Lower Control Voltages Reduce Shock Hazard

Aug. 17, 2000
Motor controllers running on 24 Vdc, such as the Cutler-Hammer Intelligent Technologies (IT) Soft Starter, weigh only 9 lb and can fit inside an 18-in.

Motor controllers running on 24 Vdc, such as the Cutler-Hammer Intelligent Technologies (IT) Soft Starter, weigh only 9 lb and can fit inside an 18-in. motor-control center. The bypass contactor and the overload relay are integrated into the controller, saving panel space and wiring costs. A kick-start feature provides additional torque at start-up while a soft-stop feature allows high-friction loads to slowly decelerate by ramping down the voltage to the motor.

The trend of adopting 24-Vdc control power on the factory floor has been accelerating over the last few years. One major reason for this shift away from traditional higher ac control voltage is that it is much safer to install and maintain. Also, 24-Vdc components cost less, are smaller, and generally have a history of higher reliability.

The migration toward 24 Vdc began with the need for safer operating voltages, simplified compliance with safety standards, and more reliable interfacing with PLC's and distributed control systems. The trend toward 24 Vdc is most apparent in sensor products. In just the past five years, there has been a shift away from ac operation, which accounted for 70% of the market, to 24 Vdc which now accounts for 60%.

Many of the obstacles that prevented greater use of 24 Vdc such as power-supply costs, transistor and hard-contact ampere ratings, and voltage drop over long cable runs, are being eliminated. As these obstacles are overcome, interest in 24-Vdc control has increased. Furthermore, the rapid development of microprocessor technology has spurred the development of 24-Vdc products. As manufacturers develop 24-Vdc solutions using microprocessors and separate control and logic circuits, control power requirements will continue to shrink. As a result, users can take advantage of the benefits of dc power, removing the limitations and added costs they incurred with ac power.

As devices such as PLC's, I/O devices, HMI machines, and valve actuators continue to be designed for 24-Vdc systems, the demand for 24-Vdc motor controls also will continue. In the field of motor control, products such as Cutler-Hammer's Intelligent Technologies (IT) 24-Vdc Soft Starter delivers all the benefits of low-voltage control. In addition to reducing power consumption, the soft starter is also physically smaller. It can be used in a wide variety of applications from motor-driven compressors and blowers to fans and pumps. It also provides the added benefit of reduced starting torque, which lowers stress on mechanical drives and couplings.

The lower voltage also simplifies compliance with regulatory standards such as those issued by NEC and OSHA. For one thing, it provides a substantial reduction over the amount of radiated noise generated by ac systems. On the international level, one common dc voltage bypasses the need to transform the different ac voltages and frequencies used globally resulting in streamlined installations and complexity. Compliance with the European Low Voltage Directive is an added benefit of 24-Vdc control power.

But perhaps the greatest benefit of 24-Vdc power is that it eliminates the hazards of using ac power. Each year, thousands of work-ers are injured to some degree while working on 120-Vac systems. This leads to costly medical exams, lost time, injuries, lawsuits, and, in some cases, death. Unlike ac power, 24 Vdc does not require workers to use safety gear like gloves, face shields, boots, and aprons when working with energized control circuit lines. Workers can hot insert failed or damaged sensors without shutting down a line.

Information for this article was provided by Matt Polk, Cutler-Hammer, 4201 N. 27th St., Milwaukee, WI 53216, (414) 449-6215, Fax: (414) 449-7319, www.cutlerhammer.com

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