Read relay ladder diagrams, match sensors and actuators, understand VFDs, program PLC logic with timers and counters, and troubleshoot, guard and maintain automated machines.

Industrial Controls & PLCs

## Why it matters

Before PLCs, machines were controlled by panels full of relays wired together, and electricians drew those circuits as **ladder diagrams**. PLC programs still use the same picture, so the start/stop circuit you learn here appears on motor starters, conveyors and pumps everywhere. If you can read it, you can follow the logic of nearly any machine.

## The ladder

A ladder diagram has two vertical **rails** and horizontal **rungs**.

- The left rail is the control power supply (for example L1 of a 120 V AC control transformer, or +24 V DC).
- The right rail is the return (L2, neutral or 0 V).
- Each rung is one circuit: **conditions** on the left (switches and contacts) and one **output** on the right (a coil, lamp or solenoid).
- Read each rung left to right: if there is a complete path of closed contacts, the output is energized.
- Rungs are numbered down the left side, and contacts carry cross-references to the rung where their coil lives.

## Contacts and coils

- A **normally open (NO)** contact, drawn `-| |-`, is open when its device is at rest and closes when it is actuated.
- A **normally closed (NC)** contact, drawn `-|/|-`, is closed at rest and opens when actuated.
- "Normal" means the device's shelf state, not the machine's usual running state.
- A **coil** `-( )-` is a relay or contactor. When it energizes, every contact with the same label changes state: its NO contacts close and its NC contacts open.

## The three-wire start/stop circuit

This is the most important rung in industrial control:

```text
   L1                                                     L2
    |                                                     |
1   |--[ STOP (NC) ]--+--[ START (NO) ]--+--[ OL (NC) ]--( M )--|
    |                 |                  |                    |
    |                 +----[ M (NO) ]----+                    |
    |                                                         |
2   |--[ M (NO) ]------------------------------------( RUN LAMP )--|
```

How it works:

1. At rest, STOP is closed (NC), START is open, the M auxiliary contact is open. M is off.
2. Press **START**: current flows through STOP, START and the overload contact, so coil **M** energizes. The contactor pulls in and the motor runs.
3. M's auxiliary NO contact, wired **in parallel** with START, closes. This is the **seal-in** (or holding, or latching) contact.
4. Release START: current now flows through the M contact instead, so M **stays on**.
5. Press **STOP**: the path breaks, M drops out, and its seal-in contact opens. Releasing STOP does not restart the motor.
6. If the motor overloads, the **OL** contact opens and drops M out the same way.

Rung 2 shows a run lamp driven by another M contact.

## Why stop buttons are normally closed

A broken wire on a normally closed stop circuit has the same effect as pressing stop: the machine stops. If the stop button were normally open and its wire broke, pressing it would do nothing. This **fail-safe** principle is why stops, overloads and safety devices use NC contacts.

The three-wire circuit also gives **low-voltage protection**: after a power cut, M drops out and the motor will not restart by itself when power returns. A **two-wire** control (a maintained switch or float switch with no seal-in) restarts automatically, which is fine for a sump pump but dangerous for a saw.

## Interlocks

An interlock stops two things from happening at once. A forward/reverse motor starter has two contactors, F and R. Energizing both would short two phases together, so each rung includes an NC contact of the other contactor:

```text
3   |--[ STOP ]--+--[ FWD PB ]--+--[ R (NC) ]--[ OL ]--( F )--|
    |            +----[ F ]-----+                            |
4   |--[ STOP ]--+--[ REV PB ]--+--[ F (NC) ]--[ OL ]--( R )--|
    |            +----[ R ]-----+                            |
```

When F is on, its NC contact in rung 4 is open, so R cannot energize. Real reversing starters also use a **mechanical interlock** between the contactors.

## Worked example: reading a fault

A conveyor starts when START is pressed but stops as soon as the button is released. Which part of rung 1 is suspect? The seal-in branch: the M auxiliary contact is not closing (a failed auxiliary block or a broken wire on that branch). The rest of the rung clearly works, because the motor runs while START is held.

## Common mistakes

- Confusing "normally closed" with "closed while running".
- Forgetting that every contact with the same label as a coil changes when that coil energizes.
- Drawing the seal-in contact in series with START instead of in parallel.
- Putting more than one output coil in series on a rung.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Rails are supply and return; each rung is conditions on the left, one output on the right.
- Seal-in: an NO contact of the coil in parallel with START keeps it on.
- Stops and overloads are NC so a broken wire fails safe.
- Interlocks use NC contacts of the opposing output.

## Try it

Open the **Do** tab and take the quiz on reading start/stop and interlock rungs.


Read a ladder diagram, including a three-wire start/stop seal-in circuit with overload protection and an interlock.

Ladder diagrams

What is Ladder Logic?

Motor Start/Stop Seal-In Circuit Explained | Ladder Logic Basics

## Why it matters

A controller is only as good as what it can sense and what it can move. Most automation faults are not in the PLC program: they are a dirty photo-eye, a proximity sensor knocked out of range, a cut cable or a sticking valve. Knowing which sensor suits which job, and how it is wired, is what lets you fix a machine quickly and replace a part with the right one.

## Discrete (on/off) sensors

**Limit switches** are mechanical: a lever, plunger or roller is pushed by the moving part, which changes NO and NC contacts. Rugged and easy to understand, but they wear and need contact with the target.

**Inductive proximity sensors** detect metal without contact, using an electromagnetic field at the sensing face. Range is short, typically a few millimeters up to a few tens of millimeters, and depends on the metal: steel is detected at the rated distance, aluminum and brass at shorter range. Uses: detecting a cylinder rod end, gear teeth for speed, a metal pallet in position.

**Capacitive proximity sensors** detect almost anything, including plastic, wood, powder and liquid, often through a non-metallic tank wall. They usually have a sensitivity adjustment. Uses: level in a plastic hopper, detecting cardboard boxes.

**Photoelectric sensors** use a light beam:

- **Through-beam**: separate emitter and receiver facing each other. Longest range and most reliable; the object breaks the beam.
- **Retroreflective**: emitter and receiver in one housing, aimed at a reflector. Easier to install; shiny objects can fool them unless the sensor is polarized.
- **Diffuse**: the light bounces off the object itself. Simplest to mount but shortest range, and affected by the object's color.

**Magnetic (reed or Hall-effect) switches** clip onto pneumatic cylinders and detect a magnet in the piston to report extended or retracted.

**Pressure, flow, level and temperature switches** change state at a set point, for example low oil pressure.

## Sinking and sourcing (NPN and PNP)

Three-wire DC sensors have power (brown, +24 V), common (blue, 0 V) and output (black).

- A **PNP (sourcing)** sensor switches +24 V onto the output wire when it detects. It needs a **sinking** PLC input.
- An **NPN (sinking)** sensor connects the output to 0 V when it detects. It needs a **sourcing** input.

Mixing them up gives an input that never turns on. Check the sensor's part number and the input card's wiring diagram before replacing one. Most sensors also have an indicator LED showing when they detect, which is the first thing to check when troubleshooting.

## Analog sensors

Analog sensors give a value, not just on/off:

- **Pressure transmitters**, **level transmitters**, **flow meters**.
- **Temperature**: thermocouples (wide range, small voltage) and RTDs (accurate resistance change, often Pt100).
- **4 to 20 mA** is the standard signal. A live zero of 4 mA means a reading of 0 mA tells you the wire is broken.

**Worked example.** A pressure transmitter is ranged 0 to 150 psi on 4 to 20 mA. The PLC reads 13.6 mA.

```text
pressure = (13.6 − 4) / (20 − 4) × 150 = 9.6 / 16 × 150 = 90 psi
```

**Encoders** count pulses as a shaft turns, giving position and speed. Incremental encoders count from a reference; absolute encoders know their position at power-up.

## Actuators

Outputs move the machine:

- **Contactors and motor starters** switch motors; **relays** switch smaller loads.
- **Solenoid valves** shift pneumatic and hydraulic valves.
- **Pilot lights, horns and stack lights** tell people what is happening.
- **Variable frequency drives** control motor speed (next lesson).
- **Servo drives and stepper drives** control precise position.

PLC output cards come as relay outputs (switch AC or DC, slower, contacts wear), transistor outputs (DC, fast) and triac outputs (AC).

## Matching sensors to applications

```text
Application                                     Good choice
Detect a steel bracket 5 mm away                Inductive proximity
Detect clear plastic bottles on a conveyor      Through-beam or a clear-object photo sensor
Detect level of grain through a plastic bin     Capacitive proximity
Know if an air cylinder is retracted            Magnetic reed switch on the cylinder
Measure tank level continuously                 4-20 mA level transmitter
Detect a door is closed (not for safety)        Limit switch or inductive sensor
Measure shaft speed                             Inductive sensor on gear teeth, or an encoder
```

Safety functions such as guard doors use special **safety-rated** devices, covered in the machine safety lesson. An ordinary sensor is not a safety device.

## Common mistakes

- Replacing a PNP sensor with an NPN one.
- Expecting an inductive sensor to see aluminum at the steel rating.
- Diffuse photo-eyes aimed at a shiny background that reflects light back.
- Reading 0 mA as "zero pressure" instead of a broken loop.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Limit switches touch; inductive sensors see metal; capacitive sensors see most materials; photo sensors use light (through-beam is most reliable).
- PNP sources +24 V, NPN sinks to 0 V; match the input card.
- 4 to 20 mA analog signals: value = (mA − 4) / 16 × span.

## Try it

Open the **Do** tab and take the quiz on matching sensors to applications.


Match limit switches, proximity, photoelectric and analog sensors to applications, and name the actuators that controls drive.

Sensors and actuators

Ep 1: What Are Industrial Sensors? The Basics Explained

What are Proximity Sensors and How Do They Work? - A Galco TV Tech Tip | Galco

## Why it matters

Most industrial motors are three-phase induction motors. Connected straight to the supply, they run at one speed. A **variable frequency drive (VFD)** lets a pump, fan or conveyor run at exactly the speed the process needs, with soft starts and big energy savings. VFDs are everywhere, and they have their own faults, settings and hazards.

## Motor speed and frequency

An induction motor's magnetic field rotates at the **synchronous speed**:

```text
synchronous rpm = 120 × frequency (Hz) / number of poles
```

A 4-pole motor on 60 Hz: 120 × 60 / 4 = **1,800 rpm**. The rotor runs a little slower because of **slip**, so the nameplate might say 1,750 rpm. On 50 Hz supplies the same motor would turn at 1,500 rpm synchronous.

Change the frequency, and you change the speed. At 30 Hz the same motor runs at about 900 rpm.

## Inside a VFD

A typical drive has three stages:

1. **Rectifier**: diodes convert the incoming three-phase AC into DC.
2. **DC bus**: large capacitors smooth the DC. On a 480 V drive the bus sits around 650 V DC.
3. **Inverter**: fast transistors (IGBTs) switch the DC bus on and off thousands of times a second. By varying the width of the pulses (**pulse width modulation, PWM**), the drive builds an output that acts like a sine wave at whatever frequency and voltage it chooses.

The drive keeps the **volts-per-hertz** ratio roughly constant so the motor's magnetic flux, and therefore its torque capability, stays steady. A 460 V, 60 Hz motor gets about 230 V at 30 Hz. More advanced drives use **sensorless vector** or **closed-loop vector** control for better torque at low speed.

## Why plants use VFDs

- **Energy**: for centrifugal pumps and fans, power falls with the cube of speed (the affinity laws). Running a fan at 80 percent speed uses roughly 0.8³ ≈ 51 percent of full-speed power, compared with throttling a damper.
- **Soft starting**: the drive ramps up, avoiding the large inrush current and mechanical shock of across-the-line starting.
- **Process control**: speed follows a setpoint from a PLC or a PID loop, for example holding pipe pressure constant.
- **Ramps and braking**: controlled acceleration and deceleration, and braking resistors for high-inertia loads.

## Control and common parameters

A VFD gets **start/stop** and **speed reference** from its keypad, hard-wired terminals (digital inputs and a 0 to 10 V or 4 to 20 mA analog input) or a network (EtherNet/IP, PROFINET, Modbus). Common parameters:

- Motor nameplate data: voltage, current, frequency, rpm, power.
- **Minimum and maximum frequency**.
- **Acceleration and deceleration times**.
- **Current limit** and motor overload settings.
- Stop mode: ramp to stop or coast.

**Worked example.** A conveyor should run between 20 and 60 Hz, with speed set by a 4 to 20 mA signal. At 12 mA the reference is halfway, so the drive runs at 20 + 0.5 × (60 − 20) = 40 Hz. On a 4-pole motor, synchronous speed is 120 × 40 / 4 = 1,200 rpm.

## Faults you will see

- **Overcurrent**: acceleration too fast, a jammed load, or a short in the motor cable.
- **Overvoltage on the DC bus**: deceleration too fast with a high-inertia load (the motor acts as a generator). Lengthen decel time or add a braking resistor.
- **Undervoltage**: supply dips.
- **Overtemperature**: blocked cooling fans or dirty heat sinks.
- **Ground fault**: damaged motor insulation or cable.

Drives keep a **fault log**: read it before resetting. Repeated resets hide a real problem.

## Side effects

The fast PWM switching causes **electrical noise**, voltage spikes on long motor cables and **shaft currents** that can damage bearings (the fluting you saw in the bearings lesson). Fixes include proper shielded cable, grounding, output reactors and shaft grounding rings. Motors on VFDs should be inverter-duty rated. Self-cooled motors at low speed also cool poorly, because their fans slow down too.

## Common mistakes

- Setting a very short deceleration time and getting overvoltage trips.
- Running a standard motor slowly at full torque for long periods, so it overheats.
- Putting a contactor between the drive and motor and opening it while running.
- Resetting faults without reading the log.

## Safety

The DC bus capacitors stay charged after the drive is switched off. Drives carry a label stating how long to wait (often several minutes) before opening the cover, and the bus voltage must be verified as zero with a properly rated meter before touching anything. A drive can also restart a motor automatically after a fault reset or power return. Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Synchronous rpm = 120 × Hz / poles; a VFD varies frequency to vary speed.
- Rectifier, DC bus, IGBT inverter with PWM; volts per hertz stays roughly constant.
- Fans and pumps save energy with the cube of speed.
- Read the fault log; respect stored energy in the DC bus.

## Try it

Open the **Do** tab and take the quiz on how VFDs control speed.


Explain how a variable frequency drive turns fixed AC into variable frequency to control an induction motor's speed.

Motor drives (VFDs)

Variable Frequency Drives Explained - VFD Basics IGBT inverter

VFD 101 Basics

Ladder diagrams, start/stop seal-in circuits and interlocks, the sensors and actuators that connect controls to a machine, and variable frequency drives.