For most applications, an asynchronous motor coasting to a stop on its own after being disconnected from the grid is sufficient. But high-inertia loads (cranes, saws, centrifuges, large fans) can keep spinning for minutes when released. This both lowers productivity and creates a safety risk: a spinning saw blade or a suspended crane load must stop immediately. This is where braking comes in. There are several ways to stop an asynchronous motor quickly and in a controlled manner; the most common are DC injection braking, dynamic (rheostatic) braking, braking resistor with a VFD, and mechanical braking. In this guide we cover how each method works, which application it suits and the correct motor selection.

DC injection braking and braking resistor panel on an asynchronous motor

Why Is Coasting Not Enough?

An asynchronous motor disconnected from the grid slows and stops gradually depending on the load inertia. On low-inertia, high-friction loads this happens in a few seconds. But on high-inertia loads (flywheeled machines, large fans, centrifuges) coasting takes very long. Moreover, in some applications the stop must be fast, repeatable and safe: a drill, a band saw or a conveyor must stop within seconds for operator safety. Coasting does not meet these requirements.

The need for braking also affects the motor's duty type and selection. A motor that starts and stops frequently and brakes at every stop is more thermally stressed. So in braking applications the duty type (S1-S6) must be carefully assessed; our electric motor duty type (S1-S6) selection article guides you. Understanding slip and actual speed also helps in grasping braking; see slip and actual speed in an asynchronous motor.

Which Applications Require a Fast Stop?

Cranes and hoists (the load stays suspended in air), band/circular saws (safety), centrifuges (process time), large fans (maintenance access), conveyors (emergency stop) and machine tools are typical applications that need a fast, safe stop. For crane and hoist motors see our crane and hoist lifting motors, and for woodworking and saw applications the woodworking workshop motor requirement list articles explain the motor needs of these applications.

DC Injection Braking

DC injection braking is the most common electrical braking method on an asynchronous motor. After the motor is disconnected from the grid, direct current (DC) is applied to the stator windings. While AC creates a rotating magnetic field, DC creates a stationary one. Because the still-spinning rotor moves within this stationary field, a braking torque is produced in the rotor and the motor decelerates and stops quickly.

Advantages of DC injection braking: it requires no extra mechanical parts, the braking torque is adjustable (proportional to the applied DC current) and it is quiet. The drawback: the braking energy turns into heat inside the motor; with frequent braking the motor heats up, so duty type and cooling matter. DC injection braking is done with a dedicated braking unit or a motor control device that has this feature (such as a soft starter or VFD). To evaluate starting and stopping methods together, see our star-delta vs soft starter and soft starter compatibility with IE3 motors articles.

Braking resistor and dynamic braking connection diagram with a VFD

Dynamic (Rheostatic) Braking

Dynamic braking dissipates the motor's kinetic energy by converting it into electrical energy and then into heat across a resistor. This method is especially common in variable-frequency-drive (VFD) systems. As the motor slows it now behaves like a generator and produces energy; this energy causes the voltage on the drive's DC bus to rise. A braking resistor and a brake chopper burn off this excess energy to keep the voltage at a safe level and rapidly decelerate the motor.

Dynamic braking is ideal in VFD applications that require a fast, controlled stop; the stopping time can be tuned by the power of the braking resistor. On high-inertia loads the braking resistor is sized larger. For all the advantages of a VFD including braking, the VFD with an asynchronous motor: when it is needed article is a comprehensive reference. With a VFD you achieve energy savings on pumps and fans while also gaining stopping control; we cover this in energy savings on pumps and fans with a VFD.

Selecting a Braking Resistor with a VFD

The braking resistor is selected by the inertia of the load to be stopped, the stopping time and the braking frequency. In a very frequently braking application (for example a centrifuge stopping every minute) the resistor must be large in both power and heat capacity; otherwise it overheats. Correct setting of the VFD parameters (braking ramp, chopper threshold) is critical here. On drive parameterisation and commissioning, the drive parameterisation and commissioning article explains the general principles. For motor selection in variable-speed and constant/variable-torque applications, see motor selection in variable-speed applications.

Braking by Counter-Current (Plugging)

Another electrical braking method is counter-current braking (plugging). In this method, while the motor is still spinning, two phases are swapped to reverse the rotation direction of the magnetic field; the motor is suddenly forced to turn in the opposite direction, producing a strong braking torque. Plugging stops very quickly but places a heavy thermal and mechanical load on the motor; it must also be disconnected by a zero-speed sensor the moment the motor reaches zero, so it does not start turning in reverse. For this reason plugging is used only in specially designed applications where a very fast stop is essential. To correctly manage the phase order and rotation direction, the motor rotation direction and phase sequence article guides you. The high current produced by plugging stresses the wiring much like inrush current; the starting current in an asynchronous motor (LRA) content helps you understand this stress.

Plugging can be done directly on squirrel-cage motors, and more controllably on slip-ring (wound-rotor) motors via the rotor resistances. We cover the difference between squirrel-cage and slip-ring motors and which suits which load in our squirrel-cage and slip-ring asynchronous motor difference article. Because braking is demanding at very high inertia and heavy duty, the motor's torque class also matters; see the asynchronous motor torque classes (Design N/H) content.

Comparison with a Mechanical Brake

Electrical braking (DC injection, dynamic) decelerates the motor but often cannot hold the load at standstill: once the motor stops, the braking torque also drops to zero. In applications such as a crane where the load must be held in the air, a mechanical brake (electromagnetic disc brake) is therefore used. A mechanical brake closes by spring force when power is cut (fail-safe principle) and mechanically locks the motor/load. Brake motors integrate this mechanism at the rear of the motor.

In practice many applications use both methods together: electrical braking decelerates quickly, and the mechanical brake holds at full stop. We cover brake motor supply for crane and conveyor applications in our IE4 brake motor supply: conveyor and crane article. For safety-critical applications such as elevators and escalators, the elevator and escalator motor supply content is also relevant.

Which Method for Which Application?

In applications such as saws, centrifuges and fans that brake frequently but do not need to hold the load, DC injection or dynamic braking is sufficient. In cranes, elevators and inclined conveyors where the load must be held after stopping, a mechanical brake is essential. For lines requiring precise speed control and frequent stops, a VFD + braking resistor is the most flexible solution. You can find flywheel effect and inertia management on high-inertia loads in the motor selection under impact load: flywheel and inertia article.

Recovering the Braking Energy

In dynamic braking the energy turns into heat in a resistor and is wasted. But in very frequently braking, high-inertia applications this energy can reach a significant amount. In that case the regenerative braking (energy feedback) option is considered: a drive with a special feedback unit returns the braking energy to the grid instead of turning it into heat. This both saves energy and reduces the panel heat load. Regenerative solutions are becoming increasingly common in plants with frequent, high-energy braking such as cranes, elevators and large centrifuges. To evaluate braking with an energy-efficiency-focused approach, see our high-efficiency motor + variable frequency drive article.

Regenerative braking is not economical for every application; the extra hardware cost pays back only if there is enough braking frequency and energy. This decision should be made by evaluating the plant's total motor inventory and energy profile; the preparing for an energy efficiency audit: motor inventory article helps with this assessment. We detail the gain that speed reduction provides via the affinity law in VFD systems in the energy savings on pumps and fans with a VFD content.

The Effect of Braking on Motor Selection

Braking increases the thermal load on the motor. Every braking cycle, especially with DC injection, produces extra heat inside the motor. So a frequently braking motor must be sized one step more carefully than its continuously running equivalent. Insulation class and temperature rise are decisive here; the temperature rise and class in an asynchronous motor article explains correct class selection. To monitor winding temperature directly, the temperature monitoring with PT100 and PTC thermistor content is recommended.

In brake motors the bearings and shaft mechanics are also stressed; the right bearing type and insulated bearing selection extend life. On this topic see our bearing type and life in an asynchronous motor article. Correct pole and speed selection also affects the braking need; the asynchronous motor purchasing guide: 2, 4, 6 poles content guides you.

Frequently Asked Questions

Does DC injection braking heat the motor?

Yes. In DC injection the braking energy turns into heat inside the motor. For one-off or infrequent braking this is no problem; but in very frequently braking applications the motor heats up. In that case you should size the motor one step more carefully considering the duty type (e.g. S4) and add temperature monitoring.

Is electrical braking alone enough on a crane motor?

No. Electrical braking decelerates the load quickly, but the braking torque drops to zero when the motor fully stops; the load cannot be held in the air. In applications such as cranes, elevators and inclined conveyors, a mechanical (electromagnetic) brake that closes by spring force when power is cut is essential. Usually electrical and mechanical braking are used together.

Is a braking resistor always needed in a VFD system?

No. On low-inertia, slowly stopping loads the drive can let the motor coast or stop it with a short ramp. But on high-inertia loads requiring a fast stop, the braking energy raises the DC bus voltage; a braking resistor and chopper are needed to burn off this energy. The resistor is sized by inertia and braking frequency.

Get a Quote

Let us determine the braking method and motor suited to your application together: DC injection, dynamic braking, VFD + braking resistor or a mechanical brake motor. Contact us to plan your fast and safe stopping solution. To get a quote now, visit our contact page or call us at +90 (532) 345 49 86. You can review our entire high-efficiency electric motors range, and for geared solutions take a look at our worm gear reducers category and our homepage.

Purchasing and Selection Checklist

For a motor application with braking, verify the following:

  • Is the load inertia high? Is the coasting stop time acceptable?
  • Is the stop safety-critical (crane, saw) or only a process requirement?
  • Must the load be held after stopping? (If so, a mechanical brake is essential.)
  • What is the braking frequency? Is the duty type S1 or frequent-start/stop S4?
  • Which suits best: DC injection, dynamic braking or VFD + braking resistor?
  • If a VFD is used, is the braking resistor power and heat capacity sufficient?
  • Are the motor's insulation class and cooling suitable for frequent braking?
  • Are temperature monitoring (PT100/PTC) and suitable bearings selected?

After completing this list, you can obtain a correct and fast quote with the steps in our information to provide when requesting a quote article.