What size motor do I need?

Answer by Hugh Falkner (Atkins)

All manufacturers supply motors in a range of standard sizes, with custom designs only being used for high volume OEM sales. Since the designer must always err on the safe side, they will opt for the “next size up”, in addition to which others in the purchasing chain may also be tempted to add on an additional safety margin.  In this way it is easy for motors to be larger than they need to be.  In response to this, motors are commonly designed with a flat efficiency between 50-100% load, and a peak usually at around 75% load.

Motor over-sizing is often seen as a problem, but the inherent gain in efficiency with motor size, combined with this flat efficiency:load characteristic, means that only in cases where motors are seriously underloaded will a reduction in motor size give a noticeable efficiency gain.

In addition, induction motors can operate for short periods at 110-115% of their nominal load, a region known as the “service factor”.  However, the efficiency in this operating region is poor, and extended operation in this region will decrease the motor lifetime. 

When undertaking measurements, the consumed power might be much less than the motor rating, but this may not necessarily indicate incorrect motor sizing.  This is because fundamentally the motor is selected to provide a torque, a larger torque giving faster acceleration of the load when starting.  High inertia loads will require a higher power during starting, but this will fall to a much lower level when the motor reaches its full operating speed.  One example of this is a fly press that will take a long time to accelerate the flywheel to full speed, and then only a much more modest current to maintain this speed until the press is used.  So care should be taken that a motor is sized based on its maximum torque requirement, which might be considerably more than shown by a single current reading.