Be careful: You may not have the overload protection you think you have. Two new requirements for overload and stall time that were added to the NEMA MG-1 Standard may be in conflict with actual performance requirements for overload relays as specified in the NEMA ICS 2 Standard.

What it's about

In 1993, NEMA published the current edition of its MG1 standard governing the operating limits of motors. It is the established standard governing the design, performance, rating, and testing of motors used in North America.

Covering motor designs A, B, C, D, and newly added Design E for extremely energy-efficient motors, the new requirements in the Test and Performance Section II, Part 12 of MG 1 specify the minimum withstand level of motors during overload conditions, and when restarting them. In both cases, the motors are at rated operating temperatures. Prior to the revision, MG 1 only specified the major operating conditions such as starting current, starting torque, power factor, and minimum efficiency. It also specified the mounting dimensions, expressed in frame sizes, facilitating motor replacements. The revisions now address the behavior of a motor at overload conditions when excess current is drawn.

Excess current

Regarding motor overload, the following paragraph was added to MG1:

Polyphase motors having outputs not exceeding 500 hp and rated voltages not exceeding 1kV shall be capable of withstanding a current equal to 1.5 times the full rated current for not less than 2 min when the motor is initially at normal operating temperature.

This would indicate that if a motor is running at full load and rated operating temperature, an overload condition of 150% could cause deterioration of the electrical insulation after 2 min. Therefore, the motor overload protective device should be of a design to trip and disconnect the motor from its power source within 2 min.

The graph below shows typical overload tripping curves for NEMA Class 10 and Class 20 overload relays. NEMA-rated overload relays are classified by how quickly they must trip at 600% of rated tripping current. Since the ultimate tripping current is generally 125% of motor full-load current per NEC Sec. 43032(a)(1), this means that a Class 20 overload relay must trip within 20 sec at 750% of full-load current, from a cold start (overload relay at ambient temperature). Similarly, a NEMA Class 10 overload relay must trip in 10 sec, and Curves 1 and 2 show this will happen.

Curves 3 and 4 represent the tripping characteristics while the motor is overloaded at operating temperature. At this point, the tripping time is approximately 25 % compared to that of a cold overload relay. Curve 3 shows that a NEMA Class 20 overload relay, even at warm condition, may not trip within 2 min of a 150% overload.

Note that none of the curves shows a tripping time under 2 min from a cold start. This is permitted because a sustained overload on a motor that is starting cold inherently can be tolerated by the motor for a longer period than after the motor windings are already at operating temperature. In contrast, Curve 4 shows an IEC Class 10A overload relay.

Testing of overload relays is based on tripping times from cold (room ambient) conditions - so are the time/current curves issued by control manufacturers. Preliminary tests conducted by the writer show that commonly available Class 20 overload relays of the bimetallic and solder pot types do not meet the criteria in the relevant paragraph (12.48) of MG 1. Thus, it's questionable whether Class 20 overload relays do in fact protect NEMA motors. Word has it that NEMA has opened a project to review this requirement.

IEC relay vs NEMA relay performance

The need to provide this protection has [TABULAR DATA OMITTED] been recognized in the IEC Standard 947, Part 4 for contactors and motor starters. Paragraph 7.2.1.5.1a of that standard specifies that an overload of the Class 10A must meet this requirement. Neither NEMA nor UL 508 have addressed this situation.

Note that a Class 10A IEC overload is different from a Class 10 IEC overload relay, which can trip in 4 min at 150% of motor current. This, in turn, is less than IEC Class 20 or Class 30 overloads, which can trip at 8 min or 12 min, respectively.

An additional point of confusion is that NEMA and IEC overload class characteristics are inconsistent. The graph (on page 54) shows a NEMA 20 overload tripping very close to the IEC 10A overload, but NEMA overloads are not routinely tested at the 150% current loading. In point of fact, a standard NEMA 20 overload might trip like an IEC Class 20; that is, 8 min, or four times as long as the motor standard would want, or somewhere in between.

The somewhat-equivalent long-time current test in UL 508 requires a trip within 8 min while carrying 200% of the rated trip current (which would normally be 250% of the motor full-load current as noted previously). A line from the 20 sec/750% mark and extended through this point (8 min/250 %) would cross the 150 % line only after an extended amount of time, as shown.

There is another variable as well. The UL tests must occur with two poles current carrying. In other words, the UL tests are based on protecting against a single-phase condition, and don't directly correlate with IEC tests, which use all three poles.

Class 10A is in accordance with IEC 34-1, 18.2, a designation for overload relays, stating that "when energized at 'C' times the current setting, tripping shall occur in less than 2 min starting from thermal equilibrium at the current setting. 'C' is a multiple of current on IEC Table III (see table on page 56).

Stall time

Paragraph 12.49 in MG 1 for NEMA design motors covers "Stall Time" as follows:

Polyphase motors having outputs not exceeding 500 hp and rated to not exceed 1 kV shall be capable of withstanding locked-rotor currents for not less than 12 seconds when the motor is initially at normal operating temperature.

As stated, this requirement appears to be an appropriate protection since overload relays would trip well within that time when they are at operating temperature. However, motors - particularly larger ones - have a much longer thermal time constant than overload relays, which can be reset within minutes after tripping. If motor restart is attempted immediately after the overload relay is reset, the motor is still near operating temperature, and the Class 20 and Class 30 overload relay would permit stall time or acceleration time that can be as high as 20 or 30 sec respectively.

This is clearly not the intent of the new requirement and needs to be evaluated. Until that happens, users may want to consider NEMA Class 10-rated overload relays, as they may provide adequate protection. And once again, the overload relays of the IEC standard Class 10A do meet the requirements at all conditions of the motor standard, during cold start and at operating temperature.

Conclusion

With the addition of these two requirements into MG 1, the respective standards specifying the characteristics and performance of motor protection devices need to be reviewed and revised where appropriate. The performance requirements for overload relays are covered in NEMA ICS 2, Standard for Industrial Control and Systems, Part 4, and in UL 508, Industrial Control Equipment. NEMA is presently in the midst of rewriting its standard for overload relays. A standard for tripping at 2 min, in accordance with current industry motor specifications, should be adopted.

Robert Schindler is Manager, Products Application, Siemens Energy & Automation, Inc., Canton, Ga.