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Tech Tip: Controls & Sensors

Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies and content from PTDA’s Power Transmission Handbook®, the definitive resource and training tool on PT/MC products.

Visit the full library  of Tech Tips for additional products.

What are proximity switches and how do they work?

Industrial sensors are devices that respond to a physical stimulus (heat, light, pressure, and flow, for example). They react by producing a signal that is used for measurement or control. Proximity sensors detect the presence of an object without physically touching it. The proximity sensor emits an electromagnetic field or a beam of electromagnetic radiation and detects a foreign object in the field by picking up a return signal. Sending distances can range from fractions of an inch to several feet. The sensors buried in the street pavement to detect cars for controlling traffic signals are usually proximity sensors.

What are encoders and what are the two main types of encoders?

Encoders are used for both position and speed feedback. Encoders produce a pulse each time a shaft rotates through a specified angle. There are two types of encoders: optical and magnetic. Magnetic encoders are less expensive and mechanically simpler, but their resolutions is limited to about 1 degree (360 pulses per revolution). By contrast, optical encoders produce up to 5,000 pulses per revolution.


What purpose do motor starter contactors serve and what do NEMA standards require for AC motor starter contactors and how do they differ from IEC standards?

The contactor portion provides the connection of the motor to the incoming power. NEMA standards require manufacturers to design all contactors for heavy service. As a result, NEMA contactors are larger than corresponding IEC ones and have more massive silver-alloy contacts. When subject to identical service, NEMA starters generally exhibit much longer mechanical and contact life than IEC contactors. 


Do VFDs have a service factor?

The answer is no. VFDs have a very specific maximum continuous output ampere rating limit without a safety factor allowance for additional continuous output amps. Note that VFDs do have a published 60 second overload rating as a percentage of the VFD's continuous output ampere rating. Variable torque or normal duty VFDs can supply 110 (or 120) percent of continuous output ampere rating for 60 seconds. Constant torque or heavy duty VFDs can supply 150 percent of continuous output ampere rating for 60 seconds as well. Beyond 60 seconds, the VFD will trip (most likely with a VFD Over temperature or Motor Overload code). An attempt by the motor to draw more than these percentage levels will result in the VFD clamping output current by reducing the amplitude and frequency of the power supplied by the motor.  Most VFDs can supply 200 percent current for one or two seconds. A quick attempt to draw a higher level of current will cause the VFD to trip (most likely with an Overcurrent code) cutting off current to the motor.

What six factors affect the proper sizing of VFD?

1. Temperature of the installation site. All VFDs can operate in a 40° C ambient. Some are rate for a 50° C ambient.
2. The ability of the VFD enclosure to dissipate heat. 
3. Total continuous loading on the VFD as a percentage of its rated ampere load capability.
4. Load type: constant torque, variable torque, constant horsepower or impact
5. Single or three-phase input power feed
6. Setting of VFD output power switching frequency. Refer to VFD selection literature for specific guidelines. 

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.

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