Thursday, April 24, 2014

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Discrete Control Elements - On/Off Electric Motor Control Circuits

An electric motor is often used as a discrete control element in a control system if driving a pump, conveyor belt, or other machine for the transportation of a process substance. As such, it is important to understand the functioning of motor control circuits.

Of all the available electric motor types, the most common found in industrial applications (by far) is the three-phase AC induction motor. For this reason, this section of the book will focus exclusively on this type of motor as a final control element.


AC induction motors

The basic principle of an AC induction motor is that one or more out-of-phase AC (sinusoidal) currents energize sets of electromagnet coils (called stator coils or windings) arranged around the circumference of a circle. As these currents alternately energize the coils, a magnetic field is produced which “appears” to rotate around the circle. This rotating magnetic field is not unlike the appearance of motion produced by a linear array of light bulbs blinking on and off in sequence: although the bulbs themselves are stationary, the out-of-phase sequence of their on-and-off blinking makes it appear as though a pattern of light “moves” along the length of the array. Likewise, the superposition of magnetic fields created by the out-of-phase coils resembles a magnetic field of constant intensity revolving around the circle. The following twelve images show how the magnetic field vector (the red arrow) is generated by a superposition of magnetic poles through one complete cycle (1 revolution), viewing the images from left to right, top to bottom (the same order as you would read words in an English sentence):


Magnetic field vector is generated by superposition of magnetic poles through one complete cycle.


If an electrically conductive object is placed within the circle on a shaft so that it is free to rotate, the relative motion between the rotating magnetic field and the conductive object induces electric currents in the conductive object, which produce magnetic fields of their own. Lenz’s Law tells us that the effect of these induced magnetic fields will be to try to oppose change: in other words, the induced fields react against the rotating magnetic field of the stator coils in such a way as to minimize the relative motion. This means the conductive object will try to rotate in sync with the stator’s rotating magnetic field. In a typical squirrel-cage induction motor design, the rotor is made up of aluminum bars joining two aluminum “shorting rings,” one at either end of the rotor. Iron fills the spaces between the rotor bars to provide a lower-reluctance magnetic “circuit” between stator poles than would be otherwise formed if the rotor were simply made of aluminum.

A photograph of a small, disassembled three-phase AC induction “squirrel-cage” motor is shown here, revealing the construction of the stator coils and the rotor:


An AC Induction squirrel cage motor revealing the construction of the stator coils and the rotor


Given the simple design of AC induction motors, they tend to be quite reliable machines. So long as the stator coil insulation is not damaged by excessive moisture, heat, or chemical exposure, they will just about run forever. The only “wearing” components are the bearings supporting the rotor shaft, and those tend to be easy to replace.

Starting a three-phase induction motor is as simple as applying full power to the stator windings. When this happens, the motor will draw a large amount of current (as much as ten times its normal running current) called the inrush current, causing the rotor to produce a large mechanical torque. As the rotor gains speed, the current reduces to a normal level, with the speed approaching the “synchronous” speed of the rotating magnetic field1.

Reversing the rotational direction of a three-phase motor is as simple as swapping any two out of three power conductor connections. This has the effect of reversing the phase sequence of the power “seen” by the motor2.


Starter (contactor) relays

Due to the large “inrush” currents at start-up time for a three-phase induction motor, a special form of electromechanical relay with high overcurrent capacity is used to make and break the three-phase line connections to the motor’s stator winding terminals. These special motor-starting relays are commonly referred to as starters or contactors.

A photograph of a motor starter, or contactor, rated at 75 horsepower (assuming 480 volt AC 3-phase power) is shown here, both assembled and with the top cover removed to reveal the three sets of high-current electrical contacts:


motor starter or contactor assembly with the top cover removed to reveal three sets of high current electrical contacts

Each contact is actually a series pair of contacts that make and break simultaneously with the actuation of an iron armature attracted by an electromagnet coil in the base of the contactor assembly. The operation of the three contact sets may be seen in this pair of photographs, the lefthand image showing the contacts in their normal (open) state, and the right-hand image showing the contacts closed by the force of my finger:


Motor contactor opened to show how its contacts work

Of course, it would be highly unsafe to touch or manually actuate the contacts of a motor starting relay with the cover removed as shown. Not only would there be an electric shock hazard from touching any one of the bare copper contacts with your finger, but the arc blast produced by closing and opening such contacts would pose a burn and blast hazard. This is why all modern motor contactors are equipped with arc shield covers.

The actual contact pads are not made of pure copper, but rather silver (or a silver alloy) designed to survive the repeated arcing and blasting action of large AC currents being initiated and interrupted.

Below the main power (line) connection terminals on this starter hide two small screw terminals providing connection points to the electromagnet coil actuating the starter:


Motor Contactor Electromagnetic Coil terminals

Like nearly every other three-phase contactor in existence, this one’s coil is rated for 120 volts AC. Although the electric motor may operate on three-phase, 480 volt AC power, the contactor coil and the rest of the control circuitry operates on a lower voltage for reasons of safety.


Motor overload protective devices

An essential component of any motor control circuit is some device to detect a condition of excessive overload and interrupt power to the motor before thermal damage will occur to it. A very simple and common overload protective device is known as an overload heater, consisting of resistive elements connected in series with the three lines of a 3-phase AC motor, designed to heat and to cool at rates modeling the thermal characteristics of the motor itself.

The following photograph shows a three-phase starter (contactor) relay joined together with a set of three “overload heaters” through which all of the motor’s current flows. The overload heaters appear as three brass-colored metal strips near a red push-bar labeled “Reset:”


Three Phase Contactor with Thermal Overload protection

Removing one of the heater elements reveals its mechanical nature: a small toothed wheel on one side engages with a lever when it is bolted into place in the overload assembly. That lever connects to a spring-loaded mechanism charged by the manual actuation of the red “Reset” push-bar, which in turn actuates a small set of electrical switch contacts:


Thermal Overload Relay disassembled


The purpose of the overload heater is to heat up as the motor draws excessive current. The small toothed wheel is held in place by a rod immersed in a solidified mass of solder, encased in a brass cylinder underneath the heater strip. The next photograph shows the underside of the heater element, with the toothed wheel and brass cylinder plainly visible:


Thermal Overload Relay Parts showing the 'gear'

If the heater element becomes too hot (due to excessive motor current), the solder inside the brass cylinder will melt, allowing the toothed wheel to spin. This will release spring tension in the overload mechanism, allowing the small electrical switch to spring to an open state. This “overload contact” then interrupts current to the motor starter’s electromagnet coil, causing the starter to de-energize and the motor to stop.

Manually pressing the “Reset” push-bar will re-set the spring mechanism and re-close the overload contact, allowing the starter to energize once more, but only once the overload heater element has cooled down enough for the solder inside the brass cylinder to re-solidify. Thus, this simple mechanism prevents the overloaded motor from being immediately re-started after a thermal overload “trip” event, giving it time to cool down as well.

A typical “trip curve” for a thermal overload unit is shown here, with time plotted against the severity of the overcurrent level:


Thermal Overload Unit graph with time against severity of the overcurrent level

In contrast to a circuit breaker or fuse – which is sized to protect the power wiring from overcurrent heating – the overload heater elements are sized specifically to protect the motor. As such, they act as thermal models of the motor itself, heating to the “trip” point just as fast as the motor itself will heat to the point of maximum rated temperature, and taking just as long to cool to a safe temperature as the motor will. Another difference between overload heaters and breakers/fuses is that the heaters are not designed to directly interrupt current by opening3, as fuses or breakers do. Rather, each overload heater serves the simple purpose of warming proportionately to the magnitude and time duration of motor overcurrent, causing a different electrical contact to open, which in turn triggers the starter relay to open.

Of course, overload heaters only work to protect the motor from thermal overload if they experience similar ambient temperature conditions. If the motor is situated in a very hot area of the industrial process unit, whereas the overload elements are located in a climate-controlled “motor control center” (MCC) room, they may fail to protect the motor as designed. Conversely, if the overload heaters are located in a hot room while the motor is located in a freezing-cold environment (e.g. the MCC room lacks air conditioning while the motor is located in a freezer), they may “trip” the motor prematurely.

An interesting “trick” to keep in mind for motor control circuit diagnosis is that overload heaters are nothing more than low-value resistors. As such, they will drop small amounts of voltage (usually quite a bit less than 1 volt AC) under full load current. This voltage drop may be used as a simple, qualitative measure of motor phase current. By measuring the voltage dropped across each overload heater (with the motor running), one may ascertain whether or not all phases are carrying equal currents. Of course, overload heaters are not precise enough in their resistance to serve as true current-measuring “shunts,” but they are more than adequate as qualitative indicators of relative phase current, to aid you in determining (for instance) if the motor suffers from an open or high resistance phase winding:


Motor Circuit Trick


As useful as thermal overload “heaters” are for motor protection, there are more effective technologies available. An alternative way to detect overloading conditions is to monitor the temperature of the stator windings directly, using thermocouples or (more commonly) RTDs, which report winding temperatures to an electronic “trip” unit with the same control responsibilities as an overload heater assembly. This sophisticated approach is used on large (thousands of horsepower) electric motors, and/or in critical process applications where motor reliability is paramount. Machine vibration equipment used to monitor and protect against excessive vibration in rotary machines is often equipped with such temperature-sensing “trip” modules just for this purpose. Not only can motor winding temperatures be monitored, but also bearing temperatures and other temperature sensitive machine components so that the protective function extends beyond the health of the electric motor.


Motor control circuit wiring

A simple three-phase, 480 volt AC motor-control circuit is shown here, both in pictorial and schematic form:


A simple three-phase, 480 volt AC motor-control circuit

Note how a control power transformer steps down the 480 volt AC to provide 120 volt AC power for the starter coil to operate on. Furthermore, note how the overload (“OL”) contact is wired in series with the starter’s coil so that a thermal overload event forces the starter to de-energize and thus interrupt power to the motor even if the control switch is still in the “on” position. The overload heaters appear in the schematic diagram as pairs of back-to-back “hook” shapes, connected in series with the three “T” lines of the motor. Remember that these “OL” heater elements do not directly interrupt power to the motor in the event of an overload, but rather signal the “OL” contact to open up and de-energize the starter (contactor).

In an automatic control system, the toggle switch would be replaced by another relay contact (that relay controlled by the status of a process), a process switch, or perhaps the discrete output channel of a programmable logic controller (PLC).

It should be noted that a toggling-style of switch is necessary in order for the motor to continue to run after a human operator actuates the switch. The motor runs with the switch in the closed state, and stops when the switch opens. An alternative to this design is to build a latching circuit allowing the use of momentary contact switches (one to start, and one to stop). A simple latching motor control circuit is shown here:


A simple three-phase, 480 volt AC motor-control circuit simple latching circuit


In this circuit, an auxiliary contact actuated by the motor starter (contactor) is wired in parallel with the “Start” pushbutton switch, so that the motor contactor continues to receive power after the operator releases the switch. This parallel contact – sometimes called a seal-in contact – latches the motor in an “on” state after a momentary closure of the “Start” pushbutton switch. A normally-closed “Stop” switch provides a means to “un-latch” the motor circuit. Pressing this pushbutton switch stops current in the coil of the contactor, causing it to de-energize, which then opens the three motor power contacts as well as the auxiliary contact that used to maintain the contactor’s energized state. A simple ladder diagram showing the interconnections of all components in this motor control circuit makes this system easier to understand:


Interconnctions of all components of motor control circuit

Most on/off motor control circuits in the United States are some variation on this wiring theme, if not identical to it. Once again, this system could be automated by replacing the “Start” and “Stop” pushbutton switches with process switches (e.g. pressure switches for an air compressor control system) to make a system that starts and stops automatically. A programmable logic controller (PLC) may also be used to provide the latching function rather than an auxiliary contact on the motor starter. Once a PLC is included in the motor control circuit, a great many automatic control features may be added to enhance the system’s capabilities. Examples include timing functions, motor cycle count functions, and even remote start/stop capability via a digital network connecting to operator interface displays or other computers.

1The rotor can never fully achieve the same rotational speed as the magnetic field produced by the stator windings, because if it did there would be zero relative motion between the rotating magnetic field and the rotating rotor, and thus no induction of currents in the rotor bars to create the induced magnetic fields necessary to produce a reaction torque. Thus, the rotor must “slip” behind the speed of the rotating magnetic field in order to produce a torque, which is why the full-load speed of an induction motor is always just a bit slower than the synchronous speed of the rotating magnetic field (e.g. a 4-pole motor with a synchronous speed of 1800 RPM will rotate at approximately 1750 RPM).

2This principle is not difficult to visualize if you consider the phase sequence as a repeating pattern of letters, such as ABCABCABC. Obviously, the reverse of this sequence would be CBACBACBA, which is nothing more than the original sequence with letters A and C transposed. However, you will find that transposing any two letters of the original sequence transforms it into the opposite order: for example, transposing letters A and B turns the sequence ABCABCABC into BACBACBAC, which is the same order as the sequence CBACBACBA.

3This is not to say overload heaters cannot fail open, because they can and will under extraordinary circumstances. However, opening like a fuse is not the design function of an overload heater.



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