Oversizing often happens when engineers try to match the inertia between the load and the motor rotor, so that they can tune the drive’s velocity, torque or position loop relatively tightly to meet the customer’s position accuracy and repeatability requirements. Think of it as a seesaw, with one person who weighs less than the other one. The person who weighs more (representing load inertia) will find it easier to push up than the person who weighs less (representing motor inertia), and vice versa. Therefore, as the engineer selects a motor inertia that most closely matches the reflected load inertia, the tighter one can tune torque, velocity or position loop to achieve more accurate control.
Another reason for oversizing could be to meet future capacity requirements for increased load and/or speed. In cases like these, determine the current and voltage characteristics of the motor and drive to see what the true torque-speed curve looks like. This means determining the continuous and peak torque and maximum motor speed required. If the application has low torque requirements, then you can use a lower-rated current drive with the larger motor.
Drive cost can be a major part of system pricing. With a lower-rated drive current to match the application’s torque requirements, as well as a larger motor to handle the inertia matching requirements, you would have an optimally sized solution based on performance and price.
It’s always important to find out the accuracy and/or repeatability requirements. Understanding the application’s rotary or linear mechanisms, with any possible compliance issues in terms of backlash, twisting power shafts or other factors will affect errors in position, torque or velocity. These issues can limit how tightly tuned the system can be between the load and motor.
Understanding the total motion profile with load masses and forces acting in each segment is a definite must, since this will provide a true picture of the RMS and peak torque requirements. Dwell time is also an important factor that not only decreases the RMS torque, but also permits a not-so-tightly-tuned servo system to get into position. Another factor when combining motors and drives is inductance. The motor’s inductance should be within the minimum and maximum inductance range for the drive or you’ll get poor control due to current loop gain and motor overheating issues.
Based on these factors, you can size a system with a typical industry standard inertia ratio of 1:1 or 10:1. An engineer experienced in sizing motors can go outside this load-to-motor inertia ratio as long as they have a thorough understanding of the mechanics and the motor is directly coupled to the mechanism. Aim to avoid motor inertia values that are larger than the reflected load inertia since all of your torque will be working to move the motor shaft, which is less efficient.
Balance Energy Use and Service Life
Robert Gradischnig, Lenze (www.lenzeamericas.com), electro-mechanical business development
People oversize motors because they think they’re building in safety. With a conveyor, for instance, they give us a larger load size to give themselves a 10 percent safety factor. Sometimes this also means they’ll have to use a larger gear motor. Since everybody in the chain of suppliers puts in a safety factor, they can end up oversizing the system by as much as 30 to 50 percent. They may get longer motor life, but component and energy costs will be significantly higher as a result.
Until recently, you could get away with that in the U.S. because energy costs are so much lower than in Europe. But the user ends up paying the price in their electricity bills. Now, with manufacturers focused on reducing operating costs, energy consumption is becoming an issue in the U.S., too. Energy efficiency has become an important selling point for OEMs, and they’re learning how to spec for efficiency. The goal should be to size components to provide the right balance between energy use and service life.
Some of the key engineering factors that need to be considered in sizing motors include:
• Maximum load
• Duty time
• Motion profile
• Pulley diameter
• Additional drive components (belt reducers, etc.)
• Ambient temperature
"Since more than 90 percent of the equipment used in packaging and production operations also include gear motors, for example, the efficiency of the gearing is critical to overall system efficiency. That means using helical bevel (95 percent efficiency) rather than worm (70 percent efficiency) gearing if your goal is operating efficiency.
Often the end user doesn’t know the facts about the performance of their equipment. In the U.S., they’re not used to discussing machine characteristics at this high level, but those are the details that are critical to know for accurate motor sizing. Providing a sketch of your drive components is the best way to help your motor supplier understand and right size your application. It can tell you more than an entire page of descriptions.
Size for Peak Efficiency
John Malinowski, Baldor Electric (www.baldor.com), general product manager
The best rule of thumb is to size for peak efficiency, roughly 80 percent of the rated load. With a grossly oversized motor, efficiency drops off dramatically, although a slight oversizing helps on temperature, motor insulation and grease. An oversized motor’s higher starting torque can wear equipment out rapidly. If you undersize, on the other hand, the motor may run too hot, the grease will not last as long, and the bearings will fail prematurely.
With a bigger motor, you’re paying more for first cost and life cost. The initial cost of a motor is only 2 percent of its actual cost; the other 98 percent is electricity. A too-large motor consumes more energy and also can result in penalties from utilities for increased low-power factor. In-rush current will also be higher. Plus other component costs are greater, such as power transformers and wiring. It just cascades, because you’ll also have higher maintenance and inventory costs.
It’s important to get away from selecting single components and design the drive train as a complete system, including starters, variable-speed drives, gearboxes or gear motors, as well as the control system. The goal is to choose the most efficient components and design them to work in harmony.
Joe Peeler, Siemens (www.usa.siemens.com), senior technical support engineer
There’s often not a good understanding of the torque requirements of the load being driven, including speed and inertia, particularly among end users. They tend to replace a motor with whatever size they already had, even if it’s been in place for 20 years. Or the motor could have been replaced at some point with a larger motor because it was the only one in stock.
The downside of oversizing is that there’s more torque when starting, so there’s more stress on equipment. High-torque motors are also not as efficient in energy use and may require larger switchgear capacity. The sweet spot is 85-90 percent of a motor’s nameplate ratings. High-inertia loads, such as large fans and grinding mills, will require a larger frame size motor.
>> Read Automation World's complete coverage on Motor Oversizing: Engineers Split on Motor Oversizing