In factory automation, the best solution isn't always the most obvious. From a design perspective it is important for the engineer to ask, "What is the end goal?" The design challenge isn't simply getting a series of off-the-shelf components to work together. Instead, it is determining the best way to integrate the power, logic, and the power-conversion devices that convert rotary motion to linear motion in the smallest, most flexible package achievable. That frequently means analyzing whether AC or DC motors represent the best option.
In the material handling industry, traditional motion control solutions generally integrate a series of off-the-shelf products such as an AC motor, speed control, gear box, and a gear-and-sprocket conveyor system, which includes custom rollers with slip drives that are tied into a programmable logic controller (PLC) or personal computer (PC). A larger factory may have a series of these lines. This type of solution also requires custom software and a significant amount of field wiring, since each AC motor must have a power drop and should be wired back to the PLC or PC.
While this solution works well for factories with pallet lines that transport heavy, high-volume product, it can be inefficient or too costly for applications that have lighter loads or periodic wait states. While there are specialized AC motors capable of addressing these applications, 24 V brushless DC motors may offer a far less costly solution.
From a design perspective, there are five areas to consider in determining whether AC or DC motors represent the best option:
- Installation cost/complexity
- Maintenance cost
AC motors must generate a magnetic field in order to operate. As a result, AC motors typically have high inrush currents. An unloaded AC motor may draw almost as much power as a fully loaded motor. To design around this most conveyor systems using AC motors have the AC gear motor running continuously. Roller slip drives, gear boxes, and air valves are some of the devices used to modify motion, but since the motor continues to run, much energy is wasted. This wasted energy ultimately transforms into heat, noise, or mechanical wear.
Comparatively, 24 V brushless DC motors have permanent magnets. Inrush current can be managed by the electronic controls, so there is minimal energy lost in high stop/start cycles. A brushless DC motor also requires a microprocessor to operate. This enables the motor to be easily programmed to shut off when it is not needed, eliminating most of the equipment required for AC motor systems, as well as the energy wasted when motion is not required.
There are times and places where AC motors are the best choice. These would include situations where the torque or power requirements exceed the limits of typical DC motors or where continuous operation is an application requirement. A large bulk-handling belt conveyor would be a perfect example. Another would be where products need to be transported up an inclined belt conveyor. There an AC gear motor might be a perfect match for the speed and torque required to get the job done.
On the other hand, more torque is generally required to get a load moving than to keep it moving. AC motors often offer starting torque that is no greater than the running torque. Brushless DC motors can provide starting torque that is two to four times their rated running torque. This can be a real advantage in systems where frequent starts and stops are required.