3.9 PRACTICE PROBLEM SOLUTIONS
1. no - a plc OUTPUT can switch a relay
2. input cards are connected to sensors to determine the state of the system. Output cards are connected to actuators that can drive the process.
3. sourcing outputs supply current that will pass through an electrical load to ground. Sinking inputs allow current to flow from the electrical load, to the common.
4. a motor starter typically has three phases
5. AC is easier, it has a zero crossing
6. it will lead to premature failure
7. by using separate modules, a PLC can be customized for different applications. If a single module fails, it can be replaced quickly, without having to replace the entire controller.
8. AC input conditioning circuits will rectify an AC input to a DC waveform with a ripple. This will be smoothed, and reduced to a reasonable voltage level to drive an optocoupler. An AC output circuit will switch an AC output with a triac, or a relay.
9. an AC output is a triac. When a triac output is turned off, it will not actually turn off until the AC voltage goes to 0V. Because DC voltages don't go to 0V, it will never turn off.
10. If a NC stop button is damaged, the machine will act as if the stop button was pushed and shut down safely. If a NO start button is damaged the machine will not be able to start.
