There are many methods to start/run the Induction motor; Through DOL starter, Star-delta starter, Variable frequency derive(VFD), & Auto transformer starter.
To start the motor; is not the only function of the starter. Along with the starting, Starters provide protection from overload, phase failure imbalance & prevention of high initial starting current.
All the starters don't provide all the protections except VFD.
All the starting methods mentioned above run the induction motor in a different way & all are having a different working principle which makes each starting method has its own merits & demerits.
For example:
In DOL starter, Initial starting current can't be control. But In Star Delta, VFD and autotransformer starter, starting current can be controlled easily. That's why DOL starters are used for small rating motors only.In this post, we will briefly talk about the Direct Online Starters.
DOL (DIRECT ONLINE STARTER)
"Direct online starters connects the motor's terminals directly with the power supply :
Without any change in motor's winding connection,
Without any change in voltage & frequency that's why known as Direct On Line Starter."
DOL starter consists of thermal overload relay, a Contactor and ON-OFF push buttons.
- A thermal overload relay is provided for short circuit protection, Overload protection & phase failure imbalance. These overload relay are available in different ampere ranges according to the motor rating.
- Contactor current rating is also depends upon the motor rating; Probably higher than the motor's current rating & its coil voltage is selectable which depend upon the control circuit voltage.
HOW DOES DOL STARTER WORKS
If ever we have started the DOL, One Important thing we observe that we need to press ON&OFF push button only once to start & stop the motor respectively.
Apart from this, Starter get disconnected when power supply goes off & we need to press the start button again to start the motor. These all the things are happening due to the 'latch circuit'.
LATCH
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| Latch circuit in ladder Logic Diagram |
As per the diagram in an image above:
As shown in an connected image above I0(NO), I1(NC) are inputs While Q0 is output. When I0 get pressed, supply get available to Q0.
Parallelly, q0 (NO contact) connected across the I1 input also changes its state & become a closed contact. Which result Supply to Q0 get hold through q0 even if we release the I0 (NO contact).
Supply to Q0 break OFF; Either series I1 (NC contact) get break or power supply get cut off.
This circuit is known As 'HOLD circuit' Which is used in DOL Starter.
LAYOUT OF DOL STARTER
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| Wiring diagram of DOL Starter| Power Diagram is marked with Blue While control diagram is marked with Green |
Power wiring
In wiring diagram above, Power wiring is marked with blue.
3phase Power supply (VLL=415) is passing through thermal overload relay first then these 3 lines are connected to the contactor & motor terminals are connected on another side of the contactor. Motor get power supply when contactor get Energized. Power supply to the motor is controlled by controlling the coil voltage to the contactor.
Control Wiring
In wiring diagram above, Control wiring is marked with Green.
Since the contactor coil voltage is 415 Volts that's why red phase is directly given to the coil. Whole the control circuit is working through the second phase which is blue.
As shown in layout above, blue phase passes through NO contact(START). Then phase is passes through both the NC contacts, NC contact of stop button & NC contact of thermal overload relay.
1 extra NO auxiliary contact of contactor which is used for HOLD; is parallelly connected with NO contact(START).
Power supply to motor get disconnected if Stop button(NC) get pressed or Overload relay's (NC) get trip.
"In this post, DOL Starter explanation is theoretical. If you are curious about the practical DOL starters which are being used in Industries, you can go through the post which are entirely based on practical working. Links are given Below:
Practical Working DOL Starter By L&THope! Post is beneficial for you. Your suggestions and queries are always welcome. Please do comments.
Tags:
Induction motor starters