Reduced-voltage motor starting: how current and torque are really affected
A starting-current limit can look completely reasonable on a one-line diagram and still leave the motor unable to accelerate the machine it's bolted to.
That trade-off sits at the heart of reduced-voltage starting. Lower the voltage applied to an induction motor and you lower the current it pulls from the supply — which can solve a genuine problem for a transformer, a generator, or a limited plant feeder. But the motor gives something back in return, and it gives up more than most people plan for.
Why reduce the starting voltage at all?
With across-the-line (ATL) starting — full-voltage, direct-on-line starting — the motor sees full supply voltage the instant the contactor closes. At standstill it draws locked-rotor current, typically 6–8 times full-load amps (FLA), while developing its full-voltage locked-rotor torque.
Plenty of installations shrug that off. Where they can't, the start can sag the local supply voltage, exceed what a transformer or generator can deliver, or slam mechanical torque into the driven equipment harder than it was built to take. On a pump, that's a water hammer. On a conveyor or gearbox, it's shock loading that shortens the life of everything downstream.
Reduced-voltage starting fixes that by applying less than full voltage while the motor comes up to speed. The catch is that current and torque don't come down at the same rate.
Current falls. Torque falls faster.
At standstill, induction-motor current drops roughly in proportion to the applied voltage. Starting torque, on the other hand, follows the square of the voltage — so a modest cut in current produces a much bigger cut in the torque you have available.
Run the numbers. Take a motor with a locked-rotor current of 600% FLA. If the starting system limits current to 300% FLA, you've cut current in half. Available starting torque, though, drops to roughly a quarter of its full-voltage value. A motor that could develop 180% of full-load torque across the line may now deliver only about 45%.
That gap — between how far current falls and how far torque falls — is exactly what a single current-limit number hides. Two motors can meet the identical current limit and leave you with wildly different amounts of torque to work with.
Breaking away is only the beginning
Getting the shaft to move is not the same as finishing the start. Once rotation begins, the motor has to keep producing more torque than the load demands, and the margin between the two is what actually accelerates the machine.
That margin doesn't hold steady. A centrifugal pump or fan needs very little torque at low speed and considerably more as it nears operating speed. A high-inertia load — a large fan, a mill, a crusher — may ask for a smaller margin but hold that demand far longer. A motor can leave standstill without complaint and still stall out later in the ramp.
It's the reason the same current limit can produce a clean start on one motor and a stalled, tripped, overheated one on another with a nearly identical nameplate.
What a current limit doesn't tell you
Knowing how much current your supply can tolerate is a real constraint — but on its own it is not a starting specification. Whether that current is enough depends on the motor's torque-speed curve, the load's torque demand across the full acceleration, and the total inertia you're moving.
Working out where those pieces meet — and what happens when they don't — is where a reduced-voltage starting specification actually gets built. That's also where the right soft starter earns its keep: instead of a single fixed voltage tap, a solid-state soft starter lets you shape the ramp, hold a current limit, and add torque control so the number on the drawing and the torque the machine actually needs stop being two separate conversations.
Where to go next
If you're sizing a start for a real application — low voltage or medium voltage — the next step is connecting your current limit to the motor's real starting characteristics and the load it has to move.
Benshaw's soft starter selection guide walks through bypass type, motor connection method, start duty, and operating conditions — the factors that turn a current limit into a start that works the first time and every time after.
Have an application you want a second set of eyes on? Talk to us. Mission-critical motor control is what we do.