Brakes for electric motors: spring-applied brakes, electronic braking methods and other brake types compared

With electric motors, a basic distinction is made between mechanical, electromagnetically released friction brakes and purely electrical braking methods. Only mechanical brakes can safely hold a motor at standstill when de-energised (fail-safe); electrical methods brake wear-free or even with energy recovery, but usually need an additional mechanical brake for the actual standstill. More on speed control itself in Speed Control (PWM).

Spring-applied brake

The spring-applied brake is the most widely used brake type on electric motors. When de-energised, preloaded compression springs in the coil housing press an axially movable armature disc against the friction-lined rotor - the resulting friction generates the braking torque. When the field coil in the electromagnet is energised, the resulting magnetic field pulls the armature disc against the spring force, releasing the brake so the rotor turns freely.

A distinction is made between the single-surface brake with one friction face (a flat, compact design, with manufacturer figures typically in the range of roughly 0.25 Nm up to a few Nm for small sizes) and the dual-surface or multi-disc brake with several friction faces, which achieves significantly higher braking torque at the same diameter - depending on the manufacturer, product lines range from a few Nm up to several thousand Nm for large versions. Further variants include brakes with or without manual release (release lever), with or without automatic wear adjustment, and dry-running or wet-running designs.

Purpose: As a fail-safe holding, operating and emergency-stop brake, the spring-applied brake reliably holds the motor on power loss - typical in hoists, cranes, overhead conveyors, roller doors, lifts and wind turbines.

Advantages:

• Fail-safe: holds reliably even on power loss.
• High achievable holding torque, rugged and simple design.
• Fits standard motor frame sizes well, with precisely adjustable braking torque.

Disadvantages:

• Mechanical wear of the friction linings, leading to recurring maintenance (checking and adjusting the air gap).
• Audible switching noise when braking and releasing.
• Braking torque can vary slightly at extreme temperatures.

Permanent-magnet holding brake

The permanent-magnet brake works on a principle similar to the spring-applied brake, but holds the armature disc when de-energised via the field of a permanent magnet rather than springs. To release it, the electromagnet generates an opposing field that cancels the permanent magnet's holding force; springs between the armature and the flange hub fully separate the armature from the excitation system. The friction pairing is usually steel on steel rather than an organic friction lining.

Advantages:

• Roughly twice the power density of a spring-applied brake - more compact and lighter for the same braking torque.
• Virtually wear-free in normal operation, since the armature lifts off completely when released.
• More temperature-stable across the full operating range, and very dynamic - well suited to servo applications.

Disadvantages:

• Not designed for high braking work or large braking energies.
• More complex construction with an air gap that must be precisely maintained on installation.
• Tends to cost more than a comparable spring-applied brake.

Typical applications: Mainly servo motors, handling technology and robotics, where compactness and dynamics matter more than high continuous braking work - while the more rugged spring-applied brake dominates in cranes, overhead conveyors and wind turbines with high braking work.

Regenerative braking

In regenerative braking, the motor acts as a generator while braking: the resulting electrical energy flows through the drive's DC link and, if the drive is designed to be regenerative (active front end), is fed back into the supply grid or buffered in a storage device. Without such a regenerative unit, the DC-link voltage rises and the energy must instead be dissipated via a brake resistor.

Advantages:

• Energy recovery instead of energy loss; depending on the application and drive technology, manufacturers report recovering a large share of the braking energy that would otherwise be dissipated as heat.
• Wear-free, since it works purely electrodynamically.

Disadvantages:

• Requires a regenerative drive or a correspondingly designed grid connection - higher investment cost.
• Not suitable as a holding function at standstill.

Typical applications: Hoists and cranes when lowering loads, conveyor systems with energy-intensive braking cycles, and electric vehicles and trains.

Resistor braking (brake chopper)

Resistor braking uses the same regenerative effect as regenerative braking, but dissipates the generated energy through a resistive brake resistor instead of feeding it back: a brake chopper - essentially a switching transistor in the DC link - connects the resistor once a threshold voltage is exceeded.

Advantages:

• Simple, rugged design with no requirement for grid regeneration capability.
• Lower investment cost than a regenerative solution.

Disadvantages:

• The braking energy is entirely lost as waste heat.
• Additional cooling effort required; less energy-efficient than regeneration.

Typical applications: Industrial drives, rail technology and wind turbines wherever grid regeneration is not provided or only occasional high braking power occurs.

Plugging (reverse-current braking)

In plugging, two motor supply leads are swapped while the motor is still turning, reversing the rotating field direction or, in DC motors, the polarity. The motor then "tries" to run in reverse, producing strong braking torque. The motor must be disconnected from the supply immediately after standstill, or it will start running in the opposite direction.

Advantages:

• A simple method for achieving high braking torque down to standstill.

Disadvantages:

• Energetically very unfavourable, since both mechanical and electrical power are fully converted to heat.
• Higher current and torque surges when switching than at direct starting; practically unsuitable for DC series motors due to brush sparking.
• Hardly suitable for large motors because of the heating involved.

Typical applications: Quickly stopping smaller three-phase motors, for example in hoists to realise a reverse-current lowering circuit.

DC-injection braking

After the rotating field is switched off, the stator winding of a three-phase motor - usually an induction motor - is connected to a DC source, which modern variable-frequency drives generate internally. The resulting stationary magnetic field induces currents in the still-turning rotor that, by Lenz's law, produce braking torque. The braking torque is strongest at higher speeds and drops to zero shortly before standstill.

Advantages:

• Available as a standard software function in practically every modern variable-frequency drive, with no extra hardware required.
• Fast braking with no requirement for grid regeneration.

Disadvantages:

• Unsuitable for large flywheel masses, since all kinetic energy is converted to heat in the rotor.
• Unsuitable for frequent, repeated braking due to the thermal load on the rotor.
• No holding torque at standstill, since braking torque drops to zero at zero speed.

Typical applications: Standard function of modern variable-frequency drives for quickly braking induction motors without grid regeneration.

Short-circuit braking

Especially with permanent-magnet DC, BLDC and servo motors, braking torque can be generated by short-circuiting the winding terminals - usually through a defined resistor. The voltage induced by the rotor's rotation then drives a short-circuit current that produces a braking counter-torque.

Advantages:

• Works even without active power supply during a mains failure, sparing mechanical brakes.
• Suitable for achieving a constant, safe lowering speed for suspended loads.

Disadvantages:

• Maximum braking torque can only be adjusted to a limited extent via the choice of resistor.
• Not suitable as a holding brake, since no voltage is induced without rotation and therefore no holding torque is produced.

Typical applications: Mains-failure protection for braking high inertias without a UPS, and defined lowering speeds in servo and BLDC drives.

Eddy-current brake

In an eddy-current brake, a rotating metal body moves relative to a magnetic field generated by coils in a fixed housing. The relative motion induces eddy currents in the metal body by Lenz's law; their own magnetic field opposes the motion, generating braking torque contactlessly - kinetic energy is converted to heat without contact.

Advantages:

• Completely wear-free, since no mechanical friction occurs.
• Low maintenance in continuous operation.

Disadvantages:

• No holding torque at standstill - braking effect drops to zero as speed decreases.
• Not suitable as a sole parking brake; an additional mechanical brake is needed for standstill.

Typical applications: Mainly vehicle and rail technology and test benches; in stationary drive technology as a wear-free auxiliary brake for high continuous braking demand, for example on long descending gradients.

Comparison at a glance

Holding brake, operating brake or emergency-stop brake?

Only the spring-applied brake and the permanent-magnet brake are suitable as a true holding brake that secures a stationary motor permanently and fail-safe - both also cover the operating and emergency-stop brake roles. All purely electrical methods, by contrast, only work while the motor is turning: regenerative braking and resistor braking are classic operating brakes, plugging and DC-injection braking are also suitable as a quick stop, and short-circuit braking mainly serves as an emergency brake during a power failure. The eddy-current brake, finally, is a pure continuous brake during operation with no holding or emergency-stop function at all.

Which motor types use which braking methods in practice depends heavily on the application: for DC motors, the spring-applied brake dominates as the mechanical holding brake, supplemented by armature short-circuit or plugging as an emergency braking method. For induction/three-phase motors, the spring-applied brake is common as the standard brake-motor solution, while variable-frequency drives provide additional electrical braking functions via DC-injection, plugging or regenerative braking. For BLDC and servo motors, the compact permanent-magnet brake is usually used as the holding brake, electrically supplemented by short-circuit or regenerative braking via the servo drive. We determine the right combination for your application together as part of the motor sizing process - more on the broader motor choice in Brushed or Brushless Motor?.

Frequently asked questions

Sources (selection): Wikipedia: electric brake, Wikipedia: plugging, Wikipedia: DC-injection braking, Wikipedia: regenerative braking, Wikipedia: brake resistor, Wikipedia: eddy-current brake, KEB Automation: spring-applied brakes, Kendrion: single-surface brake, konstruktionspraxis.vogel.de: spring-applied vs. permanent-magnet brake, servoantriebstechnik.de: short-circuit braking.