Why Motors Draw a Surge When They Start

A motor that isn't turning has very little to hold the current back. So the current rushes in. Then the motor spins up, and the rush fades.

What's going on inside

Think about a motor as a coil of wire next to a rotor. When power arrives and the rotor is still, almost nothing pushes back. A reference page on inrush current describes it. The current starts at the stalled level, then falls as the motor picks up speed¹. The spinning motor develops a "back EMF" that opposes the supply. That's a voltage the motor makes itself, pointed against the voltage feeding it.

Compare it to pushing a heavy trolley. The first shove takes real effort. Once it rolls, a light push keeps it going. The motor's first gulp of current is the shove.

For motors on alternating current, that first draw may be several times the normal full-load figure, and it lasts only a few cycles¹. Note the word "may". Not every motor surges by the same amount.

What you can notice

The effects are small but real if you pay attention:

• The lights dip. A reference page on induction motors says a squirrel-cage blower motor may cause household lights to dim upon starting². A squirrel-cage motor is a common design whose rotor looks like a hamster wheel made of metal bars.
• A short hum or growl. The motor is working against its own stillness.
• A pause before it picks up. Heavy things such as a pump full of water start slower than light things.
• A trip or a shutdown. If the supply can't deliver the burst, it may cut out.

Each sign tells you the same thing: the starting moment is costlier than the steady running that follows.

Why the load matters

The size and length of the surge depends on what the motor is pushing against. The inrush page says the starting transient lasts less time if the mechanical load is relieved until the motor has picked up speed¹. In plain words, a motor that starts with nothing to push gets going sooner and spends less time in the high-draw stage.

A motor that can't get going at all is worse. A stalled squirrel-cage motor, from an overloaded or jammed shaft, will consume current limited only by circuit resistance as it attempts to start². That page adds that overheating and damage to the winding insulation is the likely outcome unless something cuts the current off. So if a motor hums and won't turn, switch it off at once and follow your own appliance's manual.

Which things have motors that surge

Motors turn up in more places than people expect. The induction motors page lists split-phase motors as commonly used in major appliances such as air conditioners and clothes dryers². Fridges, freezers, pumps, fans and power tools are other likely suspects. Heaters, kettles and plain lamps are not motors, so they behave differently.

Compressors are a special case, and they get their own page: why a fridge needs more to start than to run.

Why the label can mislead

The label usually shows the running figure. Motors and similar loads can have a power factor well below 1, according to a reference page on the subject³. Power factor is a way of describing how much of the supplied current does useful work. A low one means the supply carries more current than the watt figure alone suggests. To see where the printed numbers sit, go to the label guide.

What to do with this

Don't try to measure or change a motor's starting behaviour yourself. Fitting devices that soften the start means working on the appliance's wiring, and that's a job for a licensed electrician. What you can do is plan around it. Know which of your appliances are motor-driven. Start them one at a time. And read your backup unit's manual to see how it treats a short burst.

The running versus starting watts page covers how to put the two figures side by side.