Whether you're selecting a motor for a new application or a replacement for one that has failed, you need a reliable way to match the capabilities and performance characteristics of various motors with the requirements of the application.

The most frequent concern about high current with a three-phase motor is high no-load current. But the broad issue of high no-load current isn’t the only three-phase motor issue to which plants should pay heed.

Too often, replacement specifications for three-phase squirrel-cage induction motors cover only basic nameplate data such as power, speed, voltage, and frame size, while overlooking other important performance characteristics such as the design letter. This can lead to misapplication of a motor, causing poor performance, inoperability, or failures that result in unnecessary downtime.

Process downtime is expensive—even more so when it’s unexpected. So, when an electric motor fails, we tend to pull, repair, or replace it, and move on as quickly as possible. In doing so, however, we may miss an opportunity to capture basic information that could help improve the reliability of the application. With a little planning, these data can be gathered with no delay in startup.

Manufacturers deploy various external connection schemes to produce three-phase induction motors for multiple voltages and/or starting methods. Be sure to follow the relevant connection diagram, which is usually affixed to the motor or contained in its manual. If the diagram is lost, damaged, or ignored, you could find yourself dealing with a costly rewind.

Correct interpretation of five operating parameters for NEMA, IEC induction motors When someone reads an electric motor nameplate, the normal assumption is that the information can be used at face value. That applies to some but not all of the nameplate information. For example, the power rating (hp or kW) and frame size are specific to the motor. However, ratings such as voltage, frequency, current, speed (rpm) and efficiency have tolerances associated with them. Our focus in this article will be to discuss the correct interpretation of each of these five operating parameters for induction motors of both NEMA and IEC design. Topics discussed include: Voltage and frequency - NEMA MG1-12.44 and IEC 60034-1.7.3 Current - NEMA MG1-12.47 and IEC 60034-1 Speed (rpm) - NEMA MG1-12.46 and IEC 60034-1-12.1 Efficiency - NEMA MG1-12.58 and IEC 60034-1 Note: The letter codes for insulation class, design and kVA code that appear on NEMA motor nameplates are addressed in "Motor Nameplate Letter Code Designations" in the March 2009 issue of Currents.

Steps to determine characteristics needed for finding a replacement A motor is received from a customer with the request that it be replaced. However, it does not have a nameplate. The steps to determine the motor characteristics needed for identifying a replacement will be described here. These same steps can also be used in the case of repair of a motor without a nameplate, so that a new nameplate with key identification characteristics can be made and attached to the repaired motor. The focus of this article will be NEMA or IEC horizontal motors in standard frame sizes.

Motors built to National Electrical Manufacturers Association (NEMA) standards use alphabetical letter codes on the nameplate to designate a number of alternating current (AC) motor characteristics. These characteristics are the code, design, and insulation class. Read the nameplate carefully as these designations are easily misinterpreted. Similarly, re-confirm these data items when your customer provides them. For example, the letter "B" could designate a design code, insulation class or kVA code (though highly improbable.) What do these different designations mean?

This article introduces a set of procedures for identifying unmarked leads of 6-lead motors with 1 or 2 windings. For most connections, the only tools required for these procedures are an ohmmeter and surge tester. These procedures will also help identify the type of connection when there is no nameplate present.

Un requerimiento frecuente al personal de soporte técnico de EASA es la solicitud de ayuda para la identi-ficaciónde los cables de salida que no están marcados. Este artículo establ-ece una serie de procedimientos para identificarlos cables no marcados en motores con 1 ó 2 bobinados que tienen 6 cables de salida. Para identificarla mayoría de las conexiones, los únicos instrumentos necesarios son un óh-metro y un equipo de onda de choque (surge tester).