Determining where to place emergency stops (E-stops), disconnect switches, and other safety devices in relation to a Variable Frequency Drive (VFD) is a critical part of designing a safe, compliant, and reliable motor control system. The question of whether to position an emergency switch (or emergency stop button) before or after the VFD often arises from concerns about proper isolation, code compliance, protection of the VFD’s sensitive electronics, and ensuring that in a genuine emergency, the equipment stops safely and predictably.
This guide will expand extensively on best practices, international standards, functional safety considerations, different types of stops, the role of Safe Torque Off (STO), and how to approach wiring to achieve both safety and equipment longevity. It will also explain why certain widely used approaches, like placing a disconnect after the VFD (on the load side), are generally discouraged.
Introduction
A VFD is an electronic device that controls the speed and torque of an AC motor by regulating the frequency and voltage of the power it supplies. VFDs bring substantial benefits: energy savings, improved process control, gentle starting and stopping, and integration with automation systems.
However, the addition of emergency stops and disconnect switches in a VFD-driven system demands careful attention. Emergency stops differ from standard operational stops and must meet stringent safety requirements. Disconnects are required for isolation during maintenance, ensuring that anyone working on the equipment can do so safely. The interplay between these safety devices and the VFD’s solid-state electronics is not as straightforward as in older, non-electronic motor control systems.
What Should Be Done First When Troubleshooting a VFD?
When troubleshooting a VFD:
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Check Power and Wiring:
Confirm that input power (voltage, phases, and grounding) is correct and stable. Inspect all wiring connections for tightness and correctness. -
Review Fault Codes and Alarms:
Most VFDs display specific alarms or fault codes that help pinpoint issues (overcurrent, undervoltage, overspeed, overtemperature). -
Environmental Conditions:
Ensure the drive’s surroundings (temperature, humidity, dust) are within the specified limits. -
Motor and Load Checks:
Verify that the motor is not mechanically jammed, overloaded, or experiencing bearing failures.
Why is this relevant to emergency stops and disconnects?
Troubleshooting may involve ensuring that isolation devices (like line-side disconnects) are present and functional so you can safely power down the system and investigate without damaging the drive or endangering personnel.
What Is the Emergency Stop Button on a VFD?
An emergency stop (E-stop)1 button is a safety device designed to quickly stop machinery to prevent injury or damage when a dangerous situation arises. Unlike a normal stop command that may follow a controlled ramp-down, an E-stop is typically a direct immediate action:
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Integration with VFD Control Inputs:
The E-stop often interfaces with the VFD’s control circuit, removing the “enable” or “run” signal. The VFD stops supplying power to the motor output almost instantly. -
Rapid Shutdown Without Disconnecting Power Under Load:
The E-stop does not (in most well-designed systems) abruptly cut the motor leads; it sends a command to the VFD to cease output, preventing voltage spikes and undue stress on the drive. -
Manual Reset:
After an E-stop is triggered, the system requires manual reset before resuming operation, ensuring that hazards are addressed.
Can You Put a Disconnect After a VFD?
General Industry Consensus: Avoid placing disconnect switches directly after the VFD (i.e., on the load side, between VFD and motor) for routine emergency stopping or isolation under load conditions.
Why?:
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Voltage Spikes and Transients:
Opening a load-side disconnect under load interrupts current flow abruptly. This can cause severe voltage spikes from the motor’s inductance, potentially damaging the VFD’s output stage (IGBTs, capacitors, filters). -
Nuisance Tripping and Equipment Damage:
The VFD monitors its output for consistent current and voltage. Abrupt load-side interruptions may trigger faults, alarms, or, in worst-case scenarios, permanent damage. -
Safe Alternatives:
Place disconnects on the line side of the VFD for isolation during maintenance. For emergencies, use E-stops or STO functionality integrated through the VFD control circuit rather than a physical load-side cut.
How Do You Use an Emergency Stop Switch?
Basic Principle:
An E-stop is typically wired into the VFD’s control logic or a dedicated safety circuit. Pressing it sends a signal that either removes the drive’s run command or activates Safe Torque Off (if available).
Good Practices:
- E-stop in Control Wiring: E-stops are part of low-voltage control circuits, not the high-voltage output lines.
- Fail-Safe Design: The E-stop circuit should be normally closed and open when pressed, ensuring a fail-safe action if wiring fails.
- Reset and Verification: The system cannot restart until the E-stop is released and any unsafe conditions are resolved.
Placing Emergency Switches Before or After the VFD
Line-Side Disconnects and Emergency Stops
A line-side disconnect2 (circuit breaker or fused switch) placed before the VFD is used to isolate power for servicing. While not intended as an operational stop, it can serve as a secondary means to ensure the system is fully de-energized during maintenance. Emergency stops, on the other hand, do not rely on physically cutting the main power but instead stop the motor electronically.
Advantages:
- Complies with electrical codes requiring a local means of isolation.
- Eliminates risk of harmful voltage spikes caused by load-side interruptions under load.
Load-Side Disconnects and the Risks
Load-side disconnecting under load operation is discouraged. If absolutely necessary (for certain maintenance procedures), one must ensure the system is not under load and the VFD is not actively driving the motor when this disconnect is operated. Consult the VFD manufacturer for approval and recommended mitigation methods (like output reactors) if load-side isolation is unavoidable.
Safe Torque Off (STO) and Other Safety Functions
Modern VFDs often have Safe Torque Off (STO)3 inputs or similar functional safety features:
- STO: Disconnects the drive’s power stage from generating torque, rapidly stopping the motor’s torque production without cutting output cables.
- Compliance with Safety Standards: STO meets SIL (Safety Integrity Level) or PL (Performance Level) requirements specified by standards such as IEC 61508 or ISO 13849.
- Preferred Method for E-Stop: Integrating E-stop with STO provides a safe, predictable stop without damaging the VFD.
Functional Safety Standards and Codes
Several standards and regulations guide how emergency stops and disconnects should be installed and used:
- IEC/EN 60204-14: Safety of machinery—electrical equipment of machines.
- ISO 138505: Specifies design and application of emergency stop functions.
- OSHA (in the U.S.): Occupational safety guidelines may influence the location and functionality of E-stops and disconnects.
- NFPA 79: Electrical standard for industrial machinery may provide guidance on control circuit design and emergency stops.
These standards emphasize that emergency stops must be easily accessible, quickly bring the equipment to a safe condition, and require manual reset before resuming normal operation.
Practical Recommendations and Compliance
- Consult with Manufacturers: Motor, VFD, and control panel manufacturers often provide recommended wiring diagrams and notes on E-stop and disconnect usage.
- Train Personnel: Operators and maintenance staff should understand how to use E-stops properly, how to lockout/tagout the system for maintenance, and how STO differs from cutting power lines.
- Regular Testing: Periodically test E-stops, STO functionality, and verify that line-side disconnects operate smoothly and safely.
Extended Example Scenarios
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HVAC Fan Application:
- VFD controls a supply fan’s speed.
- E-stop button integrated into control logic. When pressed, the VFD instantly stops output.
- A line-side disconnect upstream of the VFD allows safe maintenance on both the VFD and fan.
- No load-side disconnect is used; doing so could risk VFD damage.
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Conveyor in a Packaging Line:
- Operators positioned at intervals along the conveyor have E-stop buttons.
- Pressing an E-stop removes run enable from the VFD, halting the conveyor motor smoothly.
- For service, a line-side disconnect ensures full isolation.
- STO functionality may be employed to comply with machine safety categories required by regulatory standards.
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Machine Tool Spindle:
- High-performance servo/inverter with STO capability.
- E-stop triggers STO input, ensuring no torque is produced even though the VFD remains powered internally for diagnostics.
- High-speed load-side disconnect avoided to prevent voltage spikes at high RPM.
Visual Diagrams
Figure 1: Preferred Placement of Disconnect and E-stop Signals
Figure 2: E-stop Integration with STO
Conclusion
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Always Place Disconnects Before the VFD (Line Side): For maintenance isolation, line-side disconnects ensure the VFD can be safely powered down without risk.
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Use E-stops as Control Signals, Not Load Breakers: An emergency stop should remove the drive’s run command, or trigger STO, rather than physically cutting output cables while under load.
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Avoid Load-Side Disconnects Under Load: Opening load-side conductors on a running VFD-motor circuit can cause damaging voltage spikes and is generally not recommended.
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Standards and Advanced Functions: With modern safety functionalities like STO and adherence to IEC/EN standards, it’s easier to achieve safe, predictable emergency stops that protect both personnel and equipment.
In summary, emergency switches (E-stops) should typically interface with the VFD’s control logic or STO function rather than being placed physically after the VFD on the motor lines. Line-side disconnects are used for maintenance isolation. This approach ensures compliance with safety standards, preserves equipment integrity, and reduces operational risks.
References
- IEC/EN 60204-1: Safety of Machinery – Electrical Equipment of Machines
- ISO 13850: Safety of machinery – Emergency stop function
- NEMA, NFPA 79: Industrial machinery electrical standards
- IEEE Std 1566: Performance of Adjustable Speed AC Drives
- Manufacturer Application Notes (Siemens, Rockwell/Allen-Bradley, Yaskawa, ABB) for E-stop and STO wiring examples
Disclaimer: Always follow local codes, standards, and manufacturer recommendations. Consult qualified professionals and safety engineers to ensure compliance and safety in your specific installation.
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This resource would elaborate on E-stop functionality, ensuring readers understand its importance in industrial safety. ↩
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Helps readers understand why disconnect placement upstream of the VFD is critical for safety and compliance. ↩
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Provides insight into STO’s role in achieving safe, damage-free stops, a modern alternative to physical disconnects. ↩
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Clarifies the key safety standard referenced throughout the guide and its implications for compliance in machinery safety design. ↩
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Directs readers to details about emergency stop specifications and compliance requirements in machinery safety. ↩
