When a machine won't come up to speed, reaching for a bigger motor is an understandable first move. It's also an incomplete diagnosis. A motor's horsepower (hp) rating describes what it delivers at its normal operating point — but starting happens somewhere else on the torque-speed curve entirely, where current, motor torque, load torque, and inertia are all changing at once. Before you decide a bigger motor is the answer, it's worth finding out where the one you already have is actually running out of torque.
At standstill, the motor first has to overcome breakaway torque — static friction and whatever else is holding the shaft still. Once it's turning, the job changes. Now the motor has to supply the torque the machine needs to do its work and still have something left over to accelerate the combined motor-and-load inertia.
That surplus is acceleration torque, and it's the part that actually decides whether the machine reaches speed. If a motor produces 100 lb-ft while the load needs 70 lb-ft, roughly 30 lb-ft is left to accelerate. Push the load up to 95 lb-ft and it still accelerates — just far more slowly. Once the load demands everything the motor can produce, there's nothing left over, and speed stops climbing.
That point can land anywhere between standstill and full speed.
Different machines ask for torque differently as they accelerate, and that difference matters more than the application label lets on.
A centrifugal pump or fan usually needs relatively little torque at low speed, with the demand rising as it speeds up — so it's the narrowing margin late in the start that often decides whether it reaches operating point. A high-inertia machine — a large fan, a mill, a crusher — can look like the opposite: modest running torque, but a large amount of energy needed just to bring the rotating mass up to speed, which can stretch the start out far longer than the load alone would suggest.
“Pump,” “fan,” or “flywheel” tells you the family of problem. It doesn't tell you the actual torque curve.
Two motors with the same rated output can behave very differently once starting is involved. A larger frame brings its own locked-rotor current, locked-rotor torque, and torque-speed characteristic — not simply more of everything, in proportion.
So the more useful comparison usually isn't “this motor versus one frame size up.” It's motor torque against load torque across the actual speed range the machine has to get through. Sometimes a bigger motor is the right call. Sometimes a different starting method — a soft starter that shapes the ramp and controls torque, or a VFD — solves it. And sometimes the answer is somewhere else entirely, in the mechanical condition of the machine itself.
A difficult start gets a lot easier to reason about once the question gets specific: does the motor fail to move the load at all, does it start turning and then stop gaining speed, or does it reach full speed but take far longer than it should? Each of those points to a different place.
Tracing exactly where a marginal start loses its torque margin is what tells you whether the fix is the motor, the starter, or the load. Benshaw's soft starter sizing guide walks through matching the start to the motor and load so you're specifying against the real torque-speed picture, not just the nameplate.
Got a start that's fighting you? Explore Benshaw's motor control products, or bring us the application and we'll help you match the fix to it. Mission-critical motor control is what we do.