In most systems decisions have to be made. Does a product need to advance immediately, wait for the next section of conveyor to clear, or pause for some other operation to take place? To handle this, AC motors are typically wired to a PLC or PC. The PLC or PC must control the motor, speed control, and conveyor mechanisms. In short, a system powered by AC motors has a lot of moving parts controlled via a centralized unit. This adds complexity to both programming and field wiring.
With a 24 V brushless DC motor, intelligence is resident in a microprocessor that is part of the motor control. Todayís DC motor microprocessors can add a lot of sophisticated functionality including: direction of rotation, speed, acceleration rate, braking capabilities, and variable zone-to-zone transfer modes (i.e., zero pressure accumulation or fast transfer). These functions can be set to work automatically via distributed intelligence or controlled remotely via local network communication. Straightforward programming can be done via small switches built into the controls. Sensors trigger motor on/off modes. Simple peer-to-peer communication capability can be driven at the microprocessor level. If network control is required, an external bus can be added, or may be incorporated into the peer-to-peer communication system.
As mentioned earlier, AC motor-driven systems typically require a lot of moving parts. From an installation standpoint this increases field wiring cost significantly. Additionally, the high motor inrush current drives a need for separate circuits and power conditioning. A separate high voltage power drop is required for each motor. Building layout, power drop availability, and available space for larger motors and required accessories may add design constraints.
Comparatively, 24 V DC power is easy to attain and safer for employees working with the unit. Since torque is a factor and the motors are intelligent, the typical design strategy is to drive individual zones with small brushless DC motors. Each zone has sensors, which trigger component power supply based on preprogrammed conditions. For example, there is usually a discharge zone and receiving zone. Only the parts in the zone moving the product receive power. As the product leaves one zone, the sensors trigger power on in the next zone.
Microprocessor-based brushless DC motors can be connected and controlled wirelessly by a Bluetooth network. This minimizes field wiring requirements. Additionally, the motors have a smaller footprint than standard AC gear motors, adding flexibility to mounting options. Elimination of additional units such as gearboxes, air pressure-based braking, and slip drives further minimizes design constraints associated with building layout and space considerations.
Wasted energy can translate to faster equipment wear in AC motor-driven systems. Additionally, troubleshooting can be more challenging due to the fact that a more complex system will have a greater number of variables to check.
A 24 V brushless DC motor-driven system has fewer moving parts and spends less time running, which translates into reduced equipment wear. The microprocessor controls can even be programmed to send messages to the mobile phones of maintenance technicians when failure modes occur or when it is time for preventative maintenance. Troubleshooting is faster because it is limited to specific zones rather than the entire system.
It is important to note that specialized AC motors can be designed with increased functionality that delivers many of the same advantages listed for DC motors. Determining the best choice for the application requires designers of factory automation systems to consider system load requirements, frequency of conveyor stop/starts, layout constraints, system cost, and system operating costs carefully. Arguments can be offered for either motor technology and both are still offering new features and capabilities as the products evolve.
David Hall is vice president of Automation Controls Group, a division of MEC Companies.