UNDERSTANDING THE NEMA MOTOR-GENERATOR STANDARDS OF SECTION MG-1-1993, REVISION 3, THREE-PHASE INDUCTION MOTORS

CoDYrlaht Material IEEE Austin H. Bonnett PapeYNo. PCIC-97-24 Fellow IEEE U.S. Electrical Motors, Division of Emerson Electric St. Louis, Missouri

ABSTRACT The National Electrical Manufacturers Association (NEMA) Motor-Generator Standards (MG-1) have gone through numerous revisions and additions. Key information pertaining to motor performance, construction and operating conditions are provided for end users consideration when specifying electric motors. There still is some confusion regarding how to specify electric motors and how to interpret nameplate data on existing motors. This paper will provide the user with a condensed version of MG-1 to serve as a quick reference guide. Space will not allow any detailed analysis of the standard. Future papers could be developed on individual sections.

INTRODUCTION It is an impossible task to summarize all of the sections of MG-1 in this paper; therefore the authors have chosen the sections which contain information that is most often referred to in the application of integral horsepower (Hp) motors. Each section will also reference the applicable section in MG-1. In addition to the referenced standards, there are sections that contain suggested standards for future designs and authorized engineering information. In the development of these standards consideration was given to other standards and recommended practices developed by other societies and organizations. This summary only considers three phase integral Hp squirrel cage induction motors through 449 frame. When sections and parts are referred in this paper they are identical to the MG-1 system of numbering.

NAMEPLATE MARKINGS (NEMA 10.40) The following information shall be given on the nameplate:

a. Manufacturers type and frame designation b. Hp output c. Time rating d. Maximum ambient temperature for which

motor is designed e. Insulation system designation f. RPM at rated load g. Frequency h. Number of phases iI Rated-load amperes j. Line voltage

k.

I. m. n. 0.

P-

NOTE:

George C. Soukup, Member IEEE U S . Electrical Motors, Division of Emerson Electric St. Louis, Missouri

Locked-rotor amperes or code letter for locked-rotor kVA per Hp for motors 112 Hp or larger Design letter NEMA nominal efficiency Service factor load if other than 1 .O Service factor amps when service factor exceeds 1.1 5 For motors rated above 1 Hp equipped with over-temperature devices or systems, the words OVER TEMP PROT- followed by a type number

Section 10.39.5 and .6 provide additional information for dual voltage and frequency motors.

NOTE: IEEE 841 - A.3 requires that a corrosion resistance plate that conforms to ASTM B117-90 be used. The fasteners are usually stainless steel.

MOTOR TERMINAL MARKINGS (NEMA 2.60) The motor terminal markings are usually shown on the motor nameplate or a separate connection plate. Section 2.60 contains a variety of connections including single speed, multiple speed, part-winding start, wye- start, delta run, etc. Many of these connections can be configured either wye or delta, single or dual voltage. The phase rotation can be set on all of the options at the factory, or can be set at the installation by reversing any two phases once the direction of rotation is determined by bump starting. Tables 1 & 2 show some of the most frequently used connections for wye and delta.

TERMINAL MARKINGS AND CONNECTIONS FOR

6 LEADS

SINGLE VOLTAGE EXTERNAL

L1 L2 L3 Join 1 2 3 4 & 5 & 6

SINGLE VOLTAGE EXTERNAL DE LTA-CO N N ECTl ON

NEMA SINGLE-SPEED, THREE-PHASE MOTORS -

Y-CONNECTED

I: 3 2 5\*

L1 L2 L3 6/ T 3 - / \4 1 , 6 2, 4 3, 5 Table 1 5 2

ISBN: 0-7803-4217-8 - 225 -

97-CH36128-6/97/0000-0225 $10.00 8 1997 IEEE

TERMINAL MARKINGS AND CONNECTIONS FOR

9 LEADS

DUAL VOLTAGE Y-CO N N ECTED

Voltage L1 L2 L3 Join Low 1,7 2 , 8 3 , 9 4 & 5 & 6 High 1 2 3 4 & 7 , s & 8,6 & 9

DUAL VOLTAGE

Voltage L1 L2 L3 Join

NEMA SINGLE-SPEED, THREE-PHASE MOTORS -

DELTA- CON N ECTE D

Low I , 6 , 7 2,4, a 3 , 5 , 9 - High 1 2 3 4 & 7 , 5 & a, 6 & 9 Table 2

The motor starter sizes are governed by the voltage, Hp and enclosure. Tables 3 & 4 show the most common starter sizes and types of enclosures.

NEMA Size Starters

Type NEMA Type 1 (Indoor General Purpose)

NEMA Type 2 (Indoor DriDDrOOf)

NEMA Size

- 00 0 1 2 3 4 5 6 7

9 a

Description To prevent accidental contact of personnel with the enclosed equipment and protection against falling dirt. To exclude falling moisture and dirt.

Maximum HP Pol

NEMA Type 3 (Outdoor, Rainproof, Sleet & Ice Resistant) NEMA Type 4 (Indoor/Outdoor, Watertight/ Dusttight) NEMA Type 7 Class I, GP, A,B,C, or D (Indoor Hazardous) NEMA Type 9 Class II. GP. E.F or G

Full Voltage Starting

2OOV 230V 460V 575v

1-1/2 1-1/2 2 3 3 6

7-112 7-1/2 10 10 15 25 25 30 50 40 50 100 75 100 200 150 200 400 - 300 600 - 450 900 - 600 1500

To protect against windblown dust and water.

To protect against splashing, falling or hose directed water. (Not for submersion). For locations where combustible gases or vapors are present.

For locations where combustible dusts are Dresent.

Table 3

Auto Transf. Starting

!OOV 230V 460V 575v

- - - _ _ _ -1/2 7-1/2 10 10 15 25 20 30 50 40 50 100 75 100 200 150 200 400 - 300 600 - 450 900 - 800 1600

Ihase Motors Part Winding

Starting !OOV 230V 460V

575v - - - - - _ 10 10 15 20 25 40 40 50 75 75 75 150 150 150 350 - 300 600 - 450 900 - 700 1400 - 1300 2600

Wye-Delta Starting

200V 230V 460V 575v

- - - - _ - 10 10 15 20 25 40 40 50 75 60 75 150 150 150 300 300 350 700 500 500 1000 750 800 1500 1500 1500 3000

(Indoor Hazardous) NEMA Type 12 I Protection against flying dust, (lndoorlndustrial Use. lint and light splashing or

(Indoor Operator Stations and Pilot Devices)

12, this enclosure protects against spraying water, oil or

NOTE: Table 4 is not a NEMA MG-1 standard but may be helpful to the reader.

Table 4

FRAME DIMENSIONS (NEMA MG-1, PART 11) General purpose horizontal motors built in accordance with NEMA Standards of the same vintage will usually be interchangeable from a mounting standpoint. The center distance shaft to the base (D dimension) is determined by dividing the first two digits of the frame number by four. Table 5 provides the basic exterior motor dimensions. NEMA Standards also provide mounting dimensions for special purpose motors such as verticals and close-coupled pumps. Figure 1 shows a F-1 mounting position. For other options see NEMA Figure 4-6.

Figure 1

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Frame Dimensions (NEMA MG-1, Part 11.31 and Part 4)

Syn. RPM HP Frame

143T 145T 182T 184T 213T 215T 254T 256T 284T

284TS 286T

286TS 324T

324TS 326T

326TS 364T 364TS 365T

365TS 404T

404TS 405T

405TS

~

- -

-

-

-

- -

-

3600 1800 1200 900 ODP TEFC ODP TEFC DDP TEFC DDP TEFC

444T 444TS 445T

445TS 447T

447TS 449T 449TS

-

H 11/32 11/32 13/32 13/32 13/32 13/32 17/32 17/32 17/32 17/32 17/32 17/32 21/32 21/32 21/32 21/32 21/32 21/32 21/32

i 3/16 21/32

13/16 1311 6 13/16 1311 6 13/16 13/16 13/16 13/16 13/16 13/16 13/16

D 1 E 1 2 F 3-112 2-314 4

U BA 718 2-114 718 2-114

1-118 2-314 1-1 I8 2-314 1-318 3-112 1-318 3-112 1-518 4-114 1-518 4-114 1-718 4-314 1-518 4-314 1-718 4-314 1-518 4-314 2-118 5-114 1-718 5-114 2-118 5-114 1-718 5-114 2-318 5-718 1-718 5-718 2-318 5-718

2-718 6-518 1-718 5-718

2-118 6-518 2-718 6-518 2-118 6-518 3-318 7-1 12 2-318 7-112 3-318 7-112 2-318 7-112 3-318 7-112 2-318 7-112 3-318 7-112 2-318 7-112

1112 2 3 5

~ .~

143T 143T 145T 145T 182T 182T 184T 184T 145T 145T 145T 145T 184T 184T 213T 213T 145T 182T 182T 182T 213T 213T 215T 215T 182T 184T 184T 184T 215T 215T 254T 254T

3-112 4-112 4-112 5-114 5-114 6-114 6-114

7 7 7 7 8 8 8 8

2-112 3-118 3-1 I8 3-314 3-314 4-318

3 4-318

3

- -

-

2-314 5 3-314 4-112 3-314 5-112 4-114 5-112 4-114 7

5 8-114 5 10

5-112 9-112 5-112 9-112 5-112 11 5-112 11 6-114 10-112 6-114 10-112 6-114 12 6-114 12

5 3-112

5 3-112

7112 184T 213T 213T 213T 10 213T 215T 215T 215T 15 215T 254T 254T 254T

254T 254T 256T 256T 256T 256T 284T 284T 284T 284T 286T 286T

Table 5

20 25

FRAME ASSIGNMENTS

The Hp and synchronous speeds for three phase medium size AC motors are shown in Table 6 (NEMA Table 10-4) with the assigned frame size for HP/speeds larger than those shown, NEMA MG-13 provides frame assignments.

(NEMA MG-1, PART 10 AND MG-13)

254T 256T 256T 256T 286T 286T 324T 324T 256T 284TS 284T 284T 324T 324T 326T 326T

9 9 10 10 10 10 11 11 11 11 11 11 11 11

I I

1 1 - - I 143T 143T I 145T 145T I 182T 182T

7 12-114 7 12-114 8 12-114 8 12-114 8 13-314 8 13-314 9 14-112 9 14-112 9 16-112 9 16-112 9 20 9 20 9 25 9 25

5-510 3-112

7 4 7 4

5-114

8-114

8-114

4-112

4-112

4-112 8-114 4-112

518 518 4-114 112 112 2 314 314 5-518 112 112 2-314 314 314 5-518 112 112 2-314 718 718 6-718

718 718 6-718

718 718 6-718

518 518 3

518 518 3

518 510 3 718 718 6-718 518 518 3

\ CURRENT

30 284TS 286TS 286T 286T 40 286TS 324TS 324T 324T 50 324TS 326TS 326T 326T 60 326TS 364TS 364TS 364TS 75 364TS 365TS 365TS 365TS 100 365TS 405TS 404TS 405TS 125 404TS 444TS 405TS 444TS 150 405TS 445TS 444TS 445TS 200 444TSS - 445TS -

Table 6 Motor Current

Typically motor currents of interest are the no-load current, actual-load current, rated full-load current and service factor current. The actual-load current may range from no-load to service factor. If the service factor is 1 .O then it will also equal the full-load current, which is shown on the nameplate at rated Hp. The locked rotor currents (sometimes referred to as inrush or starting current) are shown in Table 10 and the full load values at various rated voltages are shown in Table 7. No value is normally given for the other conditions, but they can be bench marked once installed.

Figure 2 shows the typical relationship between these currents.

LOCKED ROTOR CURRENT (RMS) WIO D.C. OFF SET 6 - 8 x I

E a SERVICE P 1.2XI FACTOR CURRENT

\ FULLLOAD \ CURRENT s a I

% SPEED Typical Values of Current for a General Purpose Design 6 Motor

NOTE: There are no NEMA standards for these load points.

Figure 2

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General Purpose Design B Motor Full Load Current (National Electrical Code 1996 (NEC) Table 430 - 150)

Approx. Amps/Hp

20 - 62.1 59.4 54 27 - 22 25 - 78.2 74.8 68 34 - 27

- 2.75 2.4 1.2 .96 .24 .I4

IO-20% Down

Slightly ro u p 5% Up 10% U p l o %

Up 21%

Jp Slightly

Table 7

Occasional Excess Current (NEMA 12.48) Motors having outputs not exceeding 500 Hp (according to this part) and rated voltages not exceeding 1 kV shall be capable of withstanding a current equal to 1.5 times the full load rated current for not less than two minutes when the motor is initially at normal operating temperature.

Repeated overloads resulting in prolonged operation at winding temperatures above the maximum values given by NEMA 12.43 will result in reduced insulation life or possible damage.

3-10% Slightly UP Down

5 1 0 % Slightly

Down 10% Down 5% Down Down

10-15% Slightly Down 19% Down

Slightly Down Down

Slightly Slightly Voltage Ratings (NEMA 10.30)

The standard voltage ratings at 60 hertz three phase are 230/460/575/2300/4000 and 4600. Other options are shown in NEMA 10.30, 31.

Variations from Rated Voltage and Frequency (NEMA 12.44)

Motors shall operate successfully under running conditions at rated load with a variation in the voltage or the frequency up to the following:

a. Plus or minus 10 percent of rated voltage, with rated frequency.

b. Plus or minus 5 percent of rated frequency, with rated voltage.

c. A combined variation in voltage and frequency of 10 percent (sum of absolute values) of the rated values, provided the frequency variation does not exceed plus or minus 5 percent of rated frequency.

Performance within these voltage and frequency variations will not necessarily be in accordance with the standards established for operation at rated voltage and frequency. . .

The Electrical Apparatus Service Association (EASA) offers the following Table (8) to illustrate the effects of these variations for a typical motor.

General Effect of Voltage and Frequency Variations on Induction Motor

Characteristics

Characteristics

Starting Torque Maximum Torque Percent Slip

Efficiency , Full Load 314 Load

112 Load

Power Factor Full Load 314 Load

112 Load

Full-Load Current

Startina Current Full-Load Temoerature Rise Maximum Overload Capacity Magnetic Noise

Voltage Freqi

Down Down UD f 2% 1 0-3% I sli,gtty

0- Down Little

5-1 5%

5-1 5% 2-7% Down UD

Down UP

Slightly

Slightly

Slightly

Slightly UD

"CY 95%

up 11% ur, 11% Down 5-1 0% Down

Slightly Down

Slightly Down

Slightly Down

Slightly Down

Slightly Down

Slightly UP

Slightly

UD 5% UP

Slightly UP

Slightly UP

Slightly

Table 8

Effect of Voltage Variation on Induction Motor Characteristics (NEMA 14.35)

The effects of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a "negative sequence voltage" having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high

- 228 -

currents. A small negative-sequence voltage may produce in the windings currents considerably in excess of those present under balanced voltage conditions. The voltage unbalance in percent may be defined as follows:

With voltages of 460, 467, and 450, the average is 459, the maximum deviation from average is 9, and the percent unbalance =

100 - 9 459

x - - 1.96 percent

The locked-rotor torque and breakdown torque are decreased when the voltage is unbalanced. If the voltage unbalance is severe, the required accelerating torques might not be adequate for the application. The full-load speed is reduced slightly when the motor operates at unbalanced voltages. The locked-rotor current will be unbalanced to the same degree that the voltages are unbalanced, but the locked-rotor kVA will increase only slightly. The motor efficiency may also be significantly reduced. The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance. The performance of the machine will be effected as shown in Figure 3 and the change in loss distribution will result in higher rotor temperatures. The motor noise and vibrations may also be increased.

Medium Motor Derating Factor Due to Unbalanced Voltage

NEMA 14.35 provides the recommended derating factor shown in Figure 3 to apply to the motor load when the unbalance exceeds 1 %.

a

2 0.85 i= U ee Y

0.8

0.75

0.7 0 1 2 3 4 5

PERCENT VOLTAGE UNBALANCE

Figure 3 (NEMA Figure 14-1)

However, as stated in NEMA 12.45, for successful operation, it is recommended that the unbalance does not exceed 1 percent.

Motor Load (NEMA 14.36.1)

NEMA offers the following guidelines for the application of motors with a service factor. Motors having a service factor in accordance with NEMA 12.47 are suitable for continuous operation at rated load under the usual service conditions given in NEMA 14.02. When the voltage and frequency are maintained at the value specified on the nameplate, the motor may be overloaded up to the Hp obtained by multiplying the rated Hp by the service factor shown on the nameplate. When the motor is operated at any service factor greater than 1, it will have a reduced life expectancy compared to operating at its rated nameplate Hp. Lubrication life and bearing life can also be reduced by the service factor load.

50 Hertz Operation (NEMA 14.30)

In general, 60-hertz induction motors are not designed to operate at their 60-hertz ratings on 50-hertz power supplies, however, they are capable of being operated satisfactorily on 50-hertz power supplies if their voltage and Hp ratings are appropriately reduced to maintain a constant volts/hertz ratio. When such 60-hertz motors are operated on 50-hertz circuits, the applied voltage at 50 hertz, and the Hp load should be reduced to 5/6 of the 60-hertz Hp rating of the motor to compensate for the reduction and ventilation.

NEMA Code Letters (NEMA 10.37)

Each NEMA Code letter is a function of motor locked rotor current and is used to size switchgear and relay protection. For three phase motors the kVNHP can be calculated as follows:

Locked kVA per HP =

Locked rotor amps x line voltage

The code letters and locked rotor amps are shown in Tables 9 and 10.

When the nameplate of an alternating current motor is marked to show the locked-rotor kVA per Hp, it shall be marked with the caption Code followed by a letter selected from the table in NEMA 10.37.2.

1.73 1000 x HP

- 229 -

The letter designations for locked-rotor kVA per Hp are measured at full voltage and rated frequency are as follows:

Letter Designation

D E

kVA per HP. *

4.0 - 4.49 4.5 - 4.99

F I 5.0 - 5.59

3600 rpm -

1800 1200 900 rpm rpm rpm - - -

314 1 -

1-112 2

M I 10.0 - 11.19

- 140 50 13.4 275 175 135 60 11.9

175 250 170 135 80 10.6 170 235 165 130 100 9.9

- -

G H J

CL x HP x 1000 LRnmp = 43 x v

5.6 - 6.29 6.3 - 7.09 7.1 - 7.99

CL = Code letter kVNHP from Table 9.

3 5

7-112 10

The highest kVA/Hp is to be used unless the actual current is known.

Table 10 also provides a quick reference of torques, currents and kVA/HP for Design B motors. There are no limits specified for Design A motors, however; the torque values are the same as Design B motors.

1

160 215 160 130 128 8.6 150 185 155 130 184 7.3

254 6.7 140 175 150 125 135 165 150 125 324 6.5

Service Conditions (NEMA 14.02 and 14.03)

K

Usual Conditions: This section defines the usual ambient range from -1 5C to 40C with an altitude not to exceed 33,000 feet. The motor is to be installed on a rigid mounting surface with no significant ventilation restrictions. Motors will be designed to these conditions unless specified as unusual severe conditions.

8.0 - 8.99

Locked Rotor Torque, Current & kVA/HP for Polyphase, Squirrel-Cage Motor Design B

L

Rated H.P.

112

9.0 - 9.99

N

*Locked Rotor Current (Amps)

60 Cyl. -460V

11.2 - 12.49

40

P

*Locked kVA/HP

60 Cycle

12.5 - 13.99

100 125

105 125 125 125 2900 5.8 100 110 125 120 3630 5.8

150 I100 I 110 200 I100 I 100

120 120 I 4340 5.8 120 120 I 5800 5.8

* Current

NOTE: The values in the previous tables are rms symmetrical values, i.e. average of the three phases. There will be an one-half cycle instantaneous peak value which may range from 1.8 to 2.8 times the above values as a function of the motor design and switching angle. This is based upon an ambient temperature of 25C. For other designations, see NEMA Part 12.

Table 10

Unusual Conditions: May include exposure to severe ambient or environmental conditions, extreme mechanical loading on the motor, unusual power supply conditions, overspeed requirements, operations that subject the motor to severe dynamic loading, etc.

- 230 -

MOTOR STARTING CONDITIONS (NEMA12.44.2)

Medium motors shall start and accelerate to running speed a load which has a torque characteristic and an inertia value not exceeding that listed in NEMA 12.54 and in Table 11 with the voltage and frequency variations specified in NEMA 12.44.1.

The limiting values of voltage and frequency under which a motor will successfully start and accelerate to running speed depend on the margin between the speed-torque curve of the motor at rated voltage and frequency and the speed-torque curve of the load under starting conditions. Since the torque developed by the motor at any speed is approximately proportional to the square of the voltage and inversely proportional to the square of the frequency, it is generally desirable to determine what voltage and frequency variations will actually occur at each installation, taking into account any voltage drop resulting from the starting current drawn by the motor. This information and the torque requirements of the driven machine define the motor-speed-torque curve, at rated voltage and frequency, which is adequate for the application.

HP 1

2 1-112

Motor Inertia Limits (Loadwk2) Ib. - ft.2 NEMA Table (12 - 5)

3600 1800 1200 900 720 600 514 - 5.8 15 31 53 82 118

2.4 1 1 30 60 102 158 228 1.8 8.6 23 45 77 120 174

5

10 7-112

3 I 3.5 I 17 I 44 I 87 I 149 I 231 I 335 5.7 27 71 142 242 375 544

11 51 137 273 467 723 1048 8.3 39 104 208 356 551 798

~

25 30 40

I I

15 I 16 1 75 I 200 I 400 I 6 8 5 I 1061 I 1538 20 I 21 I 99 1 262 I 525 I 898 I 1393 I 2018

26 122 324 647, 1108 1719 2491 31 144 384 769 1316 2042 2959 40 189 503 1007 1725 2677 3881

Table 11

Stall Time (NEMA 12.49)

Motors having outputs not exceeding 500 Hp and rated voltage not exceeding 1kV shall be capable of withstanding locked-rotor current for not less than 12 seconds when the motor is initially at normal operating temperatures.

Motors specially designed for inertia loads greater than those in NEMA Table 12-5 shall be marked on the nameplate with the permissible stall time in seconds (see NEMA 10.40).

Number of Starts (NEMA 12.54)

Motors having Hp ratings given in NEMA Table 10-4 and performance characteristics in accordance with this Part 12 shall be capable of accelerating without injurious heating load Wk2 referred to the motor shaft equal to or less than the values listed in NEMA Table 12-5 under the following conditions.

a.

b.

C.

Applied voltage and frequency in accordance with NEMA 12.44.

During the accelerating period, the connected load torque is equal to or less than a torque which varies as the square of the speed and is equal to 100 percent of rated-load torque at rated speed.

Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature.

If the starting conditions are other than those stated in NEMA 12.51.1, which relates to rated voltage and frequency, the motor manufacturer should be consulted.

NEMA 12.54.3 When additional starts are required, it is recommended that none be made until all conditions affecting operation have been thoroughly investigated and the apparatus examined for evidence of excessive heating. The number of starts should be kept to a minimum since the life of the motor is affected by this severe stress.

-231 -

Motor Design Letter (NEMA MG-1, Part 12)

The motor nameplate provides the user with a design letter that characterizes the speed torque curve and associated starting current.

Figures 4 and 5 show these curves for typical (NEMA does not provide curves) Designs A, B, C, D & E motors. For a complete listing of current and torque values, refer to MG1-12. In Table 10, these values are given for Designs A & B motors.

300

g 250 a 0 E 200 0 U 5 150 -1 -1

z 100 s

50

0 0 10 20 30 40 50 60 70 80 90 100

% SYNCHRONOUS SPEED

Figure 4

300

250 3 0 E

0 4

e 200 3 150

100

50

0

-1 -1

s

0 10 20 30 40 50 60 70 80 90 100

%SYNCHRONOUS SPEEO

Figure 5

Pull-up Torque (NEMA 12.40)

The pull-irp torque of an alternating current motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed. For values of pull up and breakdown torque in % refer to NEMA, Part 12. As a general rule, all of these torque values are a function of the line voltage squared. Hence, if there is a significant voltage dip during starting, then accelerating torque is affected by at least the square of the voltage.

Motor Winding Temperature Rise (NEMA 12.43)

The allowable temperature rise is determined by the class of insulation, the ambient temperature, the service factor and the operating altitude.

The values in Table 12 are given at rated voltage and frequency. When Class F or H insulation systems are used, special consideration should be given to bearing temperatures and lubricants. The cumulative effect of the temperatures is shown in Figure 6.

Winding Temperature

INSULATION CLASS LIMIT

ALLOWABLE AVG. RISE

AF ALTITUDE FACTOR

SERVICE FACTOR

A AMBIENT

RISE = IC - ( A + SF + A F ) Figure 6

Breakdown Torque (NEMA 12.39)

The breakdown torque of a motor is the maximum torque which it will develop with rated voltage applied at rated frequency, without an abrupt, drop in speed. It is the point at which the motor will no longer carry the load.

- 232 -

Temperature Rise Vs. Insulation Class for Medium Sized Motors (NEMA 12.43)

20 25

Class of Insulation System

Time Rating (shall be continuous or any short-time rating given in 10.36)

Temperature Rise (based on a maximum ambient temperature of 40(C) Degrees C

90.2 88.5 91.0 89.5 91.0 89.5 91.0 89.5 91.7 90.2 91.7 90.2

Windings, by resistance method

1. Motors with 1 .O service factor other than those given in items a.3 and a.4

2. All motors with 1.15 or higher service factor

3. Totally-enclosed nonventilated motors with 1 .O service factor

4. Motors with encapsulated windings and with 1 .O service factor, all enclosures

The temperatures attained by cores, squirrel-cage windings, and miscellaneous parts shall not injure the insulation or the machine in any respect.

3

7.5 5

A

60

70

65

65

I

85.5 82.5 87.5 85.5 87.0 85.5

88.5 86.5 89.5 87.5 89.5 87.5 87.5 85.5 87.5 85.5 87.5 85.5

B F H

10 15 20 25 30 40

80 105 125

1

89.5 87.5 89.5 87.5 89.5 87.5 90.2 88.5 91.0 89.5 90.2 88.5 90.2 88.5 91.0 89.5 90.2 88.5 91.0 89.5 92.4 91.0 91.7 90.2 91.0 89.5 92.4 91.0 91.7 90.2 91.7 90.2 93.0 91.7 93.0 91.7

90 115 -

85 110 130

85 110 -

Table 12

Energy Efficient Motors (NEMA 12.58)

NEMA has established both nominal and minimum levels of efficiency for general purpose energy efficient motors when operating at full load. These values shown in Tables 13 & 14 are valid only at rated load, voltage, frequency and altitude and will vary somewhat as these values vary from their nominals. The Energy Policy Act (EPAC) of 1992 states that these levels will become mandatory after October 24, 1997 for general purpose ODP & TEFC 2,4 & 6 pole, Designs A & B motors through 200 Hp.

Full Load Motor Efficiencies (%)

Open Drip Proof Motors

I 2 Pole I 4 Pole 1 6 Pole

Table 13 (NEMA 12-10)

Enclosed Motors

Table 14 (NEMA 12-10)

- 233 -

Mechanical Vibration (NEMA, Part 7)

This standard applies to all NEMA motors (both horizontal and vertical), but is typically limited to 3600 rpm operating speed. These limits were established for motors when mounted on elastic pads (or suspended on a spring). For a rigid mount refer to NEMA 7.06.2. The bearing housing vibration is the peak value of the unfiltered vibration velocity in inches per second.

Vibration Limit (in/sec)

0.15

0.08

0.04

0.02 0.01

1 v I

Machine Type-General Examples

Standard industrial motors Motors for commercial/residential use Machine tool motors Mediumlarge motors with special requirements Grinding wheel motors Small motors with special requirements Precision spindle and grinder motors Precision motors with special requirements

Figure 7

Speed, rpm 3600 1800 1200 900 720 600 Standard Machines

(NEMA 7.08.1) Unfiltered vibration shall not exceed the velocity levels as shown in the top curve of Figure 8.

Rotational Velocity, in/s peak

60 0.15 (3.8) 30 0.15 (3.8) 20 0.15 (3.8) 15 0.12 (3.0) 12 0.09 (2.3) 10 0.08 (2.0)

Frequency, Hz (mmls)

Y

z n e z t

0 W

- * 0 3 2

K

> m

1 10 100 1000 10000 FREQUENCY, HZ

Figure 8

The limits at rotational frequency are as shown in Table 15.

Special Machines (NEMA 7.08.2)

For machines requiring vibration levels lower than those given in Figure 8 at limit 0.15 (standard machines), the vibration levels shall not exceed the limits shown by the various curves of Figure 8. Machines to which these lower levels apply (e.g., 0.08, 0.04, 0.02 or 0.01) shall be by agreement between manufacturer and purchaser.

NOTE - It is not practical to achieve all vibration limits in Figure 8 for all machine types in all sizes.

Limits of Relative Shaft Vibration (NEMA 7.09)

The shaft vibration limits are measured by non- contacting proximity probes. These probes are sensitive to mechanical and magnetic anomalies of the s h a f t o r on the surface of the shaft to which it responds. This is commonly referred to as electrical and mechanical probe-trace runout. The combined electrical and mechanical runout of the shaft shall not exceed 0.0005 inch peak-to-peak 6.4 pm (peak-to-peak) or 25 percent of the vibration displacement limit, whichever is greater. Caution must be exercised to use probes capable of the required sensi t ivi ty.

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Standard Machines (NEMA 7.09.1)

When specified, the limits for the relative shaft vibration of rigidly mounted, standard machines with sleeve bearings, inclusive of electrical and mechanical runout, shall not exceed the limits in Table 15.

3600 4 800

Special Machines (NEMA 7.09.2)

Special machines requiring lower relative shaft vibration levels than shown in Table 15 shall not exceed the limits in Table 16 which are inclusive of electrical and mechanical runout.

Limits for the Unfiltered Maximum Relative Shaft Displacement (Sp-p) for Standard Machines

0.0028 inches (70 Vm) 0.0035 inches (90 Vm)

Speed, rPm

Speed, rpm

Maximum Relative Shaft Displacement

(Peak-to-Peak)

Maximum Relative Shaft Displacement

3600

Table 15 (NEMA Table 7-2)

0.0020 inches (50 pm)

51 200 0.0030 inches (76 urn) ~

Table 16 (NEMA Table 7-3)

Routine Tests for Polyphase Medium Induction Motors (NEMA 12.55)

The method of testing polyphase induction motors shall be in accordance with IEEE Std. 112.

Typical tests which may be made on motors completely assembled in the factory and furnished with shaft and complete set of bearings are as follows.

Measurement of winding resistance.

No-load readings of current and speed at normal voltage and frequency. On 50 hertz motors, these readings may be taken at 60 hertz.

Current input at rated frequency with rotor at stand-still for squirrel-cage motors. This may be taken single-phase or polyphase at rated or reduced voltage. (When this test is made single- phase, the polyphase values of a duplicate machine should be given in any report.) On 50 hertz motors, these readings may be taken at 60 hertz.

Efficiency (NEMA 12.58)

Efficiency test values to demonstrate compliance with the EPAC standards should be done in accordance with IEEE 112, Method B. Tables 13 and 14 provide the nominal and minimum acceptable requirements by EPAC 92.

High Potential Test The high potential tests consist of the application of a voltage higher than the rated voltage for a specified time for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions. IEEE 112 outlines this procedure. The motor being tested shall be completely assembled (see NEMA 3.01.10). The test voltage shall be applied when, and only when, the machine is in good condition and the insulation resistance is not impaired due to dirt or moisture. (See IEEE Std 43) The specified high-potential test voltage shall be applied continuously for 1 minute. Machines for which the specified test voltage is 2500 volts or less shall be permitted to be tested for 1 second at a voltage which is 1.2 times the specified 1 minute test voltage as an alternative to the 1 minute test, if desired. To avoid excessive stressing of the insulation, repeated application of the high-potential test voltage is not recommended. Additional factory tests should have the voltage reduced to 85% or less of the factory original test, and the on site test should be limited to 75%.

The testing of accessories and components is outlined in NEMA 3.01.8. CAUTION: After the test is completed, care must be taken to discharge the winding to avoid electrical shock.

v, = (2 x v,, + 1000) x -75

Machine with Sealed Windings - Conformance Tests (NEMA 20.49)

An alternating-current squirrel-cage machine with sealed windings shall be capable of passing the following tests.

Test for Stator Which Can Be Submerged After the stator winding is completed, join all leads together leaving enough length to avoid creepage to terminals and perform the following tests in the sequence indicated.

a. The sealed stator shall be tested while all insulated parts are submerged in a tank of water containing a wetting agent. The wetting agent shall be non-ionic and shall be added in a proportion sufficient to reduce the surface tension of water to a value of 31 dyn/cm (3.1 pN/m) or less at 25C.

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Using 500 volts direct current, take a 10-minute insulation resistance measurement. The insulation resistance value shall not be less than the minimum recommended in IEEE Std 43. (Insulation resistance in megohms 2 machine rated kilovolts plus 1 .)

Subject the winding to a 60-hertz high potential test of 1 .I5 times the rated line-to-line rms voltage for 1 minute. Water must be at ground potential during this test.

Using 500 volts direct current, take a 1 minute insulation resistance measurement. The insulation, resistance value shall be not less than the minimum recommended in IEEE Std 43. (Insulation resistance in megohms 2 machine rated kilovolts plus 1).

, Remove winding from water, rinse if necessary, dry, and apply other tests as may be required.

Sound Measurement (NEMA 12.53.3)

HP 200 and smaller

250-500, incl.

Machine sound should be measured in accordance with IEEE Std 85.

Sound tests should be taken with the induction machine operating as a motor at no load so that the sound can be isolated from other sound sources. The no-load sound power levels generally do not exceed the values shown in NEMA 12.53.3.

It should be recognized that decibel readings are not exact and are subject to many external influences. For further information see NEMA Standards Publication No. MG 3.

Overspeed, Percent Synchronous of Synchronous Speed, Rpm Speed

1801 and over 25 1201 to 1800 25

1200 and below 50 1801 and over 20

1800 and below 25

Constant Speed Motors Used on Sinusoidal Bus with Harmonic Content (NEMA, Part 30)

With an ever increasing amount of harmonic content seen on the motor sinusoidal bus, because of the common use of variable voltage and variable frequency controls, it has become necessary to establish a standard which addresses these conditions. NEMA Parts 30 & 31 establishes a derating factor and defines the voltage wave form for two withstand levels. Proposed Derating for Harmonic Content of Standard Motors Operating on Sinewave Power with Harmonic Content (NEMA 30.01.2)

Harmonic currents are introduced when the line voltages applied to a polyphase induction motor include voltage components at frequencies other than nominal (fundamental) frequency of the supply. Consequently, the temperature rise of the motor operating at a particular load and per unit voltage harmonic factor will be greater than that for the motor operating under the same conditions with only voltage at the fundamental frequency applied.

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When a motor is operated at its rated conditions and the voltage applied to the motor consists of components at frequencies other than the nominal frequency, the rated Hp of the motor should be multiplied by the factor shown in Figure 9 to reduce the possibility of damage to the motor. This curve is developed under the assumption that only harmonics equal to odd multiples (except those divisible by three) of the fundamental frequency are present. It is assumed that any voltage unbalance or any even harmonics, or both, present in the voltage are negligible. This derating curve is not intended to apply when the motor is operated at other than its rated frequency nor when operated from a variable voltage or a variable frequency power supply, or both.

0 0.02 0.04 0.06 0.08 0.10 0.1:

HARMONIC VOLTAGE FACTOR (HVF)

Figure 9 (NEMA 30-1)

Overspeed for Squirrel Cage Induction Motors (NEMA 12.52.1 & 31.40.3.5)

For motors that have application to NEMA Part 30, Table 17 provides the overspeed limitation not to exceed two minutes. For higher speeds consult the manufacturer.

Table 17

-

INVERTER FED MOTORS Voltage Stress for Standard Motors

(NEMA 30.02.2.9) When operated under usual service conditions, the following limit values at the motor terminals should be observed.

- Motors with base rating voltage 5 600 volts:

Rise time 2 2 p e c

- Motors with base rating > 600 volts:

Rise time 2 .1 p e c

Vpeak 5 1 kV

Vpeak S 2.5 PU

Where Vpeak is single amplitude and one pu is peak of the line-to-ground rated base voltage

( lpu = 42 V,,/43).

See Figure 10 for a typical voltage response at the motor terminals for an illustration of Vpeak and rise time.

Typical Voltage Response at Motor Terminals

;i 100% - d 90% - 3 >

Vpeak f i STEADY-STATE

- - I I dV AV I dt - A I I 10%

1 A I k RISE TIME TIME

Figure 10 (NEMA 30-5)

Maximum Voltage Stress for Definite Purpose Inverter-Fed Motors (Part 31)

Inverters used to supply adjustable frequency power to induction motors do not produce sinusoidal output voltage waveforms. In addition to lower order harmonics, these waveforms also have superimposed on them steep-fronted, single-amplitude voltage spikes. Turn-to-turn, phase-to-phase, and ground insulation of stator windings are subjected to the resulting dielectric stresses. Suitable precautions should be taken in the design of drive systems to minimize the magnitude of these spikes.

When operated under usual service conditions stator winding insulation systems for definite purpose inverter fed motors shall be designed to operate within the following limits at the motor terminals.

Motors with base rating voltages I600 volts: Vpeak I 1600 Volts Rise time 2 0.1 ps

Motors with base rating voltage > 600 volts: Vpeak 5 2.5 pu Rise time 2 1 1s

Where Vpeak is single amplitude and 1 pu is the peak of the line-to-ground voltage at the maximum operating speed point.

CONCLUSIONS

The NEMA Standards associated with integral AC induction motors are several hundred pages in all. This paper is an attempt to summarize only those sections most often referred to when specifying and applying motors to general purpose applications. Appendix I provides the reader with a summary of categories and references the appropriate NEMA sections to the paper. There still may be times when it will be necessary to refer to the original NEMA MG-1 document. This standard can be purchased from the following address: NEMA, 1300 North 17th Street, Suite 1847, Rosslyn, VA 22209. The authors wish to thank NEMA for their assistance in preparing this paper and their unceasing effort to assure that this standard is a living document. By the end of 1997, NEMA plans to issue a more comprehensive form of a condensed version of MG-1.

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APPENDIX I Paper Outline

Page Number I Description I NEMA Section

4 4

Nameplate Markings NEMA MG-1 10.40 Terminal Markings NEMA MG-I 2.60 NEMA Size Starters NEMA Electrical Enclosure

.. Full Load Current NEC Table 430-1 50

12.44, 14.35 Voltage NEMA MG-I

3 I Frame Dimensions I NEMA MG-I 11 3 I Frame Assianments I NEMA MG-1 10

5

6 7

8 9

Impact of Voltage/ Freq. Variation Code Letter NEMA MG-1 10.37 Starting NEMA MG-I

12.44, 54 NEMA MG-1 12 Design Letter and Torque

Wind i na Tem De rat u re NEMA MG-1 12.43

Drive Multipl ier Flat Belt

APPENDIX I1 Bearings

NOTE: That in NEMA MG-1 bearing sizes or types have not been standardized because each manufacturer has developed their own unique bearing support system which may vary. However, NEMA 14.07.2 offers the following information. 14.07.2: Minimum Pitch Diameter for Drives Other than V- Belt.

To obtain the minimum pitch diameters for flat-belt, timing-belt, chain, and gear drives, the multiplier given in the following table should be applied to the narrow V-belt sheave pitch diameters in 14.41.

1.33

Chain Sprocket Spur Gear Helical Gear

Timing Belt** I 0.9 0.7 0.75 0.85 9

10 11

*The above multiplier is intended for use with conventional single-ply flat belts. When other than single-ply fiat belts are used, the use of a larger multiplier is recommended. *It is often necessary to install timing belts with a snug fit. However, tension should be no more than that necessary to avoid belt slap or tooth jumping.

Motor Efficiency NEMA 12-10 Vi bration NEMA MG-I 7 Testing NEMA MG-I

12.55. 20.49

REFERENCES

13 14

[I] NEMA Standards Publicaiion No. MGI-1993 Motors and Generators, Revision No. 3 1996

[2] NEMA Standards Publication No. 250-1 985, Enclosures for Electrical Equipment (1 000 Volts Maxi mum)

[3] NEMA Standards Publication No. MG2, Safetv Standards for Construction and Guide for Selection, Installation, and Use of Electric Motors and Generators

[4] NEMA Standards Publication No. MG10, Energv Manaaement Guide for Selection and Use of Fixed Freauencv AC Sauirrel-Caae PolvDhase Induction Motors

[5] NEMA Standards Publication No. 13-1984, Frame Assianments for Alternating Current Integral Hp Induction Motors

[6] NEMA Standards Publication No. 3, Sound Level Prediction for Installed Rotatina Electrical Machines

Harmonics NEMA MG-1 30 Inverter Applications NEMA MG-1 30, 31

The following referenced Standards were adapted, in whole or part by NEMA and are applicable to MGI.

Institute of Electrical and Electronics Engineers (IEEE)

445 Hoes Lane Piscataway, NJ 08855-1331

IEEE Std 85-1 973 Test Procedure for Airborne

Rotating Electric Machinery

Standard Test Procedure for Evaluation of Systems of Insulating Materials for Random- Wound AC Electric Machinery

(RI 980) E Sound Measurements on

IEEE Std 112-1991

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