AC motor braking torque: stopping the load and managing its energy
A spinning load carries energy. Braking torque is the shaft torque that opposes its current direction of rotation, helping bring the speed down. It is measured in newton metres (N·m). An AC motor can keep turning forwards while producing reverse torque; braking does not require reverse rotation. Torque describes a turning moment, rather than a linear force.
A useful braking specification answers three questions: how much torque is needed, where the released energy will go, and what will hold the load at rest. Motor nameplate power alone cannot answer them.
Why a shorter ramp may fail to give a shorter stop
A variable frequency drive can command opposing motor torque during deceleration. In generating operation, mechanical energy flows from the load through the motor into the drive’s DC link. Reducing frequency does not inherently mean reducing torque. Available braking depends on the motor, control method, current limits and energy-handling hardware.
The diode input stage in a conventional drive cannot export that energy to the mains. Without an adequate energy path, DC-link voltage rises. The drive may limit braking, extend the stop or trip. Setting a three-second deceleration ramp is therefore a demand on the system, not proof of a three-second stop.
Follow the energy through a resistor-braking system
Select Start for an eight-second illustration; Stop ends it. The orange marker shows the energy path as the blue speed bar falls. The load slows and its energy becomes heat in the brake resistor. This is a concept diagram, not a wiring drawing; its running time is unrelated to the numerical example below.
Choose the energy path, then the hardware
Resistor braking: A brake chopper switches energy from the DC link into a braking resistor. Dynamic braking of this type needs a suitable built-in chopper or a compatible external unit. The resistor dissipates the recovered energy as heat.
Returning energy to the supply: A suitable regenerative front end or regeneration unit can send regenerative energy back to the mains. A generating motor does not automatically make the complete drive regenerative at its supply terminals.
DC injection: DC braking applies controlled direct current to the motor windings. Rotational energy is dissipated within the motor, so current, duration and temperature limits matter. Flux braking also uses motor losses; its capacity and repetition rate must respect the motor’s thermal limits.
Mechanical braking: A mechanical motor brake may stop or hold a load, within its manufacturer’s rating. A holding brake must not be assumed suitable for repeated high-energy stops. Hoisting applications also require suitable holding torque and coordinated brake opening and closing.
A worked example: torque first
For a linear speed reduction, ignoring friction and external load torque, the braking-torque magnitude is approximately Tb = Jeq(ωinitial − ωfinal)/t. Jeq is the combined motor and load inertia referred to the same shaft, in kg·m². Convert rpm to angular speed using ω = 2πn/60, in rad/s. With a gearbox, refer load inertia to the motor shaft before using motor speed in the calculation.
Take 0.50 kg·m² of equivalent inertia at 1500 rpm and demand a stop in 3 seconds. Initial angular speed is about 157.1 rad/s, giving a braking torque of approximately 26.2 N·m. These are illustrative load assumptions; the result is not a motor or resistor recommendation.
Energy and power tell a different part of the story
The kinetic energy released is E = ½Jeq(ωinitial² − ωfinal²), or about 6.17 kJ. Average mechanical braking power over the stop is E/t ≈ 2.06 kW. With constant braking torque, instantaneous power starts at P = Tbω ≈ 4.11 kW and falls with speed. This is why torque and power are separate sizing quantities.
Cut the stop to 1.5 seconds and the ideal torque rises to 52.4 N·m. Average power becomes 4.11 kW, with an initial value of 8.22 kW. Energy remains 6.17 kJ: a faster stop must remove the same energy in less time.
Repeat the original stop once every 30 seconds and its ideal energy contribution averages about 206 W over the cycle. The resistor still has to survive each short power pulse. External torque driving the motion increases the braking requirement; friction can help. A descending load releases gravitational energy too and may need continuous braking even at steady speed.
Check the whole braking duty
For the motor and drive, check rated current, permissible short-term current, available braking torque and operating speed. For the resistor, check minimum permitted resistance, pulse energy, peak power, cycle-average power and cooling. An ohms-and-watts label is not a complete selection method. A separate brake chopper must also support the required duty.
Braking, slip and safe stopping are different issues
In an induction motor, slip is the relative speed difference between rotor and rotating magnetic field; it is not friction loss. Reversing the phase sequence of a running three-phase motor produces plugging, a different process from regenerative braking. It can impose high current and mechanical stress, so “apply reverse voltage” is not a sound general stopping instruction.
Safe Torque Off (STO) prevents motor torque generation. It does not actively brake the shaft or hold a suspended load. Where a controlled safety stop is required, functions such as SS1 are selected through the machine risk assessment. Emergency stopping, ordinary ramp stopping and stationary load holding need their own requirements. Sufficient braking torque alone does not guarantee a safe stop under every condition.