What is Direct On Line (DOL) Motor Starting?

Direct on line (DOL) motor starting is when a motor is started at full load, with full line voltage applied to motor terminals.

As this causes the motor to draw a large amount of current, it is only suitable for motor powers up to 4Kw, with a maximum motor size of 10Kw connected this way. Motors above 4Kw should be wired using a Star Delta configuration method.

If higher powered motors are wired as DOL, it can cause excessive voltage drop in the supply circuit. Many pumping stations are located near housing estates and both DOL (Direct On Line) and ASD (Automatic Star Delta) starters can cause lights to dip in properties each time motors are started.

What is Star Delta Motor Wiring?

Star Delta motor wiring is a reduced voltage starting method. The motor is configured and wired in a star and delta configuration. The motor is initially started utilizing the star wiring connections, which starts the motor at a reduced voltage and current known as soft start minimising the voltage drop across circuits.

Switching from Star to Delta is performed during operation, via a timer in the control panel enabling the motor to operate at full load.

Star Delta Advantages

1.       As the speed slowly increases it can help avoid water hammer, as a pump will not deliver a surge of fluid within the pipework by suddenly starting at full speed

2.       Little heat is produced and efficiency increases

3.       Starting current is a 1/3 compared to full motor current.

4.       Inexpensive

5.       High Torque per phase

6.       Frequent starting has little affect

Disadvantages

1. Pump can have continuous recirculation until motor speed reaches full speed

2. Starting Torque is restricted to a 1/3 which can affect successful operation of connected equipment in particular with certain designs of pump which may require a constant torque motor.

If pump operation begins with a fluid which is viscous on startup and at cold temperatures the motor may struggle to start and overheat.

3. If a motor does not reach 90% of rated speed, current peak will be higher than the DOL starting method

4. With conventional ASD (Automatic Star Delta) starting, its open transitional operation reduces the speed of the motor when the star contactor opens, before the delta contactor closes causing a second current spike.

A Wauchope ASD motor starting arrangement reduces current spikes on start up through the use of a resistor bank and contactor arrangement ensuring a closed transition method of starting.

Conventional ASD motors use 3 contactors so a closed transition Wauchope ASD is more costly with it containing 4 contactors and a three phase resistor.

Electronic soft starters and variable speed drives have now taken over to save on operational cost and reduce social interference through voltage drops.  Initial harmonic issues have been overcome, and the use of Variable Speed Drives and soft starters being widely adopted. 

Table Showing Motor Wiring Method by Motor Size