Tom Bishop, PE
EASA Senior Technical Support Specialist
Outside of North America, IEC motors are ubiquitous and if not repaired are typically replaced with IEC equipment in most applications. Comparatively, the IEC motor population in North America is continually increasing, and there are two frequent scenarios when replacing an IEC motor. The first is relatively straightforward: replacing the IEC motor with an equivalent IEC motor. The other scenario is to consider replacing the IEC design motor with a NEMA design motor.
The information in this article provides comparisons of key characteristics of IEC and NEMA motors to aid in evaluating the potential simplicity or complexity of the IEC to NEMA design conversion. Further, if desired, the information may be used to evaluate a potential NEMA to IEC motor conversion. For clarity and simplicity, all dimensions are given in millimeters. Except for the NEMA service factor (SF) and IEC duty types (S1 to S10), there are many more similarities than differences between the IEC and NEMA motor standards.
Frames and Shafts
IEC mounting (frame) and shaft extension dimensions provided in the EASA Technical Manual (TM)
- Mounting (frame) dimensions for foot mounted motors are on TM page 2-18
- Shaft extension dimensions for AC motors are on TM page 2-21
Both NEMA and IEC assign specific power ratings to certain frame sizes according to speed:
- In general, output power ratings and frame sizes are comparable
- Shaft heights, foot spacings, shaft diameters equal within 3 or 4 mm
- NEMA output shaft lengths tend to be longer
First two digits of NEMA frame size designation represent four times the actual centerline of shaft to bottom of feet (hereafter “shaft height”) in inches (Example: 11 inch shaft height x 4 = 440 frame series).
IEC motors use actual shaft height in millimeters (Example: 280 mm shaft height = 280 frame).
Most frame sizes in either NEMA (MG1) or IEC (60072-1) have a comparable equivalent in terms of shaft height:
- Example: 280 mm / (25.4 mm/in) = 11.02 inches
- Example: 11 in x 25.4 mm/in = 279.4 mm
- One exception: 100 frame IEC motor has no comparable NEMA counterpart [100 mm / (25.4 mm/in)] x 4 = 15.7 inches, ~NEMA 160 frame
IEC defines wider range of shaft height:
- 56 through 900 mm shaft heights
- No NEMA equivalents for all of them
- NEMA stops at the 680 frame series
Terminal Boxes (Enclosures)
NEMA standard terminal box location is on left hand side facing output shaft (F1):
- Optional positions on right hand side (F2) and on top (F0)
- Unrestrained (flying) leads require extra space to connect and contain inside enclosure
IEC standard is terminal box on top B3T (NEMA F3):
- Optional locations on either side – left B3L (F1) or right B3R (F2)
- Terminal box can generally be rotated 4 x 90 degrees
- Terminal posts make for easy connection of leads
IEC Duty Type Ratings (Table 2)
- Motors are designed to deliver rated power for an average lifetime
- Motor sized to the application requirements (S1-S10 ratings)
- Service factor (NEMA) is not applicable to IEC standards
IEC and NEMA frame and power rating comparisons (Figure 2 and Table 3)
Related Reference and Training Materials
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