Industrial Motor Control By Stephen L. Herman 2x5f5b

378

Chapter 47 Motor Installation

Table 430.247 Full-Load Current in Amperes, Direct-Current Motors The following values of full-load currents* are for motors running at base speed. Armature Voltage Rating* Horsepower

90 Volts

120 Volts

180 Volts

240 Volts

500 Volts

550 Volts

4.0 5.2 6.8 9.6 12.2 — — — — —

3.1 4.1 5.4 7.6 9.5 13.2 17 25 40 58

2.0 2.6 3.4 4.8 6.1 8.3 10.8 16 27 —

1.6 2.0 2.7 3.8 4.7 6.6 8.5 12.2 20 29

— — — — — — — — — 13.6

— — — — — — — — — 12.2

10 15 20 25 30 40

— — — — — —

76 — — — — —

— — — — — —

38 55 72 89 106 140

18 27 34 43 51 67

16 24 31 38 46 61

50 60 75 100 125 150 200

— — — — — — —

— — — — — — —

— — — — — — —

173 206 255 341 425 506 675

83 99 123 164 205 246 330

75 90 111 148 185 222 294

⁄4 ⁄3 1 ⁄2 3 ⁄4 1 11⁄2 2 3 5 71⁄2 1 1

*These are average dc quantities.

Figure 47– 1 Table 430.247 is used to determine the full load current for direct current motors. (Reprinted with permission from NFPA 70™, National Electrical Code®, Copyright© 2007, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented by the standard in its entirety.)

shows that a 1 horsepower motor will have a full-load current of 12.2 amperes when connected to 90 volts DC. If a 1 horsepower motor is designed to be connected to 240 volts, it will have a current draw of 4.7 amperes.

Single-Phase AC Motors The current ratings for single-phase AC motors are given in Table 430.248. Particular attention should be paid to the statement preceding the table. The statement asserts that the values listed in this table are for motors that operate under normal speeds and torques. Motors especially designed for low speed and high torque, or multispeed motors, should have their running current determined from the nameplate rating of the motor. The voltages listed in the table are 115, 200, 208, and 230. The last sentence of the preceding statement

says that the currents listed shall be permitted for voltages of 110 to 120 volts and 220 to 240 volts. This means that if the motor is connected to a 120 volt line, it is permissible to use the currents listed in the 115 volt column. If the motor is connected to a 220 volt line, the 230 volt column can be used.

EXAMPLE: A 3 horsepower single-phase AC motor is connected to a 208 volt line. What will be the full load running current of this motor? Locate 3 horsepower in the far left-hand column. Follow across to the 208 volt column. The full load current will be 18.7 amperes.