Control systems use direct current (dc) motors for many applications. Many systems displace dc with alternating current (ac) technology because ac motors are more cost effective and lower maintenance than a dc motor with brushes. But are they?
If applications require high starting torque and precision speed control, then dc is better. What options are there for replacing dc? There are brushless dc motors, which require less maintenance. Electric vehicles, robotics and automation, along with consumer electronics and smart home devices, have brushless dc motors. However, the larger dc motors still require brushes and are used in heavy industry, heavy industrial equipment, renewable energy such as wind turbines and battery-powered tools.
For industrial machines, there are dc motors that are continued to be rebuilt since the 1960s and are used in cement, paper and metals processing. Why? There is not an ac equivalent that can do the job, without large costs. How does this affect automation and controls? Sometimes dc motors and drives are necessary. DC motors and drives are not on the way out. Thus, controls engineers should be familiar with ac and dc motor/drive configurations and their differences.
Physically, dc motors and ac motors will have different frames and height and cooling requirements. On top of that, a standard ac motor cannot match the dc motor low-speed performance. A variable-frequency drive (VFD) is required to match functionality.
Size differences are related to brushes and commutators. AC motors do not use them, but dc does. DC requires a field generation and permanent magnets. AC uses stator winding and rotating magnetic fields. DC motors require more external cooling. DC drives may require more external cooling, as well. AC units may integrate cooling in the system. Size differences also are related to the application, in that heavy torque applications typically have used custom dc motors and drive set ups. When considering sizing, note that changing from dc to ac may require a different size motor/drive configuration. Understanding power matching, quadrant operation, feedback loops and load characteristics, as well as thermal management, is key.
Power matching involves matching the drive to the motor’s kilowatt (kW) output. Undersized drives will limit the motor performance. Oversized drives may waste energy or damage motors. Quadrant operation refers to monitoring in one direction, monitoring and breaking in one direction, or full control over monitoring and braking in both directions. This is based on one-quadrant, two-quadrant or four-quadrant control. More quadrants give better control. Four quadrants are needed for regeneration.
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Feedback refers to tachometers and encoders. How is the drive going to control? Is it speed regulation or torque? These characteristics are related to the application, but the drive/motor configuration will need to be defined to allow for proper feedback to control the motor for an accurate application.
Load characteristics are about understanding if the drive should be tuned for constant torque or constant horsepower. Are movements fast or slow? Sustained at high energy or in bursts? How is the system going to hold position? Does the motor have a sweet spot that the drive needs to be tuned to for the purpose of efficiency?
Thermal management is about energy loss and maintenance of the drive. Current limits should be defined based on the drive and motor specification, and the drive should be used to make sure the motor is protected. This is directly related to the load and understanding what power output needs to occur to move the application load.
In conclusion, if you are replacing dc motors with ac motors, then you must consider sizing, the drive configuration you need and control scheme required. As an integrator or machine builder or plant engineer, which dc drive might you choose? There are several dc drive manufacturers, but it’s best to consider the application and costs and the current ecosystem and then choose.
When choosing a drive manufacturer, consider scalability and power range. A drive should have a consistent dynamic response that can be utilized in the industrial environment. For example, can current and torque rise under 10 milliseconds, and do you have full torque capacity at low speeds? Drive manufacturers should have flexible configurations.
What does a flexible configuration mean? Modular design, choice on number of quadrants, input and output cards, programmability, regenerative braking, choice of communications protocols, ease of programming and integration. Drives should be robust, as well, in that their footprint should be manageable for motor control centers, and ease of drive replacement, as opposed to drive-motor system replacement. There should be tolerances for harsh environments and accommodations for nickel-plated bus bars and coated printed circuit boards (PCBs), so that the drive will stay reliable in harsh environments. Drives should be controllable from any standard controller, or programmable if standing alone.
If the application depends on a dc motor, then choosing the correct dc drive is paramount. Defining size requirements and choosing a control configuration, as well as looking at spacing, integration and the reliability will make finding the perfect drive easier or at least give knowledge for asking integration companies the right questions.
