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

Contactors and relays are small components with massive responsibility.

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

Selecting contactor based only on FLA
Ignoring AC-3 category
Underrated coil voltage
Not accounting for ambient temperature
No protection coordination review
Using PLC output directly for heavy loads
Ignoring harmonic environment
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:

What utilization category is contactor rated for?
Is AC-3 rating sufficient for motor load?
What is coil voltage?
Is Type 2 coordination achieved?
Are PLC outputs protected by relays?
Are safety relays properly specified?
Is ambient temperature considered?
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.