The induction motor draws a huge amount of current at startup. This starting current can damage the motor windings. In order to avoid any damage, we use different techniques to reduce the starting current using Motor Starter. These techniques depend on the motor ratings and the load connected to the motor. Apart from this, the motor starter also protects the motor from overloading and overcurrent.
The Direct Online or DOL starter employs full voltage or across the line starting technique where the motor is directly connected to full voltage through MCCB or circuit breaker and relays for overload protection. This is why such a starter is used with induction motors rated below 5 hp.
Protection Offered by DOL Starter:
The Motor starters not only provide the safe starting current but also provide protection to keep the motor safe during operation. It is clear that the DOL starter provides the full line voltage but it does provide the following protection:
Overcurrent Protection:
The condition that causes the flow of a faulty current in a large amount mostly due to a short circuit or ground fault is called overcurrent.
The overcurrent condition can cause damage to the motor, power lines and can be a hazard for operators. Such an amount of current is too dangerous for a brief moment.
In the DOL starter, we use a circuit breaker or fuses for protection against overcurrent. They open the circuit and breaks the current flow in an instant until the problem in the system is resolved. The fuse or circuit breaker is carefully selected with its rating kept in mind. Because we do not want the fuse to break but to tolerate the starting current as well as the heavy load current. The overcurrent breaker’s rating is kept a bit higher than the rated starting current of the motor.
Related Post: What is Soft Starter? Its Working, Diagram and Applications
Overload Protection:
The condition where the load connected to the motor increases beyond its limit and the motor draws an excessive amount of current is called overload condition. During overload, the current flow is beyond the safe limits which damage the wires as well as the motor windings. It melts the windings and may cause fire hazards.
In order to protect the motor from overloading, we use an overload relay that trips the power supply and protects the system from overheating. The overload relay monitors the current and breaks the current flow when it exceeds a certain limit for a period of time. The tripping mechanism may vary and depends on the application of motor.
Thermal Overload Relay: This type of overload relay works on the principle of expansion due to the heat generated by the current flow. A bimetallic strip is used with different thermal expansion to break or make the circuit based on the temperature.
Magnetic overload relay: such relays works on the principle of the magnetic field generated by the current flow through a coil. An excessive current drawn by the motor (that is a predetermined amount) generates enough magnetic field to trips the contact terminals and breaks the current supply.
Electronic Overload Relay: Electronic relay is a solid-state device without any movable parts or contacts. It utilizes current sensors to monitor the motor current and has an adjustable setting that allows the tripping at a wide range of current ratings.
The DOL starter is made of a circuit breaker or MCCB or fuse, an overload relay and contactor or coil. The circuit breaker is used for protection against short circuits while the overload relay protects the motor from overloading. The contactor is used for starting and stopping the motor where the green and red buttons are connected. The wiring for the start and stop button is briefly explained in this article below.Parts of DOL Starter:
A DOL starter is made of following parts:
Circuit breaker or Fuse:
The circuit breaker or fuse is directly connected to the power mains and it is used for protection against short circuits. It trips the power supply in case of short circuit to protect the system from any potential hazards.
A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the power supplied to the motor. It connects and disconnects multiple contacts conveniently by providing remote control over the operation.
The magnetic field generated by the coil is used for switching the terminals. The passing current through the coil magnetizes the iron core that is surrounded by the coil. The magnetic force pulls on the armature to close or open the contacts.
The magnetic contactors have three NO (Normally open) main contacts used for power supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep in mind that the coil used for a single-phase and three phase supply vary as the supp
ly voltages are different.
OLR or overload relay is the last part used in the DOL starter and it is used for protection against overloading of motor. It breaks the current flow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is carefully selected in such a way that its tripping current limit does not fall below the starting current range.
The excessive amount of current flow can damage the insulation of electrical wires as well as the motor winding. The motor life expectancy decrease and it can short the windings causing a risk of fire.
A simple fuse or circuit breaker cannot protect the system from overloading because they are used for overcurrent (short circuit) protection. The OLR has current sensing properties that can differentiate between the starting and overload current.
Related Post: Main Difference between Contactor and Starter
DOL Starter Wiring Diagram:
The wiring of 3 phase and single phase slightly differs from each other. Following are the wiring for 3 phase and single phase dol starter:
L1 is connected to the R phase through MCCB
L2 is connected to the Y phase through MCCB
L3 is connected to the B phase through MCCB
Point 1 is connected to R-phase while point-2 is connected to overload relay T1 point.
Point 3 is connected to Y-phase while point-4 is connected to overload relay T2 point.
Point 5 is connected to B-phase while point-6 is connected to overload relay T3 point.
2) Auxiliary NO Contacts: the auxiliary NO contact 53 and 54 closes when the coil energizes. It is connected through the green and red push button.
The point-54 is connected through the stop button.
3) Auxiliary NC Contacts: the NC contact 95 and 96 is normally closed contacts of overload relay and it opens when the current exceeds a certain limit.
The point-96 is connected to the stop button.
Relay Coil: The relay coil points A1 and A2 are connected to the voltage supply through OLR, start button and stop button.
The point-53 is connected to the point-96 start button
The Point A1 is connected to R-phase from point 1.
Point A2 is connected to the NC terminal of overload relay point 95.
Overload Relay: The overload relay has normally connected terminals T1, T2 and T3 that supply power to the motor.
The T1 is connected to the point-2 of the contactor.
The T2 is connected to the point-4 of the contactor.
The T3 is connected to the point-6 of the contactor.
Following is the basic power and control wiring diagram of starting and stopping a three-phase motor using DOL starter.
We have to use all 3 poles of the overload relay otherwise the imbalance due to the current flow in only 2 of them will cause unnecessary tripping.
Working of DOL Starter:
The DOL starter connects the 3 phase voltage supply i.e. R-phase, Y-phase and B-phase to the induction motor terminals.
There are two types of circuits in the DOL starter diagram given above; The control circuit and the power circuit.
The Control Circuit:
It is powered by only 2 phases of the power supply and it is responsible for starting and stopping the power supplied to the motor.
The green start button and red stop button is connected inside the control circuit. Pressing the green button for an instant starts the motor and the power is supplied when it is released. Pushing the red button stops the power supply and stops the motor.
Pressing the Start (Green) Button:
The Green button is connected to the B-phase power supply through point 5 and point 53 and it connects it to the point-A2 of the relay coil through OLR point 96-95.
The induction motor draws a huge amount of current at startup. This starting current can damage the motor windings. In order to avoid any damage, we use different techniques to reduce the starting current using Motor Starter. These techniques depend on the motor ratings and the load connected to the motor. Apart from this, the motor starter also protects the motor from overloading and overcurrent.
The Direct Online or DOL starter employs full voltage or across the line starting technique where the motor is directly connected to full voltage through MCCB or circuit breaker and relays for overload protection. This is why such a starter is used with induction motors rated below 5 hp.
Protection Offered by DOL Starter:
The Motor starters not only provide the safe starting current but also provide protection to keep the motor safe during operation. It is clear that the DOL starter provides the full line voltage but it does provide the following protection:
Overcurrent Protection:
The condition that causes the flow of a faulty current in a large amount mostly due to a short circuit or ground fault is called overcurrent.
The overcurrent condition can cause damage to the motor, power lines and can be a hazard for operators. Such an amount of current is too dangerous for a brief moment.
In the DOL starter, we use a circuit breaker or fuses for protection against overcurrent. They open the circuit and breaks the current flow in an instant until the problem in the system is resolved. The fuse or circuit breaker is carefully selected with its rating kept in mind. Because we do not want the fuse to break but to tolerate the starting current as well as the heavy load current. The overcurrent breaker’s rating is kept a bit higher than the rated starting current of the motor.
Related Post: What is Soft Starter? Its Working, Diagram and Applications
Overload Protection:
The condition where the load connected to the motor increases beyond its limit and the motor draws an excessive amount of current is called overload condition. During overload, the current flow is beyond the safe limits which damage the wires as well as the motor windings. It melts the windings and may cause fire hazards.
In order to protect the motor from overloading, we use an overload relay that trips the power supply and protects the system from overheating. The overload relay monitors the current and breaks the current flow when it exceeds a certain limit for a period of time. The tripping mechanism may vary and depends on the application of motor.
Thermal Overload Relay: This type of overload relay works on the principle of expansion due to the heat generated by the current flow. A bimetallic strip is used with different thermal expansion to break or make the circuit based on the temperature.
Magnetic overload relay: such relays works on the principle of the magnetic field generated by the current flow through a coil. An excessive current drawn by the motor (that is a predetermined amount) generates enough magnetic field to trips the contact terminals and breaks the current supply.
Electronic Overload Relay: Electronic relay is a solid-state device without any movable parts or contacts. It utilizes current sensors to monitor the motor current and has an adjustable setting that allows the tripping at a wide range of current ratings.
The DOL starter is made of a circuit breaker or MCCB or fuse, an overload relay and contactor or coil. The circuit breaker is used for protection against short circuits while the overload relay protects the motor from overloading. The contactor is used for starting and stopping the motor where the green and red buttons are connected. The wiring for the start and stop button is briefly explained in this article below.Parts of DOL Starter:
A DOL starter is made of following parts:
Circuit breaker or Fuse:
The circuit breaker or fuse is directly connected to the power mains and it is used for protection against short circuits. It trips the power supply in case of short circuit to protect the system from any potential hazards.
A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the power supplied to the motor. It connects and disconnects multiple contacts conveniently by providing remote control over the operation.
The magnetic field generated by the coil is used for switching the terminals. The passing current through the coil magnetizes the iron core that is surrounded by the coil. The magnetic force pulls on the armature to close or open the contacts.
The magnetic contactors have three NO (Normally open) main contacts used for power supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep in mind that the coil used for a single-phase and three phase supply vary as the supp
ly voltages are different.
OLR or overload relay is the last part used in the DOL starter and it is used for protection against overloading of motor. It breaks the current flow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is carefully selected in such a way that its tripping current limit does not fall below the starting current range.
The excessive amount of current flow can damage the insulation of electrical wires as well as the motor winding. The motor life expectancy decrease and it can short the windings causing a risk of fire.
A simple fuse or circuit breaker cannot protect the system from overloading because they are used for overcurrent (short circuit) protection. The OLR has current sensing properties that can differentiate between the starting and overload current.
Related Post: Main Difference between Contactor and Starter
DOL Starter Wiring Diagram:
The wiring of 3 phase and single phase slightly differs from each other. Following are the wiring for 3 phase and single phase dol starter:
L1 is connected to the R phase through MCCB
L2 is connected to the Y phase through MCCB
L3 is connected to the B phase through MCCB
Point 1 is connected to R-phase while point-2 is connected to overload relay T1 point.
Point 3 is connected to Y-phase while point-4 is connected to overload relay T2 point.
Point 5 is connected to B-phase while point-6 is connected to overload relay T3 point.
2) Auxiliary NO Contacts: the auxiliary NO contact 53 and 54 closes when the coil energizes. It is connected through the green and red push button.
The point-54 is connected through the stop button.
3) Auxiliary NC Contacts: the NC contact 95 and 96 is normally closed contacts of overload relay and it opens when the current exceeds a certain limit.
The point-96 is connected to the stop button.
Relay Coil: The relay coil points A1 and A2 are connected to the voltage supply through OLR, start button and stop button.
The point-53 is connected to the point-96 start button
The Point A1 is connected to R-phase from point 1.
Point A2 is connected to the NC terminal of overload relay point 95.
Overload Relay: The overload relay has normally connected terminals T1, T2 and T3 that supply power to the motor.
The T1 is connected to the point-2 of the contactor.
The T2 is connected to the point-4 of the contactor.
The T3 is connected to the point-6 of the contactor.
Following is the basic power and control wiring diagram of starting and stopping a three-phase motor using DOL starter.
We have to use all 3 poles of the overload relay otherwise the imbalance due to the current flow in only 2 of them will cause unnecessary tripping.
Working of DOL Starter:
The DOL starter connects the 3 phase voltage supply i.e. R-phase, Y-phase and B-phase to the induction motor terminals.
There are two types of circuits in the DOL starter diagram given above; The control circuit and the power circuit.
The Control Circuit:
It is powered by only 2 phases of the power supply and it is responsible for starting and stopping the power supplied to the motor.
The green start button and red stop button is connected inside the control circuit. Pressing the green button for an instant starts the motor and the power is supplied when it is released. Pushing the red button stops the power supply and stops the motor.
Pressing the Start (Green) Button:
The Green button is connected to the B-phase power supply through point 5 and point 53 and it connects it to the point-A2 of the relay coil through OLR point 96-95.
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