Servo motors used in industrial applications are as different from servo motors that control model airplanes as a commercial bakery oven is from the oven in your kitchen. Industrial servo motors are tested to the same rigorous standards as AC induction motors with the added testing of the encoder or resolver option. Most motors built in the United States meet the standards promulgated by NEMA, IEEE, NEC and Underwriters Laboratories. Most manufacturers also test to European Community (EC) standards.
Servo motors usually are found in applications that require precise and coordinated motion control. An example is the industrial robots used in welding operations on a car assembly line. Each robot might have six or more servo motors. The heavy-duty industrial welding robot can move in six axes of motion and speed simultaneously. Unreliability and unpredictability aren’t options. Taking into account the money lost if one fails, paying for servo motor reliability assumes vastly greater importance.
By their nature and usage, industrial servo motors must be engineered, designed and built to more rigorous levels of reliability than standard AC, squirrel-cage, induction motors. Otherwise, industrial users would resort to using the less expensive motor.
Harsh environments
Part of what constitutes reliability is an ability to stand up to environmental insults. Servo motor bearing failure, cable damage and aluminum oxidation and deterioration are the major areas of motor failure I’ve documented. A typical example is a food processing plant, which must be cleaner than a residential kitchen. Achieving this level of sanitation involves spraying harsh cleansers such as chlorinated compounds or alkalis on the equipment, including controls and servo motors. Then, the cleanser is rinsed off with water under pressures that can exceed 1,300 psi at the nozzle.
The cleanser attacks paint, metals and insulation — the materials from which motors, cables, plugs and other electrical equipment are fabricated. The high-pressure rinse blasts loosened paint off equipment and water into every small crack and crevice. The water can cause further deterioration of materials through oxidation.
At a minimum, any motor you use in these environments must meet IP65 or IP67 standards. But even that might be insufficient if motors engineered to meet minimum standards are subjected to environments that are harsher than the minimum levels anticipated. Servo motor deterioration and premature failure can occur under these harsh conditions. The greater costs of servo motors that can withstand these rigors are more than justified when purchasing replacements, labor costs involved in replacing the equipment, and downtime is considered.
Servo motors cost more than their more mundane counterparts. The smaller servo motors start at around $1,000 and go to $3,000 for a 10-kW servo motor. The controllers run to $10,000 and up for the main motion controller and $1,000 for each servo motor axis controller.
It’s not logical to pay $1,000 for a servo motor if it only has the same bearings, shaft seal and IP67 rating as a $250 induction motor. If you’re going to pay the premium, your servo motor should have top-of-the-line bearings, seals and paint job. If you can’t afford the higher cost, consider using an induction motor, variable-frequency drive and a resolver, all controlled through a PLC.
Protective software
The software control systems and hardware controllers used with servo motors have variables and limits that result in system faults. The servo motor is blamed for the fault when it’s usually overzealous software making certain to protect people or equipment. One example is a fault caused by exceeding an over-temperature limit on any of a machine’s main motion and axis controllers.
The over-temperature sensor embedded in the servo motor’s windings can be enabled or disabled through the software. Motor windings are encapsulated in an epoxy resin, which can’t withstand as high a temperature as unencapsulated motor windings, according to one manufacturer.
An HMI typically interfaces with a PLC program that interfaces with the Geography Markup Language (GML) program running the analog servo motors. The software and manuals document the various faults you can experience and offer ways to correct them. The HMI screen displays general faults and cycle stops. The screen displays software buttons for resetting the cycle stops and a method for resetting the 24-VDC control system that resets the "hard" faults in the GML program, unless the fault condition persists beyond the reset. If the condition isn’t corrected, the program will hard fault repeatedly.
That’s why there’s considerable time pressure on the maintenance technician to find the problem and correct it to get the line running again. That time pressure isn’t softened by the fact that the technician must retrieve a laptop PC from the shop, connect it to the faulted motion controller, and load and access the GML program before troubleshooting can start.
This takes time.
Usually, the operator and maintenance technician have already reset the control system several times while trying to get the line restarted. This often buries the initial fault in a blizzard of interlocked faults. Winnowing through the mess with a laptop computer isn’t as easy as some would believe. But it’s necessary. There are so many ways for a line to fault and so many problems that can cause the fault that using a laptop is the only way to identify the root-cause failure.
The fixes
A large, industrial robot is a compact but complex machine. Once you decide to change out a component assumed to be defective, the task can be difficult and time-consuming. Over the years, plant maintenance departments have developed methods for identifying and repairing problems in a multi-servo-motor production line. The possible faults pertaining to the servo motors and their control systems number more than 200. Isolating and identifying a particular fault requires perseverance and skill. A line will continue to run with low-level faults, but won’t restart without correcting them once a fault or cycle stop halts the production line.
A cable fault can be identified by operating the robot until a fault stops it. Locating the cable fault requires using the illustrations and cable and plug pin-outs shown in the manufacturer's manuals. Open both ends of the suspected cable. Check the continuity on each wire in the cable. One will be open or grounded/shorted.
If you’re lucky, you’ll find unused wires in the cable assembly. A quick fix is to cut and splice to replace the bad wire with a spare. It’s a temporary fix that takes less time than replacing the entire cable. If you use this approach, make sure you’ve got a spare cable on hand and it’s scheduled for replacement soon. When stress and age break one wire, others will surely follow.
A motor known to be good can be connected temporarily while it rests on the floor. If restarting the line resets the faults and the spare motor starts, the suspect motor is probably bad. Reconnect the suspect servo motor and repeat the test. If the line faults again, replace the motor. The same procedure can be applied to a cable, sensor or controller. Your maintenance department should maintain good records of faults, duration of line downtime, replacements, repairs and any other information pertaining to faults and their corrections.
Controls
The electrical waveform of servo motors has many of the characteristics of DC power. Contactors in the motor lead circuit must take this into account. Sizing a contactor to handle the amperage and load of a similarly sized AC motor won’t work. The DC arcing that occurs when the contactor opens under running conditions will destroy the contactor. The most cost-effective way to avoid arcing is to install two motor circuit contactors of the same size in series so they simultaneously open and close. The double set of contacts in each leg minimizes the severity of the DC arcing.
You can't have too much information. Manufacturer's manuals are invaluable sources of troubleshooting and repair information. Ignore these resources at your peril. Manufacturers usually maintain a 24/7 helpline. Talking through the problem and symptoms with an OEM technician or engineer can reduce downtime.
Keeping good records of what techniques work with what symptoms can be very helpful when trying to troubleshoot these complex systems. Servo motors are reliable, but because they’re used in complex systems, the systems themselves tend to become the major headache. Become familiar with the standards that apply to the equipment and the environmental constraints that limit where you can place it. Because equipment meets the minimum standards for one environment doesn’t mean it can withstand the maximum hazards present in that environment.
Larry Bush is a reliability consultant in Modesto, Calif. Contact him at [email protected], www.reliability-consultant.com and (209) 526-2427.