An electric motor will generator big electric current during starting; the sudden current may cause sudden voltage drop down, and cause other unexpected bad effects, or damage the equipment. So it’s important and necessary to choose the suitable starting method for functional and cost-effective control system.
Direct online (DOL) starter
Direct online starter is the simplest and cheapest way of electric motor starting. Magnetic starter control panels is the post popular type of Direct online starter. DOL starting is simply switching the motor on in one operation, with a direct connection from the power source. But it can only be used for small motors. For oil and gas drilling mud systems, only the motors less than 30kw will use DOL starting. For big power motors, if there is already an effective control to start and stop the motor, a DOL starter can be used as the second control.
In oil and gas drilling field, start-delta starting are normally used for motors between 30kw to 75kw, like for pumps. When we mentioned star-delta starter, the motor is first connected in ‘star’ configuration, which allows the motor to gather speed without drawing excessive current. Once the motor is up to speed (or a preset time is elapsed) it is then connected in the normal ‘delta’ configuration. This method can reduce the starting current demand by 30%, but is only suited to applications where the motor is starting without load .
Soft starters are more expensive than DOL and star-delta. A soft starter is an electronic components which controls the voltage flowing to the motor when startup. Soft starter can slowly increase the supply voltage to the motor, and start the motor smoothly without overcurrent.
Variable frequency speed drives (VFD control)
A variable frequency speed drive (VFD), also known as a VSD or VVVF, which allows complete control of the motor speed including starting and stopping. It operates by changing the frequency of the power supplied to the motor. And the motor can be run at any speed, so VFD drive control can save power, and control the driving speed. Variable frequency drives are the most expensive method for motor starters. And it can be used to control running speed of the machine, like for decanter centrifuges, vertical cuttings dryers, screw conveyors, etc…