Why DC Drives Are Still Used In Modern Industrial Applications
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5 Reasons Why DC Drives Are Still Important In Today’s Industrial Automation As you know, DC motors were introduced at the start of the 19th Century and despite the arrival of AC motors, DC motors and DC drives are still being used in a variety of industrial motor applications today. Why? The Simplicity Of How A DC Motor Is Controlled The main reason is the motor’s variable speed behaviour. When the voltage applied to the armature of a DC motor increases, the motor’s speed increases correspondingly. Another reason is the DC motor can develop full torque from zero up to its base speed. This constant-torque characteristic allows DC motors to be used on applications such as conveyor belts, elevators, cranes, extruders and mixers, amongst others. The full torque from zero speed is useful when the application needs to be stopped when fully loaded, and then got moving again. The Two Main Operating Modes Of A DC Motor: And Why They Are Beneficial A DC motor operates in one of two main operating modes. The first is the ‘constant-torque’ range, where the motor is operating from zero to base speed. As described above, as the voltage applied to the motor increases, the DC motors ability to generate full torque remains constant. This ‘operating mode’ is perfect where an industrial application must be operated at various speeds while under full load. The second is called the constant-power range, which is also known as field weakening. This operating mode is where the motor is turning faster than its base speed. This is achieved by reducing the field current when the motor is at full voltage (base speed). The increase in speed results in a reduction in the torque available to turn a load. A paper machine is a good example of where field weakening is used. The take-up rolls at the end of the machine operate in this field weakening mode. Paper comes off the machine at a fixed speed. When a new roll is started, the empty spindle must spin fastest because of its smaller diameter. As the roll starts to fill with paper, the load increases and the demand for speed lessens and the voltage to the field can be increased to maintain the required amount of torque to turn the load. Why A Smaller DC Motor Is Better Normally, a DC motor is smaller in size than its equivalent AC motor. This is advantageous in a couple of ways: The DC motor has a smaller footprint and it has a smaller rotor. Also, the DC drive usually has a smaller footprint too. The benefits of a smaller footprint are obviously gained in ease of installation and reduced material costs. A smaller rotor also has its advantages in that it creates less inertia. With less inertia, it takes less time to accelerate and decelerate meaning a DC motor can perform quicker reversals leading to shorter cycle times and a faster production line. Because they have this high power density, DC motors can produce many thousand of horse power. To control such high power applications with an AC motor requires an advanced level of equipment and specialist knowledge. A Large Installed Base For a long time, the only way to control the speed of a motor shaft was with a DC motor/drive. This means DC motors have an extremely large and far reaching installed base. With the installed base comes an installed base of knowledge amongst technicians and engineers. As DC motors and DC drives are less complicated than their AC counterparts, they are lower in cost and far more easy to repair. So when replacing components or retrofitting an existing control system, it is a easier and usually more economic to replace DC technology with DC technololgy. DC Motor Control Still Remains The Preferred Method Of Control In A Range Of Industries Despite what the media (and many AC manufacturers) will have you believe, there are still many industries that continue to choose DC motors and drives. To learn more about these industries, you can take a look at this ‘where DC drives are used’ guide. DC motors and drives can be used in today’s modern industrial applications, like steel mills www.sprint-electric.com