Troubleshooting VSD control circuits

Learning Goals ... are to:

• • understand the fundamentals of control circuits
• • understand types of control circuits
• • troubleshoot control circuits

Basic control circuits

Basic control circuits are used in starting, stopping, sequencing, and safety automatic interlocking of equipment and machines.

The control circuit consists of relays, relay contacts, contactors, timers, counters, etc.

Control circuits can also be con figured or programmed in the PLCs. This is done, using ladder logic diagram, statement lists, or control flowcharts software, by representing the logical conditions, sequences, and interlocks required for operating equipment or a machine in an automatic sequence. To understand how to troubleshoot control circuits, it’s very important to understand the working of some basic control circuits.

Basic control circuit for DOL (direct-on-line) starter

++++ a typical circuit for a DOL starter for a three-phase motor. A full-line voltage is applied across the windings with this starter. The rating of motors which can be started direct-on-line depends on the capacity of the distribution system and the acceptable bus voltage drop during starting. In large industrial systems it’s not unusual for even a 200 kW motor to be DOL started especially when fed by a transformer of 1600 kVA or higher. However, when a motor is fed by an LV emergency generator, DOL starting has to be planned with due consideration to starting voltage drop.

Main circuit shows the circuit with a three-phase power supply (L1, L2, and L3), main circuit fuses (F1), main contactor (K1), and an overload protection relay (F2) for a three phase motor.

The motor can be started by the following two methods:

1. Momentary contact control with press and release type pushbutton

2. Maintained contact control with press and latch type pushbutton.

++++1 (a) Main circuit; (b) Control circuit (Momentary contact); (c) Control circuit (Maintained contact).

Momentary contactor control:

++++1(b) shows a momentary control circuit to start and stop the three-phase motor using a DOL starter with a start and stop pushbutton S1and S0 respectively.

The control circuit consists of an overload relay (F2) NC contact, an NC contact of stop pushbutton (S0), NO contact of start pushbutton (S1) connected in series to the main contactor (K1) coil. The control supply for the circuit passes through a control fuse (F3). The main contactor coil gets the phase line (L1) through the control circuit only when all the contacts are closed. In this case, when the start pushbutton is pressed, the control circuit is closed and the main contactor is energized. An NO contact of the main contactor is connected in parallel to the start pushbutton. As the main contactor is switched on, it’s latched through this parallel NO contact (K1) even after the start pushbutton is released. The main contactor remains on and the motor continues to run until the stop pushbutton is pressed to stop the motor or the motor trips due to an overload relay operation.

Maintained contactor control:

++++1(c) a control circuit to start and stop the three-phase motor using a DOL starter with a single pushbutton (S1).

The control circuit consists of an NC contact of overload relay (F2) and NO contact of a toggle type switch (S), connected in series to the main contactor (K) coil. The control supply for the circuit passes through the control fuse (F3). The main contactor (K1) coil gets power only when all the contacts are closed. In this case, when the switch (S) is closed, the control circuit closes and the main contactor (K1) is energized.

As long as switch (S) is maintained on, the main contactor remains on and the motor continues to run until the switch (S) is opened or the motor trips due to an overload relay operation.

++++2 (a) Main circuit of star-delta starter; (b) Control circuit of star-delta starter

S0 = 'OFF' push button, S1 = 'ON' push button, K1 = Line contactor K2 = Stat contactor K3 = Delta contactor K4 = Star delta timer F2 =Overload relay F1 =Backup fuse

Star-delta 3-phase starter

The circuit is the main circuit for the star-delta starter is the control circuit.

Usually, a motor has the tendency to draw 500% higher current than the full load of normal current from the supply line during startup. This in turn increases the starting torque that is higher than normal, which can result in a mechanical damage. To avoid this, reduced voltage starters are used. Star-delta starters are also used when a weak system cannot support the DOL starting of a large capacity motor. The starting current (line) when using this method is reduced by factor of 3 (i.e. 200% in place of 600%). The starting torque however also reduces by a factor 3. This method is therefore not suitable for loads with high inertia or those that require high starting/break away torque.

During the startup in a star-delta starter, the winding is connected in a star configuration with contactor K1 and K2, which applies reduced voltage (approximately 58% of rated). Then after a while, connect the windings in delta configuration with contactor K1 and K3.

Star-delta starter working:

The main contactor K1 will energize only when the control circuit fuse (F3), backup fuse (F1), and the overload relay (F2) are healthy and the start pushbutton (S1) is pressed.

Reduced-voltage configuration (star configuration):

Star-delta timer coil (K4) gets power through fuses F3, F1, NC contact of stop pushbutton (S0), and NO contact of start push button. As start PB (S1) is pressed, the timer coil K4 will pickup and in turn energize the star contactor coil K2. The main line contactor (K1) coil gets power via the NC contact of S0, NO contact of S1, NO contact of K2 and remains latched unless the stop pushbutton (S0) is pressed.

Now, the main line contactor (K1) and the star contactor ( K2) are in a pickup condition, which will drive the motor in the star configuration.

Full voltage (delta configuration) As the time duration set on a K4 timer (star to delta timer) elapses, the contactor coil (K3) is picked up and at the same time, the star contactor ( K2) is de-energized.

Now, the main line contactor (K1) and the delta contactor (K3) are in a pickup condition, which will drive the motor in a delta configuration. When the motor trips in an overload condition either in a star or delta configuration, the control circuit always ensures that the motor restarts in a star configuration, rather than the delta configuration.

Autotransformer 3-phase starter

++++3 shows the autotransformer three-phase starter circuit. This type of starter circuit uses an autotransformer to apply reduced voltage across the windings of the motor during startup. Three autotransformers are connected in the star configuration and taps are selected, to provide an adequate starting current for the motor.

After a certain time lapse, full voltage is applied to the motor bypassing the autotransformers.

++++3 Typical main and control circuit of an autotransformer starter for a three-phase motor - S0 = 'OFF' push button S1 = 'ON' push button K1 = Star contactor K2 = Transformer contactor K3 =Main contactor K4 = Time relay K5 = Contractor relay F1 =Backup fuse F2 =Overload relay F3 =Control circuit fuse.

The working of an autotransformer:

The fuse (F1) and the overload relay (F2) provide protection to the main circuit.

Similarly, the control circuit has the fuse (F3) and overload relay (F2) NC contact.

Reduced voltage configuration In this circuit, the contactor (K5) will pickup when the start push button is pressed and will remain latched until the stop push button is pressed or the control circuit fuses or the motor trips on overload.

As K5 picks up, it will energize the timer relay (K4) coil. This in turn will energize the contactor K1 coil. The closing contactor K1 contact will energize the contactor K2 coil.

So, contactors K5, K4, K1, and K2 are in an energized condition at this stage. This will result in starting the motor through an autotransformer at a reduced voltage and with star configuration because of the contactor K1 and K2.

Full voltage configuration:

As timer relay (K4) time lapses, it de-energizes the contactor K1 coil. At the same time contactor K3 coil is energized, this will in turn de-energize the contactor K2 coil.

The motor will now run at full voltage as contactor K3 is in a pickup condition. If in the interim, the motor trips on overload, then the control circuit has to be checked, so the motor restarts in a star configuration and at a reduced voltage after the overload reset.

Ladder logic circuits

The designing, programming, testing, commissioning, troubleshooting, and maintenance of control logic are much easier using the ladder logic programs in a PLC than in hard-wired circuit.

We have discussed the ladder logic instructions before and looked at the simple ladder logic programs in Section 2. Let us consider a ladder logic program for a typical control circuit of the DOL starter for a three-phase motor as shown in ++++1(a). For a three-phase motor with a DOL starter, the following input and output signals are con figured in the PLC.

Digital inputs

1. Control voltage ON and control fuse 'F3' OK (normally open)

2. Motor overload 'F2' (normally closed)

3. Motor stop 'S0' (normally closed)

4. Motor start 'S1' (normally open)

5. Main contact on feedback (normally open).

Digital output

Main contactor on; The ladder logic program; Instructions for the DOL starter control circuit with a maintained contact control.

Remarks:

1. 'Start' push button contact is closed as push button is pressed momentarily.

2. 'Control on' input will be on, if line voltage is there and control fuse is not blown.

3. Overload relay NC contact will keep the input 'Overload' on till the relay is not operated.

4. 'Stop' input from stop push button NC contact will remain on till stop push button is not pressed.

5. Contact of ' Main on' will hold the coil output ' Main on' until the stop push button is pressed or overload relay trips.

++++4 Ladder logic program for a DOL starter and alarms

As various inputs for the control circuit in the PLC has been gathered, it will be for generating alarms. These alarms can be indicated with the help of 'Indication Lamps' that can be mounted on the motor starter panel in addition to the start and stop pushbuttons.

These alarms can help in faultfinding. The following alarms can be con figured using the inputs already available in the PLC. The PLC outputs can be generated to drive these alarm indication lamps. In case of the DOL starter for a three-phase motor, the following alarms can be con figured to indicate faults:

• Alarm indication lamp-1: Control voltage OFF

• Alarm indication lamp-2: Motor tripped on overload

• Alarm indication lamp-3: Main contactor feedback fault.

Two-wire control

The circuit can be used for an auto start operation of a motor after a power failure depending on the position of the control contact. The control contact may be a level switch contact or a temperature switch contact.

The contactor K remains energized as long as the control contact is in a close condition.

If the power failure occurs, the motor turns off.

The motor will then restart automatically, once the power is restored since the control contact is in a close condition.

This circuit type is useful for pumps, fans, blowers, etc. where an automatic restarting of the device after a power failure is desirable depending on the control contact.

++++5 Two-wire control circuit

Three-wire control - start/stop

A three-wire control circuit for start/stop operation. As shown in the diagram, NO contact of the start pushbutton and NC contact of the stop pushbutton are in series with the main contactor in the control circuit.

++++6 Three-wire control - start/stop.

The main contactor K will energize as a start pushbutton is pressed and if the stop pushbutton is not pressed, it stays latched. The latching contact K is used for the main contactor.

If a power failure occurs and the motor stops, then the motor won’t start automatically. Again, the start pushbutton has to be pressed to latch the main contactor and to start the motor. This circuit type is useful in applications where the motor is not required to start automatically after a power failure so as to prevent the occurrence of a hazard to the surroundings.

Jog/inch circuits

Start/stop/jogging circuit using push buttons

If the machine has to perform small rotations, a control circuit is required to accomplish the 'Inching' motion of a motor.

To serve the purpose, a 'Jog/Inch' control circuit for a three-phase motor is designed such that when the jog pushbutton is pressed, the motor runs and when the pushbutton is released the motor stops. Generally, this type of movement is used in machine tools.

++++7 Start/stop/jogging circuit using pushbutton: F1 = Backup fuse; F2 =Overload relay; F3 =Control circuit fuse.

Operation:

As shown in the control circuit, a double-contact jog pushbutton is used with one NC contact and one NO contact. Therefore, when the jog button is pressed, the latching circuit to the starter coil (K) is opened by the NC contacts of the jog pushbutton. Therefore, the starter coil (K) won’t lock in; rather it remains energized as long as the jog button is fully pressed. Thus, an 'Inching/Jogging' action can be achieved.

If the jog pushbutton were released suddenly, then if the NC contacts closes before the starter maintaining the contacts (K) open, the motor would continue to run. This in turn can prove hazardous to the surrounding workers and machinery. A mechanical device can be installed which ensures that the starter-holding circuit is not reestablished if the jog button is released too rapidly.

Start/stop/jogging circuit using selector switch

++++8 shows the use of a selector switch in the control circuit to obtain jogging.

The start button performs twin functions; it works as a jog button as well as a start button.

++++8 Start/stop/jogging circuit using selector switch --- F2 =Overload relay F3 = Control circuit fuse

To operate the motor in a run mode, the selector switch should be in the 'RUN' position. The K coil circuit is completed if the start button is pressed and it remains latched because of the latching contact K1 and the selector switch.

To operate the motor in the jog mode, the selector switch should be in the jog position. If the start button is pressed, the K coil circuit is completed but as soon as the start button is released, the K coil is de-energized because the latching circuit of K1 coil is open.

Sequence start and stop

In large industrial plants, there are a large number of machines and drives. They are required to start and stop in a predetermined sequence. In such cases, it’s impossible to start/stop each drive with an individual start/stop control circuit. Moreover, it’s impossible to monitor each drive, follow the sequence of start/stop for the drives, and stop drives as per the process inputs.

To perform this kind of operation, sequencers are used; these may be mechanical or electromechanical.

With 'Time' sequencers, depending on the time duration and base sequencers, a number of outputs are switched on-off in a predefined sequence.

In the case of 'pulse' sequencers, a number of outputs are switched on-off in predefined sequences, depending on the pulses received from the process (pulses may be either derived from the 'proximity' switches or the 'limit' switches, etc.). To understand this, consider a sequence of the three-belt conveyor system carrying raw coal from an inlet vibro feeder to storage hopper. To start the material conveying from the inlet vibro feeder to the hopper, first start the belt conveyor-3 (BC-3). Once the belt conveyor-3 is running, start the belt conveyor-2 (BC-2) and then start the belt conveyor-1 (BC-1). After the three conveyer belts are started sequentially, then start the inlet vibro feeder, which drops the material onto the belt conveyors.

Sequence of belt conveyors --- Inlet vibro feeder BC-1 BC-2 BC-3 Storage hopper Level high switch

During the material feeding, if BC-3 or BC-2 trips, the upstream belt conveyors must also be stopped immediately. In addition, when the raw material hopper is filled to a high level, the belt conveyors must stop in the reverse sequence.

The sequence for start and stop for the 'Belt Conveyor System'.

Note: The purpose of not stopping conveying equipment immediately on receiving a STOP command is to avoid material remaining on the conveying equipment.

Therefore in some installations, the material feed (such as a gate) is closed immediately on receipt of a STOP command and then the whole line is stopped after a preset delay adequate for the entire material remaining in the system to be cleared.

This scheme avoids the necessity of multiple timer circuits required for a sequential stopping.

++++10 --- Start and stop sequence for the belt conveyors system.

Automatic sequence starting

It’s difficult to manually control (start/stop) multiple drives simultaneously, while depending upon the various interlocks. Sometimes a large number of machine drives, in a plant or section of a plant, are required to run in a predefined sequence. When these are interlocked with each other, it can be quite difficult to start/stop the drives manually in sequence, depending on the interlocking that may be required for safety or due to the process devices. A typical automatic belt conveyor sequence system. An automatic sequence start and stop of the belt conveyors is done by using a single start push button, a stop push button, a high level switch in the hopper, and timers for auto sequencing.

Reversing circuit

An interchange of any two phases reverts the direction of a three-phase motor. This will run the motor in the reverse direction.

To accomplish this reversal, two different types of control circuits are detailed as follows.

++++11 Automatic sequence starting and stopping: (a) Start sequence; (b) Stop sequence.

Jog type for/rev/off circuit using selector switch

Interchanging any two leads to a three-phase induction motor will cause it to run in the reverse direction. A three-phase reversing starter shown in the main circuit shows two contactors K1 and K2 (forward and reverse, respectively). The selector switch is of the spring return-type with the center off.

The contactor coil K1 is energized keeping the selector switch in a forward position. The contactor K1 connects the supply leads (L1, L2, and L3) to the motor leads (U, V, and W) in the same phase sequence. This causes the motor to run in a forward direction. Keeping the selector switch in a reverse position energizes the contactor coil K2. The contactor K2 connects the supply leads L1 to W, L2 to V, and L3 to U changing the phase sequence of L1 and L2. This causes the motor to run in a reverse direction. Putting the selector switch in an off position turns off the motor.

The motor is protected from a short-circuit condition if an overload relay, a backup fuse, and a control circuit fuse provide both the forward and reverse protection to the motor.

Latch type for/rev/stop circuit using pushbuttons

The circuit discussed earlier was of the jog type, fwd/rev circuit. The circuit is of the latch type, fwd/rev control.

Pressing the forward pushbutton will energize the contactor K1 coil. This in turn will connect the supply leads to the motor leads in the same phase sequence causing the motor to rotate in a forward direction.

The contactor K1 will remain energized because of the latching contact of K1. The motor will continue to run in a forward direction until the stop/reverse pushbutton is pressed or the motor trips on overload or the protection fuse blows.

++++12 Jog type fwd/rev/off circuit using selector switch. S= For/rev/off selector switch

K1 = For. Contactor

K2 = Rev. contactor

F1 =Backup fuse

F2 =Overload relay

F3 =Control circuit fuse

++++13 Latch-type fwd/rev/off circuit using push buttons.

S0 =Stop P.B.

S1 = For. P.B.

S2 =Rev. P.B.

K1 = Fwd. Contactor

K2 = Rev. contactor

F1 = Backup fuse

F2 =Overload relay

F3 = Control fuse

Pressing the reverse pushbutton will energize the contactor K2 coil simultaneously by de-energizing the K1 coil. This will connect the supply leads to the motor leads in a different phase sequence causing the motor to rotate in a reverse direction. The contactor K2 will remain energized because of the latching contact of K2. The motor will continue to run in a reverse direction until the stop/forward pushbutton is pressed or the motor trips on overload or the protection fuse blows.

The stop button need not be pressed before changing the direction of the rotation.

Plug stop and anti-plug circuits

To halt a motor or to stop a running motor, a common method is to remove the supply voltage and allow the motor and load to come to a stop.

Nevertheless, in some applications, the motor must be stopped instantaneously or held in position by some sort of a braking device.

This is achieved using the electric braking circuit. It uses the windings of the motor to produce a retarding torque. The kinetic energy of the rotor and the load is dissipated as heat in the rotor bars of the motor.

The following are the two different means of electric braking:

1. Plugging

2. Dynamic braking

Plugging

In this method, a motor is connected to run in a reverse direction. This is done while the motor is still running in a forward direction, resulting in motor stoppage.

In order to achieve this, a switch, or a contact is used which gives the status of the motor. Depending on the running of a motor and its speed, the switch status changes from NO to NC. This switch is called the zero-speed switch or the plugging switch. A zero-speed switch prevents a motor from reversing after it has come to a stop.

A 'zero-speed' switch is physically coupled to a moving shaft on the machinery, the motor of which is to be plugged. As the zero-speed switch rotates along with the machine, the centrifugal force causes the contacts of the switch to open or close, depending on its intended use.

Each zero-speed switch has a rated operating speed range, within which the contacts will be switched, example, 10-100 rpm. The control schematic shows one method of plugging a motor to stop from one direction only.

As the start (forward) pushbutton is pressed, it energizes forward the contactor coil K1.

Therefore, the motor runs in the forward direction. The contactor K1 is latched through its latching contact.

As the motor runs in the forward direction, the NC contact F (zero switch) opens the circuit of the reverse contactor coil K2. If the stop pushbutton is pressed, it will de-energize the forward contactor K1. This in turn will help the reverse contactor K2 to energize because the forward contact on the speed switch is also in a closed condition.

As the reverse contactor is energized, the motor is plugged. The motor starts decreasing in speed rapidly up to the setting of the speed switch, at which point its contact opens and de-energizes the reverse contactor K2.

++++14 Plugging circuit for three-phase motor

S0 = Stop P.B.

S1 = For P.B.

S2 = Fwd. zero speed switch

K1 = Fwd. Contactor

K2 = Rev. contactor

F1 = Backup fuse

F2 =Overload relay

F3 = Control circuit fuse

This contactor is used, only to stop the motor, using the plugging operation. It’s not used to run the motor in reverse. Many machines require the motor be able to reverse.

Most small machines are not adversely affected by reversing the motor, before coming to a stop.

This is not true of larger pieces of equipment. The sudden reversing torque applied when a large motor is reversed (without slowing the motor speed) could damage the motor.

The driven machinery and the extremely high current could affect the distribution system. Plugging a motor for more than five times a minute requires the motor starter to be de-rated.

Anti-plugging

Anti-plugging protection is necessary, when a motor with large inertia is connected suddenly, in a reverse direction, while the motor is still running in a forward direction.

Anti-plugging protection prevents the application of a counter torque, until the motor speed is reduced to an acceptable value. In the anti-plugging circuit, the motor can be reversed but not plugged. Pressing the forward pushbutton completes the circuit for the K1 contactor coil causing the motor to run in the forward direction. It continues to run because of the latching contacts of K1. With the NC contact F (zero speed switch contact) in reverse, the contactor K2 opens, because of the forward rotation of the motor.

Pressing the stop button de-energizes the K1 contactor coil, which opens the latching contact of K1 also, causing the motor to slow. Pressing the reverse button won’t complete a circuit for the K2 contactor coil until the F (zero-speed switch) contact re-closes (i.e., when the speed is reduced below switch setting). Therefore, only when the motor reaches a near-zero speed, can the reverse circuit be energized. The motor now runs in a reverse direction.

++++15 Anti-plugging circuit for three-phase motor

S0 = Stop P.B.

S1 = For P.B.

S2 =Rev P.B.

K1 = For.contactor

K2 = Rev.contactor

F1 =Backup fuse

F2 =Overload relay

F3 =Control ckt. Fuse

Two-speed motor control

At times, it’s required to run equipment at two different speeds. This is usually so in certain industrial applications, such as mixer motor speeds, ventilating pumps, and batch control processes. Particularly in batch control, while feeding the batch components in a mixer, these components are fed into the mixer at two different feed rates - fast coarse feeding and slow fine feeding. This is done to feed the components accurately and to avoid overshoots. To achieve this, two-speed motors are employed.

A typical control circuit for a two-speed motor. There are two electrically separate windings housed in the motor. The control circuit connects the windings in different configurations causing the speed to change from one rpm to another. Each winding can deliver the motor's horsepower at a rated speed.

Two contactors are incorporated for low and high rpm. They should not be activated at the same time electrically. To protect both of them separately, an independent overload relay protection is provided.

Overload protection

Protection is provided to protect the motor from excessive overheating due to a motor overload. The overload protection protects the motor from excessive loading and at the same time protects starter components and conductors from overheating.

The working principle being, to sense the current flowing through the motor, that being a direct measure of heating and load to the motor. For overload protection in a motor circuit, bimetal relays are commonly used. The bimetal relays are triple-pole adjustable overload relays with built-in single-phasing protection. They provide accurate and reliable protection to the motors against overload and single phasing.

++++16 Typical control circuit for two-speed motor.

The bimetal relays provide an accurate overload and accelerated single-phasing protection for the motors. They incorporate the duel slider principle for accelerated tripping under the single-phasing protection. They also provide protection against severe unbalanced voltages.

The thermal relay is connected in series with the motor conductors, as excessive current passes through it for a preset time interval, a contact (in series) gets operated resulting in a motor trippage. The relay has settings (generally 3-14% of full load current) for various current ratings of the motor. These can be set accordingly.

Following the curve of a thermal overload, the relay shows the relationship between current and time. The relay won’t trip at a rated current, but at twice the rated current after a time duration of 45 s approximately. The bimetal relays protect themselves against overloads of up to 10 times the maximum setting. Beyond this limit, they have to be protected from short-circuits. It’s mandatory to use backup fuses for this purpose. The typical operating characteristics of bimetal relays. In high intertia drives, the starting time of the motor may be high (30-60 s). Use of standard thermal overload relays will cause the motor to trip before it achieves rated speed. In such cases, special thermal overload relays with a saturable CT may be required and are available from several vendors. A backup protection may be also provided in the form of a speed switch and a timer to detect abnormal starting condition and trip the motor.

Troubleshooting examples

In the above sections, various basic and complex control circuits for a three-phase motor have been dealt with.

The following is an example of troubleshooting a control circuit. Consider the control circuit for a three-phase motor with a DOL starter with a maintained-contact control.

++++17 Operating characteristics of a bimetal relay ---3-f characteristics curve (Typical) (Starting from cold)

Motor startup or running problems are listed below:

1. The motor starts running as the start pushbutton is pressed, but stops as soon as the start pushbutton is released.

2. The motor starts running and trips 2 min after the start pushbutton is released.

Let us assume the main circuit fuses are not blown. The following is the solution to the above-listed problems:

• Since the motor starts running as the start pushbutton is pressed, it indicates that the main contactor (K1) coil gets control supply when the circuit is completed on pressing the start pushbutton. However, the motor trips as soon as the start pushbutton is released.

• In the control circuit, as soon as the main contactor is switched on, the NO contact parallel to the start pushbutton contact must also close and hold the control circuit on until the stop pushbutton is pressed or the overload relay trips and its NC contact is open. To troubleshoot the problem, perform the following steps: (i) Check the control supply (L1), check control voltage between L1, and neutral (N). (ii) Check the control fuse (F3) with the multimeter. If the control fuse (F3) is blown, change the fuse and restart the motor, the motor must start if control fuse (F3) blown was the only problem. (iii) If the control fuse is OK, check whether the overload relay has tripped.

Check this with the help of a multimeter. Check voltage between the neutral terminal and the outgoing terminal of the overload relay contact, connected to the stop pushbutton. If the overload relay has not tripped and the multimeter shows that the control voltage between the two points is OK, then go to point (iv). (iv) Check the control voltage at the stop pushbutton outgoing terminal to the start pushbutton. If the voltage is OK, then go to point (v). (v) If two NO contacts are connected in parallel to each other, and the motor runs only when the start pushbutton is pressed, it indicates that, the NO contact of the main contactor must close as soon as the main contactor is switched on. It also indicates that the contactor that holds the control circuit on is not closing. The wires connected in parallel from the NO contact to the start pushbutton NO contact may be closed, or the NO contact of the main contactor is not closed, due to a faulty contact. To confirm this, take a loop of insulated wire, and short the contact K1; if the motor starts, it confirms that the NO contact is faulty. Change the NO contact block of the main contactor.

If the motor runs and trips after 2 min, to troubleshoot this perform the following steps: (i) Check the control supply (L1), check control voltage between L1 and neutral (N). (ii) Check the control fuse (F3) with the multimeter. If the control fuse (F3) is blown change the fuse and restart the motor. The motor must start if the control fuse (F3) blown was the only problem. (iii) If the control fuse is OK, check whether the overload relay has tripped.

Check this with the help of a multimeter, by checking the voltage between the neutral terminal and the outgoing terminal of the overload relay contact connected to the stop pushbutton. If the overload relay has tripped, you won’t get control voltage between the two points. Reset the overload relay and look into the reasons for which the motor tripped on overload. If there is a voltage between the two points, look for a loose contact or loose wiring at the subsequent contacts in the control circuits.

Troubleshooting strategies

Strategies for troubleshooting of control circuits and 'Ladder Logic Circuits':

1. It’s important to have the control circuit drawings, details of devices, their interconnection and interlocking while troubleshooting the control circuits. To troubleshoot a machine or equipment problem, it’s good to have the 'Manufacturer's Operation and Maintenance Manual', as well as the ' Troubleshooting Instructions'.

2. 'Block Interlocking Diagram' and 'Control Sequences' of the equipment/ machine operations should be available during troubleshooting.

3. Drawings and details of the power circuit of the equipment or the machine, control devices, contactors, timers, counters, safety, and protection devices, etc. are needed for troubleshooting the root cause.

4. Appropriate test and measurement instruments required for testing the control and power circuit of the equipment, or the machine must be available.

5. Switch OFF the main power supply to the equipment/machine and switch control supply ON, to avoid any mishaps or accidents while troubleshooting control circuits because of the sudden starting of the equipment.

6. As control circuits are different from equipment-to-equipment and machine to-machine, it’s not be possible to formulate a single or common strategy for troubleshooting control circuits. However, exemplary/standard engineering and trade practices must be followed while troubleshooting the control circuits.

General document checklist for troubleshooting

Control circuit drawings; Manufacturers operations and maintenance manuals and troubleshooting instructions; Block interlocking diagram and control sequences involving the equipment/ machine

Drawings and details of power circuits of the equipment/machine; Details of devices, control devices, contactors, timers, counters, safety/protection; Power circuit of the equipment or the machine.

++++18 Sequence start stop --- Consider the example of a drill machine and conveyor table.

Drill machine; Up limit switch; Down limit switch; Stopper; Material; Conveyor table

Limit switch:

The sequence is as listed below:

1. The conveyor table should run as long as material strikes the conveyor limit switch that is provided.

2. The 'Stopper and Drill' should go up and down until it strikes the 'Down' limit switch. This makes a hole in the stationary material for 1 s. Again, the 'Drill' should go up until it strikes the 'Up' limit switch.

3. Then the conveyor starts again until the next material strikes the conveyor limit switch.

4. Outputs from PLC can be con figured for conveyor start/stop, drill machine start/stop, drill up/down and stopper up/down.

5. Inputs to the PLC are the conveyor limit switch and drill machine up/down limit switch, conveyor table on/off.

Ladder logic design exercise; Prepare a 'PLC Ladder Logic Control Circuit' from the above example.