A Comprehensible Guide to Servo Motor Sizing

The Importance of servo motor sizing should not be underestimated. Proper motor sizing will not only result in significant cost savings by saving energy, reducing purchasing and operating costs, reducing downtime, etc.; it also helps the engineer to design better motion control systems. However, the knowledge of mechanical systems and their influence on motor speed, inertia and torque requirements seems to decline in a world where modern technology aspects, such as tuning and programming, seem to be the main focus. The motor sizing process involves a number of mathematical equations, which are most certainly documented, but not necessarily with the motor sizing process in mind.

The servo motor represents the most influential cost factor in the motion control system design, not only during the purchasing process, but especially during operation. A high-torque motor will require a stronger and thus more expensive amplifier than smaller motors. The combination of higher torque motor plus amplifier results not only in higher initial expenses, but will also lead to higher operational costs, in particular increased energy consumption. It is estimated, that the purchase price represents only about 2% of the total life cycle costs; about 96% is electricity. Proper servo motor sizing will not only assure best system performance; it also provides considerable cost savings.

Oversizing a motor is naturally more common than undersizing. An undersized motor will consequently not be able to move the load adequately (or not at all) and, in extreme cases, may overheat and burn out, especially when it can’t dissipate waste heat fast enough. Larger motors will stay cool, but if they are too large, they will waste energy during inefficient operation. After all, the motor sizing process can also be seen as an energy-balancing act.

This book focuses primarily on servo motor sizing and it documents in detail the inertia and torque calculations of standard mechanical components and the motor selection process.