Reduced-voltage starting explained: the current vs. torque trade-off
A starting-current limit can look perfectly reasonable on an electrical drawing and still leave the motor unable to accelerate the machine.
That is the trade-off at the centre of reduced-voltage starting. Lowering the voltage applied to an induction motor reduces the current it draws from the supply, which can solve a real problem for a transformer, generator, or local network. But the motor gives something up in return, and it gives up more than most people expect.
Why reduce the starting voltage?
With direct-on-line (DOL), or across-the-line (ATL), starting, full supply voltage is applied to the motor immediately. At standstill, the motor draws its locked-rotor current while developing its full-voltage locked-rotor torque.
Many installations handle that without difficulty. Where they cannot, the start may pull down the local supply voltage, exceed the current available from a transformer or generator, or apply mechanical torque more abruptly than the driven equipment can tolerate.
Reduced-voltage starting addresses that by applying less than full voltage while the motor accelerates. The difficulty is that current and torque do not fall at the same rate.
Current falls. Torque falls faster.
At standstill, induction-motor current reduces roughly in proportion to the applied voltage. Starting torque follows a square relationship with voltage, so a modest reduction in current produces a much larger reduction in available torque.
Take a motor with a locked-rotor current of 600% of full-load current. If the starting system limits current to 300% FLC, the current has been halved. Available starting torque, however, falls to only about a quarter of its full-voltage value. A motor that could develop 180% of full-load torque at full voltage may now develop only around 45%.
That gap between the current reduction and the torque reduction is the part a single current-limit number does not show. Two motors can satisfy the same current limit and still leave you with very different amounts of torque to work with.
Breaking away is only the beginning
Getting the shaft to move is not the same as completing the start. Once rotation begins, the motor has to keep producing more torque than the driven machine requires, and the margin between the two is what accelerates the load.
That margin does not stay constant. A centrifugal pump or fan may need very little torque at low speed and considerably more as it approaches operating speed. A high-inertia load can ask for a smaller margin, but for much longer. A motor can leave standstill comfortably and still run into trouble later in the acceleration.
This is why the same current limit can produce a successful start on one motor and a stalled one on another with a very similar nameplate.
What a current limit does not tell you
Knowing how much current the supply can tolerate is a genuine constraint. It is not, on its own, a starting specification. Whether that current is enough depends on the motor's torque-speed characteristic, the load's torque requirement across the full acceleration, and the total inertia being moved.
Working out where those pieces meet, and what happens when they do not, is where a reduced-voltage starting specification is actually built.
That is where Get Your Motor Running goes next. It works through how to connect a current limit to a motor's real starting characteristics and the load it has to move, so the number on the electrical drawing and the torque the machine actually needs stop being two separate conversations.
WANT THE FULL EXPLANATION?
Part 1 of Get Your Motor Running takes a deeper look at reduced-voltage starting, motor characteristics, and available starting torque.
