VFD for Cranes and Hoists: Regenerative Braking
What is a VFD for cranes and hoists? A variable frequency drive controlling a crane or hoist motor is a vector-controlled inverter sized for four-quadrant operation, Heavy Duty overload (150% of rated current for 60 s per IEC 61800-2), and Safe Torque Off per IEC 61800-5-2. Skip vector control and the motor cannot hold torque at zero speed, so the load drops or creeps the instant the mechanical brake releases. This article covers four-quadrant loading, vector versus V/f control, dynamic versus regenerative braking, Heavy Duty overload sizing, STO and brake-coordination logic, and where ABB, Siemens and Schneider drives fit hoist duty.
Why Crane and Hoist Loads Force Four-Quadrant Operation
A pump only pushes power one way: from the drive into the fluid. A hoist does not. Lifting a load draws power from the line, motoring the motor in the normal direction. Lowering it does the opposite. Gravity spins the motor, and the motor becomes a generator, feeding power back toward the DC bus. Traverse and trolley motors add two more quadrants, reversing direction under load in both travel senses. A drive sized only for motoring will see its DC bus voltage climb every time the load descends, and without somewhere for that energy to go, it will trip on overvoltage mid-lower.
What we see in the field: a hoist retrofit sized on nameplate kW alone, with no braking resistor and no regen unit, ran fine on bench tests with no load, then tripped on overvoltage the first time it lowered a rated load at full speed. The fix was not a bigger drive. It was a braking resistor sized for the descent, not the lift.
Vector Control Holds the Load at Zero Speed
Scalar V/f control keeps volts-per-hertz roughly constant and runs open loop. It is inexpensive, and it is fine on a fan. It cannot hold torque near zero speed, because it has no way to measure or command torque directly. A hoist needs exactly that: full rated torque at zero speed, sustained for the instant between mechanical-brake release and load movement, or the load free-falls before the motor catches it.
Vector control (FOC, sensorless or with an encoder) decouples flux-producing and torque-producing current, so the drive commands torque directly, even at a standstill. ABB's Direct Torque Control does the same job without a modulator stage, with a faster torque-response loop. Either method, not V/f, is the baseline for a hoist motor. See VFD control methods: V/f, vector and DTC for how the three compare on torque response.
Dynamic vs Regenerative Braking for the Descent
Lowering a rated load returns real mechanical power to the drive. The formula below gives a first-pass estimate of that power for sizing a braking resistor or regen unit.
Formula: Power returned while lowering a load — Source: mechanical hoisting fundamentals (rate of potential-energy change)
Plower = (m × g × v) / η
| Symbol | Description | Unit |
|---|---|---|
| m | Load mass | kg |
| g | Gravitational acceleration (9.81) | m/s² |
| v | Lowering speed | m/s |
| η | Mechanical efficiency of hoist gearbox and rope system | ratio, less than 1 |
That power has to go somewhere. Two options, covered in more detail in VFD braking: dynamic, regenerative and DC injection: dynamic braking dumps it as heat through a chopper and resistor bank sized for the worst-case descent; regenerative braking, via an active front end, pushes it back onto the line instead. A single occasional lower, on a light-duty hoist, rarely justifies the cost of an AFE. A duty-cycle crane lowering rated loads dozens of times a shift is a different calculation — the resistor bank gets large, hot, and the wasted energy is not trivial over a year.
Heavy Duty Overload Sizing for Cranes
Cranes and hoists are constant-torque, high-inertia loads, not variable-torque loads like pumps and fans. That puts them on the Heavy Duty (HD) overload curve, typically 150% of rated current for 60 s, with some drives allowing 200% for 3 s on top, rather than the Normal Duty 110%/60 s curve built for centrifugal loads. A given drive frame carries a lower continuous kW rating on the HD curve than on the ND curve at the identical frame size. Size by duty class first, nameplate kW second. Full comparison in VFD overload: Normal Duty vs Heavy Duty ratings.
Starting torque matters more here than average torque. Breaking a stalled load free, or accelerating a fully loaded hook from zero, draws well above rated current for a second or two. Undersize the overload margin and the drive trips on the first heavy lift, not the tenth.
Safe Torque Off and Mechanical Brake Coordination
Every hoist has a mechanical brake — usually a spring-set, electrically-released disc or shoe brake on the motor shaft — because a VFD alone is not a certified holding device. IEC 61800-5-2 covers Safe Torque Off (STO) and related safety functions built into the drive. STO removes torque-producing energy from the motor on a safety trip without opening the main contactor, which shortens restart time on a busy crane.
Sequencing is where hoist commissioning goes wrong. The drive must ramp to full torque and confirm it before the brake releases; on stop, it must hold torque until the brake has mechanically set, confirmed by a brake-monitoring contact, before the drive removes torque. Skip either step and the load drops a few millimeters at the top of every lift and every stop — small, but it fatigues the rope and rattles the operator. Some drives run this as a built-in motor brake control function rather than external PLC logic, which removes one source of timing error.
Where ABB, Siemens and Schneider Fit Hoist Duty
Schneider's Altivar Machine ATV340 is built for exactly this kind of load: the range is specified for packaging, material handling and hoisting duty, with fast current-loop dynamics and safety functions on board. ABB's ACS880 is the DTC flagship, with active-front-end and regenerative options at the higher power ratings a bridge crane or gantry hoist can need. Siemens S120 sits at the high-performance/servo end, running multi-axis coordinated motion — hoist, trolley and bridge on one shared DC bus, so a lowering hoist axis can feed a motoring trolley axis directly across the bus before either touches a resistor or the line. All three sit within Stoklink's variable frequency drives range.
This depends on the crane duty cycle more than on brand preference. A shared-bus, multi-axis architecture pays off on a high-cycle production crane. A single dynamic-braking hoist drive is enough for an occasional-use maintenance hoist.
Frequently Asked Questions
Does a crane or hoist VFD need a braking resistor?
Most rated hoist duty does. An occasional, light-load hoist can sometimes ride through a short overvoltage on DC bus capacitance alone, but any hoist lowering rated loads regularly needs a resistor sized for that descent power. Regenerative units only pay off at high duty cycle, where the wasted heat becomes a real energy cost.
What overload rating should I pick for a hoist drive?
Heavy Duty, typically 150% of rated current for 60 s, with some drives adding 200% for 3 s. Normal Duty (110%/60 s) is built for variable-torque loads like pumps and fans and undersizes a constant-torque hoist.
Can I run a hoist on V/f control?
Not reliably. V/f control cannot hold torque near zero speed, and a hoist needs full torque at zero speed the instant before the mechanical brake releases. Vector control or Direct Torque Control is the correct baseline.
What is Safe Torque Off and why does it matter on a crane?
Safe Torque Off (STO), defined in IEC 61800-5-2, is a drive function that removes torque-producing energy from the motor on a safety trip without opening the main contactor. It shortens restart time after a safety stop, which matters on a crane running frequent cycles.
How is a crane VFD sequenced with the mechanical brake?
The drive ramps to full torque and confirms it before the brake releases. On stop, the drive holds torque until the brake has mechanically set, confirmed by a monitoring contact, before removing torque. Reversing this order lets the load drop or creep.
Is a shared DC bus worth it for a multi-axis crane?
On a high-cycle production crane, yes: a lowering hoist axis can feed a motoring trolley or bridge axis directly across the bus, cutting how much energy ever reaches a braking resistor. For an occasional-use maintenance hoist, a single dynamic-braking drive is enough.
Conclusion
A crane or hoist VFD is not a general-purpose drive with a bigger nameplate. It needs vector or DTC control for zero-speed torque holding, a braking path sized for the actual descent power rather than nameplate kW, Heavy Duty overload margin, and STO wired into a brake-release sequence that never lets the load move before the motor is ready. Get the duty cycle right first, then choose between dynamic and regenerative braking, and size the drive frame on starting current, not average load. For the full power-stage and control background behind these choices, see the VFD engineering guide.
