Servomotors are smaller than other motors of comparable power. A 1-hp brushless servo is only 3.5 in. on a side compared to dc brush (4 in.), SCR-rated (5 in.), and standard ac induction motors (around 6 inches). The smaller size, a reflection of a smaller rotor, means less rotor mass and inertia and greater acceleration. It also means servomotors are more efficient.

As the cost of electricity goes up, energy efficiency becomes more and more relevant as a design parameter. This is even the case in motion/position control applications, where servomotors are widely used. Unfortunately, many people overlook servomotor efficiency because the motors themselves are usually fairly streamlined. However, a misapplied servomotor can overheat and waste energy just like any other motor. It’s the application that makes the difference, and if you think it through you’ll get it right.

Defining features

First, however, let’s get on the same wavelength by defining what a “servomotor” is. The single most noticeable feature about a properly designed servomotor is that it is physically compact. For a given output, a servomotor can be anywhere from 25 to 50% smaller than other motors. For example, an ordinary 1-hp ac induction motor is typically around six inches in diameter. By comparison, a 1-hp SCR-rated motor is about five inches in diameter. A brush-type servomotor is four inches, and a brushless servo is three and a half inches square. The servomotor’s small size reduces rotor inertia, providing the fast response necessary for motion control applications. It also conserves energy.

Another characteristic feature is a feedback device. Servos require feedback to operate properly. This allows for closed-loop control and improved accuracy.

Compared to other motors, servos are also more carefully manufactured. More wire is wound onto the laminations, thoroughly filling the space between the teeth. Slot fills range from 75 to 80%, compared to less than 65% for most other motors. Although it presents a production challenge, a higher slot fill provides additional torque and greater efficiency.

A servomotor designed by these standards is typically over 85% efficient. Anything less, and the motor is probably not fit for a “servo” application.

Efficiency gauge

Since true servomotors are already efficient, the real measure of efficiency is the application itself. Every servomotor application is described by a set of performance plots. Each motor has a unique set of curves containing information about speed, torque, voltage, and thermal conditions. These curves, when considered in light of the intended load and duty cycle, tell how efficient an application is likely to be.

Torque curves — which everyone should be familiar with — let you determine operating speed from load torque and voltage. Thermal curves tell you under what circumstances a motor will run at its maximum operating temperature (usually 155°C). If the motor operates continuously on either side of the curve, it will run cooler or hotter. Inside (or under) the curve is the safe operating area. Outside the curve the motor will overheat (if operated continuously).

Efficiency curves also provide valuable information. They show the difference between input and output power over the recommended operating range. In motion control applications, servomotors should operate within this range to achieve high efficiencies.

Red zones

In general, there are two operating areas where servomotor efficiency tends to decline. One is at high torque and the other, low voltage. Both extremes are encountered in everyday applications, and you need to be aware of them.

Servomotors are most often employed because of their ability to produce high peak torque, thus providing rapid acceleration. But in achieving high torque, servomotors typically have to run two to three times higher than their normal (continuous) torque range. Here, servomotors must sacrifice efficiency by an amount depending on the design.

Low voltage losses can also be an efficiency factor in servomotor applications. Servos are designed to operate over a wide range of voltages. This is how their speed is varied. But when the voltage drops so does efficiency.

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