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
157514001225105087570052535017587.535
1/60.50.150.133.117.100.083.067.050.033.017.008.003
1/40.75.225.200.175.150.125.100.075.050.025.013.005
1/31.00.300.267.233.200.167.133.100.067.033.017.007
1/21.50.450.400.350.300.250.200.150.100.050.025.010
3/42.25.675.600.525.450.375.300.225.150.075.038.015
13.00.900.800.700.600.500.400.300.200.100.050.020
1-1/24.501.3501.2001.050.900.750.600.450.300.150.075.030
26.001.8001.6001.4001.2001.000.800.600.400.200.100.040
39.002.7002.4002.1001.8001.5001.200.900.600.300.150.060
515.004.5004.0003.5003.0002.5002.0001.5001.000.500.250.100
7-1/222.506.7506.0005.2504.5003.7503.0002.2501.500.750.375.150
1030.009.0008.0007.0006.0005.0004.0003.0002.0001.000.500.200
1545.0013.50012.00010.5009.0007.5006.0004.5003.0001.500.750.300
2060.0018.00016.00014.00012.00010.0008.0006.0004.0002.0001.000.400
2575.0022.50020.00017.50015.00012.50010.0007.5005.0002.5001.250.500
3090.0027.00024.00021.00018.00015.00012.0009.0006.0003.0001.500.600
40120.0036.00032.00028.00024.00020.00016.00012.0008.0004.0002.000.800
50150.045.00040.00035.00030.00025.00020.00015.00010.0005.0002.5001.000
60180.0054.00048.00042.00036.00030.00024.00018.00012.0006.0003.0001.200
75225.0067.50060.00052.50045.00037.00030.00022.50015.0007.5003.7501.500
100300.0090.00080.00070.00060.00050.00040.00030.00020.00010.0005.0002.000
125375.00112.500100.0087.50075.00062.50050.00037.50025.00012.5006.2502.500
Percent of
Base Speed
908070605040302010502

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 

Print Friendly