Adjustable Speed Drives –
(a.k.a. Variable Speed Drives)

What They Are, How They Work

Application Information

AC DRIVES – PRINCIPLES OF OPERATION

Adjustable frequency AC motor drive controllers frequently termed inverters are typically more complex than DC controllers since they must perform two power section functions, that of conversion of the AC line power source to DC and finally an inverter change from the DC to a coordinated adjustable frequency and voltage output to the AC motor. The appeal of the adjustable frequency drive is based upon the simplicity and reliability of the AC drive motor, which has no brushes, commutator or other parts that require routine maintenance, which more than compensates for the complexity of the AC controller. The robust construction, and low cost of the AC motor makes it very desirable for a wide range of uses. Also, the ability to make an existing standard constant speed AC motor an adjustable speed device simply by the addition of an adjustable frequency controller creates a very strong incentive for this type of drive.

TABLE 3. HORSEPOWER OUTPUT AT VARIOUS MOTOR SPEEDS WITH 1750 RPM BASE SPEED CONSTANT TORQUE DRIVES

Rated HP
At 1750 RPM
Base Speed
Rated Torque
At All Speeds
Lb.-Ft. (1)(2)
HP Rating at Various Motor RPM
1575 1400 1225 1050 875 700 525 350 175 87.5 35
1/6 0.50 .150 .133 .117 .100 .083 .067 .050 .033 .017 .008 .003
1/4 0.75 .225 .200 .175 .150 .125 .100 .075 .050 .025 .013 .005
1/3 1.00 .300 .267 .233 .200 .167 .133 .100 .067 .033 .017 .007
1/2 1.50 .450 .400 .350 .300 .250 .200 .150 .100 .050 .025 .010
3/4 2.25 .675 .600 .525 .450 .375 .300 .225 .150 .075 .038 .015
1 3.00 .900 .800 .700 .600 .500 .400 .300 .200 .100 .050 .020
1-1/2 4.50 1.350 1.200 1.050 .900 .750 .600 .450 .300 .150 .075 .030
2 6.00 1.800 1.600 1.400 1.200 1.000 .800 .600 .400 .200 .100 .040
3 9.00 2.700 2.400 2.100 1.800 1.500 1.200 .900 .600 .300 .150 .060
5 15.00 4.500 4.000 3.500 3.000 2.500 2.000 1.500 1.000 .500 .250 .100
7-1/2 22.50 6.750 6.000 5.250 4.500 3.750 3.000 2.250 1.500 .750 .375 .150
10 30.00 9.000 8.000 7.000 6.000 5.000 4.000 3.000 2.000 1.000 .500 .200
15 45.00 13.500 12.000 10.500 9.000 7.500 6.000 4.500 3.000 1.500 .750 .300
20 60.00 18.000 16.000 14.000 12.000 10.000 8.000 6.000 4.000 2.000 1.000 .400
25 75.00 22.500 20.000 17.500 15.000 12.500 10.000 7.500 5.000 2.500 1.250 .500
30 90.00 27.000 24.000 21.000 18.000 15.000 12.000 9.000 6.000 3.000 1.500 .600
40 120.00 36.000 32.000 28.000 24.000 20.000 16.000 12.000 8.000 4.000 2.000 .800
50 150.0 45.000 40.000 35.000 30.000 25.000 20.000 15.000 10.000 5.000 2.500 1.000
60 180.00 54.000 48.000 42.000 36.000 30.000 24.000 18.000 12.000 6.000 3.000 1.200
75 225.00 67.500 60.000 52.500 45.000 37.000 30.000 22.500 15.000 7.500 3.750 1.500
100 300.00 90.000 80.000 70.000 60.000 50.000 40.000 30.000 20.000 10.000 5.000 2.000
125 375.00 112.500 100.00 87.500 75.000 62.500 50.000 37.500 25.000 12.500 6.250 2.500
Percent of
Base Speed
90 80 70 60 50 40 30 20 10 50 2

NOTE: ON SHADED AREA: Motors may require supplemental cooling when operated continuously at rated load at reduced speeds. See Specifications 9100, 9200, 9300, 9700.
NOTE: (1) lb-in = lb – ft x 12
NOTE: (2) Torque ratings for other base speed motors:
2500 RPM Motor = 1750 RPM Torque x.7 Approx.
1150 RPM Motor = 1750 RPM Torque x 1.52 Approx.
850 RPM Motor = 1750 RPM Torque x 2.06 Approx.

Adjustable Speed Drive Application Information provided by: FINCOR Automation 

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