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.
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)
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.
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
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
No. Motor loads require AC-3 or appropriate utilization category.
Underrating, high inrush current, or short-circuit event.
Only if output rating allows. Often interposing relay is safer.
Improper sizing and overheating.
Indirectly yes — increased current and heating can reduce life.
During scheduled maintenance, especially in high-duty systems.
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.
Copyright 2026 © Machine Matcher.