Current affairs
March 1st 2006

Current monitoring improves machine and process automation says Loic Moreau, business segment manager Macro Components Products (MCP), LEM

Measuring the current drawn by motors in various processes has long been seen as a useful technique. Until now, however, the main strategy has only been to protect and/or monitor the heater, motor or other load.

The monitoring sensor has been set to trip at a pre-set level, and a relay is used to switch off the defective unit and possibly also switch in a back-up.

Now that PLC prices are coming down - miniature ones can be purchased for 150- 200 Euro - it is realistic from a cost point of view to use a current transducer that communicates with the PLC and offers much greater functionality, including much more versatile control functions, plus recording and analysis facilities. There is a clear trend in industry to take this path, as the following examples illustrate.

Usually, automated processes are regulated by parameters such as temperature, pressure, torque, position and others, but measuring these directly can be inefficient and inaccurate. Measuring the input current of a load provides much more information than just the current value and has a number of advantages:

• Speed - load changes are detected immediately, which can prevent equipment failure and process disruption.
• Ease of use - a current transducer or switch can simply be snapped over a cable, without the need to screw or weld on complex brackets.
• Reliability - solid-state transducers are more robust than electromechanical devices.

Current transducers

Current transducers convert monitored current to a proportional AC or DC voltage or milliamp signal. There are two principal types. Inductive transducers are easier to install because they are two-wire, selfpowered (0-5 or 0-10 VDC outputs) or looppowered (4-20mA output) instruments. Hall Effect transducers are generally four-wire devices and require a separate power supply. Because both types can be connected directly to data systems and display devices, they are ideal for monitoring motors, pumps, and any electrical load that requires an analogue representation over a wide range of currents.

Most current transducers are available in solid or split-core configurations to facilitate installation. The typical transducer uses field-adjustable span pots. More advanced devices feature jumperselectable ranges to eliminate calibration labour. Typical transducer ranges are 0-2A up to 0-2000A, with apertures of 0.5 inches to over 3 inches (12-76mm).

Current switches

Designed for monitoring and switching AC and DC circuits, current-operated switches integrate current sensing and signal conditioning with a limit alarm. The switch output is activated when the current level sensed by the limit alarm exceeds a userselectable threshold. Inductive current switches generally feature solid-state output switches. They are self-powered and, consequently, are a good choice for retrofits, renovations, and temporary monitoring. Hall Effect current switches have either a solidstate or relay output. Their high power requirements preclude a self-powered design, and the need for a separate power source increases their installation cost.

Some current switches are shipped with a fixed setpoint. Newer designs provide field-adjustable setpoints with a potentiometer and LED or LCD feedback.

Setpoint ranges span from 0-5A up to 0- 2000A. For relay logic systems, switches should be equipped with integral time delays to allow for start-up surge and momentary sags or swells.

In the plastic injection moulding process, the transducer is mainly used to monitor the heating elements that heat the mould and plastic. It is necessary to monitor these because, if a heater element fails, for whatever reason, the heat to the mould will become unbalanced. This nonuniform heating will have a detrimental effect on product quality. In some cases, the failure can be gradual, so it is important to monitor the current in each heater to spot any trends. A single temperature sensor is also used to monitor the overall temperature. In this application, humidity in the chamber can also cause quality problems. A transducer can be used to monitor earth leakage current, which will increase with humidity.

A transducer provides the user with options. It can be used just to stop the oven or switch in back-up heating element, but this basic operation could have been provided by a current-operated switch.

Where the transducer brings a major advantage is the ability to monitor and record the currents so that any drift can be spotted immediately. This allows maintenance intervention to be made at the right time, ensuring that the oven never operates on product while being out-ofspecification.

In some oven applications, such as semiconductor production, the limits are very tight and it is important to see any performance drift as soon as it starts.

The entire stone crushing process (see Fig. 2) can be managed with a combination of current switches and transducers. The current to the input conveyor motor can be monitored by a simple current switch (relay) if the only parameter that needs to be checked is a conveyor stall. Alternatively, if there is a need to avoid the input of oversize stones, a current transducer can sense the change in motor load. A transducer on the input to the crusher motor senses how hard the unit is working and, as in previous examples, can also provide an early indication of a developing problem. Finally, a current switch on the output conveyor motor can sense if this conveyor is jammed, and therefore switch off the input conveyor.

As systems become more complex, so do their failure modes. Monitoring the current drawn by many different types of electrical load can give an indication of a wide range of problems - often before they cause a system failure or have a noticeable effect on product quality. The advent of lowcost PLCs and a growing range of current transducers has brought this powerful monitoring and diagnostic technique to a much broader range of applications.

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