Electric motors are critical assets in many applications, so access to nameplate ratings and terminal markings can save time and money as a motor is moved in and out of service for storage, maintenance, repair, or replacement. While there are many types of electric machines, this piece will focus on standard, 3-phase, squirrel cage induction machines. Most of these are built to NEMA or IEC standards, and most of their nameplate information is standardized.
Nameplate information
NEMA and IEC standards mandate that motor nameplates provide such details as the manufacturer’s name, motor type, and frame designation. Manufacturers assign unique types to identify motor applications and specifications. Frame designations define standardized dimensions critical for mounting and coupling motors to driven equipment. This information is available in NEMA Std. MG 1 and IEC Std. 60072-1.
Motor nameplates generally include the rated power, base speed, voltage, frequency, and full-load current. This basic information is often documented by the end user either on schematics or in asset management systems. Other important characteristics like insulation class, ambient temperature, and duty type are often overlooked but the two characteristics we’ll focus on here relate to starting current and accelerating torque.
Starting current
The starting current, also known as locked-rotor current (LRA), may appear on the nameplate of NEMA motors, but usually, a NEMA code letter indicates a permissible range. IEC Std. 60034-12 limits locked rotor apparent power using design letters. When end-users procure replacement motors without considering this starting characteristic, starting problems often arise.
Newer machines built to higher efficiency standards generally have low-resistance rotor cages, which means higher starting current. So, replacing an older kVA code G motor with a new kVA code J model could increase the starting current by as much as 43%. To some extent, this situation is unavoidable, but it can be anticipated, which helps when planning.
Accelerating torque
The NEMA design letter specifies the torque profile as the motor accelerates to full speed direct-on-line at rated voltage and frequency (see Figure). Design B machines (the most common) are used in applications like pumps, fans, and blowers. Design C machines are usually found in applications like conveyors and crushers. High-inertia load applications like punch presses use Design D machines, often with a flywheel in the system. The rotor cage design determines the speed-torque curve profile, so a stator winding redesign cannot facilitate a design letter change. Trying to replace a Design C or Design D motor with a Design B motor because of availability is a common error that almost always results in rapid failure. IEC Std. 60034-12 defines the common IEC design letters N and H.
The nameplate may indicate the motor’s design characteristics. Keeping a photo of each nameplate on file is useful when repairs or replacements are needed.
Terminal markings
NEMA Std. MG-1 and IEC Std. 60034-8 provide standardized terminal markings for 3-phase machines. It is important to retain documentation for the number of motor leads and how they are marked. A 3-lead machine is simple to connect to the power supply. It gets more complicated with 6-, 9- or 12-lead machines that may have several design configurations, including multiple voltages, multiple speeds, and special starters like wye-delta or part-winding. Machines designed for multiple speeds at power frequency by changing the external connection may have more than one winding as well.
Service centers have reliable procedures for identifying and marking the leads of 6- and 9-lead machines, but they’re not trivial and can require special equipment not always available to the end-user. For this reason, the service center usually sends personnel to the site or transports the motor to the service center. It is difficult to identify and mark 12-lead machines without disassembly if most (or all) of the markings are gone.
The bottom line: Having procedures in place to document and preserve the external connections can save time and prevent mistakes when motor repair or replacement is necessary. If a service center rewinds a motor and changes the number of leads, be sure the change is well documented.
Summing it up
Including complete motor nameplate data and terminal marking information in your asset management system and ensuring that procedures are in place to preserve terminal markings on the motor are very simple ways to reduce downtime and streamline replacement should a machine need to be removed from service for repair or replacement.
