DOL Starter Basics & Manual

Cheatsheet Content

1. Direct Online (DOL) Starter Overview A DOL starter is the simplest and most economical method of starting a 3-phase squirrel cage induction motor. It connects the motor directly to the full supply voltage. Application: Suitable for small to medium-sized motors where the starting current doesn't cause excessive voltage drop in the supply. Advantages: Simple design and operation Low cost Easy maintenance High starting torque Disadvantages: High starting current (5-8 times full load current) Sudden mechanical stress on motor and driven equipment Not suitable for large motors due to voltage dip issues 2. Components of a DOL Starter Component Function Molded Case Circuit Breaker (MCCB) / Fuse Provides short-circuit and overload protection for the main circuit. Contactor (KM) An electromagnetically operated switch that connects/disconnects the motor from the supply. Consists of main contacts and auxiliary contacts. Thermal Overload Relay (F) Protects the motor from sustained overcurrents by tripping the contactor. Has bimetallic strips that heat up with current. Start Push Button (NO) Momentary normally open (NO) contact to energize the contactor coil. Stop Push Button (NC) Momentary normally closed (NC) contact to de-energize the contactor coil. Auxiliary Contact (NO) Normally open contact of the main contactor used for "latching" or "holding-in" the contactor coil. Motor (M) The 3-phase squirrel cage induction motor being controlled. 3. DOL Starter Control Circuit Diagram The control circuit typically operates at a lower voltage (e.g., 24V, 110V, 230V AC) for safety and uses auxiliary contacts. L1---(F1)---(STOP NC)---(START NO)---(KM NO)---(KM Coil)---(F2 NC)---N | ^ | | +-----------------------+ (Holding Contact) F1: Control circuit fuse/MCB STOP NC: Normally Closed Stop Push Button (breaks circuit) START NO: Normally Open Start Push Button (makes circuit) KM NO: Normally Open Auxiliary Contact of Contactor KM (holding contact) KM Coil: Contactor Coil (energizes main contacts) F2 NC: Normally Closed Auxiliary Contact of Thermal Overload Relay (trips on overload) 4. DOL Starter Power Circuit Diagram Connects the 3-phase supply to the motor through the main contacts of the contactor and overload relay. R --- (MCCB/Fuse) --- (KM Main Contact 1) --- (Overload F1) --- Motor U Y --- (MCCB/Fuse) --- (KM Main Contact 2) --- (Overload F2) --- Motor V B --- (MCCB/Fuse) --- (KM Main Contact 3) --- (Overload F3) --- Motor W 5. Operating Principle Motor OFF: All main and auxiliary contacts of the contactor (KM) are open, except for the NC contacts of the Stop button and overload relay. Press START: The Start (NO) push button closes, completing the circuit to the contactor coil (KM Coil). The KM Coil energizes, pulling in the contactor armature. All main contacts of KM close, connecting the motor to the full 3-phase supply. The motor starts at full voltage. The KM (NO) auxiliary contact (holding contact) also closes. Release START: The Start (NO) push button returns to its open position. However, the KM (NO) auxiliary contact, which is now closed, maintains the circuit to the KM Coil. This is called "latching" or "holding-in" and keeps the motor running. Press STOP: The Stop (NC) push button opens, breaking the circuit to the KM Coil. The KM Coil de-energizes, and the contactor armature releases. All main contacts of KM open, disconnecting the motor from the supply. The motor stops. The KM (NO) auxiliary contact also opens, resetting the control circuit. Overload Trip: If the motor draws excessive current for too long, the thermal overload relay (F) heats up. The bimetallic strips inside F bend, causing its NC auxiliary contact (F2 NC) in the control circuit to open. This breaks the circuit to the KM Coil, de-energizing it and stopping the motor, protecting it from damage. The overload relay typically has a reset button after it cools down. 6. Manual Wiring Procedure (Practical Steps) 6.1 Pre-wiring Checks & Safety Safety First: ALWAYS ensure power is disconnected and locked out before starting any wiring. Use appropriate PPE. Identify Components: Familiarize yourself with the terminals on the MCCB, contactor, overload relay, and push buttons. Tools: Screwdrivers, wire strippers, crimping tool (for ferrules/lugs), multimeter. Wire Sizing: Use appropriate wire gauges for both power and control circuits based on motor current and control voltage. 6.2 Power Circuit Wiring MCCB to Contactor: Connect the R, Y, B phases from the MCCB output terminals to the main input terminals (L1, L2, L3 or 1, 3, 5) of the contactor. Contactor to Overload Relay: Connect the main output terminals (T1, T2, T3 or 2, 4, 6) of the contactor to the input terminals of the thermal overload relay. For most modern overloads, they directly mount to the bottom of the contactor. Overload Relay to Motor: Connect the output terminals (T1', T2', T3') of the thermal overload relay to the motor terminals (U, V, W). Ensure correct phase sequence for motor rotation. Earth Connection: Connect the motor body to the main earth busbar. 6.3 Control Circuit Wiring Assume control voltage is derived from one phase (e.g., R-phase) and Neutral (N). Control Circuit Fuse/MCB: Connect one side of the control voltage supply (e.g., R-phase) to an input terminal of a control circuit fuse/MCB (e.g., F1). Overload NC Contact: Connect the output of F1 to one terminal of the NC auxiliary contact of the thermal overload relay (e.g., 95). Connect the other terminal (e.g., 96) of the overload NC contact to the Stop push button. Stop to Start: Connect the output terminal of the Stop (NC) push button to the input terminal of the Start (NO) push button. Start to Contactor Coil: Connect the output terminal of the Start (NO) push button to one terminal of the contactor coil (e.g., A1). Holding Contact (Latching): Connect a wire from the output terminal of the Stop (NC) push button (or input of Start NO) to one terminal of the NO auxiliary contact of the main contactor (e.g., 13). Connect the other terminal of the contactor's NO auxiliary contact (e.g., 14) to the same terminal as the Start (NO) push button output (i.e., A1 of the contactor coil). Contactor Coil to Neutral: Connect the other terminal of the contactor coil (e.g., A2) to the Neutral (N) terminal of the control voltage supply. Grounding: Ensure all metallic enclosures and panels are properly grounded. 7. Commissioning & Testing Visual Inspection: Check all connections for tightness, correct polarity, and insulation. Continuity Test (Power OFF): Use a multimeter to check continuity for all circuits: Control circuit: Verify continuity from F1 to A2 when Start is pressed and holding contact is made. Main contacts: Verify continuity through main contacts when contactor is manually pressed. Overload NC: Check continuity of overload NC contact (95-96). Insulation Resistance Test: Use a megger to test insulation resistance between phases, phase to earth, and control circuit to earth. Power ON (Control Circuit Only): Switch ON control circuit supply. Press Start: Contactor should pull in and hold. Press Stop: Contactor should drop out. Manually trip Overload: Contactor should drop out. Reset overload. Power ON (Full System): Switch ON main power supply. Press Start: Motor should start. Check for correct rotation (if critical, temporarily disconnect load or run briefly). Press Stop: Motor should stop. Monitor motor current (using clamp meter) to ensure it's within limits. Test overload trip by setting it to a slightly lower value or using a test button if available. 8. Troubleshooting Common Issues Problem Possible Cause(s) Solution Contactor doesn't pull in when START is pressed No control voltage; Fuse blown; Loose connection; Faulty Start button; Faulty Overload NC contact; Faulty Contactor Coil Check control supply, fuse, wiring, test push buttons, check overload reset, measure coil resistance. Contactor pulls in but drops out immediately after START is released Faulty holding contact (KM NO); Incorrect wiring of holding contact Check holding contact continuity when contactor is energized; re-wire holding contact as per diagram. Motor doesn't start (Contactor pulls in) No main power supply; Blown main fuse/tripped MCCB; Faulty main contacts; Open circuit in motor winding; Incorrect motor wiring Check main supply, fuses/MCCB; check continuity of main contacts; check motor winding resistance. Motor trips on Overload Motor overloaded; Low supply voltage; Faulty motor; Incorrect overload setting; Phase imbalance; Mechanical fault in driven machine Check motor load; measure supply voltage; insulation test motor; adjust overload setting (if motor current is within limits); check for mechanical issues. Motor runs in wrong direction Incorrect phase sequence Swap any two of the three phase connections at the motor terminals or at the contactor output.