversing interlock that is absent from this circuit.
Trace one branch completely
- From the fused supply, find the healthy EOCR contact and PB0 stop contact first. The control supply is L1/L3, not L1/neutral.
- PB1 can energize MC1 and T1 through the series X1/X2 condition and T1's timed NC path.
- T1's instantaneous contact supplies the holding path around PB1. Releasing START does not remove the need for the sensor and stop conditions.
- When T1 expires, the timed NC path opens and MC1/T1 drop out. Releasing either LS also breaks the series condition.
- Read the PB2 branch with X1/X2 in parallel instead. T2 expiry or release of both LS inputs stops MC2/T2.
MC auxiliary contacts also control RL/GL and standby lamp WL. Not every NC contact is a reversing interlock: follow what load is connected after it.
Test the example
The saved site example sets T1 and T2 to five seconds each. Do not interpret the official t1/t2 notation as a universal five-second setting.
| Action | Expected result |
|---|---|
| Breaker ON, before starting | Standby indicator WL |
| Both LS detected; press/release PB1 | MC1/T1 and M1/RL operate |
| T1 expires or either LS releases | MC1 stops |
| One LS detected; press/release PB2 | MC2/T2 and M2/GL operate |
| T2 expires or both LS release | MC2 stops |
| Press PB0 while running | Operating circuit stops |
| Trip EOCR while running | Operation stops; YL alarm |
| Reset EOCR after addressing the cause | Return to initial state; verify a new start command |
Work backwards from a fault
| Symptom | Possible mistake | Question to check |
|---|---|---|
| PB1 works with only one LS | X1/X2 wired in parallel | Are both NO contacts in the same series path? |
| PB2 also requires both LS | X1/X2 wired in series | Can one parallel branch complete the condition? |
| Stops as soon as START is released | Holding path omitted | Does the timer's instantaneous contact bypass START? |
| Keeps running after timer expiry | Timed NC bypassed or wrong contact used | Must coil supply pass through the timed NC? |
Exercise and answer
Question: With only LS1 detected, why do PB1 and PB2 behave differently? Answer: PB1 has an open contact in its series condition. PB2 has one closed parallel branch, which is sufficient. Compare input logic, not motor direction.
Open practical projects →. After signing in, use a problem-14 project and change the sensors in the order above. The link does not preload a completed solution. These are virtual testing steps, not instructions to energize equipment during an exam.
References and limits
Q-Net public problems, 2025-08-04 edition, problem 14 pages 11-7 to 11-9: circuit, operation and component diagrams. The original logic illustration here is a learning summary, not a replacement circuit drawing. The simulator calculates virtual contacts; it does not model actual overload current or mechanical motion.
Test the contact logic here
Open the interactive exercise below →. No sign-in is needed; the instructions and results explain exactly which part of the circuit is being modeled.
Try without signing in: AND/OR conditions with two sensors
Detect LS1 alone and compare both paths. Then detect LS2 too. This exercise shows start permissions only; it does not model running motors, timers or stopping.
A contact-logic learning model, not a hardware wiring or ratings check. Active buttons are highlighted.
- PB1 → MC1 (AND)
- Start blocked
- PB2 → MC2 (OR)
- Start blocked
Read the holding-path explanation · Open the homepage wiring simulator