Sizing a generator for an electric winch

Running an electric winch from a generator is common on sites without mains power, but it catches people out, because a winch is not a steady load. An electric motor draws a heavy surge of current as it starts, far more than it draws when running, and a generator sized only for the running load will sag or trip the moment the winch starts under load. Sizing the genset for that starting surge, and matching the phase and voltage, is what lets a winch run cleanly on generator power, and it ties into the wider picture of how a winch draws power that our note on the power supply and voltage covers.

Why starting current is the catch

An electric motor draws far more current to start than to run. As it breaks the load into motion and gets up to speed, the inrush current can be several times the running current, a brief but heavy surge. Once running, the draw settles to a much lower steady figure. A generator must supply that brief surge as well as the steady load, and this is where sizing on the running figure alone goes wrong: the genset can comfortably run the winch but cannot supply the surge to start it, so it sags or trips at the moment of starting, exactly when the winch is asked to do its hardest work.

Sizing for the surge, not just the load

The right way to size a generator for a winch is to allow for the starting surge, not just the running load. The genset must be able to supply the inrush current the motor draws at start up while holding its voltage, which usually means it is rated well above the winch's running power. A common mistake is to read the winch's running power, pick a genset of about that size, and find it trips every time the winch starts under load. Sizing for the surge, with margin, avoids this, so the winch starts cleanly every time rather than stalling the generator at the worst moment.

Voltage dip and stability

When a winch starts, the surge it draws pulls the generator's voltage down briefly, a voltage dip, and if the genset is too small the dip is deep enough to stall the start or trip the protection. A generator with enough margin holds its voltage through the surge, so the winch starts smoothly and the dip is small and brief. This is why sizing is not only about the average power but about the genset's ability to ride through the surge without sagging. A stable supply on starting is what a winch needs, and an undersized genset cannot give it however well it runs the steady load.

How soft start and VFD help

The most effective way to ease the demand on a generator is to reduce the starting surge itself, and that is what a soft start or a variable frequency drive does. By ramping the motor up gently rather than letting it draw a full inrush, a soft start or VFD greatly cuts the starting current, as our note on variable speed and soft start explains. This lets a smaller generator run the winch, because it no longer has to supply a heavy surge, only a gentle ramp. On limited generator power, a soft start is often the cleanest solution, letting a modest genset start a winch it could otherwise never bring to life.

Matching phase and voltage

Beyond the size, the generator must match the winch's phase and voltage. A three-phase winch needs a three-phase generator, and the voltage must match what the winch expects, the subject of our note on three-phase versus single-phase winches. A mismatch means the winch cannot run, or runs poorly, regardless of the genset's size. So sizing a generator is not only about power but about supplying the right kind of power, and the phase and voltage are settled alongside the rating. Getting these right is part of planning the supply and the winch together rather than discovering a mismatch on site with a genset that cannot run the machine.

Margin for a healthy supply

A generator run constantly at its limit ages quickly and supplies a poorer quality of power, so sizing with sensible margin is worth it beyond just starting the winch. A genset with margin starts the winch cleanly, holds its voltage, runs cool and lasts, and copes if the duty is a little heavier than expected. An undersized genset run hard to start and feed a winch struggles, trips, and wears. The margin is not waste; it is what gives a reliable, stable supply that starts the winch and runs it well, which matters on a site where the generator is the only power and a failure stops the work.

Planning the genset and winch together

The honest approach is to size the generator and the winch as one. Establish the winch's running power, its starting surge, its phase and voltage, and whether a soft start or VFD is fitted, then choose a genset that supplies the surge with margin and matches the phase and voltage. Where the available generator is fixed, a soft start may be what makes the winch usable on it. Settled this way before anything is bought, the winch and the genset arrive matched, and the winch starts and runs cleanly, rather than tripping a generator chosen on the running figure alone, which is how generator power catches a winch out. The same care applies if a genset already on site is to power the winch, because then the winch and its starting must be matched to the generator that exists, not the other way round.

Matching a winch to your generator with us

We help match an electric winch to the generator that will power it, including the soft start that often makes a smaller genset work. See the range in our winch catalogue, and read how the power supply, the soft start and the phase and voltage bear on it. Tell us the generator you have or plan, and the winch and duty, and we will size and specify so the winch starts and runs cleanly rather than tripping the genset.

Frequently asked questions

Why size a generator above the winch running power?

Because an electric motor draws several times its running current as a brief surge when it starts. A generator sized only for the running load can run the winch but cannot supply the starting surge, so it sags or trips when the winch starts under load. Sizing for the surge, with margin, avoids this.

What is the starting inrush?

The heavy surge of current an electric motor draws to break the load into motion and get up to speed, often several times the running current, lasting only briefly. Once running, the draw settles much lower. The generator must supply this surge as well as the steady load.

Can a soft start let me use a smaller generator?

Yes. A soft start or variable frequency drive ramps the motor up gently rather than letting it draw a full inrush, greatly cutting the starting current. This lets a smaller generator run the winch, because it no longer has to supply a heavy surge, which is often the cleanest solution on limited generator power.

Does the generator have to match the winch phase?

Yes. A three-phase winch needs a three-phase generator, and the voltage must match what the winch expects. A mismatch means the winch cannot run or runs poorly regardless of the genset size, so the phase and voltage are settled alongside the rating when sizing the generator.