Braking, Safe Stopping and Troubleshooting
Mechanical and dynamic braking, safe torque off and stop categories, commissioning, and a symptom table for motion faults.
Brakes: holding and dynamic
There are two kinds of braking, and they are often confused. A holding brake is a mechanical device that locks the shaft when power is off; a dynamic (electrical) brake slows the motor under control, using the drive.
| Brake | How it works | Use | Limit |
|---|---|---|---|
| Spring-applied, power-off holding brake | Springs clamp the discs; a coil releases them when powered | Holds vertical loads and parks the axis when power is lost | Not for repeated dynamic stops: the friction lining wears. Rated for a few emergency stops |
| Permanent-magnet brake | Magnets clamp; a coil cancels them to release | Compact holding brake for servo motors | Holding torque only; no wear in normal use |
| Dynamic braking resistor | The drive switches the regenerated energy into a resistor | Fast decelerations, lowering a load | Heat: the resistor must be rated for the energy and the repetition |
| Regenerative drive | Returns the energy to the supply | Frequent braking, large energies | Costlier; needs a suitable supply |
| DC injection braking | Direct current in the motor stops it | Stopping induction motors without a resistor | Heats the motor; works only at low speeds and low inertia |
| Short-circuit (motor leads shorted) | Back-EMF drives a current that opposes motion | A simple stop for small servo motors if the drive fails | Does not hold at standstill |
Worked example: sizing a holding brake
- The vertical axis of Sizing a Motion Axis holds a 20 kg load on a 10 mm lead ball screw (90 percent efficient): the static torque is 196.2 × 0.010 ÷ (2π × 0.9) = 0.347 N·m.
- A common rule is to choose a holding brake with at least 1.5 times the static torque: 1.5 × 0.347 = 0.520 N·m. Compare the brake’s rating with the motor’s own peak torque too: a brake that is much stronger than the motor can be damaged by a runaway motor.
- Emergency braking energy: the rotary table of the same example has an inertia of 2.20 × 10-4 kg·m² at the motor and turns at 600 rpm (62.8 rad/s): stopping it dissipates ½ J ω² = 0.4 J. With 20 stops an hour that is 9 J per hour: trivial, but at larger inertias the brake’s energy rating (J per stop and per hour) decides the life.
- Always apply the brake only when the axis is stopped in normal operation, and keep the motor torque on until the brake has closed (a delay of 50 to 200 ms), or the load sags.
Stopping time and distance
How far the axis travels after a stop command is a safety distance as well as a mechanical one. The distance traveled is the speed times the reaction time plus the braking distance v² ÷ (2a).
Worked example: stopping distance of the ball-screw axis
- At the cruise speed of 0.667 m/s a controlled stop at 5.0 m/s² takes v ÷ a = 0.13 s and covers v² ÷ (2a) = 0.044 m = 44 mm.
- If the torque is simply removed (coast, STO) and only friction of about 0.5 m/s² decelerates the load, the distance is 0.444 ÷ (2 × 0.5) = 444 mm: nearly half a meter, and the axis could hit the end of travel. This is why a stop on a screw axis often needs a brake.
- With a spring brake that reacts in 50 ms and decelerates at 8 m/s², the distance is 0.667 × 0.05 + 0.444 ÷ 16 = 61 mm.
- Safety distance to a guard (ISO 13855) adds the speed of approach of a person times the total stopping time of the machine, so a shorter stop means a closer, more convenient guard.
Safe stopping functions
Modern drives include certified safety functions defined in IEC 61800-5-2. They are implemented in the drive’s safety hardware, with their own inputs, so that a software fault or an unwanted command cannot override them.
| Function | What it does | Equivalent stop |
|---|---|---|
| STO: Safe Torque Off | Removes the energy that can produce torque: the motor cannot be driven. It does not brake; the load coasts | Stop category 0 |
| SS1: Safe Stop 1 | A monitored controlled deceleration to zero, then STO | Stop category 1 |
| SS2: Safe Stop 2 | A monitored controlled deceleration, then the drive holds the position under supervision | Stop category 2 |
| SOS: Safe Operating Stop | The drive holds position with the torque on, and monitors it | Maintenance with the power on |
| SLS: Safely Limited Speed | Monitors the speed against a limit | Set-up and jog with guards open |
| SBC / SBT: Safe Brake Control and Test | Controls the holding brake safely and tests it | Vertical axes |
| SDI, SLP | Safe direction, safely limited position | Machine zones |
- STO is wired with two channels (two inputs, from a dual-channel safety relay or a safety PLC) and the drive reports a fault if they disagree. It is the cheapest way to remove the energy, and it replaces the output contactor.
- A stop by the program or a normal stop command is not a safety function. It relies on software and on the drive working, which a safety function must not.
- Brakes need a safe control (SBC) where a falling load could injure; test them regularly.
- A drive with built-in safety over a fieldbus (Safety over EtherCAT / FSoE, CIP Safety, PROFIsafe) gives safe functions without extra wiring; see Industrial Ethernet.
Emergency stop, categories and performance level
| Term | Meaning |
|---|---|
| Stop category 0 (EN 60204-1) | Immediate removal of power to the actuators: an uncontrolled stop |
| Stop category 1 | A controlled stop with power kept to the actuators until standstill, then power removed |
| Stop category 2 | A controlled stop with power left on at standstill |
| Emergency stop | A complementary protective measure that must stop the machine quickly. It must be category 0 or 1, and not need the control software. A red mushroom button on a yellow background; latching, needs a deliberate reset |
| Performance Level (PL a to e) and SIL | The reliability required of a safety function, from a risk assessment (ISO 13849-1, IEC 62061). Many emergency stops need PL d, with redundancy and monitoring |
| Risk assessment | Identifies the hazards and the risk reduction needed. It comes first: the safety functions follow from it |
Design safety with certified components (safety relays, safety PLCs, drives with STO) and with proper integration of the whole chain: the sensor, the logic and the actuator. A guard door with a safety interlock switch, a light curtain and an emergency stop all feed the same safety logic, which controls the drive’s STO and the brake. See PLC Troubleshooting, Safety and Documentation for the PLC side. This guide is not a substitute for the standards or for a qualified safety engineer.
Commissioning an axis
- Verify the safety first: the emergency stop, STO and any limit switches work, with the motor uncoupled where possible.
- Check the wiring and parameters: motor and encoder data, the supply voltage, the drive’s current limits, the direction of the feedback and of the command.
- Move with the torque and speed limited, in jog mode, with a hand near the stop button. Confirm the direction and the scaling (does 10 mm commanded give 10 mm of travel?).
- Set the limits: software travel limits inside the hardware limit switches, inside the mechanical stops. Test each.
- Home the axis and check the repeatability (see Encoders, Resolvers and Feedback).
- Auto-tune or tune (see Servo Systems and Tuning) with the real load, then check the step response and the following error at full speed.
- Test the brake and the stops: hold test, and an emergency stop at full speed with a safe stopping distance.
- Run a hot test at full duty and read the motor temperature and the drive’s torque against the calculation (see Sizing a Motion Axis).
- Record everything: parameter file, wiring, test results and the safety validation, then save a backup of the drive and controller programs.
Lockout and stored energy
- Isolate the energy (electrical, pneumatic, hydraulic, gravity, springs) and lock and tag the isolators before working on or in a machine. Try to start it to check.
- Wait for the drive’s DC bus to discharge (often five minutes; follow the label) and measure it before touching a drive.
- Support raised loads mechanically before working under them; a brake or a drive can fail.
- Release springs and vent air or oil pressure before opening a circuit.
- Safety standards: OSHA 1910.147 (lockout/tagout, US), ISO 14118 (preventing unexpected start-up), ISO 12100 (risk assessment).
Troubleshooting motion faults
| Symptom | Likely cause | First check |
|---|---|---|
| Axis does not move, no fault | Not enabled, a limit or STO input open, a brake not released, no command | Drive status and enable, the safety circuit, the brake voltage, the command source |
| Moves the wrong way or runs away | Reversed feedback or command, wrong commutation | Feedback direction and phasing, motor wiring (see Encoders, Resolvers and Feedback) |
| Following-error fault | Overload, jam, low gains, current limit, brake dragging | Free the axis by hand; torque and current limits; brake release |
| Overcurrent or overload fault | Too short a ramp, a heavy or jammed load, wrong motor data | Ramps, the mechanics, motor parameters |
| Overvoltage on deceleration | Regeneration into the bus | Longer deceleration or a braking resistor (see AC Motors and Variable-Frequency Drives) |
| Vibration, noise, buzz | Gains too high, resonance, a loose coupling | Lower the gain, add notch filters, tighten the coupling (see Servo Systems and Tuning) |
| Motor or drive too hot | Torque above the continuous rating, poor cooling, high ambient temperature | RMS torque from the drive, airflow, ambient temperature |
| Position errors that grow | Lost counts or steps, slipping coupling, backlash | Encoder signals, coupling, mechanical lash (see Mechanical Transmission: Screws, Belts and Gearboxes) |
| Vertical axis drops when disabled | No brake, or the brake is worn or not applying | Brake holding test, brake wiring and sequence |
| Intermittent faults | Noise, a poor connection, a failing cable | Shielding and grounding, connectors, cable flex points |
Related
- Motion Control and Actuators: the overview of this guide.
- Sizing a Motion Axis: sizing brakes and drives into the axis.
- AC Motors and Variable-Frequency Drives and Servo Systems and Tuning: braking and tuning in the drive.
- PLC Troubleshooting, Safety and Documentation and Workshop Safety.
- Sensors and Field Devices: limit switches and safety sensors.
FAQ
Is the emergency stop button enough to make a machine safe?
No. It is one protective measure. A risk assessment decides what else is needed (guards, interlocks, light curtains, safe speed), and the whole chain must reach the required performance level.
Do I need STO if I have an emergency stop contactor?
A contactor in the motor supply works, but an STO input is cheaper, faster, has no contact wear and keeps the DC bus charged for a quick restart. Many standards accept either when correctly wired and validated.
Why does my vertical axis sag before the brake closes?
The brake takes time (tens of milliseconds) to engage. Keep the torque on until the brake has closed, and release it only after the torque is established when starting.
How often should brakes be tested?
Follow the manufacturer and the risk assessment: commonly at each start-up of a vertical axis or at set intervals, by applying a test torque with the brake closed and checking for no movement.
References
- IEC 61800-5-2, Adjustable speed electrical power drive systems: safety requirements, functional.
- ISO 13849-1 (safety-related parts of control systems), IEC 62061, EN 60204-1 (electrical equipment of machines), ISO 12100 and ISO 13855.
- Drive and brake manufacturers’ safety manuals (Siemens SINAMICS Safety Integrated, Rockwell, Kollmorgen, Warner Electric brakes).
Last reviewed: October 2026. This page is an introduction, not a substitute for the applicable standards or a qualified safety assessment of your machine.