Strip Accumulator Motor Control Systems in Roll Forming Lines (Loop & Tension Control Guide)

Strip Accumulator Motor Control Systems

Loop, Tension & Synchronisation Control in Roll Forming Machines

Strip accumulators are used in roll forming and coil processing lines to:

• Maintain continuous production during coil change

• Absorb speed fluctuations

• Stabilize strip tension

• Prevent line stoppage

Without proper motor control design, accumulators cause:

• Strip snapping

• Loop collapse

• Excessive tension

• Slack strip entering roll former

• Length inaccuracies

• Coil change instability

The accumulator motor control system must coordinate:

• Uncoiler

• Entry pinch rolls

• Accumulator carriage or pit

• Main roll forming drive

This guide explains how strip accumulator motor systems are electrically controlled, synchronized, and protected.

1) Purpose of a Strip Accumulator

Accumulator creates buffer between:

Coil supply section → Roll forming section

During coil change:

Roll former continues running
Accumulator feeds strip from stored loop

Motor control must ensure smooth tension transition.

2) Types of Strip Accumulators

1) Overhead Loop Accumulator

Strip forms hanging loop.

2) Pit Accumulator

Strip forms loop inside floor pit.

3) Carriage-Type Accumulator

Moving carriage controls strip storage.

Each type requires different motor control strategy.

3) Core Electrical Components

Typical accumulator motor control includes:

• VFD-controlled motor

• Loop position sensor (ultrasonic, dancer arm, encoder)

• PLC control logic

• Limit switches (upper/lower travel)

• Emergency stop integration

• Tension feedback input

Precise speed coordination is required.

4) Word-Based Power Circuit (VFD-Controlled Accumulator)

• 3-Phase Supply →
• MCCB →
• VFD →
• Accumulator Motor

Motor PE → Earth Bar

VFD provides speed control and torque management.

DOL starting is not recommended for accumulator systems.

5) Loop Position Feedback Integration

Common sensors:

• Dancer arm with potentiometer

• Ultrasonic loop sensor

• Linear encoder on carriage

Word-Based Signal Flow:

• Loop Sensor → PLC Analog Input →
• PLC Calculates Position Error →
• PLC Sends Speed Reference to VFD

Closed-loop position control stabilizes strip loop.

6) Control Philosophy – Loop Height Control

Basic control logic:

If loop height too high → Slow accumulator motor
If loop height too low → Speed up accumulator motor

PLC continuously adjusts VFD speed to maintain mid-loop position.

Improper tuning causes oscillation.

7) Tension Control Considerations

Accumulator motor must not:

• Pull strip aggressively

• Allow slack into roll former

Tension control methods:

• Speed matching

• Torque limiting in VFD

• Dancer arm feedback

• PID control loop

Structural lines require more precise tension control than light roofing lines.

8) PID Control in Accumulator Systems

PLC often uses PID loop:

• Setpoint = Desired loop height
• Process Variable = Measured loop height
• Output = Speed correction to VFD

Improper PID tuning causes:

• Hunting (oscillation)

• Slow response

• Overcorrection

• Strip snap

Tuning must match mechanical inertia.

9) Synchronisation with Main Drive

Accumulator speed must coordinate with:

Main roll forming drive speed.

Word-Based Integration:

• Main Drive Speed Feedback → PLC
• Accumulator Loop Error → PLC
• PLC Combines Signals → VFD Speed Command

If main drive accelerates rapidly, accumulator must respond smoothly.

10) Coil Change Mode

During coil change:

Uncoiler stops
Accumulator feeds strip

Control logic shifts from:

Loop control mode → Feed-out mode

Electrical logic must manage mode transition safely.

Improper logic causes loop collapse.

11) Travel Limit Protection

Accumulator carriage systems include:

Upper limit switch
Lower limit switch

Word-Based:

Limit Switch → PLC Input →
PLC Stops Accumulator Motor

Prevents mechanical overtravel.

Safety integration must override speed command.

12) Motor Protection & Overload Setup

Accumulator motor often runs dynamically.

Overload must:

• Protect motor

• Allow short torque peaks

Trip class must consider dynamic operation.

Electronic overload or VFD internal protection preferred.

13) Regenerative Braking Considerations

If accumulator decelerates quickly:

Motor becomes generator.

Energy returns to VFD DC bus.

Brake resistor may be required.

Improper braking setup causes overvoltage trips during loop correction.

14) Cable & Shielding Requirements

Accumulator motor cable must be:

• Shielded (VFD systems)

• Properly grounded

• Routed separately from signal cables

Loop sensor analog wiring must be:

• Shielded

• Twisted pair

• Separated from motor cables

Noise causes unstable loop control.

15) Common Accumulator Motor Problems

1. Loop oscillation

2. Strip slack entering rolls

3. Excess tension snapping strip

4. VFD overcurrent during correction

5. Overvoltage during deceleration

6. Encoder noise (carriage systems)

7. Voltage sag during rapid correction

Most problems are tuning-related.

16) Commissioning Checklist

1. Verify motor rotation

2. Confirm loop sensor calibration

3. Tune PID loop gradually

4. Test slow acceleration

5. Test rapid deceleration

6. Simulate coil change

7. Verify limit switches

8. Confirm no oscillation at production speed

Commission at full line speed.

17) Differences: Roofing vs Structural Accumulators

Roofing lines:

• Higher speed

• Lower tension

• Faster response required

Structural lines:

• Heavier gauge

• Higher torque demand

• Slower but stronger control needed

Electrical tuning differs significantly.

18) Buyer Strategy (30%)

Before purchasing a roll forming machine with accumulator, verify:

1. VFD-controlled accumulator motor

2. Closed-loop PID control implemented

3. Shielded analog feedback wiring

4. Brake resistor installed if required

5. Travel limit protection integrated

6. Coil change mode tested

7. Tension control verified under load

8. Commissioning performed at max production speed

Red flag:

“Accumulator runs open-loop without feedback.”

That design causes tension instability.

6 Frequently Asked Questions

1) Why does loop oscillate up and down?

PID tuning likely incorrect.

2) Can accumulator use DOL motor?

Not recommended for precise loop control.

3) Why does strip snap during coil change?

Improper tension transition or speed mismatch.

4) Does accumulator require brake resistor?

If deceleration rapid, yes.

5) Why is analog sensor unstable?

Likely shielding or grounding issue.

6) What is most common accumulator mistake?

Poor PID tuning and lack of proper feedback integration.

Final Engineering Summary

Strip accumulator motor control systems in roll forming machines require:

• VFD-based speed control

• Closed-loop loop-height feedback

• Proper PID tuning

• Shielded signal wiring

• Overload protection

• Regenerative braking management

• Travel limit integration

• Mode transition control

Improper electrical integration causes:

• Strip tension instability

• Coil change failure

• Production downtime

• Scrap

In modern roll forming production, accumulator motor control is a precision electrical coordination system — not just a simple drive.