Selecting Contactors and Relays for Roll Forming Machines (Sizing, Ratings & Protection)

Selecting Contactors and Relays

Engineering Guide for Roll Forming & Coil Processing Control Panels

Contactors and relays are small components with massive responsibility.

In roll forming machines, they:

• Enable motor starting

• Control hydraulic pumps

• Switch auxiliary systems

• Interface PLC outputs

• Execute safety shutdown

• Control braking circuits

When selected incorrectly, they cause:

• Welded contacts

• Overheating

• Nuisance trips

• Control instability

• Coil burnout

• Unexpected downtime

This guide explains how to properly select contactors and relays for industrial roll forming environments.

1) Contactors vs Relays — Understanding the Difference

Contactors

Used for switching:

• High current loads

• Motors

• Heaters

• Power circuits

Typically rated in:

• AC-1 (resistive load)

• AC-3 (motor load)

• AC-4 (frequent motor starting/jogging)

Relays

Used for:

• Control signals

• Low current switching

• PLC interface

• Interlocking

• Timing logic

Rated for lower current and lighter duty.

Contactors handle power.
Relays handle logic and signal-level switching.

2) Understanding Contactor Ratings

Most roll forming motor loads require AC-3 rating.

AC-3 rating means:

• Starting induction motor

• Breaking motor current during normal operation

Do not size contactor based only on motor full load current (FLA).

Always check:

• Motor rated current

• Duty cycle

• Ambient temperature

• Utilization category

3) Motor Load Categories in Roll Forming Machines

Common motor loads:

• Main forming motor

• Hydraulic pump motor

• Uncoiler motor

• Recoiler motor

• Conveyor motors

Hydraulic pump motors often have high starting current and pressure load.

Main forming motors experience:

• Frequent speed variation

• Variable load

• High torque peaks

Contactors must be selected accordingly.

4) Coil Voltage Selection

Contactor coil voltage must match control voltage.

Common coil voltages:

• 24VDC

• 110VAC

• 230VAC

Modern systems often use 24VDC for safety and PLC compatibility.

Incorrect coil voltage leads to:

• Chatter

• Overheating

• Coil burnout

• Intermittent shutdown

Always confirm:

Control PSU output matches coil rating.

5) Short-Circuit Coordination

Contactors must be protected by:

• MCCB

• MPCB

• Fuses

Protection coordination types:

• Type 1 coordination (may allow damage under short circuit)

• Type 2 coordination (no damage under short circuit)

For industrial roll forming machines, Type 2 coordination is preferred for critical motors.

Protection must be engineered with upstream device.

6) Thermal Considerations

Contactors generate heat during:

• Coil energization

• Load switching

• High duty cycles

Cabinet heat management directly affects contactor lifespan.

High ambient + high duty = reduced contact life.

7) Contact Material Selection

Contacts typically made of:

• Silver alloy

In motor applications, contact wear is affected by:

• Switching frequency

• Current magnitude

• Power factor

Hydraulic pump contactors with frequent cycling require higher durability.

8) Auxiliary Contacts & Interlocking

Contactors include auxiliary contacts for:

• Feedback to PLC

• Interlocking logic

• Safety chains

Example word-based interlock:

Main Drive ON → Aux Contact Closed → PLC Confirms Motor Running

Hydraulic Pump OFF → Aux Contact Open → Prevent Shear Activation

Interlocking prevents mechanical damage.

9) Relay Selection Criteria

Relays must be selected based on:

• Contact current rating

• Coil voltage

• Switching frequency

• Electrical life

• Mechanical life

PLC outputs often drive interposing relays to:

• Protect PLC transistor outputs

• Provide isolation

• Switch higher current loads

10) Interface Relays in Roll Forming Systems

Common uses:

• Solenoid valves

• Indicator lights

• Small motors

• Alarm outputs

Typical path:

PLC Output → Relay Coil → Relay Contact → Solenoid Valve

Relays protect PLC from direct load switching.

11) Safety Relays (Not Standard Relays)

Safety relays are specialized devices.

Used for:

• Emergency stop circuits

• Guard interlocks

• Light curtains

Must meet safety integrity requirements.

Never substitute standard relay for safety relay.

12) Common Selection Mistakes

1. Selecting contactor based only on FLA

2. Ignoring AC-3 category

3. Underrated coil voltage

4. Not accounting for ambient temperature

5. No protection coordination review

6. Using PLC output directly for heavy loads

7. Ignoring harmonic environment

8. Overcrowding causing overheating

These errors lead to early failure.

13) Word-Based Example: Main Motor Switching Circuit

MAIN MCCB → BUSBAR → MPCB → CONTACTOR → OVERLOAD RELAY → MOTOR

Control circuit:

24VDC PSU → E-STOP LOOP → PLC OUTPUT → CONTACTOR COIL

Aux contact feedback:

CONTACTOR AUX → PLC INPUT

This structure ensures protection, control, and monitoring.

14) Mechanical Life vs Electrical Life

Contactor datasheets specify:

• Mechanical life (no load switching)

• Electrical life (rated load switching)

High-frequency jogging reduces electrical life drastically.

Roll forming lines with frequent start-stop require higher-rated devices.

15) Export Considerations

When exporting machines:

• Confirm voltage compatibility

• Confirm frequency rating

• Ensure contactor coil voltage suits local control voltage

• Check short-circuit level compatibility

• Verify ambient temperature rating

North American short-circuit levels are often higher than other regions.

Contactor interrupt capacity must match.

16) Buyer Strategy (30%)

Before purchasing or approving a roll forming machine, ask:

1. What utilization category is contactor rated for?

2. Is AC-3 rating sufficient for motor load?

3. What is coil voltage?

4. Is Type 2 coordination achieved?

5. Are PLC outputs protected by relays?

6. Are safety relays properly specified?

7. Is ambient temperature considered?

8. What is short-circuit rating of panel?

Red flag:

“It’s a standard contactor.”

There is no such thing as “standard” without rating context.

6 Frequently Asked Questions

1) Can I use AC-1 rated contactor for motor load?

No. Motor loads require AC-3 or appropriate utilization category.

2) Why do contactors weld shut?

Underrating, high inrush current, or short-circuit event.

3) Should PLC drive contactor directly?

Only if output rating allows. Often interposing relay is safer.

4) What is biggest failure cause?

Improper sizing and overheating.

5) Does harmonic environment affect contactors?

Indirectly yes — increased current and heating can reduce life.

6) How often should contactors be inspected?

During scheduled maintenance, especially in high-duty systems.

Final Engineering Summary

Selecting contactors and relays in roll forming control systems requires:

• Correct utilization category

• Proper current rating

• Appropriate coil voltage

• Protection coordination

• Thermal margin

• Interlocking logic integration

• Compatibility with local short-circuit levels

Correct selection ensures:

• Reliable motor control

• Stable hydraulic operation

• Safe shutdown

• Reduced downtime

• Long-term panel durability

Improper selection guarantees production instability.