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Introduction
Industrial control, in its broadest sense, encompasses all the methods used to control the performance of an electrical system. When applied to machinery, it involves the starting, acceleration, reversal, deceleration, and stopping of a motor and its load. In this section we will study the electrical (but not electronic) control of 3-phase alternating current motors. Our study is limited to elementary circuits because industrial circuits are usually too intricate to explain briefly. However, the basic principles covered here apply to any system of control, no matter how complex it may appear to be.
Control devices
Every control circuit is composed of a number of basic components connected together to achieve the desired performance. The size of the components varies with the power of the motor, but the principle of operation remains the same. Using only a dozen basic components, it’s possible to design control systems that are very complex. The basic components are the following:
- 1. Disconnecting switches
- 2. Manual circuit breakers
- 3. Cam switches
- 4. Pushbuttons
- 5. Relays
- 6. Magnetic contactors
- 7. Thermal relays and fuses
- 8. Pilot lights
- 9. Limit switches and other special switches
- 10. Resistors, reactors, transformers, and capacitors
The ensuing list of Basic Components for Control Circuits illustrates these devices, and states their main purpose and application. Fuses are not included here because they are protective devices rather than control devices. The symbols for these and other devices are given below.
BASIC COMPONENTS FOR CONTROL CIRCUITS
Disconnecting switches
A disconnecting switch isolates the motor from the power source. It consists
of 3 knife-switches and 3 line fuses enclosed in a metallic box. The
knife-switches can be opened and closed simultaneously by means of an
external handle. An interlocking mechanism prevents the hinged cover from
opening when the switch is closed. Disconnecting switches (and their fuses)
are selected to carry the nominal full-load current of the mo tor, and to
withstand short-circuit currents for brief intervals.
Manual circuit breakers
A manual circuit breaker opens and closes a circuit, like a toggle switch. It trips (opens) automatically when the current exceeds a predetermined limit. After tripping, it can be reset manually. Manual circuit breakers are often used instead of disconnecting switches because no fuses have to be replaced.
above: Three-phase circuit breaker, 600V, 100A.
Cam switches
A cam switch has a group of fixed contacts and an equal number of moveable contacts. The contacts can be made to open and close in a preset sequence by rotating a handle or knob. Cam switches are used to control the motion and position of hoists, callenders, machine tools, and so on.
above: Three-phase surface-mounted cam switch..
Push-buttons
A pushbutton is a switch activated by finger pressure.
Two or more contacts open or close when the button is depressed. Push-buttons are usually spring loaded so as to return to their normal position when pressure is re moved.
--4. Mechanical-interlocked push buttons with NO (normally open) and NC (normally closed) contacts; rated to interrupt an ac current of 6 A one million times.
Control relays
A control relay is an electromagnetic switch that opens and closes a set of contacts when the relay coil is energized. The relay coil produces a strong magnetic field which attracts a movable armature bearing the contacts.
Control relays are mainly used in low-power circuits.
They include time-delay relays whose contacts open or close after a definite
time interval. Thus, a time-delay closing relay actuates its contacts after
the relay coil has been energized. On the other hand, a time-delay opening
relay actuates its contacts some time after the relay coil has been de-energized.
--5. Single-phase relays: 25A, 115/230V and 5A, 115V.
Thermal relays
A thermal relay (or overload is a temperature sensitive device whose contacts open or close when the motor current exceeds a preset limit. The current flows through a small, calibrated heating element which raises the temperature of the relay. Thermal relays are inherent time-delay devices because the temperature cannot follow the instantaneous changes in current.
above: Three-phase thermal relay with variable current setting, 6A to 10A.
Magnetic contactors
A magnetic contactor is basically a control relay designed to open and close a power circuit. It possesses a relay coil and a magnetic plunger, which carries a set of movable contacts. When the relay coil is energized, it attracts the magnetic plunger, causing it to rise quickly against the force of gravity. The movable contacts come in contact with a set of fixed contacts, thereby closing the power circuit. In addition to the power contacts, one or more normally open or normally closed auxiliary contacts are usually available, for control purposes. When the relay coil is de-energized, the plunger falls, thereby opening and closing the respective contacts. Magnetic contactors are used to control motors ranging from 0.5 hp to several hundred horsepower. The size, dimensions, and performance of contactors are standardized.
Three-phase magnetic contactor rated 50 hp, 575V, 60 Hz. Width:
158 mm; height: 155 mm; depth: 107 mm; weight: 3.5 kg. (Siemens)
Pilot lights
A pilot light indicates the on/off state of a remote component in a control system.
--8. Pilot light, 120 V, 3 W mounted in a start-stop push button station. (Siemens)
Limit switches and special switches: A limit switch is a low-power snap-action device that opens or closes a contact, depending upon the position of a mechanical part. Other limit switches are sensitive to pressure, temperature, liquid level, direction of rotation, and so on.
--9a Limit switch with one NC contact; rated for ten million operations; position accuracy: 0.5 mm. (Square D)
--9b Liquid level switch. (Square D)
Proximity detectors: Proximity detectors are sealed devices that can detect objects without coming in direct contact with them.
Their service life is independent of the number of operations. They are wired to an external dc source and generate an alternating magnetic field by mean" of an internal oscillator. When a metal object comes within a few millimeters of the detector, the magnetic field decreases, which in turn causes a dc control current to flow. This current can be used to activate another control device, such as a relay or a programmable logic controller.
Capacitive proximity detectors, based on a similar principle but generating an ac electric field, are able to detect nonmetallic objects, including liquids.
--10 Proximity detector to monitor the loading of a conveyor belt (Telemecanique, Groupe Schneider)
In order to understand the sections that follow the legends should be read before proceeding further.
Normally open and normally closed contacts
Control circuit diagrams always show components in a state of rest, that is, when they are not energized (electrically) or activated (mechanically). In this state, some electrical contacts are open while others are closed. They are respectively called normally open contacts (NO) and normally closed contacts (NC) and are designated by the following symbols: normally open contact (NO) normally closed contact (NC)
Relay coil exciting current
When a magnetic contactor is in its de-energized or open position, the magnetic circuit has a very long air gap, compared to when the contactor is closed.
Consequently, in the case of an ac contactor the inductive reactance of the relay coil is much lower when the contactor is open than when it’s closed.
Because the coil is excited by a fixed ac voltage, the magnetizing current is much higher in the open than in the closed contactor position. In other words, a considerable inrush current is drawn by the relay coil at the moment it’s excited. This places a heavier than expected duty on auxiliary contacts that energize the coil.
Example +++1
A 3-phase NEMA size 5 magnetic contactor rated at 270 A, 460 V possesses a 120 V, 60 Hz relay coil.
The coil absorbs an apparent power of 2970 VA and 212 VA, respectively, in the open and closed contactor position. Calculate the following:
a. The inrush exciting current
b. The normal, sealed exciting current
c. The control power needed to actuate the relay coil compared to the power handled by the contactor Solution
a. The inrush current in the relay coil is:
I=S/E = 2970/120 = 24.75 A
b. The normal relay coil current when the contactor is sealed (closed) is:
I= S/E 2121120 = 1.77 A
c. The steady-state apparent control power needed to actuate the relay coil is 212 VA. The apparent power that the contactor can handle is
S = EI sqr-rt (3 ) = 460 X 270 sqr-rt (3 )
= 215 120 VA
===
GRAPHIC SYMBOLS FOR ELECTRICAL DIAGRAMS
1. terminal; connection 2; conductors crossing 3. conductors connected 4. three conductors 5. plug; receptacle 6. separable connector 7. ground connection; arrester 8. disconnecting switch 9. normally open con tact (NO) 10. normally closed contact (NC) 11. pushbutton NO; NC 12. circuit-breaker 13. single-pole switch; three-way switch 14. double pole double throw switch 15. fuse 16. thermal overload element 17. relay coil 18. resistor 19. winding, inductor or reactor 20. capacitor; electrolytic capacitor 21. transformer 22. current transformer; bushing type 23. potential transformer 24. dc source (general) 25. cell 26. shunt winding 27. series winding; commutating pole or compensating winding 28. motor; generator (general symbols) 29. dc motor; dc generator (general symbols) 30. ac motor; ac generator (general symbols) 32. 3-phase squirrel-cage induction motor; 3-phase wound-rotor motor 33. synchronous motor; 3-phase alternator 34. diode 35. thyristor or SCR 36. 3-pole circuit breaker with magnetic overload device, drawout type 37. dc shunt motor with commutating winding; permanent magnet dc generator 38. magnetic relay with one NO and one NC contact. 39. NPN transistor 40. PNP transistor 41. pilot light
===
Thus, the small control power (212 VA) can control a load whose power is 215 120/212 = 1015 times greater.
Control diagrams
A control system can be represented by four types of circuit diagrams. They are listed as follows, in order of increasing detail and completeness:
- • block diagram
- • one-line diagram
- • wiring diagram
- • schematic diagram
--11 Block diagram of a combination starter.
600 V / 3-phases --> fused disconnecting switch --> thermal overload relay
--12 One-line diagram of a combination starter.
--13 Wiring diagram of a combination starter.
--14 Schematic diagram of a combination starter.
A block diagram is composed of a set of rectangles, each representing a control device, together with a brief description of its function. The rectangles are connected by arrows that indicate the direction of power or signal flow. A one-line diagram is similar to a block diagram, except that the components are shown by their symbols rather than by rectangles. The symbols give us an idea of the nature of the components; consequently, one-line diagrams yield more information.
A list of typical symbols is displayed in Table 20A. The lines connecting the various components represent two or more conductors. A wiring diagram shows the connections between the components, taking into account the physical location of the terminals and even the color of wire. These diagrams are employed when in stalling equipment or when troubleshooting a circuit.
A schematic diagram shows all the electrical connections between components, without regard to their physical location or terminal arrangement.
This type of diagram is indispensable when troubleshooting a circuit or analyzing its mode of operation ---14). In the sections that follow, this is the kind of diagram we will be using.
The reader should note that the four diagrams all relate to the same control circuit. The symbols used to designate the various components are given in Table 20B.
Starting methods
Three-phase squirrel-cage motors are started either by connecting them directly across the line or by applying reduced voltage to the stator. The starting method depends upon the power capacity of the supply line and the type of load.
Across-the-line starting is simple and inexpensive. The main disadvantage is the high starting current, which is 5 to 6 times the rated full-load current.
It can produce a significant line voltage drop, which may affect other customers connected to the same line. Voltage-sensitive devices such as incandescent lamps, television sets, and high-precision machine tools respond badly to such voltage dips.
Mechanical shock is another problem that should not be overlooked. Equipment can be seriously dam aged if full-voltage starting produces a hammer-blow torque. Conveyor belts are another example where sudden starting may not be acceptable.
In large industrial installations we can some times tolerate across-the-line starting even for motors rated up to 10000 hp. Obviously, the fuses and circuit breakers must be designed to carry the starting current during the acceleration period.
A motor control circuit contains two basic components: a disconnecting switch and a starter. The disconnecting switch is always placed between the supply line and the starter. The switch and starter are sometimes mounted in the same enclosure to make a combination starter. The fuses in the disconnecting switch are rated at about 3.5 times full load current; consequently, they don’t protect the motor against sustained overloads. Their primary function is to protect the motor and supply line against catastrophic currents resulting from a short circuit in the motor or starter or a failure to start up.
Under normal start-up conditions, the fuses don’t have time to blow, even though the initial current is 6 to 7 times full-load current. The fuse rating, in amperes, must comply with the requirements of the National Electric Code.
In some cases the disconnecting switch and its fuses are replaced by a manual circuit breaker.
Manual across-the-line starters
Manual 3-phase starters are composed of a circuit breaker and either two or three thermal relays, all mounted in an appropriate enclosure. Such starters are used for small motors (10 hp or less) at voltages ranging from 120 V to 600 V. The thermal relays trip the circuit breaker whenever the current in one of the phases exceeds the rated value for a significant length of time.
--15 Manual starters for single-phase motors rated 1 hp (0.75 kW); left: surface mounted; center: flush mounted; right waterproof enclosure. (Siemens)
Single-phase manual starters are built along the same principles but they contain only one thermal relay. The thermal relays are selected for the particular motor that is connected to the starter.
Magnetic across-the-line starters
--16a Three-phase across-the-line magnetic starter, 30 hp, 600 V, 60 Hz.
--16b Schematic diagram of a 3-phase across-the-line magnetic starter.
--17 Typical curve of a thermal overload relay, showing trip ping time versus line current. The tripping time is measured from cold-start conditions. If the motor has been operating at full-load for one hour or more, the tripping time is reduced about 30 percent.
--18 Three-phase across-the-line combination starter, 150 hp, 575 V, 60 Hz. The protruding knob controls the disconnecting switch; the pushbutton station is set in the transparent polycarbonate cover. (Klockner-Moeller)
Magnetic across-the-line starters are employed whenever a motor has to be controlled from a remote location. They are also used whenever the power rating exceeds 10 kW. ---16 shows a typical magnetic starter and its associated schematic diagram. The disconnecting switch is external to the starter. The starter has three main components: a magnetic contactor, a thermal relay, and a control station. We now de scribe these components.
1. The magnetic contactor A possesses three heavy contacts A and one small auxiliary contact Ax. As can be seen, these contacts are normally open.
Contacts A must be big enough to carry the starting current and the nominal full-load current without overheating. Contact Ax is much smaller because it only carries the current of relay coil A. The relay coil is represented by the same symbol (A) as the contacts it controls. Contacts A and Ax remain closed as long as the coil is energized.
2. The thermal relay T protects the motor against sustained overloads.
* The relay comprises three individual heating elements, respectively connected in series with the three phases. A small, normally closed contact T forms part of the relay assembly. It opens when the thermal relay gets too hot and stays open until the relay is manually reset.
The current rating of the thermal relay is chosen to protect the motor against sustained overloads. Contact T opens after a period of time that depends upon the magnitude of the overload current. Thus, the tripping time as a multiple of the rated relay current. At rated current (multiple I), the relay never trips, but at twice rated current, it trips after an interval of 40s. The thermal relay is equipped with a reset button enabling us to re close contact T following an overload. It’s preferable to wait a few minutes before pushing the button to allow the relay to cool down.
3. The control station, composed of start-stop push buttons, may be located either close to, or far away from the starter. The pilot light is optional.
Referring to the schematic, to start the motor we first close the disconnecting switch and then depress the start button. Coil A is immediately energized causing contacts A and Ax to close. The full line volt age appears across the motor and the pilot light is on.
When the pushbutton is released it returns to its normal position, but the relay coil remains excited be cause auxiliary contact Ax is now closed. Contact Ax is said to be a self-sealing contact.
To stop the motor, we simply push the stop button, which opens the circuit to the coil. In case of a sustained overload, the opening of contact T produces the same effect.
It sometimes happens that a thermal relay will trip for no apparent reason. This condition can occur when the ambient temperature around the starter is too high. We can remedy the situation by changing the location of the starter or by replacing the relay by another one having a higher current rating. Care must be taken before making such a change, because if the ambient temperature around the motor is also too high, the occasional tripping may actually serve as a warning.
---18 shows a typical combination starter.
---19 shows another combination starter equipped with a small step-down transformer to ex cite the control circuit. Such transformers are always used on high-voltage starters (above 600 V) because they permit the use of standard control components, such as pushbuttons and pilot lights while reducing the shock hazard to operating personnel.
---20 shows a medium-voltage across-the line starter for a 2500 hp, 4160 V, 3-phase, 60 Hz squirrel-cage motor. The metal compartment houses three fuses and a 3-phase vacuum contactor.
The contactor can perform 250 000 operations at full-load before maintenance is required. The 120 V holding coil draws 21.7A during pull-in, and the current drops to 0.4A during normal operation.
Closing and opening times of the main contactor are respectively 65 ms and 130 ms.
---21 shows a special combination starter that can be reset remotely following a short-circuit. Its distinguishing feature is that it’s programmable and requires no fuses. The sophisticated contactor is designed to interrupt short-circuit currents in less than 3 ms, which is comparable to that offered by HRC fuses. The contactor acts also as a disconnecting switch and consequently the overall size is much smaller than more conventional combination starters.
--19 Three-phase across-the-line combination starter rated 1 00 hp, 575 V, 60 Hz. The isolating circuit breaker is controlled by an external handle. The magnetic contactor is mounted in the bottom left-hand corner of the waterproof enclosure. The small 600 V/120 V transformer in the lower right-hand corner supplies low voltage power for the control circuit. (Square D)
--20 Three-phase 5 kV starter for a 2500 hp cage motor.
The medium- and low-voltage circuits are completely isolated from each other to ensure safety. The compartment is 2286 mm high, 610 mm wide, and 813 mm deep. The entire starter weighs 499 kg. (Square D, Groupe Schneider)
Inching and jogging
In some mechanical systems, we have to adjust the position of a motorized part very precisely. To accomplish this, we energize the motor in short spurts so that it barely starts before it again comes to a halt.
A double-contact pushbutton J is added to the usual start/stop circuit. This arrangement permits conventional start-stop control as well as jogging, or inching. The following description shows how the control circuit operates.
If the jog button J is in its normal position (not depressed) relay coil A is excited as soon as the start button is depressed. Sealing contact Ax in the main contactor closes and so the motor will continue to turn after the start button is released. Thus, the control circuit operates in the same way.
Suppose now that the motor is stopped and we depress the jog button. This closes contacts 3, 4 and relay coil A is excited. Contact Ax closes, but contacts 1, 2 are now open and the closure of Ax has no effect. The motor will pick up speed so long as the jog button is depressed. However, when it’s re leased, coil A will become de-energized and contactor A will drop out, causing Ax to open. Thus, when contacts 1, 2 are again bridged, the motor will come to a halt. Thus, by momentarily depressing the jog button we can briefly apply power to the motor.
Jogging imposes severe duty on the main power contacts A because they continually make and break currents that are 6 times greater than normal.
It’s estimated that each impulse corresponds to 30 normal start-stop operations. Thus, a contactor that can normally start and stop a motor 3 million times, can only jog the motor 100000 times, because the contacts have to be replaced. * Furthermore, jogging should not be repeated too quickly, because the intense heat of the breaking arc may cause the main contacts to weld together. Repeated jogging will also overheat the motor. When is required, the contactor is usually selected to be one NEMA size larger than that for normal duty.
Reversing the direction of rotation
We can reverse the direction of rotation of a 3-phase motor by interchanging any two lines. This can be done by using two magnetic contactors A and B and a, manual 3-position cam switch as shown in ---23.
When contactor A is closed, lines L1 , and L3 are connected to terminals A, B, C of the motor. But when contactor B is closed, the same lines are connected to motor terminals C, B, A In the forward direction, the cam switch engages contact 1, which energizes relay coil A, causing contactor A to close. **
--21 Special self-protected starter rated at 40 hp, 460 V, 60 Hz. In addition to a short-circuit capability of 42 kA at 460 V, it features adjustable thermal and magnetic trip settings. Overall dimensions: 243 mm high, 90 mm wide, 179 mm deep. (Telemecanique, Groupe Schneider)
--22 Control circuit and pushbutton station for start-stop job operation. Terminals 8, L3 correspond to terminals 8, L3 in ---13.
--23a Simplified schematic diagram of a reversible magnetic starter.
--23b Three-position cam switch in ---23a. (Siemens)
--23c Emergency stop pushbutton in ---23a. (Square D)
--
A magnetic contact has an estimated mechanical life of about 20 million open/close cycles, but the electrical contacts should be replaced after 3 million normal cycles.
The contacts and relay coils may be designated by any appropriate letters. Thus, the letters F and R are often used to designate forward and reverse operating components. In this guide we have adopted the letters A and B mainly for reasons of continuity from one circuit to the next.
--
--24a Simplified schematic diagram of a starter with plugging control.
To reverse the rotation, we move the cam switch to position 2. However, in doing so, we have to move past the off position (0). Consequently, it’s impossible to energize coils A and B simultaneously.
Occasionally, however, a mechanical defect may prevent a contactor from dropping out, even after its relay coil is de-energized. This is a serious situation, because when the other contactor closes, a short circuit results across the line. The short-circuit current could easily be 50 to 500 times greater than normal, and both contactors could be severely dam aged. To eliminate this danger, the contactors are mounted side by side and mechanically interlocked, so as to make it physically impossible for both to be closed at the same time. The interlock is a simple steel bar, pivoted at the center, whose extremities are tied to the movable armature of each contactor.
During an emergency, pushbutton U, equipped with a large red bull's-eye, can be used to stop the motor (---23c). In practice, operators find it easier to hit a large button than to turn a cam switch to the off position.
--24b Typical zero-speed switch for use in ---24a.
Plugging
We have already seen that an induction motor can be brought to a rapid stop by reversing two of the lines (Section 14.8). However, to prevent the motor from running in reverse, a zero-speed switch must open the line as soon as the machine has come to rest. The circuit of ---24a shows the basic elements of such a plugging circuit. The circuit operates as follows:
1. Contactor A is used to start the motor. In addition to its 3 main contacts A, it has 2 small auxiliary contacts Ax I and
2. The start pushbutton has one NO contact 1, 2 and one NC contact 3, 4 which operate together. Thus, contact 3, 4 opens before contact 1, 2 closes.
3. Contactor B is used to stop the motor. It’s identical to contactor A, having 2 auxiliary contacts Bx_1 and Bx_2 in addition to the 3 main contacts B.
4. The stop pushbutton is identical to the start pushbutton. Thus, when it’s depressed contact 7, 8 opens before contact 5, 6 closes.
5. Contact F-C of the zero-speed switch is normally open, but it closes as soon as the motor turns in the forward direction. This prepares the plugging circuit for the eventual operation of coil B.
6. Contacts Ax 1 and Bx 1 are sealing contacts so that pushbuttons A and B have only to be pressed momentarily to start or stop the motor.
7. Contacts Ax2 and Bx2 are electrical interlocks to prevent the relay coils A and B from being ex cited at the same time. Thus, when the motor is running, contact Ax2 is open. Consequently, relay coil B cannot become excited by de pressing pushbutton B until such time as contactor A has dropped out, causing contact Ax_1 to reclose.
Several types of zero-speed switches are on the market and ---24b shows one that operates on the principle of an induction motor. It consists of a small permanent magnet rotor N, S and a bronze ring or cup supported on bearings, which is free to pivot between stationary contacts F and R. The permanent magnet is coupled to the shaft of the main motor. As soon as the motor turns clockwise, the permanent magnet drags the ring along in the same direction, thereby closing contacts F-C. When the motor stops turning, the brass ring returns to the off position. Because of its function and shape, the ring is often called a drag-cup.
---24c shows another zero-speed switch that operates on the principle of centrifugal force.
Reduced-voltage starting
Some industrial loads have to be started very gradually. Examples are coil winders, printing presses, conveyor belts, and machines that process fragile products. In other industrial applications, a motor cannot be directly connected to the line because the starting current is too high. In all these cases we have to reduce the voltage applied to the motor either by connecting resistors (or reactors) in series with the line or by employing an autotransformer. In reducing the voltage, we recall the following: I. The locked-rotor current is proportional to the voltage: reducing the voltage by half reduces the current by half.
2. The locked-rotor torque is proportional to the square of the voltage: reducing the voltage by half reduces the torque by a factor of four.
--25a Simplified schematic diagram of the power section of a reduced-voltage primary resistor stator.
--25b Control circuit of ---25a.
--25c Control circuit of ---25a using an auxiliary relay RA.
--26a Typical torque-speed curves of a 3-phase squirrel-cage induction motor: (1) full-voltage starting; (2) primary resistance starting with voltage reduced to 0.65 pu.
--26b Typical current-speed curves of a 3-phase squirrel-cage induction motor: (1) full-voltage starting; (2) primary resistance starting with voltage reduced to 0.65 pu.
Primary resistance starting
Primary resistance starting consists of placing three resistors in series with the motor during the start-up period. Contactor A closes first and when the motor has nearly reached synchronous speed, a second contact B short-circuits the resistors.
This method gives a very smooth start with complete absence of mechanical shock. The voltage drop across the resistors is high at first, but gradually diminishes as the motor picks up speed and the current falls. Consequently, the voltage across the motor terminals increases with speed, and so the electrical and mechanical shock is negligible when full voltage is finally applied (closure of contactor B). The resistors are short-circuited after a delay that depends upon the setting of a time-delay relay.
The schematic control diagram reveals the following circuit elements: A, B: magnetic contactor relay coils A,: auxiliary contact associated with A RT: time-delay relay that closes the circuit of coil B after a preset interval of time As soon as the start pushbutton is depressed, relay coils A and RT are excited. This causes the contacts A and Ax to close immediately. However, the contact RT only closes after a certain time delay and so the relay coil of contactor B is only excited a few seconds later.
If the magnetic contactors A, B are particularly large, the inrush exciting currents could damage the start pushbutton contacts if they are connected as shown. In such cases, it’s better to add an auxiliary relay having more robust contacts.
Thus, the purpose of auxiliary relay RA is to carry the exciting currents of relay coils A and B. Note that the start pushbutton contacts carry only the exciting current of relay coils RA and RT. Other circuit components are straightforward, and the reader should have no difficulty in analyzing the operation of the circuit.
How are the starting characteristics affected when resistors are inserted in series with the stator?
---26a shows the torque-speed curve 1 when full voltage is applied to a typical 3-phase, 1800 RPM induction motor. Corresponding curve 2 shows what happens when resistors are inserted in series with the line. The resistors are chosen so that the locked-rotor voltage across the stator is 0.65 pu.
The locked-rotor torque is, therefore, (0.65)1 0.42 pu or only 42% of full-load torque. This means that the motor must be started at light load.
---26b shows the current versus speed curve 1 when full voltage is applied to the stator. Curve 2 shows the current when the resistors are in the circuit.
When the speed reaches about 1700 rpm, the resistors are short-circuited. The current jumps from about 1.8 pu to 2.5 pu, which is a very moderate jump.
Example 2 ______
A 150 kW (200 hp). 460 Y, 3-phase 3520 RPM, 60 Hz induction motor has a locked-rotor torque of 600 N m and a locked-rotor current of 1400 A. Three resistors are connected in series with the line so as to reduce the voltage across the motor to 0.65 pu.
Calculate:
a. The apparent power absorbed by the motor under full-voltage. locked-rotor conditions
b. The apparent power absorbed by the motor when the resistors are in the circuit
c. The apparent power drawn from the line, with the resistors in the circuit
d. The locked-rotor torque developed by the motor Solution
a. At full voltage the locked-rotor apparent power is
b. The voltage across the motor at 0.65 pu is
The current drawn by the motor decreases in proportion to the voltage:
I = 0.65 X 1400 910A
The apparent power drawn by the motor is:
c. The apparent power drawn from the line is = 724 kVA
Thus, percentagewise, the apparent power is only 724 kVAlll14 kVA = 65% of the apparent power under full-voltage conditions.
d. The torque varies as the square of the voltage: T = 0.652 X 600
0.42 X 600 = 252 N m (~186 ft·lbf) The results of these calculations are summarized.
Example 3: In Example 2, if the locked-rotor power factor of the motor alone is 0.35, calculate the value of the series resistors and the power they dissipate.
Solution: We will solve this problem by considering active and reactive powers and using the power triangle method.
The apparent power drawn by the motor at reduced voltage is Sm = 471 kVA (from Example +++2)
The corresponding apparent power drawn by the line is SL = 724 kVA
The active power drawn by the motor is Pill = Sm cos e 471 X 0.35 165 kW The reactive power absorbed by the motor is Q/n 441 kvar
The resistors can only absorb active power in the circuit. Consequently, the reactive power supplied by the line must be equal to that absorbed by the motor: QL 441 kvar The active power supplied by the line is PL 574 kW The active power absorbed by the three resistors is 574 - 165 409kW The active power per resistor is P PR/3 409/3 = 136 kW The current in each resistor is: I=910 A)
The value of each resistor is: P = PR 136 000 = 910^2 x R
R = 0.164 ohm
The three resistors must therefore each have a resistance of 0.164-ohm and a short-term rating of 136 kW. The physical size of these resistors is much smaller than if they were designed for continuous duty.
This is an interesting example of the usefulness of the power triangle method in solving a relatively difficult problem. The results are summarized.
Autotransformer starting
Compared to a resistance starter, the advantage of an autotransformer starter is that for a given torque it draws a much lower line current. The disadvantage is that autotransformers cost more, and the transition from reduced-voltage to full-voltage is not quite as smooth.
Autotransformers usually have taps to give output voltages of 0.8, 0.65, and 0.5 pu. The corresponding starting torques are respectively 0.64, 0.42, and 0.25 of the full-voltage starting torque.
Furthermore, the starting currents on the line side are also reduced to 0.64, 0.42, and 0.25 of the full voltage locked-rotor current.
---29 shows a starter using two autotransformers connected in open delta. A simplified circuit diagram of such a starter is given in ---30. It has two contactors A and B. Contactor A has five NO contacts A and one small NO contact Ax. This contactor is in operation only during the brief period when the motor is starting up.
--29 Reduced-voltage autotransformer starter, 100 hp, 575 V, 60 Hz. (Square D)
--30 Simplified schematic diagram of an autotransformer starter.
Contactor B has 3 NO contacts B. It’s in service while the motor is running.
The autotransformers are set on the 65 percent tap. The time-delay relay RT possesses three contacts RT1 RT2, RT3. The contact RTI in parallel with the start button closes as soon as coil RT is energized. The other two contacts RT2, RT3 operate after a delay that depends upon the RT relay setting.
Contactors A and B are mechanically interlocked to prevent them from closing simultaneously.
Contactor A closes as soon as the start button is depressed. This excites the autotransformer and reduced voltage appears across the motor terminals.
A few seconds later, contact RT2 in series with coil A opens, causing contactor A to open. At the same time, contact RT3 causes contactor B to close.
Thus, contactor A drops out, followed almost immediately by the closure of contactor B. This action applies full voltage to the motor and simultaneously disconnects the autotransformer from the line.
In transferring from contactor A to contactor B, the motor is disconnected from the line for a fraction of a second. This creates a problem because when contactor B closes, a large transient current is drawn from the line. This transient surge is hard on the contacts and also produces a mechanical shock.
For this reason, we sometimes employ more elaborate circuits in which the motor is never completely disconnected from the line.
--- compare the torque and line current when autotransformer starting (3) and resistance starting (2) is used. The locked-rotor voltage in each case is 0.65 pu. The reader will note that the locked-rotor torques are identical, but the locked-rotor line current is much lower using an autotransformer (2.7 versus 4.2 pu). However, when the motor reaches about 90 percent of synchronous speed, resistance starting produces a higher torque because the terminal voltage is slightly higher than the 65 percent value that existed at the moment of start-up. On the other hand, the line current at all speeds is smaller when using an autotransformer.
Because the autotransformers operate for very short periods, they can be wound with much smaller wire than continuously rated devices. This enables us to drastically reduce the size, weight, and cost of these components.
--31 a Typical reduced voltage (0.65 pu) torque-speed curves of a 3-phase squirrel-cage induction motor: (2) primary resistance starting; (3) autotransformer starting.
--31 b Typical reduced voltage (0.65 pu) current-speed curves of a 3-phase squirrel-cage induction motor: (2) primary resistance starting; (3) autotransformer starting.
Example 4 ___
A 200 hp (150 k W), 460 V, 3-phase, 3520 rpm, 60 Hz induction motor has a locked-rotor torque of 600 N·m and a locked-rotor current of 1400 A (same motor as in Example +++2). Two autotransformers, connected in open delta, and having a 65 percent tap, are employed to provide reduced-voltage starting.
Calculate:
a. The apparent power absorbed by the motor
b. The apparent power supplied by the 460 V line
c. The current supplied by the 460 Y line
d. The locked-rotor torque Solution
a. The voltage across the motor is E 0.65 X 460 299 Y
The current drawn by the motor is I 0.65 X 1400 910 A.
The apparent power drawn by the motor is Sm DEI X 299 X 910 471 kVA
b. The apparent power supplied by the line is equal to that absorbed by the motor because the active and reactive power consumed by the autotransformers is negligible (Section 12.1). Consequently, SL Sm 471 kVA
c. The current drawn from the line is I SL I (D El (8.9)
= 471 000/( 1.73 X 460) 592 A Note that this current is considerably smaller than the line current (910 A) with resistance starting.
d. The locked-rotor torque varies as the square of the motor voltage: T = 0.652 X 600
= 0.42 X 600 252 N m
The results of these calculations are summarized. It’s worthwhile comparing them with the results in ---27.
--33 Part-winding starting of an induction motor.
Other starting methods
In addition to resistors and autotransformers, several other methods are employed to limit the current and torque when starting induction motors. Some only require a change in the stator winding connections. The part-winding starting method can be used when the induction motor has two identical 3-phase windings that operate in parallel when the motor is running. During the starting phase, only one of these 3-phase windings is used. As a result the impedance is higher than if the two windings were connected in parallel. After the motor has picked up speed, the second 3-phase winding is brought into service so that the two windings operate in parallel. ---33 shows how two 3-pole contactors A and B can be arranged for part-winding starting.
Contactor A closes first thus energizing windings 1, 2, 3. Shortly after, contactor B closes, bringing windings 7, 8, 9 in parallel with windings 1,2,3.
There are many different types of part-winding connections and some larger motors have specially designed windings so that the starting performance is optimized.
In wye-delta starting, all six stator leads are brought out to the terminal box. The windings are connected in wye during start-up, and in delta during normal running conditions. This starting method gives the same results as an autotransformer starter having a 58 percent tap. The reason is that the volt each wye-connected winding is only ( = 0.58) of its rated value.
Finally, to start wound-rotor motors, we progressively short-circuit the external rotor resistors in one, two, or more steps. The number of steps depends upon the size of the machine and the nature of the load.
--35 Schematic diagram of a cam switch permitting for ward-reverse and stop operation of a 3-phase motor.
--34
a. Cam switch external appearance.
b. Detail of the cam controlling contact 1 in the stop position.
c. Table listing the on-off state of the five contacts; cam shown in off position
Cam switches
Some industrial operations have to be under the continuous control of an operator. In hoists, For example, an operator has to vary the lifting and lowering rate, and the load has to be carefully set down at the proper place. Such a supervised control sequence can be done with cam switches.
---34 shows a 3-position cam switch de signed for the forward, reverse, and stop operation of a 3-phase induction motor. For each position of the knob, some contacts are closed while others are open.
This information is given in a table, usually glued to the side of the switch. A cross (X) designates a closed contact while a blank space is an open contact. In the forward position, For example, contacts 2, 4, and 5 are closed and contacts 1 and 3 are open. When the knob is turned to the stop position, all contacts are open.
---34b shows the shape of the cam that controls the opening and closing of contact 1. The schematic diagram shows how to connect the cam switch to a 3-phase motor. The state of the contacts (open or closed) is shown directly on the diagram for each position of the knob.
The 3-phase line and motor are connected to the appropriate cam-switch terminals. Note that jumpers J1, J2, 13, J4 are also required to complete the connections. The reader should analyze the circuit connections and resulting current flow for each position of the switch. For example, when the switch is in the forward position, contacts 2, 4, 5 are closed and L I is connected to T1, L2 to T2, and L3 to T3.
Some cam switches are designed to carry several hundred amperes, but we often prefer to use magnetic contactors to handle large currents. In such cases a small cam switch is employed to control the relay coils of the contactors. Very elaborate control schemes can be designed with multi contact cam switches.
--36 Electric drives can operate in four distinct quadrants, generator or brake; motor
Computers and controls
The control devices we have covered in this section are used throughout the industry. However, with the advent of computers, it’s now possible to simulate the behavior of many relay coils and relay contacts.
Furthermore, the connections between these devices can also be simulated. As a result, it’s possible to make very complex control circuits by simply using a keyboard, a monitor; and a computer. Thus, instead of using real relays, contacts, and time-delay dash pots, we simply program these devices (and their wiring) on a computer. The computers used for this purpose are called Programmable Logic Controllers (PLCs). Their construction and basic principle of operation are covered in Section 31.
Also see: Generating Electrical Power