Effects of Oversizing or Undersizing an Electric Motor for a Job

FLUID POWER - Design Data Sheet 49

Motor information given here pertains only to 3-phase squirrel cage induction motors, Design B, the type used most often to drive hydraulic pumps.

Our suggestions for limits on overloading electric motors were given in Data Sheet 2. Information in this issue considers other factors which may be pertinent to motor loading.

Optimum results are obtained if motor HP rating very closely matches load HP requirement, being neither too far oversized nor undersized. Some effects of mismatch are:

Oversized Motor. Using a 20 HP motor on a system which requires only 10 HP, for example, will give good results as far as running the system is concerned, but will consume more electric power than a 10 HP motor and will cause the power factor of the electric system in the plant to become poorer, especially during periods when the motor is idling. Idling current of a 20 HP motor is about half the full load current of a 10 HP motor, so a great deal of power is wasted during periods in the cycle when the motor is idling.

Undersized Motor. Using a 20 HP motor on a system which requires 25 HP for peaks is quite possible, but during overload periods the line current of such a motor may be about twice the line current of a 25 HP motor. Again, there may be a high waste of power during peak times in the cycle. But the smaller motor may save considerable power during periods it is idling or working at less than full rating.

In view of the above factors, on a hydraulic system where peak consumption occurs less than 10% of total running time, it may be good design to use an undersize motor provided it will not be overloaded by more than 25%.

Power companies charge for power by the number of kilowatt-hours used. But in some plants, the rate at which each kilowatt-hour is charged is based on the maximum current flow occurring during the metering period. A poor power factor within the plant increases the line current (and the rate) without consumption of more power. Therefore, either oversizing or undersizing of electric motors can increase the cost of electric power to the plant.

Measuring HP Load on an Electric Motor
The line current can be measured to see whether a motor is working at less or at more than its full load rating. Measurements are made with a loop ammeter without disconnecting any wires. The ammeter loop is clipped around any one of the 3-phase feeder wires any place in the circuit, in the motor junction box or in the motor starter or fuse box. The current reading is compared to nameplate current rating. Since line current is not exactly proportional to HP, the chart in the opposite column may be used to estimate how nearly the motor is operating at its full load rating.

Values in the body of the chart are the amps which the motor will draw at various percentages of full loading. F.L. (full load) amps will vary slightly between brands. Amps are shown at 230 volts, and for 440 volt operation will be about half the values shown.

After measuring motor current, interpolate between values in the chart to estimate per cent of full loading.

 

 Illustration 49_1

Measure Motor Line Current With a Loop Ammeter.


Example: Suppose the measured current of a 40 HP, 1800 RPM motor was 55 amps. Interpolation between the 63.1 amps shown for 60% loading and the 47.4 amps shown for 40% loading will indicate that the motor is operating at about 50% of its full load capability. This same machine could be run with less current and with a better power factor with a 20 HP motor. And, if the 55 amps was peak current which occurred no more than 10% of total running time, a 15 HP motor would probably do the job.

 

Amperage/Loading Chart for Design B Induction Motors
Amperage values are shown in the body of the chart

HP RPM Frame F.L. Amps 80% Load 60% Load 40% Load 20% Load 0% Load
20 1,200 286T 52.8 41.2 33.0 24.8 16.5 8.24
20 1,800 256T 51.2 40.0 32.0 24.0 16.0 7.99
20 3,600 254T 50.4 39.7 31.5 23.6 15.8 7.86
25 1,200 324T 65.6 51.2 41.0 30.8 20.5 10.2
25 1,800 284T 64.8 50.6 40.5 30.4 20.3 10.1
25 3,600 256T 60.8 47.5 38.0 28.5 19.0 9.48
30 1,200 326T 78.8 61.5 49.3 37.0 24.7 12.3
30 1,800 286T 75.6 59.0 47.3 35.5 23.7 11.8
30 3,600 284T 73.7 57.6 46.1 34.6 23.1 11.5
40 1,200 364T 102 79.7 63.8 47.8 31.9 15.9
40 1,800 324T 101 78.9 63.1 47.4 31.6 15.8
40 3,600 286T 96.4 75.3 60.3 45.2 30.2 15.0
50 1,200 365T 126 98.4 78.8 59.1 39.4 19.7
50 1,800 326T 124 96.8 77.5 58.2 38.8 19.3
60 1,200 404T 150 117 93.8 70.4 47.0 23.4
60 1,800 364T 149 116 93.1 69.9 46.0 23.2
75 1,200 405T 184 144 115 86.3 57.6 28.7
75 1,800 365T 183 143 114 85.8 57.3 28.5
100 1,200 444T 239 187 149 112 74.8 37.3
100 1,800 404T 236 184 148 111 73.9 36.8

 

DECIMAL EQUIVALENTS
of Fractional and Letter Size Drills, Common Fractions, and Metric Sizes

Size Decimal
Inches
  Size Decimal
Inches
  Size Decimal
Inches
  Size Decimal
Inches
  Size Decimal
Inches
97 0.0059 1.35 mm 0.0531 26 0.1470 F 0.2570 Z 0.4130
96 0.0063 54 0.0550 3.75 mm 0.1476 6.6 mm 0.2598 10.5 mm 0.4134
95 0.0067 1.4 mm 0.0551 25 0.1495 G 0.2610 27/64 0.4219
94 0.0071 1.45 mm 0.0571 3.8 mm 0.1496 6.7 mm 0.2638 11 mm 0.4331
93 0.0075 1.55 mm 0.0591 24 0.1520 17/64 0.2656 7/16 0.4375
92 0.0079 53 0.0595 3.9 mm 0.1535 6.75 mm 0.2657 11.5 mm 0.4528
0.2 mm 0.0079 1.55 mm 0.0610 23 0.1540 H 0.2660 29/64 0.4531
91 0.0083 1/16 0.0625 5/32 0.1562 6.8 mm 0.2677 15/32 0.4688
90 0.0087 1.6 mm 0.0630 22 0.1570 6.9 mm 0.2717 12 mm 0.4724
0.22 mm 0.0087 52 0.0635 4 mm 0.1575 I 0.2720 31/64 0.4844
89 0.0091 1.65 mm 0.0650 21 0.1590 7 mm 0.2756 12.5 mm 0.4921
88 0.0095 1.7 mm 0.0669 20 0.1610 J 0.2770 1/2 0.5000
0.25 mm 0.0098 51 0.0670 4.1 mm 0.1614 7.1 mm 0.2795 13 mm 0.5118
87 0.0100 1.75 mm 0.0689 4.2 mm 0.1654 K 0.2810 33/64 0.5156
86 0.0105 50 0.0700 19 0.1660 9/32 0.2812 17/32 0.5312
85 0.0110 1.8 mm 0.0709 4.25 mm 0.1673 7.2 mm 0.2835 13.5 mm 0.5315
0.28 mm 0.0110 1.85 0.0728 4.3 mm 0.1693 7.25 mm 0.2854 35/64 0.5469
84 0.0115 49 0.0730 18 0.1695 7.3 mm 0.2874 14 mm 0.5512
0.3 mm 0.0118 1.9 mm 0.0748 11/64 0.1719 L 0.2900 9/16 0.5625
83 0.0120 48 0.0760 17 0.1730 7.4 mm 0.2913 14.5 mm 0.5709
82 0.0125 1.95 mm 0.0768 4.4 mm 0.1732 M 0.2950 37/64 0.5781
0.32 mm 0.0126 5/64 0.0781 16 0.1770 7.5 mm 0.2953 15 mm 0.5906
81 0.0130 47 0.0785 4.5 mm 0.1772 19/64 0.2969 19/32 0.5938
80 0.0135 2 mm 0.0787 15 0.1800 7.6 mm 0.2992 39/64 0.6094
0.35 mm 0.0138 2.05 mm 0.0807 4.6 mm 0.1811 N 0.3020 15.5 mm 0.6102
79 0.0145 46 0.0810 14 0.1820 7.7 mm 0.3031 5/8 0.6250
1/64 0.0156 45 0.0820 13 0.1850 7.75 mm 0.3051 16 mm 0.6299
0.4 mm 0.0158 2.1 mm 0.0827 4.7 mm 0.1850 7.8 mm 0.3071 41/64 0.6406
78 0.0160 2.15 mm 0.0846 4.75 mm 0.1870 7.9 mm 0.3110 16.5 mm 0.6496
0.45 mm 0.0177 44 0.0860 3/16 0.1875 5/16 0.3125 21/32 0.6562
77 0.0180 2.2 mm 0.0866 4.8 mm 0.1890 8 mm 0.3150 17 mm 0.6693
.5 mm 0.0197 2.25 mm 0.0886 12 0.1890 O 0.3160 43/64 0.6719
76 0.0200 43 0.0890 11 0.1910 8.1 mm 0.3189 11/16 0.6875
75 0.0210 2.3 mm 0.0906 4.9 mm 0.1929 8.2 mm 0.3228 17.5 mm 0.6890
0.55 mm 0.0217 2.35 mm 0.0925 10 0.1935 P 0.3230 45/64 0.7031
74 0.0225 42 0.0935 9 0.1960 8.25 mm 0.3248 18 mm 0.7087
0.6 mm 0.0236 3/32 0.0938 5 mm 0.1968 8.3 mm 0.3268 23/32 0.7188
73 0.0240 2.4 mm 0.0945 8 0.1990 21/64 0.3281 18.5 mm 0.7283
72 0.0250 41 0.0960 5.1 mm 0.2008 8.4 mm 0.3307 47/64 0.7344
0.65 mm 0.0256 2.45 mm 0.0965 7 0.2010 Q 0.3320 19 mm 0.7480
71 0.0260 40 0.0980 13/64 0.2031 8.5 mm 0.3346 3/4 0.7500
0.7 mm 0.0276 2.5 mm 0.0984 6 0.2040 8.6 mm 0.3386 49/64 0.7656
70 0.0280 39 0.0995 5.2 mm 0.2047 R 0.3390 19.5 mm 0.7677
69 0.0292 38 0.1015 5 0.2055 8.7 mm 0.3425 25/32 0.7812
0.75 mm 0.0295 2.6 mm 0.1024 5.25 mm 0.2067 11/32 0.3438 20 mm 0.7874
68 0.0310 37 0.1040 5.3 mm 0.2087 8.75 mm 0.3445 51/64 0.7969
1/32 0.0312 2.7 mm 0.1063 4 0.2090 8.8 mm 0.3465 20.5 mm 0.8071
0.8 mm 0.0315 36 0.1065 5.4 mm 0.2126 S 0.3480 13/16 0.8125
67 0.0320 2.75 mm 0.1083 3 0.2130 8.9 mm 0.3504 21 mm 0.8268
66 0.0330 7/64 0.1094 5.5 mm 0.2165 9 mm 0.3543 53/64 0.8281
0.85 mm 0.0335 35 0.1100 7/32 0.2188 T 0.3580 27/32 0.8438
65 0.0350 2.8 mm 0.1102 5.6 mm 0.2205 9.1 mm 0.3583 21.5 mm 0.8465
0.9 mm 0.0354 34 0.1110 2 0.2210 23/64 0.3594 55/64 0.8594
64 0.0360 33 0.1130 5.7 mm 0.2244 9.2 mm 0.3622 22 mm 0.8661
63 0.0370 2.9 mm 0.1142 5.75 mm 0.2264 9.25 mm 0.3642 7/8 0.8750
0.95 mm 0.0374 32 0.1160 1 0.2280 9.3 mm 0.3661 22.5 mm 0.8858
62 0.0380 3 mm 0.1181 5.8 mm 0.2283 U 0.3680 57/64 0.8906
61 0.0390 31 0.1200 5.9 mm 0.2323 9.4 mm 0.3701 23 mm 0.9055
1 mm 0.0394 3.1 mm 0.1220 A 0.2340 9.5 mm 0.3740 29/32 0.9062
60 0.0400 1/8 0.1250 15/64 0.2344 3/8 0.3750 59/64 0.9219
59 0.0410 3.2 mm 0.1260 6 mm 0.2362 V 0.3770 23.5 mm 0.9252
1.05 mm 0.0413 3.25 mm 0.1280 B 0.2380 9.6 mm 0.3780 15/16 0.9375
58 0.0420 30 0.1285 6.1 mm 0.2402 9.7 mm 0.3819 24 mm 0.9449
57 0.0430 3.3 mm 0.1299 C 0.2420 9.75 mm 0.3839 61/64 0.9531
1.1 mm 0.0433 3.4 mm 0.1339 6.2 mm 0.2441 9.8 mm 0.3858 24.5 mm 0.9646
1.15 mm 0.0453 29 0.1360 D 0.2460 W 0.3860 31/32 0.9688
56 0.0465 3.5 mm 0.1378 6.25 mm 0.2461 9.9 mm 0.3898 25 mm 0.9843
3/64 0.0469 28 0.1405 6.3 mm 0.2480 25/64 0.3906 63/64 0.9844
1.2 mm 0.0472 9/64 0.1406 E 0.2500 10 mm 0.3937 1 1.0000
1.25 mm 0.0492 3.6 mm 0.1417 1/4 0.2500 X 0.3970    
1.3 mm 0.0512 27 0.1440 6.4 mm 0.2520 Y 0.4040    
55 0.0520 3.7 mm 0.1457 6.5 mm 0.2559 13/32 0.4062    

 

Download a PDF of Fluid Power Design Data Sheet 49 - Effects of Oversizing or Undersizing an Electric Motor for a Job.

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.

 

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