Adjustable Speed Drives –
(a.k.a. Variable Speed Drives)
What They Are, How They Work
Application Information
- Adjustable Speed Drives – Application Information
- DC Drives – Principles of Operation
- DC Drive Types
- DC Motor Control Characteristics
- AC Drives – Principles of Operation
- AC Controller Types
- AC Motor Control Characteristics
- Motor Selection
- AC vs. DC Drive Comparison
- Basic Mechanics
- Other Application Factors
- Measuring Machine Torque
- Mechanical Formulas
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