How to Select a VFD for a Crane (Three Drives, Not One)

Select a VFD for a crane by starting with the duty class, not the motor nameplate. A crane is not one drive. It is three, and the hoist, the bridge and the trolley each want a different answer.

Eero learned that on a 30 kW hoist at a steel mill in Finland. The overhead crane VFD had been chosen from the motor nameplate and a vendor’s guidance to size at 1.25 times full-load current. It tripped on the first lift of hot metal because the duty class was C rather than B. The drive had been picked off the normal-duty current column, and a class C crane does not live on the normal-duty column.

Most crane VFD material starts with the product. Crane-specific drives do carry functions a general-purpose drive does not, and that argument is correct. It is also not where selection starts, because it means deciding what to buy before working out what the crane does. Classify first, split the three motions, then size.

Key Takeaways

• A crane needs three drives, not one. The hoist needs closed-loop vector control with an encoder and roughly 150 percent overload for 60 seconds. Bridge and trolley travel usually run on V/f or sensorless vector.
• Crane duty classification comes first. CMAA Specification 70 and ISO 4301-1 both grade cranes by how hard they work, and the class, not the motor, sets the overload the drive must survive.
• The published sizing multipliers do not actually contradict each other. They are stated against different baselines, and most sources do not say which. A frame publishes a normal-duty current and a lower heavy-duty current, and that gap is most of the apparent disagreement.
• Crane motor derating is the step most guides skip. ABB’s crane duty catalogue permits well under nameplate output, and the ratio is not constant across frames, so no single percentage works.
• A setting that merely changes motor speed on a fan can be a safety problem on a hoist.

The whole selection reduces to two inputs: the crane’s duty class and each motor’s full-load current. If you have those and want the frame selections checked before ordering, send them to our engineers.

Why a Crane Is Three Drives, Not One

Treating a crane as one application is the most common selection error. It shows up as a trolley drive two frames oversized and a hoist drive one frame undersized.

The hoist lifts against gravity and lowers back down. Lowering is what changes the selection: a descending load is overhauling, driving the motor rather than being driven by it, which makes the hoist a four-quadrant application whether the drive is ready for it or not. The drive must also produce high torque at near-zero speed to hold a load as the mechanical brake releases, and survive breakaway current at the start of every lift.

Bridge and trolley travel move mass horizontally. Gravity is not reversing them, so both are inertial loads and the work is done in getting started. That is a materially easier job.

	Hoist	Bridge / long travel	Trolley / cross travel
Load character	Overhauling, four-quadrant	Inertial, mostly motoring	Inertial, light
Overload the drive must hold	Highest: breakaway plus held load	Moderate	Lightest
Control mode that suffices	Closed-loop vector with encoder	V/f or sensorless vector	V/f or sensorless vector
Encoder	Required	Only for long spans or precise positioning	Usually not
Braking path	Regeneration or a resistor, sized for the descent	Resistor, sized from inertia	Small resistor
Safety-critical functions	Brake integrity, torque proving, slack rope, overload	Travel limits, anti-shock, anti-skew	Travel limits

All three motions share the duty class, the supply and the operator. They also share the consequence of a mistake, and that consequence is not symmetrical. A mis-sized trolley drive gives you poor positioning. A mis-sized hoist drive gives you a load on the floor.

Start With the Duty Class, Not the Motor Nameplate

Crane duty classification describes how hard a crane works over its life. It is not a rating of capacity. A 20 tonne crane that lifts twice a day and one that lifts twice a minute may have identical nameplates and need different drives.

CMAA Specification 70 defines six service classes:

Class	Description	Typical duty
A	Standby or infrequent	Power houses, utilities. Slow, precise handling with long idle periods
B	Light	Repair shops, light assembly. 2 to 5 lifts per hour, loads averaging near 50 percent of rated
C	Moderate	Machine shops, paper mill machine rooms. 5 to 10 lifts per hour, loads averaging 50 percent of rated, not over half at rated capacity
D	Heavy	Foundries, steel warehouses, container yards. 10 to 20 lifts per hour, loads approaching 50 percent constantly, not over 65 percent at rated capacity
E	Severe	Magnet and bucket cranes, scrap yards, cement mills. 20 or more lifts per hour, loads approaching rated throughout
F	Continuous severe	Specialty cranes critical to total production

The class is derived, not guessed. CMAA 70 weights a load spectrum by the fraction of lifts at each load, using the cube of the load fraction, then reads the class from a matrix of load class against total operating cycles.

ISO 4301-1 and FEM 1.001 do the same job outside North America, combining load spectrum with total operating cycles to produce the A3 through A8 groups.

The two systems are related but not identical, and the cross-reference is not stable across publications. A common mapping runs ISO A3 to CMAA B, A4 to C, A5 to D, A6 to E and A7 to F. Consulted for this article, two sources gave M6 as equivalent to CMAA Class E while a third gave M6 as Class D. Confirm which class the drive has to satisfy rather than assuming two published classes describe the same duty.

The class sets the overload event the drive must survive and the output the motor may deliver. A class B crane lifting 2 to 5 times an hour spends most of its life idle, and a drive sized for that will rarely see a sustained overload. A class E crane handling scrap is near rated capacity continuously.

The Two Deratings the Other Guides Skip

Two things get reduced when a motor runs on a crane, and only one is usually mentioned.

One frame, two current ratings

A VFD frame publishes at least two continuous currents. The normal-duty column carries a lighter overload for a longer period. The heavy-duty column carries a heavier overload for a shorter one, at a lower continuous current on the same hardware.

That single fact explains most of the apparent disagreement in published crane sizing advice.

Source	Stated rule	What it actually measures
Crane builder guidance	Size the hoist VFD at 100 to 125 percent of motor full-load amps	Continuous rating, on a lightly loaded column
Integrator guidance	1.25 times FLA normal duty, 1.5 times for hoist duty	Continuous rating on the heavy-duty column, roughly two frames more conservative than the row above
Magnetek sizing bulletin	Regenerative units sized 1:1 to total motor FLA	Continuous rating on the normal-duty column, for a regenerative inverter rather than a hoist drive
Yaskawa CR700	Drive output current times a coefficient of 0.6 to 0.9 must exceed motor rated current	A control-mode and configuration coefficient applied to the drive’s own rating

Read without the baseline, these look like a contradiction. They answer different questions, and the honest rule is to name the quantity before quoting a number.

The motor is derated further than the drive is upsized

The drive gets upsized for crane duty. The motor gets derated for it, and by more. ABB’s crane and hoist duty catalogue shows the effect, publishing a nominal motor rating and a lower maximum permissible mechanical output per frame.

Frame (6-pole)	Nominal rating	Permissible at 25 to 40% CDF	Permissible at 100% CDF
KHX+132SMC6	3.7 kW	2.9 kW	2.5 kW
KHX+160MLC6	7.5 kW	5.8 kW	4.9 kW
KHX+200MLC6	22 kW	16.8 kW	14.3 kW
KHX+225SMC6	30 kW	25 kW	20 kW

Read that with its basis attached, because the basis is narrow. The catalogue is an Indian-market document at 415 V, 50 Hz, with a 50 degrees C reference ambient, a declared 10 to 100 percent speed range, and Indian Standards alongside IEC 60034-1. That reference is not the 40 degrees C most other markets use, so the top row does not mean a crane motor needs no ambient derating in a plant rated at 40.

The finding worth carrying is the shape, not a number. Across the frames sampled, permitted output runs from about 0.59 to about 0.83 of nominal, and falls as cyclic duration factor rises. The ratio is not constant, which is why the catalogue publishes a chart per frame instead of a rule.

Sunita hit this in a foundry in Pune. The crane-duty motor was nameplated at 22 kW and everyone read that as 22 kW of available output. At the plant’s cyclic duration factor the chart permitted 16.8 kW, so the frame was a size short and the motor ran hot.

The nameplate was not wrong. It was a nominal rating, not a promise about that duty.

Size the drive from motor current and the overload the duty class requires, then verify the motor separately. The motor fails first because it cannot be replaced in an afternoon.

Sizing the Drives: Hoist, Then Travel

Hoist VFD sizing

Hoist VFD sizing starts with current, not horsepower. Horsepower on a nameplate is a mechanical output rating, and converting it to a drive frame requires an efficiency and power factor that vary by motor. Full-load current is the quantity the drive actually supplies. The method matches any motor calculation (how to size a VFD for a motor), with one crane addition: the overload event is not occasional.

Magnetek’s bulletin states 150 percent overload for one minute on the heavy-duty rating for an active front end, where both motoring and regenerating power flow through the inverter, and 120 percent for one minute on a regenerative-only unit where only regenerating power does. The practical window for a hoist drive is roughly 150 percent for 60 seconds with a short higher peak. Invertek’s Optidrive P2 publishes 150 percent for 60 seconds as standard plus 200 percent for 4 seconds, with a hoist mode offering up to 200 percent torque from zero speed. Verify the figures for your drive rather than assuming a number.

The hoist needs closed-loop vector control with encoder feedback for torque at zero speed and the ability to hold a load without the brake. The comparison between modes is explained in VFD control modes. Open-loop control on a hoist gives poor low-speed torque and allows the load to droop. For why constant-torque loads need the closed-loop options at all, see VFD selection by load type.

Bridge and trolley travel

Bridge and trolley are inertial loads with modest overload requirements, so V/f or sensorless vector is adequate and keeps the panel simpler. Two conditions push you further: long spans need an encoder so the two ends stay synchronised, and precise positioning benefits from tighter speed regulation.

Bridge travel often uses two motors, one at each end. Running both from one drive is common and acceptable, but the drive must be sized for the sum of the motor currents. Yaskawa’s crane documentation handles this with a coefficient method, where the inverter’s rated output current times a coefficient must exceed combined motor current, and that coefficient drops for multiple motors on one inverter.

An anti-sway control VFD suppresses load swing electronically, rather than leaving it to the operator’s timing, which cuts cycle time and improves positioning. Specify the function, not the trademark: the same idea appears under different product names, and naming one vendor’s term narrows your supply options for no benefit.

Nkem ran into a related problem at a container terminal in Lagos. A bridge travel motion used two motors on one drive. Both motors were correct and both cables were correct, but nothing had been done about skew, and the crane crabbed under load, wearing the wheel flanges on one side of the span. Anti-sway and anti-skew are different functions, and buying the first does not get you the second.

What a General-Purpose Drive Cannot Do

A general-purpose drive can physically be configured to run a crane, and made safe with enough engineering time. Whether that time is worth spending is a risk question, not a cost question.

Columbus McKinnon frames it precisely: “a fan, pump, or conveyor is dramatically different than an overhead crane or hoist”, because “the forces of gravity and inertia impact these applications differently”. Their central point is the one to carry:

“a certain setting may make the motor spin faster. While this may not be a concern for something like a fan, it can cause a safety issue on a hoist.”

On a fan, a mis-set parameter wastes energy. On a hoist, it changes how a suspended load behaves.

The functions a crane-specific drive adds, in their terms:

• Brake integrity check at the start and end of every lift, which “minimizes the chance of a load falling to the floor due to a brake failure”. On detected slip the drive “will electrically hold the load and alert the operator to lower the load to the ground immediately”.
• Electrical integrity check throughout operation. “The VFD knows the safe limits for itself and the motor and will prevent overloading and overheating.” If the line loses a phase, sags, or ripples, the drive stops.
• Slack cable detection against excessive rope unwrap, and overload detection, since “without a load cell, it may be unknown whether a load exceeds the safe capacity of the crane”.
• Travel limits, so the crane does not hit a wall and the hook does not hit the floor, plus anti-shock protection against shock loading by the operator.
• Micro-speed for fine positioning and hoist synchronization.

Two deserve emphasis. Brake integrity checking addresses the failure that actually drops loads, and it happens at every lift rather than on a maintenance schedule. Slack rope detection catches an event that damages the rope and drum and is invisible to the operator until the damage is done.

Send us the duty class and the motor nameplate. Our engineers will tell you which of these functions your application needs and which you can leave out. Talk to our engineers.

The Motor, the Cable and the Panel

A VFD output is a switched waveform with fast voltage transitions, and those transitions stress motor insulation in ways a direct-on-line supply does not. Motors intended for inverter duty are rated for it, and NEMA MG1 Part 31 becomes the practical requirement once cable runs get long. Verify the rating against a NEMA-derived source for your motor rather than a vendor page; the full picture is in motor compatibility with VFD.

Long cable runs raise the peak voltage at the motor terminals, and past a length that varies by drive and cable an output filter becomes necessary rather than optional. dv/dt filter vs sine wave filter covers which one addresses which problem. The panel then has to survive the same environment as the motor: ABB’s crane duty range declares IP55 and IC411 cooling with a Class F insulation system and Class B temperature rise, and allows 100 percent output at 50 degrees C, 95 percent at 55 and 90 percent at 60. A drive specified for an air-conditioned electrical room and installed in a mill bay will derate, and that derating is not in the motor’s numbers.

Braking Capacity: How to Size It

Sizing braking capacity is part of selecting the drive. Choosing the method is a separate subject.

Magnetek’s guidance gives the number that matters: regenerative power is roughly 55 to 65 percent of the motoring horsepower required, the difference being mechanical and electrical losses. A regen unit is sized from that reduced figure, on the normal-duty column by default, at 1:1 to total motor full-load current on the drive output. Where there is no gear reduction or efficiency exceeds 80 percent, the heavy-duty rating applies. A supply with interphase voltage imbalance above 2 percent needs a frame size one larger, and supply kVA should exceed the drive’s rated input capacity.

A crane can share one active front end across several drives, since only one rectifier can be used. Motor powers can be added, and drives interlocked so they cannot run simultaneously can be left out of the sum. Magnetek’s worked example has a 100 HP main hoist, a 40 HP auxiliary hoist, a 25 HP bridge and a 10 HP trolley, with the two hoists interlocked. Since the hoists cannot run together, the active front end is sized from the 100 HP hoist plus the 25 HP bridge and 10 HP trolley, selecting a 150 HP unit rather than one sized for all four.

For the choice between a braking resistor and a regenerative unit, including the duty cycle that makes regeneration economic, see regenerative drive vs braking resistor. One constraint from that comparison belongs here: regeneration does not stop a load when the power fails, so the mechanical brake remains the stopping device whichever method you choose.

How to Select a VFD for a Crane, End to End

1. Classify the crane. Establish CMAA or ISO duty class from the load spectrum and operating cycle count. Do not accept a class because it is on an old drawing.
2. Split the three motions and list hoist, bridge and trolley separately with their motor ratings.
3. Establish the overload event each drive must survive: highest for the hoist, moderate for bridge travel, lightest for the trolley.
4. Work in current. Take motor full-load current, confirm the overload window and its duration, and select the frame off the rating column that matches the duty class.
5. Verify the motor separately against the manufacturer’s crane duty chart for its own cyclic duration factor, ambient and speed range.
6. Assign control modes. Closed-loop vector with encoder on the hoist, V/f or sensorless vector on the other two.
7. Size braking capacity from the reduced regenerative figure, and confirm the method against the duty cycle and grid rules.

Document each step. A crane drive schedule that records why a frame was chosen is the document that settles the argument in year three.

Frequently Asked Questions

How do I select a VFD for a crane?

Classify the crane under CMAA Specification 70 or ISO 4301-1 first, then treat the hoist, bridge and trolley as three separate drives. Size the hoist for its breakaway overload with closed-loop vector control, size travel on V/f or sensorless vector, and verify the motor against the manufacturer’s crane duty chart before ordering.

Why do sizing guides give different multipliers for crane VFDs?

Because they measure different quantities. Some state a multiple of motor full-load current against a normal-duty rating column, some against a heavier-duty column on the same frame, and some give a control-mode coefficient applied to the drive’s own rating. Confirm which baseline a published rule uses before applying it.

Can I use a general-purpose VFD on a crane?

It can be made to work, but the safety functions differ. Crane-specific drives add brake integrity checking at every lift, slack rope detection, overload calibration without a load cell, travel limits and micro-speed positioning. A parameter that merely changes motor speed on a fan can change how a suspended load behaves on a hoist.

How many drives does an overhead crane need?

One per motion, in most cases. A typical overhead crane has a hoist, a bridge travel motion and a trolley travel motion, so three drives. Bridge travel sometimes uses two motors on a single drive, sized for the sum of the motor currents.

Is anti-sway control worth specifying?

On trolley and bridge travel, usually yes. Suppressing load swing in the drive reduces cycle time and improves positioning without depending on the operator’s timing. Note that anti-sway and anti-skew are different functions: anti-sway controls how the load swings, anti-skew keeps the two ends of a bridge travelling together.

Conclusion

Selecting a VFD for a crane comes down to four things.

• Classify first. The duty class, not the nameplate, sets the overload the drive survives and the output the motor may deliver.
• Three drives, three answers. Hoist on closed-loop vector with an encoder, bridge and trolley on V/f or sensorless vector.
• Check the baseline behind any multiplier. The published crane sizing rules disagree on paper because they measure different quantities on differently loaded rating columns.
• Verify the motor as well as the drive. Crane motor derating is larger than the upsizing on the drive side, and it is the more commonly missed of the two.

Once you have the duty class and the three motor currents, the rest is arithmetic. The expensive mistakes happen before that, when a drive is chosen from a nameplate and a rule of thumb.

Ask our engineers to check the drive schedule. Send us the duty class, the three motor nameplates and the supply conditions, and we will review the frame selections and braking capacity before anything is ordered.