DOL vs Star Delta: Which Motor Control is Right for You?

DOL (Direct online) and star-delta starting are the most applicable and widely-used starting methods in the industrial area due to their economic reasons. But there are lots of differences between DOL and star-delta. Let’s check them one by one.

Differences Between DOL and Star Delta

Differences between DOL and star-delta are the following:

1. Relative starting current

The disadvantage of DOL is that it gives the highest possible starting current. It does not reduce the starting current of the motor. A normal value is between 6 to 7 times the rated motor current but values of up to 9 or 10 times the rated current exist. The received starting current of a star-delta starter is about 30% of the starting current during a direct online start. i.e., typically to 2…2.5 Ie.

2. Relative starting torque

During a DOL start, the starting torque is very high and is usually higher than required for most applications. The starting torque of a star-delta starter is reduced to about 30% of the torque available at a DOL start. The star-delta method only works when the application is lightly loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.

3. Operating principle

In the DOL starter, it is possible to start a motor with a simple circuit. The direct online motor starter (DOL) consists of a contactor and an overload relay for protection. The contactor can be opened by the thermal overload relay under fault conditions. Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, to hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.

In the star-delta starting method, the wiring connection from the power supply source to the motor is connected from the star (wye) to the delta connection. The motor is started in a star configuration and then it is transferred to the delta configuration. This allows the full voltage to be applied to the motor during its running so as to get the full torque output. This can be further explained that in a Star-Delta starter, the motor is started as a star connection and when the motor starts running the connection is changed to the delta. With star connection, the motor takes times less voltage. However, as the torque is proportional to the square of the voltage, the starting torque also reduces.

4. Current peak

In the star-delta starting method, when starting up, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approximately 80-85% of the motor-rated speed the load torque is equal to the motor torque and the acceleration ceases. To reach the rated speed, a switch over to the delta position is necessary, and this will result in high transmission and current peaks. In some cases, the current peak can reach a value that is even bigger than for a DOL start.

5. Mechanical stress

Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipment, etc. leading to premature failure and plant downtimes. In the DOL starter, mechanical stress cannot be reduced. On the other hand, a star-delta starter reduces mechanical stress.

6. Areas of application

The starting current at the moment of switching DOL can be as high as 6-8 times the rated current. Due to this reason, the electricity boards have laid down general regulations that direct-on-line starting of squirrel cage motors shall be allowed only for small motors-up to 7.5 or 10 HP (5.5 or 7.5 kW) Star delta starter can be used up to 75 H.P (55kW)

7. Cost

DOL is the most economical and cheapest starter option. Star delta is more expensive than DOL. Moreover, star-delta requires more space in the enclosure.

8. Starting time

The starting time of DOL is shorter than the star-delta. The run-up time of the DOL is approx. 0,2….5s. The run-up time of star-delta is approx. 2…15s. (These values are longer in heavy-duty start)

9. Component

The components used in DOL starting consist of only a main contactor and thermal overload relay. On the other hand, the components used in star-delta starting consist of three contactors, an overload relay, and a timer for setting the time in the star position. The motor must be delta connected during a normal run to be able to use the star-delta starting method.

10. Wiring

To enable a switch-over from star to delta, the six ends of the motor winding are connected to terminals. The contactors of a star-delta starter switch over the windings accordingly. In the DOL starter, only one set of cables is required from the starter to the motor. That’s why DOL is easier to understand and troubleshoot.