9.2 Motor Control Devices: Pilot Devices, Control Relays & Timers

9.2 Motor Control Devices: Pilot Devices, Control Relays & Timers

Quick Answer: The contactor does the switching; the control devices decide when. Red Seal sub-tasks D-22.03 and D-22.04 cover everything between the operator's hand or the process variable and the starter coil: pushbuttons, selector switches, emergency stops, pressure and float and limit switches, control relays, timing relays and the interposing relays that let a PLC command a 600 V starter. The three ideas that separate a professional job from a working job: safety contacts must open mechanically even if they weld, contact ratings describe what the contact can switch, not just carry, and every inductive coil gets suppression at the coil.

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1. Manual Pilot Devices

Pushbuttons

Mounting sizes. The two standards in Canadian plants are 22 mm (IEC) and 30 mm (NEMA heavy-duty). They are not interchangeable, and a plant that mixes them accumulates a stock problem — identify the existing standard before ordering replacements. Heavy-duty 30 mm devices are the traditional choice in mills and foundries; 22 mm devices dominate modern OEM machinery.

Contact blocks stack on the back of the operator. A single operator can carry several NO and NC blocks; the mechanical operator and the electrical contacts are separate parts and are ordered separately.

Direct-opening-action contacts — the safety concept that matters

An ordinary NC contact is held closed by a spring and opened by an actuator pushing against that spring. If the contacts weld, the spring cannot part them and the "stop" button does nothing while looking and feeling completely normal.

A direct-opening-action contact — also called positive-opening operation, marked with the arrow-in-circle symbol and defined in IEC 60947-5-1 — uses a rigid mechanical linkage between the actuator and the moving contact. Pressing the button physically drives the contact apart with mechanical force, tearing welded contacts open.

Every emergency stop device and every safety interlock switch must use direct-opening-action contacts. This is not a preference; it is the basis on which the device is certified for safety duty.

Selector switches

• Two-position (HAND/OFF, LOCAL/REMOTE) and three-position (HAND/OFF/AUTO) are the industrial standards.
• Maintained (stays where you put it) or spring-return (returns to centre or to a specified position).
• Key-operated where the selection must be restricted — MAINTENANCE/RUN, BYPASS enable.
• The cam arrangement determines which contacts close in which position; the manufacturer's contact development chart (a table of positions versus contacts) is read, not guessed.

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2. Automatic Pilot Devices

These are the devices that let the process, rather than a person, close the circuit. The RSOS range of variables for motor control devices lists exactly this family.

Deadband is what stops the chattering

A pressure or level switch with its cut-in and cut-out too close together will cycle the motor continuously as the process variable hovers at the setpoint — short-cycling, which destroys contactors and motors. The differential is the gap between the operate and release points, and setting it deliberately is the whole skill. A sump pump set to start at 600 mm and stop at 580 mm will hammer; the same pump set to start at 600 mm and stop at 250 mm runs a sensible number of times per hour.

Limit switch selection

Mount limit switches so the machine cannot overtravel into the switch body: use a cam with a gradual rise and a mechanical stop beyond the switch's rated overtravel. Half the failed limit switches in a plant were destroyed by the machine they were watching.

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3. Control Relays

A control relay switches control circuits. A contactor switches motor and power loads and has arc chutes to prove it. Using a control relay where a contactor belongs is a fire; using a contactor where a relay belongs is a waste of panel space.

Electromechanical relays

Latching (impulse) relays hold their last state without coil power — used where a control power interruption must not change the machine state, and specifically not used where it must.

Contact ratings — carry versus switch

A contact's continuous current rating tells you what it can carry. Whether it can switch a given load is described by the utilization category:

The North American equivalent is the NEMA contact designation: A600 means AC control circuit, 600 V maximum, with defined make and break VA; B600 is a lower-current version; Q600 covers DC control circuits. When an engineering drawing specifies "A600 contacts", it is specifying make/break capability, not just voltage.

Why DC is harsher than AC on the same contacts. A DC contact rating is always far lower than the AC rating of the same device, because a DC arc has no current zero to help it extinguish. A relay rated 10 A at 250 V AC may be rated 0.5 A or less at 250 V DC.

Solid-state relays (SSRs)

An SSR switches with a triac or back-to-back SCRs, optically isolated from the input.

• Zero-crossing (zero-fire) SSRs turn on at the next voltage zero — low electrical noise, ideal for resistive heater control.
• Random-fire (instant-on) SSRs turn on immediately — required for inductive loads and phase-angle control.
• Heat sinking is not optional. An SSR drops roughly 1 to 1.6 V across its output; at 25 A that is 25 to 40 W of continuous heat that must leave through a properly sized, thermally compounded heat sink.
• Leakage current flows through the snubber even when the SSR is off — typically a few milliamperes, which is enough to keep a small neon indicator glowing or to hold a sensitive input on. A bleeder resistor across the load fixes it.
• An SSR fails shorted far more often than open. A safety circuit therefore never relies on an SSR alone to remove power; a series electromechanical contactor provides the positive break.

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4. Timing Relays

Contact marking on a timing relay (IEC)

• 15/16/18 — instantaneous changeover contact (transfers as soon as the coil energizes)
• 17/18 or 27/28 — delayed contacts
• A1 / A2 — coil terminals

Reading the timing diagram in the manufacturer's data rather than assuming is essential; "off-delay" relays differ in whether they need control voltage present during timing (two-wire versus three-wire off-delay), and wiring a three-wire off-delay as though it were two-wire produces a relay that never times.

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5. Interposing Relays: The Bridge Between the PLC and the Power

A PLC discrete output card is a low-power device. A typical transistor output sources a few hundred milliamperes at 24 V DC; a relay output card is often rated 2 A resistive at 250 V AC. A 600 V motor starter coil presents:

• an inrush VA several times its sealed VA while the magnetic gap is open;
• a coil voltage that is not the PLC's voltage — 120 V AC or 600 V AC, not 24 V DC;
• a substantial inductive kickback at de-energization.

An interposing relay solves all three: the PLC output drives a small 24 V DC relay coil, and that relay's contacts — chosen for the correct utilization category and voltage — switch the starter coil.

Additional reasons interposing relays are standard practice in industrial panels:

• Isolation. A fault on a 120 V control circuit cannot reach the PLC backplane.
• Field replaceability. A blown ice-cube relay is a two-minute, no-tools swap; a blown output card is a downtime event.
• Voltage translation between a 24 V DC logic system and legacy 120 V AC control devices.
• Contact multiplication. One PLC output, several independent contacts.
• A physical test point. The relay's LED and manual override let a technician confirm whether the fault is upstream or downstream in seconds.

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6. Coil Suppression

Every time an inductive coil is de-energized, the collapsing field generates a voltage spike governed by $v = -L,di/dt$ that can reach hundreds or thousands of volts. That spike erodes the switching contacts, radiates interference into nearby signal cables, and destroys semiconductor outputs.

Suppress at the coil, not at the contact. Suppression works by giving the collapsing field a local path; putting the device at the far end of a long control wire leaves the inductance of that wire unsuppressed and radiating.

The safety-circuit subtlety: a plain flyback diode can double or triple a contactor's dropout time. In a safety circuit where stopping time is calculated for guard-door interlock distances, that delay is a real hazard. Use a diode-plus-Zener or a TVS so the dropout stays fast while the spike stays clamped.

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7. Troubleshooting a Control Circuit With a Voltmeter

The disciplined method is half-stepping with the circuit energized and full PPE for the incident energy present.

1. Read the ladder drawing first. Identify the rung, the devices in series, and the wire numbers on each side of every device.
2. Confirm control power. Measure across L1 and L2 of the control transformer secondary. No control power is the most common "complex" fault.
3. Measure from one side of the control supply to a point partway along the rung. Full voltage means the circuit is intact to that point.
4. Split the remaining segment in half again and repeat, so each measurement eliminates half of the remaining suspects.
5. Measure across each suspect device. A closed contact shows 0 V; an open contact in a live circuit shows full control voltage across it.
6. Confirm before condemning. Read the coil voltage at the coil terminals. A coil at full voltage that will not pull in is a mechanically jammed or open-circuit coil; a coil at low voltage points upstream to a high-resistance connection or an undersized control transformer.

The ghost-voltage trap

A high-impedance digital multimeter will read substantial induced voltage on a conductor that is actually open, because the meter draws almost no current. Confirm anything suspicious with a low-impedance (LoZ) meter setting or a solenoid-type tester, which loads the circuit and collapses a phantom reading to near zero. Condemning a good device because of a ghost voltage is one of the most common ways to turn a one-hour fault into a shift-long fault.

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8. Maintaining Control Devices (RSOS D-22.04)

• Do not file contacts. Modern silver-cadmium-oxide and silver-tin-oxide contacts develop a dark oxide film that is a normal, conductive condition. Filing removes the plating and shortens life. Replace contacts when the silver is worn through to the base metal, or when the tips are badly pitted or misaligned.
• Replace contacts as a set, movable and stationary together, and replace springs with the kit.
• Check coil resistance against the manufacturer's value and inspect for a scorched or swollen coil bobbin.
• Chattering contactor — look for low control voltage, a broken shading coil in the pole face, dirt or rust in the magnetic gap, or a mechanically binding armature.
• Clean the magnetic pole faces with a lint-free cloth; never with abrasive, which changes the gap and creates noise and chatter.
• Verify all emergency stops and interlocks function on a documented schedule, and test them by operating the device — not by jumpering it.
• Retorque control terminations. Vibration loosens small terminals faster than large ones, and a loose control terminal produces the intermittent faults that consume the most troubleshooting hours in a plant.