Motors, Drives

What’s in a nameplate?

Information helps the selection of the right motor regardless of application.
By Jim Bryan November 9, 2018
Figure 3. Typical speed torque curves. Courtesy: EASA

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.

Fortunately, motors that conform with NEMA Std. MG 1-2016 or IEC Std. 60034-8:2007 must include all nameplate data that the respective standards require. What this entails will vary with motor type and size, so for example, rated field and armature current data would be required for direct current (dc) motors but not for alternating current (ac) motors. The focus here is on how the required nameplate data for NEMA and IEC motors can be helpful for selecting the right motor for an application.

Figure 1. AC nameplate. Courtesy: EASA

Figure 1. AC nameplate. Courtesy: EASA

Table 1 lists the basic requirements for most NEMA and IEC motor nameplates. However, not all motors will comply with these requirements, including those built before the standards were implemented or those outside the jurisdiction of the standards agencies. Some motors, such as synchronous and wound rotor motors, will have additional requirements. To cover all of these is beyond the scope of this article.

Figure 2. DC nameplate. Courtesy: EASA

Figure 2. DC nameplate. Courtesy: EASA

Figures 1 and 2 show generic metal nameplates that repairers sometimes use to replace damaged or altered originals. The same data requirements exist for such replacements.

Identification

Both NEMA and IEC require manufacturers to include such information as name, type, and frame. The name and type are determined by the manufacturers, each of which may have unique type designations that identify a family of motor applications and specifications.

Table 1. Nameplate requirements. Courtesy: EASA

Table 1. Nameplate requirements. Courtesy: EASA

NEMA frame designations (commonly called “NEMA frames”) are standardized through 449 frame. These frames can be identified by dividing the first two digits by four to get the shaft height (“D” dimension, center of shaft to bottom of foot); for example, 44/4 = 11 inches. For motors larger than 449 frame (termed “above NEMA”), manufacturers can develop their own frame designations. Consequently, for these machines the same apparent frame number may represent different frame dimensions for different manufacturers.

IEC Std. 60072-1:1991 defines the frame designations for metric motors. For foot-mounted models, the frame designation is the shaft height in millimeters and ranges from 56 to 400 mm. The designation for flange-mounted motors is the bolt circle hole, which ranges from 55 to 1080 mm.

Power

Power output is the power level in horsepower (hp) or kilowatts (kW) for which the motor design is optimized. The motor will respond to the load connected to the shaft and try to provide the necessary torque. At this load point, the motor current for NEMA motors should be within ±10% of the rated load amps on the nameplate; IEC does not provide a tolerance. If the load is less than the rated value, the motor load current will decrease.

This will have little impact on performance if the load is above 75% of the rated load; at less than this efficiency and power factor will decrease, negatively affecting the cost of operation. If the load requires more power, the load current will increase, while also increasing the heating of the motor and thus reducing its life. If the motor has limits on how long it can remain at full load, this will be designated in the time rating or duty cycle.

Maximum ambient

Maximum ambient is the highest environmental (room) temperature that the temperature rise produced by the motor will allow within its insulation class. Table 2 shows the total winding temperature for each insulation class (A, B, F, and H) and, based on the ambient, the temperature rise contribution of the motor. Temperature rise varies based on motor size and enclosure, so this table is a simplification.

Table 2. Temperature classes in degrees Celsius. Courtesy: EASA

Table 2. Temperature classes in degrees Celsius. Courtesy: EASA

Classes A and B are no longer used in the motor manufacturing process, but occasionally a motor with Class F insulation will be designed for Class B rise. This conservative approach ensures the motor will operate at a cooler temperature and have an extended service life. If the motor is to operate in a higher ambient temperature, the design will have less allowable temperature rise.

Speed

Rated revolutions per minute (rpm) and frequency are directly related. The synchronous speed of a squirrel cage induction motor (SCIM) can be determined by the formula:

 

 

As the load on a SCIM increases, speed will decrease. This is called slip. The nameplate speed of a SCIM should be within ± 20% of the actual slip for both NEMA and IEC motors. The rated speed of a SCIM or a DC motor is its speed at rated load. (Note: For dc motors, speed is determined by the designs of the armature and field windings, not by the number of poles.)

Phase and voltage

Single-phase and 3-phase motors are the most common. Although rare, some 2-phase motors are still in operation, as well as some special 6-phase motors. Voltage determines the power supply requirements for operating the motor. The motor must be capable of operating at ±10% or ±5% of rated voltage, for NEMA and IEC respectively, and deliver the rated power output. But as stated earlier, a lower voltage will require higher current to do the same work, resulting in increased temperature and shortened life.

Figure 3. Typical speed torque curves. Courtesy: EASA

Figure 3. Typical speed torque curves. Courtesy: EASA

Code letter

The locked rotor amps (LRA) may be included on the nameplate, but often a NEMA code letter is used. IEC’s approach is to limit the locked rotor apparent power based on a series of design letters in IEC Std. 60034-12. LRA also may be called starting current or inrush current. To determine the upper and lower limits of LRA to expect for a given code letter, plug each of the kVA/hp values in Table 3 into the formula provided. This range will accommodate manufacturing variations within the same motor design and allow for the proper selection of the motor controls.

Table 3. Locked rotor amps. Courtesy: EASA

Table 3. Locked rotor amps. Courtesy: EASA

Design letter

The NEMA design letter defines the profile of the torque developed as the motor accelerates to full speed (see Figure 3). Most applications use Design B; Designs C and D have much higher starting torques for applications that require it. The Design A profile is similar to Design B but does not meet the inrush current requirements in NEMA Std. MG 1. The primary factor in developing the different design profiles is the rotor cage. Therefore, the stator winding cannot be redesigned to change the design letter of a motor.

Table 4. Degrees of protection. Courtesy: EASA

Table 4. Degrees of protection. Courtesy: EASA

IEC design letters are N, NY, H, and HY. IEC Std. 60034-12 lists torque characteristics, locked rotor apparent power, and starting requirements but gives no direct relationship with LRA. Figure 3 shows that the Design N profile is similar to NEMA Designs A and B, and that Design H resembles NEMA Design C. The NY and HY designs designate that the motors are suitable for wye start-delta run applications.

Efficiency

An important factor in motor purchase decisions for many years, efficiency is the ratio of the output power to the input power. The difference is termed motor losses, most of which produce useless heat rather than the work required to drive the load. The lower the losses that a design can produce, the higher its efficiency. This translates to lower utility operating costs.

Service factor

Service factor is a multiplier that defines the load point at which a motor can operate thermally. Although the definition provides no time limit, operating continually at the service factor load significantly will increase the operating temperature and shorten the expected life. Operating at a 1.15 service factor has been shown to increase the temperature as much as 30°C; and each 10°C increase reduces the life expectancy by half. In this case, operating continuously at service factor would reduce the life of the motor to one-eighth of that at full load.

IEC Std. 60034-1, Sec. 4 defines 10 duty types that combine elements of NEMA’s duty cycle and service factor. S1 is continuous running duty at rated voltage, frequency, and load. S2 through S10 describe various short-time or periodic-duty cycles that the purchaser defines so that the motor can be designed accordingly.

DC motors

To operate at rated speed, dc motors must meet additional requirements for field and armature current and rated volts. If the field voltage is reduced (field weakening), the motor will accelerate. The maximum safe speed is the mechanical limit at which it is safe to operate the motor. It also can be limited by potential field instability. The nameplate should identify the motor as dc and indicate the winding type: shunt, series, compound, or stabilized shunt.

Figure 4. Power triangle. Courtesy: EASA

Figure 4. Power triangle. Courtesy: EASA

Shunt motors have field windings that are parallel to the armature. In series motors, the fields are in sequence with the armature. Compound windings combine both to take advantage of the good speed control of the shunt and high torque throughout the speed range of the series motors. The percent of compounding is determined by the ratio of the series field ampere-turns over the total (shunt + series) ampere-turns. If this is less than 20%, it is called a stabilized shunt.

IEC has additional nameplate requirements for both ac and dc machines, including the manufacturer’s identification number and year of manufacture. The pertinent standard is the one that applies to the motor; often this is IEC Std. 60034-x or 60079-x.

Another requirement is the IEC degree of protection, which NEMA also has adopted but does not require on its nameplates. This two-digit code follows “IP” and defines the level of protection provided to prevent contaminates from entering the motor (see Table 4). For instance, IP54 means the machine is protected from dust and splashing.

Power factor

Power factor is the ratio of the true power to the apparent power. Because a motor is a large inductive load, the voltage and current are out of phase. The apparent power is the voltage times the current that must be delivered to the motor to accomplish the true power. This is represented by the vector diagram in Figure 4.

The utility must provide the apparent power, but the true power is the watts for which the customer pays. Utilities often have a power factor demand charge to compensate for this. The power factor can be corrected by installing capacitors to counteract the effect of the motor’s inductance.

Altitude

Altitudes above 3,300 ft (1,000 m) reduce the cooling effect of the motor. The altitude on the nameplate is the maximum elevation at which the motor will receive sufficient cooling at rated load. A word of caution is in order here for motors used at higher altitudes. It is not uncommon to see the nameplate horsepower downsized for operation at a high elevation, in which case the manufacturer should adjust the kilovolt ampere code to a higher letter to reflect the inrush current of the actual power rating.

While the nameplate requirements listed in Table 1 provide a consistent basis of comparison for matching motor performance characteristics and capabilities with those of the application, neither NEMA nor IEC standards exclude anything. As a result, manufacturers sometimes include additional information (e.g., bearing nomenclature, to help users reduce downtime by acquiring replacement bearings before they take a motor off line for service).

There are also other specifications that may have different nameplate requirements for specific motor designs. In either case, it’s always helpful to keep a photograph of each nameplate on file for ready reference when the time comes to repair or replace a machine.

Jim Bryan recently retired from his position as a technical support specialist at EASA, a CFE Media content partner, an international trade association of nearly 1,800 firms in about 80 countries that sells and services electromechanical apparatus.


Jim Bryan
Author Bio: EASA