Selection of Short-Circuit Protection and Control for Design E Motors -- Drives and Controls

The 1996 National Electrical Code [Article 430-7 (9)] introduced the Design E motor to the electrical industry (National Fire Prevention Association, 1996). A new motor designation was necessary because the Design E motor has higher efficiency, higher locked-rotor current, and reduced torque requirements compared to existing NEMA motor designs. Design E motors provide increased energy savings that are important to many users. But the possible additional costs of protective and control equipment as well as more horsepower must be part of a cost-savings calculation.

This section is intended to give users of Design E motors electrical application information concerning the selection of the appropriate motor disconnection means; motor branch-circuit short-circuit protection; motor circuit conductor, motor controller, and motor overload protection; and guidance concerning torque characteristics. It describes how motor efficiency is increased and how the design changes related to increased efficiency affect the mechanical and electrical characteristics of motors. The section reviews the requirements of Article 430 of the 1996 National Electrical Code and provides generic application tables that compare the protection and control requirements of Design E motors with those of Designs B, C, and D.

1 Definitions

Breakdown torque The maximum torque the motor will develop under rated volt-age and frequency which won't result in an abrupt decrease in speed. This torque level typically represents a significant overload situation, and the motor should be operated at this level for only short periods of time and very low duty cycles to prevent overheating.

Full-load torque The torque necessary to produce rated power at full-load speed.

Locked-rotor torque The minimum torque the motor can produce at locked rotor (zero speed) at any angular position with rated voltage and frequency applied.

Pull-up torque The minimum torque developed by the motor during acceleration from rest to the speed corresponding to breakdown torque. The load must require less than this value to obtain reliable starting.

Slip The difference between the synchronous speed of the motor and the actual motor speed. It is usually designated as a percent of synchronous speed. In an induction motor, the slip increases with increasing load.

X/R ratio The ratio of inductive reactance to resistance measured at the motor terminals during starting. This ratio is proportional to the electrical time constant during starting.

2 Origins of the Design E Motor

Motor Characteristics and Performance. The Comprehensive Energy Policy Act (Public Law 102-486) of 1992 specifies that motor manufacturers meet an efficiency standard for "energy efficient" polyphase squirrel-cage induction motors as defined by NEMA Standard MG 1-1993, Table 10-9. Motors designed to the previous NEMA Standard (MG 1-1987, Table 12-6B) were also labeled "energy efficient." However, the motors that meet the old standard are generally referred to as "standard efficiency" motors in today's nomenclature. Therefore, in the context of this section, standard efficiency refers to motors that meet the old (1987) NEMA requirements, whereas energy efficient or premium efficiency refers to motors that meet the new (1993) NEMA requirements. Design E is a special category of energy-efficient motors whose efficiency design requirements exceed those of other NEMA-design motors.

Premium-Efficiency Motors. Higher efficiencies are typically obtained by using more material, better materials, and better manufacturing processes, which results in a higher-cost motor. Energy-efficient motors have lower operating costs than standard-efficiency motors because they use less power; therefore, the higher initial cost can be offset by the lower operating costs over the life of the motor. However, the payback period is very much dependent on the application. E.g., a pump motor that's run at full load for weeks without stopping will have a shorter payback period than an underloaded pump motor that's run occasionally.

How Efficiency Is Increased. The efficiency of the induction motor is defined as the mechanical output power measured at the motor shaft divided by the electrical input power measured at the motor terminals. The difference is due to losses in the motor. Motor efficiency is increased by decreasing these losses. There are five components of losses in the induction motor, typically referred to as (1) stator I 2 R cop-per losses, (2) rotor I 2 R copper losses, (3) iron (core) losses, (4) windage and friction losses, and (5) stray load losses. The premium-efficiency motor addresses each one of these loss components.

The stator copper losses are caused by the resistance of the stator windings. These losses are reduced by increasing the cross section of the stator conductors and there-fore reducing the stator resistance. Adding cross section to the stator conductors adds more copper and cost to the motor.

The rotor copper losses are caused by the resistance of the conductors in the rotor-the rotor cage and end rings. Reducing the resistance of these conductors could also reduce losses, but at the expense of the starting torque and locked-rotor current. The starting torque is also proportional to the rotor resistance, and the locked-rotor current is inversely proportional to the rotor resistance. Since the motor usually has to meet NEMA requirements for the minimum starting torque and maximum locked-rotor current, there is a limit to the amount the rotor resistance can be reduced. But motor designers can optimize the shape of the rotor bars to minimize locked-rotor current and maximize efficiency.

The core losses are caused primarily by hysteresis and eddy current losses in the stator iron. The hysteresis is improved by reducing the flux density in the stator iron by increasing the stator stack length. It is also improved by using a higher grade of stator lamination material. The eddy current losses are reduced by optimization of the quality and thickness of the stator lamination material. These changes all increase the cost of the motor.

The windage and friction losses are due to the rotation of the motor. In fan-cooled motors, the fan is connected directly to the rotor shaft and provides circulating air to limit the motor temperature due to motor losses via convection cooling. An energy-efficient motor requires less cooling than a standard-efficiency motor because it has lower losses and therefore requires a smaller fan and as such requires less power to operate. The friction losses are improved by incorporating better bearings.

The stray losses are more difficult to identify but are very important in terms of efficiency. These losses are minimized by careful motor design and manufacturing practices.

Effect of Efficiency Improvements on Motor Performance. The motor design modifications done to increase efficiency have significant effects on other motor performance characteristics, as shown in Table7. Table7 shows the typical performance characteristics of the Design E motor compared to an energy-efficient Design B motor. In any particular case, the actual comparison depends on how the motor efficiency is obtained. However, NEMA provides limits for some of the values listed in Table7, and the modifications necessary for obtaining increased efficiency tend to push the performance in a direction that may exceed these limits. E.g., an effort to increase efficiency by reducing the resistance of the stator windings is limited by restrictions on locked-rotor amperage (LRA). Therefore, the high-efficiency design becomes a compromise between efficiency and performance with respect to the NEMA requirements.

TABLE7 Design E Motor Performance Characteristics Relative to energy-efficient design B motor (Average change Range) Characteristic |

Nominal efficiency, 1% 1 to 4% Maximum locked-rotor current 0.42% 15 to 54% Minimum locked-rotor torque  1%  16 to 13% Minimum pull-up torque  3%  20 to 0% Minimum breakdown torque  6%  20 to 5% Full-load current None Full-load torque None

* In addition, the power factor and slip of Design E motors tend to decrease relative to Design B motors; the X/R ratio tends to increase relative to that of Design B motors.

3 NEMA Motor Designs

Designs A, B, C, D, and E. NEMA designates letter classifications for induction motors according to the performance characteristics of locked-rotor torque, break-down torque, locked-rotor current, and full-load slip. Table8 shows the typical values for the different classifications. Note that the Design A motor isn't specifically defined in the table but has characteristics similar to the Design B except for higher locked-rotor starting current. NEMA designates limits for minimum locked-rotor torque, minimum breakdown torque, minimum pull-up torque, and maximum locked-rotor current for each of these motor classifications. NEMA also requires minimum efficiencies for Design B and Design E motors.

Design B Energy-Efficient Motors. Many Design B motors had been redesigned to meet the 1993 NEMA efficiency standards given in NEMA MG 1, Table 12-10. Efficiency gains come at the expense of other performance characteristics. Whereas the standard-efficiency motors typically fall within the NEMA limits with room to spare, the energy-efficient motors tend to push the NEMA limits. These motors are typically physically longer and heavier and are more expensive than the standard-efficiency motors. It isn't clear what will happen to the standard-efficiency motor products.

Design E. The Design E motor is intended to provide a satisfactory solution to most of the general-purpose applications provided by the Design B motor but at a higher efficiency. Because of the modifications needed to obtain this higher efficiency, the Design E motor possibly cannot meet the Design B NEMA performance requirements. Therefore, NEMA has defined new requirements for the Design E motor. The efficiency requirements are included in Table8, and Table9 com-pares the NEMA LRA, efficiency, and torque requirements of Design E and Design B motors at 460 V.

Comparison of Designs E and B. ill27 compares the nominal efficiency of Designs E and B. ill28 compares the locked-rotor amperage (LRA) to full-load amperage (FLA) ratios of Designs E and B. ill29 compares the LRAs of Designs E and B. ill30 compares the locked rotor torque, breakdown torque, and pull-up torque of Design E and B motors.

TABLE8 Motor Designs Classification | Locked- rotor torque (% rated- load torque) | Breakdown torque (% rated- load torque) | Locked- rotor current (% rated- load current) | Slip, % | Typical applications | Relative efficiency Design B- normal locked- rotor torque and normal locked- rotor current Design C- high locked- rotor torque and normal locked- rotor current Design D- high locked- rotor torque and high slip Design E- IEC 34- 12 Design N locked- rotor torques and currents Fans, blowers, centrifugal pumps and compressors, motor generator sets, etc., where starting torque requirements are relatively low Conveyors, crushers, stirring machines, agitators, reciprocating pumps and compressors, etc., where starting torque under load is required High peak loads with or without flywheels such as punch presses, shears, elevators, extractors, winches, hoists, oil- well pumping and wire drawing machines Fans, blowers, centrifugal pumps and compressors, motor generator sets, etc., where starting torque requirements are relatively low

Note. Design A motor performance characteristics are similar to those for Design B, except that the locked- rotor starting current is higher than the values shown.

* Higher values are for motors having lower horsepower ratings.

TABLE 9 460- V Four- Pole, Open- Frame Design B and E Motors- Comparison of NEMA- Defined Performance Full-load amperes (FLA) per NEC Table 430- 150 | Maximum locked- rotor amperes (LRA) per NEMA MG 1, Tables 12.35 and 12.35A | LRA/ FLA ratio | Nominal full- load efficiency per NEMA MG 1, Tables 12- 10 and 12- 11 (open 4- pole), % | Efficiency ratio | Minimum locked- rotor torque per NEMA MG 1, Tables 12- 2 and 12.38.4, % | Minimum breakdown torque per NEMA MG 1, Tables 12.39.1 and 12.39.3, % | Minimum pull-up torque per NEMA MG 1, Tables 12.40.1 and 12.40.3, %

TABLE10 200- V Design B, C, D or E Motors- Branch- Circuit Short- Circuit Protection and Controller Selection

TABLE11 208- V Design B, C, D, or E Motors- Branch- Circuit Short- Circuit Protection and Controller Selection

TABLE12 230- V Design B, C, D, or E Motors- Branch- Circuit Short- Circuit Protection and Controller Selection

TABLE13 460- V Design B, C, D, or E Motors- Branch- Circuit Short- Circuit Protection and Controller Selection

TABLE14 575- V Design B, C, D, or E Motors- Branch- Circuit Short- Circuit Protection and Controller Selection

FIGURE27 Design E to Design B nominal efficiency ratio.

FIGURE28 LRA/FLA ratios of Design E (upper curve) and Design B (lower curve).

FIGURE29 Design E to Design B LRA ratio.

FIGURE30 Design E to Design B locked rotor torque (upper curve), breakdown torque (middle curve) and pull-up torque (lower curve) ratios.

4 Electrical and Mechanical Considerations Regarding the Application of Design E Motors

Modifications were made to the 1996 National Electrical Code to accommodate Design E motor applications. Article 430-7 (9) acknowledges the Design E marking on motors. Table 430-151B includes maximum locked-rotor current values for Design E motors. Table 430 152 includes branch-circuit short-circuit protective devices for Design E motors.

Motor Disconnection Means. Article 430-109 Exception 1 specifies a derating factor on motor-circuit switches that are not marked "rated for use with a Design E motor." The marking may appear on the motor-switch nameplate or in the instruction literature. If there is no marking, the motor-circuit switch "shall have a horsepower rating not less than 1.4 times the rating of a motor rated 3 through 100 horsepower, or not less than 1.3 times the rating of a motor rated over 100 horsepower." In certain cases, as shown in Tables10 through 14, a larger motor-circuit switch is required with Design E motors. The horsepower ratings of the motor-circuit switches are based on the NEMA KS 1-1990 Standard, Table 4-3.

Motor Branch-Circuit Short-Circuit Protection. Article 430-52 (3) Exception 1 allows instantaneous-trip circuit-breaker settings for Design E motors to be in the range of 1100 to 1700 percent of full-load current. Settings for Design B motors are limited to a range of 800 to 1300 per-cent of full-load current. This higher rating for Design E motors avoids nuisance tripping during start-up due to higher transient and steady-state LRA. In certain cases, as shown in Tables10 through14, a larger instantaneous-trip circuit breaker will be required to obtain the necessary current range to start Design E motors.

In the case of thermal-magnetic circuit breakers, the nonadjustable magnetic trip is typically set by the manufacturer at 1600 percent of full-load current. So to avoid nuisance tripping during start-up, thermal-magnetic circuit breakers should be chosen near the maximum (400 percent of FLA for FLA 100 A, 300 percent of FLA for FLA  100 A) allowed by Article 430-52 (c)(1)(c) of the National Electrical Code. In certain cases, as shown in Tables10 through14, a larger thermal-magnetic trip circuit breaker than the one normally recommended by the circuit-breaker manufacturer is required with Design E motors.

In the case of time-delay fuses, there is a potential danger of nuisance fuse blowing on start-up due to the higher locked-rotor currents of Design E motors. So, time-delay fuses used with Design E motors should be chosen near the maximum (225 percent of FLA ) allowed by Article 430-52 (c)(1)(b) of the National Electrical Code. In certain cases, as shown in Tables10 through14, a larger fuse rating than the one normally recommended by the fuse manufacturer is required with Design E motors.

Motor Circuit Conductor. Article 430-22 (a) specifies that "Branch-circuit conductors supplying a single motor shall have an ampacity not less than 125 percent of the motor full-load current rating." Article 430-6 (a) specifies that Table 430-150 be used to determine the motor full-load current. In Table 430-150, motor full-load cur-rents for Designs B, C, D, and E are equal. Therefore, no conductor derating is required when Design E motors are used.

Motor Controller. Article 430-83 (a) Exception 1 specifies that a controller for a Design E motor must either be marked "Rated for use with a Design E motor," or the controller horsepower rating must be derated. The marking may be on the controller nameplate or in the instruction literature. If there is no marking, the controller

"shall have a horsepower rating not less than 1.4 times the rating of a motor rated 3 through 100 horsepower, or not less than 1.3 times the rating of a motor rated over 100 horsepower." In certain cases, as shown on Tables10 through14, a larger-size controller is required with Design E motors. The horsepower rating of the controllers is based on the NEMA ICS 12-1993 Standard, Table 2-4-1 .

Motor Overload Protection. Article 430-32 specifies that overload devices are selected based on motor nameplate full-load current. Full-load currents for Design E and B motors are equal, but service factor is a consideration when selecting overload heater elements. While open-type Design A, B, and C motors are required by NEMA to have service factors of 1.15 (NEMA MG 1 Standard, Table 12-4), Design E may have a service factor of 1.0. Normal practice is to select heaters one size lower for 1.0 service factor applications. Class 20 (standard-trip) overload relays provide ample motor protection for general-purpose applications of Design B, C, D, or E motors unless specified otherwise by the motor control manufacturer. The locked-rotor cur-rent of Design E motors may necessitate an increase in overload class from Class 20 to Class 30 or may require that the heater size be increased as allowed by NEC Article 430-34 in order to start the motor. Consult with the motor control manufacturer regarding applications such as hermetic refrigerant motor compressors that may require Class 10 (fast-trip) or applications such as high-inertia ball mills, reciprocating pumps, or loaded conveyors that may require Class 30 (slow-trip) protection.

Application Example. Using the retrofit of a four-pole Design B 460-V 50-hp motor having a minimum efficiency of 93 percent with a four-pole Design E 460-V 50-hp motor having a minimum efficiency of 95.4 percent as an example, each of the protection and control requirements must be considered.

1. Switch and fuse combination. The Design B motor requires a 100-A disconnect switch. A Design E motor requires a 200-A disconnect switch unless the existing disconnect switch is marked "Rated for use with a Design E motor." A 90-A time-delay fuse is typically recommended for use with a 50-hp Design B motor. This fuse is at 138 percent of the full-load current. To avoid nuisance fuse blowing, a 125 A time-delay fuse at 192 percent of full-load current can be recommended. If the 125-A fuse is used, 200-A fuse clips are required in place of the original 100-A fuse clips used on this application.

2. Thermal-magnetic breaker. The Design B motor requires a 125-A circuit breaker. The same circuit breaker can be used with a Design E motor.

3. Magnetic circuit breaker. The Design B motor uses a 100-A magnetic circuit breaker with a typical setting range of 462 to 1538 percent of full-load current. A 150-A magnetic circuit breaker with a typical setting range of 692 to 2308 percent of full-load current is required with the 50-hp Design E motor.

4. Motor starter. The Design B motor requires a NEMA size 3 motor starter. A size 4 motor starter is required with a Design E motor unless the original size 3 motor starter is marked "Rated for use with a Design E motor."

5. Overload relay. A Class 20 (standard-trip) overload relay is typically used with the normal 1.15 service factor Design B motor. The same Class 20 overload may be used with the Design E motor, but one lower heater rating is recommended due to the 1.0 service factor.

6. Locked-rotor torque. The minimum locked-rotor torque of the 50-hp Design B motor is 140 percent of the full-load torque. The minimum locked-rotor torque of the Design E motor is 130 percent of the full-load torque. If this 7 percent reduction in torque is indicated by the motor manufacturer, it must be considered in the mechanical application of the motor.

7. Breakdown torque. The minimum breakdown torque of the 50-hp Design B motor is 200 percent of the full-load torque. The minimum breakdown torque of the Design E motor is 190 percent of the full-load torque. Such a 5 percent reduction in torque must be considered in the mechanical application of the motor.

8. Pull-up torque. The minimum pull-up torque of the 50-hp Design B motor is 100 percent of the full-load torque. The minimum pull-up torque of the Design E motor is also 100 percent of the full-load torque.

Summary. As the preceding example shows, increased locked-rotor current, and reduced locked-rotor torque, breakdown torque, and pull-up torque of Design E motors can result in costly errors in retrofit or replacement situations if all of the parameters are not thoroughly studied. If a Design E motor is being used to replace an existing general-purpose Design B motor, torque characteristics, existing protective device rating, and controller size must be considered.

5 Availability of Design E Motors

Motor manufacturers are not marketing Design E motors at this time. Among the reasons for the lack of market demand are market satisfaction with the efficiency levels of today's energy-efficient Design B motors, failure of manufacturers to offer switches and controllers rated for use with Design E motors, application issues related to the adjustment requirements of magnetic circuit breakers used with energy-efficient motors, and application issues related to the torque provided by Design E motors.

6 Conclusion

The engineering solution used to calculate the viability of Design E motors to an application must balance the energy savings with the possible increased cost of more horsepower and increased cost of protection and control equipment. Tables10 through14 give a guide as to the relative protection and control equipment requirements of NEMA Design B and E motors. The user of this information must contact the control equipment manufacturer to determine specific product requirements.

The user of this information must also verify that the suggested combination of equipment meets all of the valid safety codes. Applications using IEC (Design N) metric motors and controls have already demonstrated that the technology for control and protection of Design E motors has been proven in European applications where reduced-voltage starting is more common than in the United States . New equipment using Design E motors will comply with the protection and control standards referenced in this section. But care must be taken that the proper protection and control be applied in motor replacement situations.