February 2013


Innovations in Comfort, Efficiency, and Safety Solutions.

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Will DelsmanEMAIL INTERVIEW Will Delsman and Ken Sinclair

Will Delsman, Technical Sales Manager, NK Technologies

Will Delsman has worked in the electrical industry since 1974, holding positions in purchasing and inventory control management, and sales and marketing management. He has been with NK Technologies since 1999 as senior technical support specialist, and now holds the position of inside and technical sales manager.

Current Switches

Most current switches are designed to be used as signaling devices only, in conjunction with a programmable logic controller or building energy management system.

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Sinclair:  Why use current operated switches?

Delsman:  The first product developed by Nielsen-Kuljian, Inc. (now NK Technologies) was an AC current operated device which provided a solid state contact that would change state (open or close) when AC current passed through the sensor.

From what I was told, the application driving the design was monitoring air handling fans in a tomato processing plant. After washing, the produce was stored in bins and the fans kept air circulating to reduce the formation of mold or mildew. If a drive belt broke, the fan would stop but the motor starter mechanical interlock contacts would continue to indicate that all was working properly. With the motor running in an open shaft condition, there was a significant drop in the motor current. At the time there was no simple means to detect under current conditions. What was needed was a separate current transformer over a motor lead which had to be connected to a current relay through a shorting block, and the relay required an external power source.

The one piece, self-powered solid state current switch provided a much safer and simpler solution. There are multitudes of applications for these devices, from monitoring air handling equipment in buildings and detecting fan belt breakage or slippage (as in the tomato processing plant), to detecting conveyor jams or overloading of industrial equipment. A jam on a conveyor will cause the current use of the drive motor to spike and standard motor overload protection using bimetal or eutectic elements will not operate quickly enough to reduce damage to the conveyance system, or the product being transferred.

Sinclair:  Many BAC systems utilize differential pressure switches to monitor air flow in buildings. Is there any advantage to monitoring current rather than air pressure?

Delsman:  Differential pressure in air handling applications is detected using two inlets, one measuring pressure on each side of the fan blades. Their installation requires cutting into the duct to access the air flow, mounting the pressure switch in an area of low vibration and running conduit and wiring from the controller or motor starter to the pressure switch. Both intake and discharge pressure settings must be adjusted so the switch output is in the required state (open or closed) when the fan is working properly.

A current switch can be mounted at the controls eliminating the need for conduit to the fan, and can be adjusted easily without a ladder or scaffolding; only a screwdriver is needed. A visible LED mounted on the current switch shows the condition of the output contact, whether normal (shelf state) or tripped.

Sinclair:  Can a current switch be used to detect both AC and DC current?

Delsman:  NK Technologies manufactures products which can detect AC current and others designed to sense DC circuits. AC current can be detected to as low as 0.003 amps up to 1200 before the output changes state. DC current can be detected down to 0.75 amps and up to 100 amps. DC current sensors manufactured by NK Technologies require an external power supply, while most AC switches are powered from the monitored circuit current.

Sinclair:  Can the switch be used to turn a motor on and off?

Delsman:  Most current switches are designed to be used as signaling devices only, in conjunction with a programmable logic controller or building energy management system.

There are models utilizing relay contacts rated up to ten amps. As an example, this product could be used to interlock a dust collector when a machine is operated or to start a booster fan in a duct system when a clothes dryer is running. The second application is quite common in apartment and condominium complexes.

Care should be exercised when using solid state contacts for switching inductive loads like motors and transformers due to the possibility of overloading during inrush current upon energization.

Reliable Controls Sinclair:  Will a current switch detect phase loss?

Delsman:  Monitoring current applies only when there is a load being driven (power being consumed), and will not detect the presence or absence of a phase until the command to start is given. A current sensor will detect a phase loss while the load is running but not before it is energized.

Sinclair:  Why are there so many choices to select from?

Delsman:  Since the early 1980’s when Maynard Kuljian and David Neilsen designed the original one piece current switch, the varied applications for the device have shown the necessity for various versions.

Adding a delay before operation of the output helps to reduce nuisance tripping when monitoring large grinding or shredding machines, or to bypass inrush current, and our customers asked for this. Some applications require very close tolerances while others are better served having wider hysteresis. There are models available to suit any need, and if NK doesn’t make exactly what you need, just let us know and our design engineers will do their best to work out the details with you. 


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