Analogue to PLC

An analog signal is an instantaneous reading from a device to a PLC or from a PLC to a device. You can read an earlier post that defines a PLC if necessary.

The signal to or from the PLC can be in many forms . The typical signal would be 4-20MA (could also be 0-20MA, 0-10VDC). Other examples would be an RTD, a thermocouple, millivolts, etc.

When this signal is an input to a PLC it would be used to monitor devices in the field, such as a flow meter, a pressure transducer, level transmitters, feed back from drives, or any other of the many devices used to measure a process.

When the signal is an output from a PLC it would be used to give a speed reference to a device in the field, such as an inverter or dc drive, a positioning control, regulating valve or any other device that can accept an analog signal as a input to control it.

Analog signals are usually used in a PLC with a PID loop which is a closed loop control.

A typical example of a PID closed loop control would be an input to the PLC from a flow meter and an output from the PLC to an ac drive. The operator or PLC programmer would have entered a set point for the flow into the PLC program. The PLC will compare the flow measurement from the flow meter with the set point and depending upon whether the flow is above or below the set point, will increase or decrease the signal to the drive. The drive in this case is activating a pump and the increase or decrease in signal will result in and increasing or decreasing of flow to bring it closer to the desired flow rate. The further the input is from the set point, the greater the change in output will be.

More about PID loops in another discussion. If you need help with any control challenges, why not ASK JIM?

 

 

 

Just what is a PLC?

Those in the control industry, and those looking for control solutions, bandy the term PLC about regularly, but what is a PLC?

PLC is the acronym for Programmable Logic Controller. Okay, we can see where the PLC comes from, but then what is a programmable logic controller?

A PLC is a computer similar in concept to, but different from, a desk top or laptop computer. It has internal programs, data storage and input devices like the desktop or laptop computer, but it provides more capabilities than just those basic computers.

The PLC will have the ability to accept incoming information from a variety of devices on a piece of machinery, not just from the mouse or keyboard found on the home computer. The incoming information might be from push buttons, timers, toggle switches, proximity switches; the incoming signal can be from many different sources and these sources can feed information to the PLC through the PLC’s Inputs.

Once the PLC receives an input from somewhere, it executes the control program that has been programmed into it by the PLC programmer. This program then may create an output signal of some sort.

The Outputs are part of the PLC as well, and allow a broad range of equipment to be connected to them. As the CPU executes the program, it will send information to the correct PLC Output (if it’s been programmed correctly), and that output signal will then cause some other external device on the machine to function.

Even though the PLC has a central CPU (central processing unit) just like the home computer, it differs by having more than just two inputs (mouse & keyboard), and it differs by being able to send control commands out from itself to initiate action somewhere else on a machine.

PLC’s are often quite small (in comparison to their P.C. cousins) though those with large numbers of Inputs or Outputs (I.O. points) can get quite large. They are installed inside of a control panel enclosure on or near the machine they control. Wires leading from the various Input devices to the PLC, and those from the Output’s of the PLC, enter or exit the enclosure through holes drilled in the enclosure wall.

 

Control panel enclosures

Control panel enclosures can be manufactured out of a variety of materials, and the material selected will be dictated by the environment into which the panel is to be installed.

Control panel enclosures can be made of aluminum, mild steel, stainless steel or fiberglass.

For more information on the various materials, and the gaskets commonly used for seals, please click here and follow the links to Rantro Tips.

 

NEMA ratings for panel enclosures - the last post!

NEMA panel ratings run from 1-13, with ratings 1, 4 and 12 the most commonly applied to control panel enclosures.

We’ve been reviewing the specifications of the various NEMA ratings in this series of blogs, and this is the last, where we’ll review ratings 6-13.

If you need more information about machine control panels and accessories, why not Ask Jim?

NEMA 6

NEMA 6 enclosures have to be constructed for use either indoors or out, must provide a degree of protection against persons accidentally being able to come in contact with equipment inside the pane. NEMA 6 also means the panel will provide protection against falling dirt, hose-directed water, and will prevent the entry of water into the panel should it be temporarily submerged at a limited depth. Contents and the panel must remain undamaged by the build up of external ice on the panel enclosure.

NEMA 6P

All the same conditions as NEMA 6, but this panel must prevent the ingress of water if the panel has a prolonged submersion at a limited depth.

NEMA 7

A NEMA 7 enclosure is used indoors in locations with classifications as Class I, Groups A,B,C, or D, and shall be capable of withstanding the pressures resulting from the internal explosion of specified gases, and must contain such explosion sufficiently that an explosive gas-air mixture existing in the atmosphere surrounding the panel enclosure will not be ignited. Any enclosed heat generating devices shall not cause external surfaces of the panel enclosure to reach temperatures capable of igniting gas-air mixtures in the surrounding atmosphere. NEMA 7 enlcosures shall meet explosion, hydro-static and temperature design tests.

NEMA 9

The are panel enclosurs intended for use indoors, in locations classified as Class II, Groups E, F, or G, and shall be prevent the entrance of dust. Any enclosed heat generating devices shall not cause external surfaces of the panel enclosure to reach temperatures capable of igniting or discoloring dust on the surface of the enclosure, or igniting dust-air mixtures in the surrounding atmosphere. Enclosures must meet dust penetration, temperature design tests, and aging of gaskets if used.

NEMA 12

These are enclosures construction (without knockouts) to be used indoors, and to provide a degree of protection to persons from incidental contact with enclosed equipment. They must also provide a degree of protection against falling dirt, circulating dust, against lint fibers, and dripping or splashing of liquids.

NEMA 12K

Offers all the same protections as NEMA 12, these enlcosures have knockouts.

And last but not least….

NEMA 13

These are enclosures for indoor use, and to provide a degree of protection to persons from incidental contact with enclosed equipment. They must also provide a degree of protection against falling dirt, circulating dust, against lint fibers, and against the spraying, splashing, and seepage of water, oil and non-corrosive coolants.

 

NEMA ratings for control panel enclosures

NEMA

In our continuing blogs about NEMA ratings for electrical enclosures, today we’ll look at the NEMA 4 and 5 ratings. NEMA rates panels on a scale of specification numbered 1 through 13, with numbers 1, 4 and 12 normally used for panels.

NEMA 4

To be classified as a NEMA 4 panel, the enclosure must be constructed for either indoor or outdoor use, to provide a degree of protection to prevent incidental contact to enclosed equipment by any personnel, to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, and splashing or hose directed water. The enclosure must remain undamaged by the external formation of ice on it. NEMA 4 panels are normally constructed of painted carbon steel.

NEMA 4X

To be classified as NEMA 4X, the panel must provide all the protection as noted in NEMA 4, as well as provide protection of the panel enclosure itself from corrosion. NEMA 4X panels will normally be constructed of stainless steel, or fiberglass.

NEMA 5

A NEMA 5 classified panel provides a degree of protection to personnel against incidental contact with the enclosed equipment, and to provide a degree of protection against falling dirt, settling airborne dust, lint, fibers, and dripping or splashing of liquids.

Need more information about machine control panels? Click here.

 

More info about NEMA & Machine Control Panels

In this series of blogs we are looking at the various NEMA ratings, and what they mean for selecting the enclosure for a control panel.

We know that NEMA has ratings of 1 through 13 for panel enclosures, and also that normally it’s NEMA ratings 1, 4 and 12 that usually apply when selecting a control panel.

NEMA 3

Control panel enclosures that are rated NEMA 3 are constructed for use either indoors or outdoors, provide a degree of protection to personnel against incidental contact with the enclosed equipment; provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and ensure that those control panel enclosures will be undamaged by the external formation of ice on that enclosure.

NEMA 3R

A control panel built to NEMA 3R standards provides all of the protection of the class 3 rating, except that it does not provide protection for the contents against wind blown dust.

NEMA 3S

Controls panels with this rating have all the protection that is included in NEMA 3, as well as ensuring that any control panel external mechanisms will remain operable when the control panel is ice laden.

 

NEMA

NEMA is the acronym for the National Electrical Manufacturing Association. To quote from their web site “NEMA is the trade association of choice for the North American electrical manufacturing industry, and is the largest trade association for manufacturers of electrical products in the United States,” and also for Canada.

We use a NEMA rating to identify the particular protection level of Machine Control Panel Enclosures.

NEMA ratings for control panel enclosures number 1 through 13 although the common ratings for enclosures are NEMA 1, 4 and 12. In this blog we will review the various control panel enclosure ratings.

NEMA 1
This is the lowest NEMA protection rating for a control panel enclosure. In order for the panel box to be considered as NEMA 1, it must provide a basic level of protection from persons coming into incidental contact with the contents of the panel box, and it must provide protection from the panel box contents being exposed to incidental contact with falling dirt.

Panels with NEMA 1 rating are general purpose because they lack gaskets. They should only be used indoors in a dry location.

NEMA 2
A panel with NEMA 2 rating must provide the protection as outlined under the NEMA 1 standard as shown just above, as well as providing protection for the panel contents from dripping or splashing liquids.

In the next blog we’ll look at other NEMA ratings for motor control panel enclosures.

 

More Electric Motor Data Plate Information

The plate on your electric motor might have a box that indicates the motor’s efficiency. Not all motors have this box, so if yours doesn’t, don’t be alarmed.

When electric motors are operated they generate heat. Some motors generate lots of heat (probably those that don’t show the motor efficiency on the plate) and some less so.

Heat generation by the motor indicates a of loss of energy, as the electricity is being used to generate heat rather than rotation. The more heat the motor generates, the less efficient it is in turning electricity into rotary motion.

Motors with higher efficiency ratings likely have more metal in their construction. Insulated metal is placed between the laminations to reduce eddy currents that produce energy wasting heat.

If you have an efficiency rating on the motor data plate of your motor, you will see a rating in percent, as in 86%, or you might see it as a decimal, as in .86.

When purchasing a motor, higher efficiency motors will reduce operating costs.

 

Electric Motor information plate…

In earlier blogs we talked about Voltage and FLA’s (Full Load Amps) and how this information could be used in better operating an electric motor.

This entry is about Locked Rotor Amps.

Locked Rotor Amps are the amount of power that this motor will draw when it’s starting or when it’s attempting to start but cannot.

It might not start because the bearings have seized, for example.

An electric motor is considered “locked” any time the rotor isn’t rotating. If a start is attempted but the motor cannot for a mechanical reason rotate, the motor will be in locked rotor amp.

Therefore, the LRA (locked rotor amperage) of an electric motor is the maximum amount this motor will draw when it’s attempting to start, regardless of the condition of that motor.

 

What’s FLA on the motor info panel mean?

FLA is the acronym for “Full Load Amps”.

FLA is the current that this particular motor will draw when the load on the motor reaches it’s rated horsepower.

The motor will draw less than it’s FLA if the motor is loaded at less than it’s rated horsepower, and will draw more FLA’s when it’s loaded higher than it’s rated horsepower.

Measuring the amps a motor is drawing and comparing those to it’s FLA will quickly tell you if the motor is being overloaded in the application.
If you have a question about control panels, motor controls, industrial wiring, get help from a pro. Ask Jim!

Next blog we’ll look at Locked Rotor Amps.