ac-motor-control-circuits
The most challenging aspect of interpreting ladder diagrams, particularly for individuals familiar with electronic schematic diagrams, is the representation of electromechanical relays. The operation of a motor control circuit should be explained, detailing what occurs when the "Run" switch is actuated and what happens when it is released. The simplest and least expensive method of electric motor control is the across-the-line starter. This motor control circuit functions by directly connecting the motor to the power supply. The term "starter" in this context refers to the device that initiates motor operation. While across-the-line motor control circuits are straightforward and cost-effective, they are not ideal for starting large motors. Reduced voltage starting is an alternative method. Reasons for the undesirability of across-the-line starting for large electric motors include high inrush current and mechanical stress. Overcurrent protection devices, such as overload heaters, are commonly used in motor control circuits. These devices are connected in series with the motor conductors and heat up slightly under normal conditions, serving a different purpose than fuses. The function of overload heaters is to protect electric motors from burnout due to overcurrent conditions. The operation of overload heaters differs from that of fuses or circuit breakers, and their presence does not eliminate the necessity for circuit breakers or fuses. The process of reversing motor direction can be achieved with two different motor starters, M1 and M2, and it is noted that there is only one set of overload heaters instead of two. The purpose of normally-closed contacts in series with each starter coil should also be explained. Motor control assemblies are often located in motor control centers (MCC) away from the motor, necessitating diagnostic checks from the MCC. One such check involves measuring line current to detect open motor windings, known as single-phasing. If a three-phase motor winding fails, the motor will not operate correctly. A clamp-on ammeter can be used to check line current on all three phases while the starter is energized. If a technician lacks a clamp-on ammeter, an alternative test involves measuring AC millivolts across each overload heater element with a digital multimeter (DMM). This method can indicate whether the motor is drawing equal current on all three phases, though it has limitations compared to a clamp-on ammeter. Variable frequency drives (VFDs) are popular for controlling AC induction motors by varying the frequency of power supplied, which offers advantages in speed control and energy efficiency. Reset times for overload heater contacts after a trip differ from circuit breakers and fuses, which can be reset immediately. This characteristic is intentional, as overload heaters require a rest period after several successive starts to prevent overheating. Protective relays are devices that automatically open or close circuit breakers in large electric power systems, including features like start-up lockout to prevent excessive re-starts of large motors. The need for a rest period after multiple starts is crucial for preventing damage to the motor. Electromechanical relays, or contactors, used for high-power electric motors can pose a risk of arc flash due to their construction and operation. Reduced-voltage starting methods, such as the autotransformer method, are commonly used, with a specific sequence of contacts connecting power to the motor. The operation of this motor control circuit should be explained, including the function of the "Run" switch. A latch circuit, often a "start-stop" relay, is used in motor controls, typically involving a low-voltage control circuit and a three-phase motor. The operation from the "Start" switch to the "Stop" switch should be detailed, including the role of the normally-open M1 contact, referred to as a seal-in contact. Ladder diagrams are an alternative to conventional schematic diagrams in AC power control systems, with power conductors represented as vertical lines and loads and switch contacts depicted between them. The symbolism in ladder diagrams may differ from electrical schematic diagrams, necessitating improvisation in certain scenarios.
The motor control circuit operates by using a combination of components to ensure safe and efficient motor operation. When the "Run" switch is actuated, power is supplied to the motor, allowing it to start. The contactor, which acts as a relay, closes its contacts to connect the motor directly to the power supply. Once the "Run" switch is released, the circuit must be designed to maintain power to the motor, typically through the use of a seal-in contact that latches the circuit in the "on" position. If the circuit is not properly designed, releasing the "Run" switch could cause the motor to stop immediately, indicating a potential fault in the latch mechanism.
In the context of overload protection, the overload heaters serve to monitor the current flowing to the motor. Under normal operating conditions, these heaters heat up slightly, but they are not designed to function as fuses. Instead, they provide a measure of protection by opening the circuit if the current exceeds a certain threshold for an extended period. This is critical in preventing motor burnout due to excessive current, which can occur during overload conditions.
The reversal of motor direction is achieved by using two separate motor starters, M1 and M2, which control the direction of current flow to the motor windings. This setup allows for forward and reverse operation while utilizing a single set of overload heaters. The normally-closed contacts in series with each starter coil ensure that if one starter is engaged, the other is disabled, preventing potential damage from simultaneous operation.
The diagnostic checks performed at the motor control center (MCC) are essential for maintaining motor functionality. The use of a clamp-on ammeter is the preferred method for measuring line currents, as it provides accurate readings without interrupting the circuit. However, when such equipment is unavailable, measuring the voltage drop across the overload heaters using a DMM can provide an indication of the motor's operational status, though it may not be as reliable as direct current measurement.
Variable frequency drives (VFDs) enhance motor control by allowing for the adjustment of motor speed and torque through frequency modulation. This capability is particularly advantageous in applications requiring variable speed operation, improving energy efficiency and reducing mechanical wear.
Overall, the design and operation of motor control circuits encompass various components and principles that ensure safe and effective motor operation, emphasizing the importance of proper protective measures and diagnostic capabilities.Perhaps the most challenging aspect of interpreting ladder diagrams, for people more familiar with electronic schematic diagrams, is how electromechanical relays are represented. Compare these two equivalent diagrams: Also, explain the operation of this motor control circuit. What happens when someone actuates the "Run" switch What happens when t hey let go of the "Run" switch The simplest and least expensive style of electric motor control is the so-called across-the-line starter. Describe how this motor control circuit functions, and also define the word G tarter" in this context.
Although G¤cross-the-line" motor control circuits are simple and inexpensive, they are not preferred for starting large motors. An alternative to across-the-line motor starting is reduced voltage starting. Identify some of the reasons across-the-line starting is undesirable for large electric motors. A special type of overcurrent protection device used commonly in motor control circuits is the overload heater.
These devices are connected in series with the motor conductors, and heat up slightly under normal current conditions: Although the "heater" elements are connected in series with the motor lines as fuses would be, they are not fuses! In other words, it is not the purpose of an overload heater to burn open under an overcurrent fault condition, although it is possible for them to do so.
The key to understanding the purpose of an overload heater is found by examining the single-phase (L1 / L2) control circuit, where a normally-closed switch contact by the same name (G–L") is connected in series with the motor relay coil. How, exactly, do overload heaters protect an electric motor against "burnout" from overcurrent conditions How does this purpose differ from that of fuses or circuit breakers Does the presence of overload heaters in this circuit negate that need for a circuit breaker or regular fuses Explain your answers.
Explain how the reversal of motor direction is accomplished with two different motor starters, M1 and M2. Also, explain why there is only one set of overload heaters instead of two (one for forward and one for reverse).
Finally, explain the purpose of the normally-closed contacts in series with each starter coil. The starter and overload heater assembly for an industrial electric motor is often located quite a distance from the motor itself, inside a room referred to as a motor control center, or MCC: Since it is impossible for a technician to be in two places at once, it is often necessary to perform diagnostic checks on a malfunctioning electric motor from the MCC where the technician has access to all the control circuitry. One such diagnostic check is line current, to detect the presence of an open motor winding. If a three-phase motor winding fails open, the motor will not run as it should. This is called single-phasing. A good way to check for this condition is to use a clamp-on (inductive) ammeter to check line current on all three lines while the starter is energized.
This may be done at any location where there is physical access to the motor power conductors. Suppose, though, you are working on a job site where single-phasing is suspected and you do not have a clamp-on ammeter with you. All you have is a DMM (digital multimeter), which does not have the ability to safely measure the motor`s current.
You are about to head back to the shop to get a clamp-on ammeter when a more experienced technician suggests an alternate test. He takes your DMM, sets it to the AC millivolt range, then connects the test probes to either side of each overload heater element, one heater at a time like this: Across each overload heater element he measures about 20 mV AC with the starter engaged.
From this he determines that the motor is not single-phasing, but is drawing approximately equal current on all three phases. Explain how this diagnostic check works, and why this determination can be made. Also describe what limitations this diagnostic procedure has, and how a clamp-on ammeter really is the best way to measure motor line current.
A popular strategy for AC induction motor control is the use of variable frequency drive units, or VFDs. Explain what varying the frequency of power to an AC induction motor accomplishes, and why this might be advantageous.
Why is there any time required to re-set an overload heater contact after a "trip" Circuit breakers can be re-closed mere moments after a trip with no problem, and fuses (of course) can be replaced moments after blowing. Is this an intentional design feature of overload heaters, or just an idiosyncrasy Also, explain why the reset curve starts to decrease for currents above 300% of the motor`s full-load rating.
Why doesn`t the reset time curve continue to increase with increasing fault current magnitudes Protective relays are special power-sensing devices whose job it is to automatically open or close circuit breakers in large electric power systems. Some protective relays are designed to be used directly with large electric motors to provide sophisticated monitoring, shut-down, and start-up control.
One of the features of these motor-oriented protective relays is start-up lockout. What this means is the relay will prevent someone from attempting too many successive re-starts of a large electric motor. If the motor is started and stopped several times over a short period of time, the relay will prevent the person from starting it again until a sufficient "rest" time has passed.
Explain why a large electric motor would need to "rest" after several successive start-up events. If electric motors are perfectly capable of running continuously at full load for years on end, why would a few start-ups be worthy of automatic lock-out Electromechanical relays used to start and stop high-power electric motors (called "contactors" or G tarters") must be considered a possible source of arc flash. Explain why this is. What is it about the construction or operation of such a relay that invites this dangerous phenomenon There are several different methods of providing reduced-voltage starting for electric motors.
One of them is the autotransformer method. Here is a diagram showing how this works: "L1, " "L2, " and "L3" represent the three phase power supply conductors. Three sets of contacts (R, S, and Y) serve to connect power to the motor at different times. The starting sequence for the motor is as follows: Also, explain the operation of this motor control circuit.
What happens when someone actuates the "Run" switch What happens when they let go of the "Run" switch Identify at least one fault that would cause the motor to turn off immediately once the "Start" pushbutton switch was released, instead of "latch" in the run mode as it should: A very common form of latch circuit is the simple G tart-stop" relay circuit used for motor controls, whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. In this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor: Explain the operation of this circuit, from the time the "Start" switch is actuated to the time the "Stop" switch is actuated.
The normally-open M1 contact shown in the low-voltage control circuit is commonly called a seal-in contact. Explain what this contact does, and why it might be called a G eal-in" contact. An alternative to the conventional schematic diagram in AC power control systems is the ladder diagram.
In this convention, the "hot" and "neutral" power conductors are drawn as vertical lines near the edges of the page, with all loads and switch contacts drawn between those lines like rungs on a ladder: As you can see, the symbolism in ladder diagrams is not always the same as in electrical schematic diagrams. While some symbols are identical (the toggle switch, for instance), other symbols are not (the solenoid coil, for instance).
Yes, the "Run" switch shown in the diagram is a SPST, but the switch shown in the illustration is a SPDT. This is a realistic scenario, where the only type of switch you have available is a SPDT, but the wiring diagram calls for something different.
It is your job to improvise a solution! Examine this three-phase motor control circuit, where fuses protect against overcurrent and a three-pole relay (called a contactor) turns power on and off to the motor: After years of faithful service, one day this motor refuses to start. It makes a "humming" sound when the contactor is energized (relay contacts close), but it does not turn.
A mechanic checks it out and determines that the shaft is not seized, but is free to turn. The problem must be electrical in nature! You are called to investigate. Using a clamp-on ammeter, you measure the current through each of the lines (immediately after each fuse) as another start is once again attempted. You then record the three current measurements: Determine at least two possible faults which could account for the motor`s refusal to start and the three current measurements taken.
Then, decide what your next measurement(s) will be to isolate the exact location and nature of the fault. Working on a job site with an experienced technician, you are tasked with trying to determine whether the line currents going to a three-phase electric motor are balanced.
If everything is okay with the motor and the power circuitry, of course, the three line currents should be precisely equal to each other. The problem is, neither of you brought a clamp-on ammeter for measuring the line currents. Your multimeters are much too small to measure the large currents in this circuit, and connecting an ammeter in series with such a large motor could be dangerous anyway.
So, the experienced technician decides to try something different - he uses his multimeter as an AC milli-voltmeter to measure the small voltage drop across each fuse, using the fuses as crude shunt resistors: 🔗 External reference
The motor control circuit operates by using a combination of components to ensure safe and efficient motor operation. When the "Run" switch is actuated, power is supplied to the motor, allowing it to start. The contactor, which acts as a relay, closes its contacts to connect the motor directly to the power supply. Once the "Run" switch is released, the circuit must be designed to maintain power to the motor, typically through the use of a seal-in contact that latches the circuit in the "on" position. If the circuit is not properly designed, releasing the "Run" switch could cause the motor to stop immediately, indicating a potential fault in the latch mechanism.
In the context of overload protection, the overload heaters serve to monitor the current flowing to the motor. Under normal operating conditions, these heaters heat up slightly, but they are not designed to function as fuses. Instead, they provide a measure of protection by opening the circuit if the current exceeds a certain threshold for an extended period. This is critical in preventing motor burnout due to excessive current, which can occur during overload conditions.
The reversal of motor direction is achieved by using two separate motor starters, M1 and M2, which control the direction of current flow to the motor windings. This setup allows for forward and reverse operation while utilizing a single set of overload heaters. The normally-closed contacts in series with each starter coil ensure that if one starter is engaged, the other is disabled, preventing potential damage from simultaneous operation.
The diagnostic checks performed at the motor control center (MCC) are essential for maintaining motor functionality. The use of a clamp-on ammeter is the preferred method for measuring line currents, as it provides accurate readings without interrupting the circuit. However, when such equipment is unavailable, measuring the voltage drop across the overload heaters using a DMM can provide an indication of the motor's operational status, though it may not be as reliable as direct current measurement.
Variable frequency drives (VFDs) enhance motor control by allowing for the adjustment of motor speed and torque through frequency modulation. This capability is particularly advantageous in applications requiring variable speed operation, improving energy efficiency and reducing mechanical wear.
Overall, the design and operation of motor control circuits encompass various components and principles that ensure safe and effective motor operation, emphasizing the importance of proper protective measures and diagnostic capabilities.Perhaps the most challenging aspect of interpreting ladder diagrams, for people more familiar with electronic schematic diagrams, is how electromechanical relays are represented. Compare these two equivalent diagrams: Also, explain the operation of this motor control circuit. What happens when someone actuates the "Run" switch What happens when t hey let go of the "Run" switch The simplest and least expensive style of electric motor control is the so-called across-the-line starter. Describe how this motor control circuit functions, and also define the word G tarter" in this context.
Although G¤cross-the-line" motor control circuits are simple and inexpensive, they are not preferred for starting large motors. An alternative to across-the-line motor starting is reduced voltage starting. Identify some of the reasons across-the-line starting is undesirable for large electric motors. A special type of overcurrent protection device used commonly in motor control circuits is the overload heater.
These devices are connected in series with the motor conductors, and heat up slightly under normal current conditions: Although the "heater" elements are connected in series with the motor lines as fuses would be, they are not fuses! In other words, it is not the purpose of an overload heater to burn open under an overcurrent fault condition, although it is possible for them to do so.
The key to understanding the purpose of an overload heater is found by examining the single-phase (L1 / L2) control circuit, where a normally-closed switch contact by the same name (G–L") is connected in series with the motor relay coil. How, exactly, do overload heaters protect an electric motor against "burnout" from overcurrent conditions How does this purpose differ from that of fuses or circuit breakers Does the presence of overload heaters in this circuit negate that need for a circuit breaker or regular fuses Explain your answers.
Explain how the reversal of motor direction is accomplished with two different motor starters, M1 and M2. Also, explain why there is only one set of overload heaters instead of two (one for forward and one for reverse).
Finally, explain the purpose of the normally-closed contacts in series with each starter coil. The starter and overload heater assembly for an industrial electric motor is often located quite a distance from the motor itself, inside a room referred to as a motor control center, or MCC: Since it is impossible for a technician to be in two places at once, it is often necessary to perform diagnostic checks on a malfunctioning electric motor from the MCC where the technician has access to all the control circuitry. One such diagnostic check is line current, to detect the presence of an open motor winding. If a three-phase motor winding fails open, the motor will not run as it should. This is called single-phasing. A good way to check for this condition is to use a clamp-on (inductive) ammeter to check line current on all three lines while the starter is energized.
This may be done at any location where there is physical access to the motor power conductors. Suppose, though, you are working on a job site where single-phasing is suspected and you do not have a clamp-on ammeter with you. All you have is a DMM (digital multimeter), which does not have the ability to safely measure the motor`s current.
You are about to head back to the shop to get a clamp-on ammeter when a more experienced technician suggests an alternate test. He takes your DMM, sets it to the AC millivolt range, then connects the test probes to either side of each overload heater element, one heater at a time like this: Across each overload heater element he measures about 20 mV AC with the starter engaged.
From this he determines that the motor is not single-phasing, but is drawing approximately equal current on all three phases. Explain how this diagnostic check works, and why this determination can be made. Also describe what limitations this diagnostic procedure has, and how a clamp-on ammeter really is the best way to measure motor line current.
A popular strategy for AC induction motor control is the use of variable frequency drive units, or VFDs. Explain what varying the frequency of power to an AC induction motor accomplishes, and why this might be advantageous.
Why is there any time required to re-set an overload heater contact after a "trip" Circuit breakers can be re-closed mere moments after a trip with no problem, and fuses (of course) can be replaced moments after blowing. Is this an intentional design feature of overload heaters, or just an idiosyncrasy Also, explain why the reset curve starts to decrease for currents above 300% of the motor`s full-load rating.
Why doesn`t the reset time curve continue to increase with increasing fault current magnitudes Protective relays are special power-sensing devices whose job it is to automatically open or close circuit breakers in large electric power systems. Some protective relays are designed to be used directly with large electric motors to provide sophisticated monitoring, shut-down, and start-up control.
One of the features of these motor-oriented protective relays is start-up lockout. What this means is the relay will prevent someone from attempting too many successive re-starts of a large electric motor. If the motor is started and stopped several times over a short period of time, the relay will prevent the person from starting it again until a sufficient "rest" time has passed.
Explain why a large electric motor would need to "rest" after several successive start-up events. If electric motors are perfectly capable of running continuously at full load for years on end, why would a few start-ups be worthy of automatic lock-out Electromechanical relays used to start and stop high-power electric motors (called "contactors" or G tarters") must be considered a possible source of arc flash. Explain why this is. What is it about the construction or operation of such a relay that invites this dangerous phenomenon There are several different methods of providing reduced-voltage starting for electric motors.
One of them is the autotransformer method. Here is a diagram showing how this works: "L1, " "L2, " and "L3" represent the three phase power supply conductors. Three sets of contacts (R, S, and Y) serve to connect power to the motor at different times. The starting sequence for the motor is as follows: Also, explain the operation of this motor control circuit.
What happens when someone actuates the "Run" switch What happens when they let go of the "Run" switch Identify at least one fault that would cause the motor to turn off immediately once the "Start" pushbutton switch was released, instead of "latch" in the run mode as it should: A very common form of latch circuit is the simple G tart-stop" relay circuit used for motor controls, whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. In this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor: Explain the operation of this circuit, from the time the "Start" switch is actuated to the time the "Stop" switch is actuated.
The normally-open M1 contact shown in the low-voltage control circuit is commonly called a seal-in contact. Explain what this contact does, and why it might be called a G eal-in" contact. An alternative to the conventional schematic diagram in AC power control systems is the ladder diagram.
In this convention, the "hot" and "neutral" power conductors are drawn as vertical lines near the edges of the page, with all loads and switch contacts drawn between those lines like rungs on a ladder: As you can see, the symbolism in ladder diagrams is not always the same as in electrical schematic diagrams. While some symbols are identical (the toggle switch, for instance), other symbols are not (the solenoid coil, for instance).
Yes, the "Run" switch shown in the diagram is a SPST, but the switch shown in the illustration is a SPDT. This is a realistic scenario, where the only type of switch you have available is a SPDT, but the wiring diagram calls for something different.
It is your job to improvise a solution! Examine this three-phase motor control circuit, where fuses protect against overcurrent and a three-pole relay (called a contactor) turns power on and off to the motor: After years of faithful service, one day this motor refuses to start. It makes a "humming" sound when the contactor is energized (relay contacts close), but it does not turn.
A mechanic checks it out and determines that the shaft is not seized, but is free to turn. The problem must be electrical in nature! You are called to investigate. Using a clamp-on ammeter, you measure the current through each of the lines (immediately after each fuse) as another start is once again attempted. You then record the three current measurements: Determine at least two possible faults which could account for the motor`s refusal to start and the three current measurements taken.
Then, decide what your next measurement(s) will be to isolate the exact location and nature of the fault. Working on a job site with an experienced technician, you are tasked with trying to determine whether the line currents going to a three-phase electric motor are balanced.
If everything is okay with the motor and the power circuitry, of course, the three line currents should be precisely equal to each other. The problem is, neither of you brought a clamp-on ammeter for measuring the line currents. Your multimeters are much too small to measure the large currents in this circuit, and connecting an ammeter in series with such a large motor could be dangerous anyway.
So, the experienced technician decides to try something different - he uses his multimeter as an AC milli-voltmeter to measure the small voltage drop across each fuse, using the fuses as crude shunt resistors: 🔗 External reference
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