ALearning Material
Between the PLC's low-power control signals and the high-power motors and equipment of a plant sits essential electrical hardware: relays and motor starters. These are how a small control signal safely switches large loads: a fundamental piece of industrial automation that every system relies on, and the bridge between control logic and the physical power that drives machinery.
A relay is an electrically-operated switch: a small control current energizes a coil, which mechanically closes (or opens) contacts that switch a separate, often much larger, circuit. The key idea is isolation and amplification of control: a low-power signal (the PLC output, a few mA at 24 V) controls a high-power circuit (a motor at 480 V, many amps) without the control side touching the power side:
PLC output (24V, low current) -> relay coil -> contacts switch -> motor circuit (480V, high current)
small signal large load
A motor starter is the purpose-built assembly for switching a motor on/off safely: a contactor (a heavy-duty relay sized for motor loads) plus overload protection (which trips if the motor draws too much current for too long, preventing burnout). Pressing start energizes the contactor coil (often through a PLC output and a seal-in), the contactor's main contacts connect the motor to power, and the overload relay protects it. The concepts: relays/contactors isolate and switch power (control side separate from power side), a motor starter = contactor + overload protection, and the control logic (PLC) drives the coil, while the contacts switch the actual load. The disciplines: use a relay/contactor to switch a load bigger than the control signal can handle, a motor starter adds overload protection (don't switch a motor without it), and understand control-side vs power-side separation. This hardware is how control logic actually moves machinery.
The seal-in circuit, and the same circuit as a ladder rung. A start pushbutton is momentary, so something has to hold the contactor in after the operator lets go. That something is one of the contactor's own auxiliary contacts, wired in parallel with the start button, and the arrangement is called a seal-in or latch:
L1 L2
| STOP START |
+----|/|----+----| |----+------------------( K1 )---------------+
| | | coil |
| | K1 aux |
| +----| |----+ <- the SEAL-IN: once K1 pulls in, its own
| auxiliary contact keeps the coil energised
Press START and the coil energises; K1's auxiliary contact closes and now supplies the coil through its own path, so releasing START changes nothing. Press STOP and the series path breaks, the coil drops out, the auxiliary contact opens, and the circuit stays off. It latches on, and it fails off.
Syntax you need here. That relay drawing is also, exactly, a ladder logic rung. Ladder is the relay schematic redrawn as a program, which is why it is the language of the plant floor and why an electrician can read a PLC program on sight. This is the notation you need to read and sketch a rung, and the PLC topic teaches it properly later:
THE SYMBOLS
--| |-- examine if CLOSED true when the bit is 1 (drawn as a NO contact)
--|/|-- examine if OPEN true when the bit is 0 (drawn as a NC contact)
--( )-- output coil set the bit to the rung's result
in SERIES = AND in PARALLEL = OR
ADDRESSES (Siemens style; other vendors differ in spelling, not in idea)
I0.0 a physical input Q0.0 a physical output M0.0 an internal memory bit
THE SAME SEAL-IN, AS A RUNG
| ESTOP STOP START MOTOR |
|----| |------| |------| |----+--------------------------------( )----|
| I0.0 I0.1 I0.2 | Q0.0 |
| |
| MOTOR |
|----| |---------------------+ <- the seal-in branch, in parallel
| Q0.0 with START
A TIMER, as a block on a rung
| RUNNING +---------------+ |
|----| |----------------------| TON T1 |----------( )----------|
| | PT = 12s | DONE |
| +---------------+
TON on-delay: DONE after the input has been true for PT continuously
TOF off-delay: DONE stays true for PT AFTER the input goes false
A COUNTER, the same shape. Count ONE-SHOTS, never a raw level, or it
increments once per scan for as long as the input is on.
| PART_DONE +---------------+ |
|----| |----------------------| CTU C1 |----------( )----------|
| (a one-shot, see below) | PV = 10 | DONE |
| +---------------+
| RESET |
|----| |----------------------( RES C1 )------------------------------|
A ONE-SHOT (rising edge): true for exactly ONE scan when the input goes 1
| BOTTLE_PE PE_MEM PE_PULSE |
|----| |----------|/|------------------------------------------( )-----|
| I0.4 M0.3 M0.4 |
| BOTTLE_PE PE_MEM |
|----| |---------------------------------------------------------( )---|
| I0.4 THIS RUNG SECOND |
Two rules make the difference between logic that works and logic that is dangerous. The first is
that a rung is solved left to right, top to bottom, once per scan, which is why the one-shot's
two rungs must be in that order: reversed, PE_MEM is already set when the first rung is solved and
the pulse never happens.
The second is the fail-safe convention, and it is the one that reads backwards until it clicks.
A stop button and an e-stop are wired normally closed in the field, so a healthy circuit presents
a 1 to the input and a broken wire, a pulled connector or a lost 24 V presents a 0. In the program
you therefore examine them with --| |--, "examine if closed", because true means healthy. Wire an
e-stop normally open and examine it with --|/|-- instead: the ladder looks identical on screen, the
machine tests fine, and a broken wire silently disables the e-stop rather than stopping the machine.
Whether a device is wired NC and examined for ON, or NO and examined for OFF, is a safety decision,
not a style one.
Why it exists. A PLC's outputs are low-power signals that can't directly switch a plant's high-power motors and equipment, and connecting them directly would be impossible or dangerous. Relays and contactors bridge this gap (a small control signal safely switches a large, isolated power circuit) and motor starters add the overload protection a motor needs, making them the essential interface between control logic and the physical power that drives machinery.
Mental model. A relay is like a light switch operated by a tiny remote: your small remote signal flips a big, heavy switch that handles the real power. You never touch the high-voltage circuit directly. A motor starter is that heavy switch built specially for motors, with a built-in circuit-breaker-like protector (the overload) that cuts power if the motor is straining too hard, so it doesn't burn out.
Common misunderstandings.
- "The PLC output can drive the motor directly." No. PLC outputs are low-power; you use a relay/contactor to switch the high-power motor circuit. The PLC drives the coil; the contacts carry the load.
- "A contactor alone is a motor starter." A motor starter is a contactor plus overload protection, switching a motor without overload protection risks burning it out on a fault/overload.
- "Control and power are the same circuit." Relays isolate the low-power control side from the high-power load side
- that separation is the whole point (safety and capability).
Connections. Relays/contactors are what the PLC outputs and motor-control logic (PLC topic) actually switch. The coil is the PLC output, the seal-in/interlocks drive it; overload protection extends the safety/fail-safe thinking; and this control-vs-power-side hardware is the physical layer beneath the automation pyramid and the bottling/water projects.
BImmediate Active Recall
QUERYWhat is a relay, and what is its key function?
REVEAL
A relay is an electrically-operated switch: a small control current energizes a coil that mechanically closes/opens contacts switching a separate circuit. Its key function is isolation and amplification of control. A low-power signal (a PLC output) switches a high-power circuit (a motor) without the control side touching the power side. The control drives the coil; the contacts switch the load.
QUERYWhat is a motor starter, and what two parts make it up?
REVEAL
A motor starter is the purpose-built assembly to switch a motor on/off safely. It's a contactor (a heavy-duty relay sized for motor loads) plus overload protection (an overload relay that trips if the motor draws too much current for too long, preventing burnout). The contactor switches the motor power; the overload protects the motor.
QUERYWhy can't a PLC output drive a motor directly, and how is the gap bridged?
REVEAL
Because PLC outputs are low-power signals (a few mA at 24 V), while a motor is high-power (e.g. 480 V, many amps). The output can't switch that load, and connecting them directly is impossible/dangerous. The gap is bridged by a relay/contactor: the PLC output energizes the coil (low power), and the contacts switch the high-power motor circuit (isolated from the control side).
QUERYWhat is the purpose of overload protection in a motor starter?
REVEAL
To protect the motor from drawing too much current for too long (an overload, e.g. a jammed load, a failing motor), which would overheat and burn it out. The overload relay trips (opening the circuit) when excessive current persists, cutting power before damage. That's why you never switch a motor without overload protection. A contactor alone isn't enough.
CConceptual Questions
Answer each in your own words in the box, then reveal the model answer to compare. These ask why, not how, and your answers are saved.
Why is the isolation between the low-power control side and the high-power load side the fundamental value of a relay/contactor, and what does it make possible?
REVEAL MODEL ANSWER
The isolation is the whole point because it cleanly separates two electrically incompatible worlds (the delicate, low-power control logic and the dangerous, high-power load) while still letting one command the other. A PLC output, or any control signal, operates at low voltage and tiny current; a plant's motors and equipment run at high voltage and large currents. You cannot connect them directly: the control electronics can't supply or survive the power-side energy, and bridging them would be both impossible (insufficient drive) and dangerous (high voltage on the control side, fault energy, no isolation). A relay/contactor resolves this by making the control signal energize only a coil, which mechanically (or via solid state) operates separate contacts that switch the power circuit, so the control side and power side are electrically isolated, touching only through the coil's magnetic field. This makes several essential things possible. Capability: a tiny signal can switch an enormous load, because the contacts are sized for the load, not the signal. Amplification of control authority. Safety: the high-voltage, high-energy power circuit is kept away from the control electronics and the people/logic on that side; a fault on the power side doesn't directly reach the control side. Flexibility: the control logic (PLC) just drives coils, so it can command any load by choosing an appropriately-sized relay/contactor, and the same low-power logic interfaces to wildly different power circuits. This control-coil / power-contacts separation is the foundational interface of all industrial control. It's how the 'brain' (low-power logic) safely commands the 'muscle' (high-power machinery), and recognizing that the PLC drives the coil while the contacts carry the load is the key to understanding how control logic actually moves real equipment.
Why is overload protection an inseparable part of a motor starter rather than an optional extra, and what does its absence risk?
REVEAL MODEL ANSWER
Overload protection is integral because a motor is a costly, failure-prone load with a specific, dangerous failure mode, drawing excessive current, that switching alone does nothing to prevent, so a starter without it is incomplete and unsafe by design. A motor draws current in proportion to the mechanical load it's working against; if it's jammed, overloaded, running with a failing bearing, or stalled, it draws far more current than rated, and that excess current heats the windings. Sustained overcurrent overheats the motor until its insulation fails and it burns out. Destroying an expensive machine and potentially causing a fire. A bare contactor will happily keep the motor connected to power straight through such an overload, because its job is only to switch, not to judge. The overload relay is what watches for this condition: it senses current over time and trips (disconnects) when the current is high enough for long enough to threaten the motor: tolerating the brief, normal inrush at startup but cutting power on a genuine sustained overload. That time-current behaviour is precisely matched to what motors can withstand, which is why it's purpose-built into the starter rather than improvised. Its absence risks exactly the catastrophic, common failure it exists to prevent: a motor burning out on a jam or overload, with the downtime, cost, and fire hazard that entails. So 'motor starter = contactor + overload protection' is not a convention but a requirement. The switching and the protection are two halves of safely operating a motor, and providing one without the other leaves the motor exposed to its most likely destructive failure. It's the same fail-safe, protect-the-equipment thinking that runs through industrial design, applied at the power interface.
DPractice Problems
P1 (easy). Draw the control circuit for a start-stop station with seal-in, as a ladder rung, for a 480 V three-phase motor controlled from a 24 V DC PLC output. Label every device.
Then draw the power circuit separately, and state which devices appear in both drawings and which appear in only one. Say what the 24 V PLC output is actually switching.
P2 (medium). A motor nameplate reads 480 V, 3-phase, 27.0 A full-load amps, service factor 1.15.
Size the starter: the overload relay's trip setting, the contactor's AC-3 current rating, the branch-circuit conductor ampacity, and the short-circuit protective device. Give the rule behind each number. Then state which of these four protects the motor and which protects the wiring, and why they are different devices.
P3 (harder). Trace a fault. The motor runs, then stops, and the contactor has dropped out. Determine which of these it is, in an order that minimises risk: overload trip, control transformer fuse, a stop button contact, the seal-in contact, a lost phase.
Give the measurement for each, its expected value, and what it rules out. State the one measurement you would make first for safety reasons, and the one failure on this list that the overload relay is supposed to catch and often does not.
Solutionsclick to reveal
P1. The control circuit, 120 V AC control voltage derived from a control transformer:
L1 (120 V control) N
| |
| OL STOP START |
+---] [-----]/[----+---] [----+--------------------( M )-------------+
(aux NC (NC | | contactor coil
contact) push) | |
| M aux |
+---] [---+ <- the SEAL-IN, an auxiliary
(NO) contact on the contactor itself
From the PLC: the 24 V DC output drives an INTERPOSING RELAY (CR),
whose NO contact replaces or parallels the START pushbutton:
+---] [---+ CR contact, 120 V rated, in parallel with START
(CR)
The power circuit:
L1 L2 L3 (480 V, 3-phase)
| | |
[Q1] disconnect / fused switch or circuit breaker (short-circuit protection)
| | |
[M ] [M ] [M ] the CONTACTOR's three main power poles
| | |
[OL][OL][OL] the OVERLOAD relay's three heating elements
| | |
T1 T2 T3 --> MOTOR
Which devices appear in both:
- The contactor, M. Its coil is in the control circuit and its main poles are in the power circuit. It is the bridge between them, and that is its entire purpose
- The overload relay, OL. Its heating elements carry motor current in the power circuit; its auxiliary normally-closed contact sits in the control circuit, in series with everything else, so that an overload drops the coil and opens the main poles
Which appear in only one:
- Control only: the stop and start pushbuttons, the seal-in auxiliary contact, the interposing relay, the control transformer
- Power only: the disconnect, the short-circuit protective device, the motor itself
What the 24 V PLC output is actually switching.
It is switching the coil of an interposing relay, drawing perhaps 20 to 50 milliamps at 24 V DC, about one watt.
That relay's contact then closes the 120 V control circuit, energising the contactor coil, which draws perhaps 10 volt-amps.
The contactor's main poles then carry 480 V at the motor's full load current, tens of amps.
So the chain is 1 W, then 10 VA, then 20 kW, in three stages, and each stage exists because the one before it cannot do the next one's job. The PLC output is a small transistor or relay rated for 0.5 A at 24 V DC: it cannot switch 480 V, it cannot carry 27 A, and it cannot interrupt the arc that breaking 27 A at 480 V produces.
A note on why the interposing relay is there at all, when many PLCs have 120 V AC relay outputs that could drive a contactor coil directly. Three reasons, and they are practical rather than theoretical: it keeps high voltage out of the PLC's I/O wiring and out of the rack; it makes the PLC output easy to replace without touching the motor circuit; and it gives a visible, testable break between the control system and the machine, which is what lets a technician lock out the motor circuit and still exercise the PLC.
P2.
| Item | Rule | Value |
|---|---|---|
| Overload relay trip | Set at 115 to 125 percent of FLA for a service factor of 1.15 or greater | , up to . Set at about 31 A, and adjust upward only if the motor nuisance-trips on a legitimate load |
| Contactor, AC-3 rating | AC-3 is the utilisation category for a squirrel-cage motor started and stopped while running. Choose a contactor whose AC-3 rating at 480 V exceeds the FLA | Next standard size above 27 A, typically a 32 A AC-3 contactor. Do not size from the AC-1 figure, which is for a resistive load and is much higher for the same device |
| Branch-circuit conductors | 125 percent of FLA for a continuous-duty motor | 33.75 A, so conductors rated at least 34 A at the installed temperature and derating. Typically 8 AWG copper |
| Short-circuit protective device | An inverse-time circuit breaker at up to 250 percent of FLA for a design B motor, higher for some types | 67.5 A, so a 70 A breaker. It must be large enough not to trip on the starting inrush, which for a direct-on-line start is roughly , about 162 A, for a few seconds |
Which protects what.
The overload relay protects the MOTOR. It responds to sustained current a little above full load: a jammed conveyor, a failed bearing, a lost phase, a motor running in an unventilated enclosure. Its characteristic is slow, deliberately: it must ignore the 162 A of starting inrush for several seconds and still trip on 35 A sustained for a minute. What it is really protecting is the winding insulation, whose life halves for roughly every 10 degrees of temperature rise, and which is damaged by tens of seconds of overcurrent rather than by milliseconds.
The short-circuit device protects the WIRING and the equipment. It responds to a fault: a short between phases, a phase to earth, a failed contactor welded across a fault. Those currents are hundreds or thousands of amps, and the device must interrupt them in milliseconds, before the conductors reach a temperature that destroys their insulation and before the fault energy damages the switchgear.
Why they cannot be the same device.
Look at the two requirements together:
- Must ignore 162 A for several seconds (starting)
- Must trip on 35 A within about a minute (overload)
- Must trip on 2000 A within milliseconds (short circuit)
A single time-current curve cannot do all three. A device fast enough to clear the short circuit would trip on every start; a device slow enough to permit the start would let a 35 A overload cook the motor for hours, and would take far too long to clear a genuine fault.
So the protection is split by timescale: the overload relay owns the region from about 1.15 to perhaps , over seconds to minutes, and the breaker owns everything above that, in milliseconds. Their curves overlap at the top of the overload relay's range, and co-ordination is the requirement that the faster device clears first in the overlap so the correct one operates.
The one number to sanity-check before anything else: the 6x inrush. Direct-on-line starting draws roughly six times FLA, and if the branch is shared, or the supply is weak, the resulting voltage dip can drop out contactors elsewhere in the plant. That is what makes soft starters and VFDs attractive for larger motors, and it is a system-level consequence of a starter-level decision.
P3. The measurement to make first, for safety: verify the disconnect is open and the power circuit is dead, then decide whether the diagnosis requires it live.
Most of the list below is diagnosed in the control circuit at 120 V, with the 480 V power circuit isolated and locked out. Doing it that way means the only live parts within reach are at control voltage, and it removes the possibility of contacting 480 V while probing a coil terminal. Only the lost-phase check needs the power circuit energised, and it goes last.
The sequence.
| # | Check | Measurement | Expected | What it rules out |
|---|---|---|---|---|
| 1 | Overload relay tripped? | Look at it. Every overload relay has a mechanical trip indicator and a reset button | Indicator showing "tripped" | This is free, takes two seconds, and if the flag is out you have your answer. Do not just reset it: it tripped for a reason, and the reason is still there |
| 2 | Control voltage present? | Meter across the control transformer's secondary | 120 V AC | If absent: the transformer's primary or secondary fuse, or the transformer itself. The whole control circuit is dead, which explains the dropout and nothing else on this list applies |
| 3 | Voltage across the control circuit, rung by rung | Meter from the neutral to each point along the rung, working from the supply end | 120 V at each point up to the break, 0 V after it | The first point reading 0 V is immediately after the open device. This single technique finds items 3, 4 and 5 on the original list in one pass, which is why it comes here rather than being split into separate tests |
| 4 | Stop button and OL auxiliary contact | Both are normally closed and in series. From step 3, whichever one has 120 V on its input and 0 V on its output is the open one | Continuity when the button is released | Distinguishes a failed stop button from an overload auxiliary contact that has opened without the mechanical flag showing, which happens |
| 5 | Seal-in contact | Press start and hold. If the motor runs while held and stops when released, the seal-in path is open | The M auxiliary contact closes when the contactor pulls in | This symptom is diagnostic on its own and needs no meter: "runs while I hold the button" means the seal-in, every time |
| 6 | Lost phase. Power circuit energised, all guards in place | Meter phase-to-phase at the motor terminals: L1-L2, L2-L3, L1-L3 | 480 V on all three pairs | One pair reading 0 or low means a blown fuse, an open contactor pole, or a broken conductor |
The failure the overload relay is supposed to catch and often does not: a lost phase.
When one of three phases is lost, the motor keeps running (it cannot start, but a running motor continues on single phase) and the remaining two phases carry roughly 1.73 times their normal current to deliver the same torque. That is a genuine overload and a thermal overload relay should see it.
Two reasons it frequently does not:
A lightly loaded motor may not exceed the trip setting. A motor running at 50 percent load draws 13.5 A per phase normally; on single phase it draws about 23 A in two phases, which is below the 31 A trip setting. The motor overheats steadily because the current is unbalanced and the negative-sequence component heats the rotor far more than the current magnitude suggests, and the overload relay sees 23 A and is satisfied.
Eutectic and bimetallic relays respond to heat, slowly. By the time the relay reaches its trip point, the motor's winding has been running unbalanced for minutes.
The fix is a relay that measures what actually matters: an electronic overload relay with phase-loss and phase-imbalance detection, which trips on the difference between phases rather than on the magnitude of any one of them, typically within seconds. On any motor where a single-phase condition is credible, which is any motor fed through fuses, that capability is worth its small extra cost, and it is the standard argument for electronic over thermal overload relays.
EFeynman Exercise
Explain to a beginner, using a tiny remote that flips a big heavy switch, plus a built-in protector: (1) why your small remote signal can control a big power circuit without you ever touching the high voltage, (2) why this lets a weak control signal switch a powerful load, and (3) why the version for motors has a built-in protector that cuts power if the motor strains too hard.
REVEAL MODEL ANSWER
A relay (and its big brother the contactor) is like a tiny remote that flips a big, heavy switch. Imagine the real power, the 480-volt motor circuit, runs through a heavy switch you should never touch directly. Instead, you have a little remote: a small, safe, low-power signal. When you press it, it energizes a coil that mechanically flips that big heavy switch for you, so your small remote signal controls the big power circuit without you ever touching the high voltage. That's the magic of a relay: the control side (your remote) and the power side (the heavy switch and motor) are kept completely separate, linked only through the coil. This is what lets a weak control signal switch a powerful load: the PLC's little output can't run a motor itself, but it can flick the remote, and the heavy switch (sized for the motor) does the real work. Now, the version built for motors adds something important: a built-in protector (the overload). Motors can strain, if one jams or works too hard, it gulps far too much current and starts to overheat, and left alone it would burn out. So the motor's heavy switch comes with a protector that watches how hard the motor is straining and cuts the power if it's too much, for too long, like a safety cut-out that saves the motor from cooking itself. A tiny remote flipping a big isolated switch, so weak signals control powerful loads safely, with a built-in protector for motors: that's relays, contactors, and motor starters.
FError Analysis Framework
- Trying to drive a high-power motor directly from a PLC output. Why: the PLC output commands the motor. Recognise: the low-power output can't switch the load; it's dangerous. Avoid: use a relay/contactor: the PLC drives the coil, contacts switch the load.
- Switching a motor with a contactor but no overload protection. Why: the contactor turns the motor on/off. Recognise: an overload (jam/stall) burns out the motor. Avoid: use a motor starter (contactor + overload protection).
- Mixing the control side and power side in one circuit. Why: it's all one motor control. Recognise: no isolation. Unsafe and beyond the control signal's capability. Avoid: keep control side (coil) isolated from power side (contacts/load).
- Thinking the PLC output carries the motor current. Why: the output controls the motor. Recognise: the output is low-power; the contacts carry the load current. Avoid: the PLC drives the coil; the contactor's contacts carry the motor current.
GMini Challenge
Design the electrical interface for a PLC to start/stop a large industrial pump motor safely: specify the hardware (relay/contactor, motor starter, overload), how the control signal switches the motor, and the control-side/power-side separation. Explaining how this safely connects control logic to the physical motor.
REVEAL MODEL ANSWER
Hardware interface:
PLC output (24V) -> motor-starter contactor COIL (control side)
|
contactor MAIN CONTACTS -> [overload relay] -> 480V pump MOTOR (power side)
- A motor starter = a contactor (heavy-duty relay sized for the pump motor's load) + an overload relay (sized to the motor's full-load current).
- The PLC's start/stop logic (start, seal-in, e-stop/interlocks) drives the PLC output, which energizes the contactor coil (low power, control side).
- The energized coil closes the contactor's main contacts, connecting the 480 V motor to power (power side); the overload relay in series watches the current.
How the control signal switches the motor: on a valid start, the PLC output (24 V, low current) energizes the coil; the coil magnetically closes the heavy main contacts; the motor runs. On stop/e-stop/interlock or an overload trip, the coil de-energizes (or the overload opens), the contacts open, and the motor stops.
Control-side / power-side separation: the control side (PLC output, contactor coil, ~24 V) is electrically isolated from the power side (main contacts, 480 V motor), linked only through the coil's magnetic field. This is essential because (1) the low-power PLC output could never switch the 480 V load directly (the contacts (sized for it) do) and (2) the dangerous high-power circuit is kept away from the control electronics and people; a power-side fault doesn't reach the control side.
Overload protection: the overload relay trips on sustained overcurrent (a jammed pump, stalled motor), cutting power before the motor burns out, the essential protection a contactor alone lacks.
Result: the PLC's control logic safely commands the physical pump: the brain (low-power logic) drives the coil, the muscle (heavy contacts) switches the motor, isolation keeps it safe, and the overload protects the motor. This is the physical interface beneath all the motor-control logic.
Quiz Check
A quick auto-graded check, separate from the recall cards above. Your score is pooled with the recall cards into this module's Mastery score, and completing this lesson requires the quiz submitted with pooled mastery at 80% or above.