Standard Industrial Controls...
Let's talk about the "definition" of "Standard Industrial Controls"...
First of all, the term is synonymous with "Motor Controls" or "Motor Controller". With that in mind, we looked up the definition on Wikipedia.org and here's what we came up with the following...
A "Motor Controller" is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.
Taking that definition a step further, the article also states...
"Every electric motor has to have some sort of controller. The motor controller will have different features and complexity depending on the task that the motor will be performing.
The simplest case is a switch to connect a motor to a power source, such as in small appliances or power tools. The switch may be manually operated or it may be a relay or contactor, connected to some form of sensor to automatically start and stop the motor. The switch may have several positions to select different connections of the motor. This may allow reduced-voltage starting of the motor, reversing control or selection of multiple speeds. Overload and over current protection may be omitted in very small motor controllers, which rely on the supplying circuit to have over current protection. Small motors may have built-in overload devices to automatically open the circuit on overload. Larger motors have a protective overload relay or temperature sensing relay included in the controller and fuses or circuit breakers for overcurrent protection. An automatic motor controller may also include limit switches or other devices to protect the driven machinery.
More complex motor controllers may be used to accurately control the speed and torque of the connected motor (or motors) and may be part of closed-loop control systems for precise positioning of a driven machine. For example, a numerically controlled lathe will accurately position the cutting tool according to a preprogrammed profile and compensate for varying load conditions and perturbing forces to maintain tool position."
Some Basic Control Components
Contactor - The contactor is an important starting point since it is used as the main "starting/stopping" device for electric motors. The contactor is an "electro-mechanical" device. it has wire terminals on an "incoming power" side and a second set on the "outgoing power" side. Then there is a "moving" crossbar with a "bridging member" that is the proper size, such that when it is pressed/pulled (forced) against the appropriate incoming and outgoing terminal, it completes an electrical path across those terminals, much like a bridge connects towns on opposite sides of a river.
The pulling force that moves the "crossbar" (and therefore the bridge contacts) is created by an electromagnetic coil. When the coil is energized, the force of the magnet pulls the crossbar and "moving contacts" in the direction, toward the coil, causing them to mate with the "stationary" (incoming/outgoing) terminal contacts. With this action, an electrical circuit is completed and the incoming power is connected to the outgoing terminals, and ultimately to the load (in our case, an electric motor).
The Electric Motor Starter is an "expansion" of the contactor.
An electric motor is used to power some load. Well, that load can become TOO GREAT for the motor to operate, and the motor becomes "overloaded".
So to handle that situation, the "contactor" is paired with an "overload relay". The short version of this story is... the "overload" relay is a device that monitors the current going to the motor. When it becomes "more than normal", the overload is designed to "trip", breaking the circuit to the coil of the contactor, thus canceling the magnetic field, and dropping out the crossbar of the contactor. The moving contacts disengage from the line and load contacts (terminals), and the motor stops. It is PROTECTED from the overload, rather than burning up and/or causing damage or a fire.
This type of motor starter is referred to as an "Across The Line" starter. That simply means that when the device is energized, and the contacts close, the electrical voltage on the line side is connected directly to the load at "FULL VOLTAGE". Other starters are "Reduced Voltage Starters" where the starting power to the motor is "reduced" by various means for a specific period and then transitions to "Full Voltage" so the motor runs at Full Voltage. A reduced voltage starter is used to reduce the mechanical shock on the motor and load and also to reduce the inrush of current on higher horsepower motors. We'll discuss Reduced Voltage Starters in a separate topic and webpage.
The motor starter is the key to good motor control operation and protection.
The Overload Relay
There are three (3) types of overload relays in the industrial market today. The older technology belongs to the "melting alloy" overload, then the "bi-metal" overload, and finally the "electronic" overload relay. It can probably be said that each of them has a place but rest assured... the electronic overload will most likely take over the industry as time marches on.
We'll attempt to address each of them here, with as accurate a description as we can muster to help you understand how they work. But first let's get the basics out of the way... all of the overload relays are used to protect the load (usually an electric motor) from excessive electrical current. So all of the types and brands monitor the CURRENT passing through them to the load (motor). With the Melting Alloy and Bi-metal types of devices, the current passes through a "heater" made usually of "Nichrome" wire (an alloy of Nickel and Chromium). The properties of the Nichrome wire are consistent and the heat generated by an excessive current is measurable. The electronic type of overload monitors the current, also, but it senses the current going through electronic circuits within the device and triggers an "overload" when pre-programmed values are reached.
Melting Alloy (Solder Pot) Overload
With the Melting Alloy overload, the electrical contact that completes the circuit to the contactor coil is held "CLOSED" by spring pressure applied to a ratchet mechanism. One design has the ratchet mechanism being held firmly in this position by the fact that the shaft of the ratchet passes through a tube that is filled with a low melting point "solder". The heater, through which the motor current passes, is wrapped around this tube. Under normal operating conditions, the solder remains solid and the ratchet mechanism remains under pressure by the spring. When the load current increases and remains "over normal" for some time, the heater applies excessive heat to the tube. At a given point, the solder will "melt" and release the hold on the shaft of the ratchet. The spring pressure will overcome the holding force and the ratchet will release, allowing the electrical contact to OPEN, thus opening the circuit to the contactor coil and releasing the magnetic field holding the power contacts and crossbar of the contactor closed. The circuit opens, and the motor stops... protected from overload.
Once the overload disappears, the heater cools down, the solder solidifies and the ratchet mechanism can be "reset" by pressing the reset button on the overload and reapplying spring pressure to the contact... ready for another run.
Bi-Metal Overload Relay
The Bi-Metal overload relay performs the same function as the Melting Alloy device. Instead of a melting alloy, however, this design uses a "bi-metal" strip. A "bi-metal" strip is just as it says... 2 strips of metal, in this case, bonded together lengthwise. The metals are chosen such that the "linear expansion" rate (when indirect heat is applied to them) is different. When this happens, the side of the bi-metal strip with the metal that expands "faster", will push the lower expanding side and bend the strip. So that's the principal...!
We use the same principle as before, by applying heat. The heater, through which the load current is passing, is close to the bi-metal strip and when it heats (above its normal operating limit), the heat is applied to the bi-metal strip by convection, and the strip bends. When it bends far enough, it pushes on the circuit contact and opens the electrical circuit to the coil of the contactor. The contactor opens the power poles and the motor stops since the voltage was interrupted, and the motor has been protected from a dangerous overload.
Today's units, rather than purchasing separate "heaters"... are manufactured with a "range" of current that can be handled by that particular device. When the unit is purchased, one needs to know the full load current of the motor to which it will be connected. Then when the installation is complete, the installer simply "dials in" the proper full load current setting on the overload relay and it's done.
Electronic Overload Relay
Electronic Overload Relays rely on the circuitry designed within the housing to protect the motor. They monitor the current going to the motor just like any other overload relay, but this device has no heaters or solder-pot to make it work. The "Electronic Digital Overload Relay", containing a microprocessor, may be used, especially for high-value motors. These devices model the heating of the motor windings by monitoring the motor current. They can also include metering and communication functions.
The photo shown above displays the simple design of a lower current device. The line and load terminals go directly into the device. On higher current units, the device is designed with "current transformers" in the housing so that the internal circuitry of the overload relay is subjected to a small current (maybe 5 amps) rather than the hundreds of amps that may be flowing to the motor. The current transformer simply drops the current down to a value that ends up being a ratio of the current flowing to the current transformer. Again, depending on the device, the current transformer ratio might be 100:5, or 200:5, or 500:5, meaning that if 100, 200, or 500 amps flows in the power circuit, a maximum of only 5 amps will be transferred to the internal circuit of the overload relay.
The additional features and sophistication of these electronic devices have become phenomenal. With the use of computers and PLCs in today's industrial setting, these overloads have certainly "come into their own', to say the least. The ability to sense overloads if the motor is now only a small portion of what they can do. Sensing "under-current" (light loading), single-phasing, unbalanced load (another layer of single-phasing), heat sensing, and other features are things that the old melting alloy and bi-metal overloads simply didn't do well if they did it at all!
And as a new feature on some manufacturer's designs... the offering of "communications" via an Ethernet network, or the internet is now a reality. Think about how convenient it would be to have your "overload relay" send you a message telling you the main motor on your most important piece of equipment is experiencing an OVERLOAD!!! How cool would that be?
So talk to us about these Standard Control items when your next project surfaces. We'll be there to help you out.