The most basic, inexpensive and straightforward method of starting a 3 phase squirrel cage induction motor is the DOL starter. Direct online starter (DOL) connects 3-phase motors directly across 3-phase supply, and the beauty of this type of starter is in spite of motor connected straight, it doesn’t cause any harm to the motor. Mainly it consists of the main contacts and protective device.
Let us see one by one its construction, working and its advantages. Since the starter connects the motor directly to the main supply line, the motor draws very high current (5-8 times) compared to the full load current of the motor and the value of large current decreases as the motor reaches to its rated speed.
As we know, the equation for armature current in the motor.
The value of back emf (E) depends upon speed (N), i.e. E is directly proportional to N.
At starting, the value of E is zero. So starting current is very high. In a small rating motor, the rotor has more considerable axial length and small diameter. So it gets accelerated fastly. Hence, speed increases and thus the value of armature current decreases rapidly. Therefore, small rating motors smoothly run when it is connected directly to a 3-phase supply. If we connect a large motor directly across 3-phase line, it would not run smoothly and will be damaged, because it does not get accelerated as fast as a smaller motor since it has short axial length and larger diameter more massive rotor. However, for large rating motors, we can use oil immersed DOL starter for the purpose.
Construction of DOL Starter
It consists of two buttons, GREEN for starting and RED for stopping purpose of the motor. The DOL starter comprises of an MCCB or circuit breaker, contactor and an overload relay for protection. These two buttons, i.e. Green and Red or start and stop buttons control the contacts. To start the motor, we close the contact by pushing Green Button, and the full line voltage appears to the motor. Contactor can be of 3 poles or 4-pole. Below given contactor is of 4-pole type. It contains three NO (normally open) contacts that connect the motor to supply lines, and the fourth contact is “hold on contact” (auxiliary contact) which energises the contactor coil after the start button is released. If any fault occurs, the auxiliary coil gets de-energised, and hence the starter disconnects the motor from supply mains.
When motor draws excessive current to meet the load requirement and If load requirement goes beyond the rated limit, termed as Overload. Overload protection is a type of security when motor draws over current or excessive current and causes overheating of the equipment. Overload is also the type of over current. So overload relays are employed to limit the amount of current drawn. But that does not mean that protects the short circuit. Fuse or MCB used in the system protects the over current. Overload protection opens a circuit at relatively low currents that are a little higher than the rating of the motor.Overload currents are likely to damage if they persist for a long time, i.e. it will not trip if a high value of current flows for a short period such as starting of the motor. We often provide the overload protection by an overload relay. Overload relays may be solid-state devices with adjustable trip setting also called as the electronic relay or by interacting with related temperature sensors called as a thermal relay or if only operates for excess current flow then called as a magnetic relay. For most motors, the maximum rating of the overload protection device is 125% of the full load ampere rating.
Working of DOL Starter
The above wiring connection is of DOL starter. The DOL starter connects the 3-phase main with the motor. The control circuit is connected to any two phases and energised from them only. When we press the start button, the current flows through contactor coil (Magnetizing Coil) and control circuit also. The current energises the contactor coil and leads to close the contacts, and hence 3-phase supply becomes available to the motor. If we press the stop button, the current through the contact becomes discontinued, hence supply to the motor will not be available, and the similar thing will happen when overload relay operates. Since the supply of motor breaks, the machine will come to rest. The contactor coil (Magnetizing Coil) gets supply even though we release start button because when we release start button, it will get supply from the primary contacts as illustrated in the diagram of the Direct Online Starter.
Advantages of DOL Starter
- Simple and most economical starter.
- More comfortable to design, operate and control.
- Provides nearly full starting torque at starting.
- Easy to understand and troubleshoot.
- DOL starter connects the supply to the delta winding of the motor.
Disadvantages of DOL Starter
- High starting current (5-8 times of full load current).
- DOL Starter causes a significant dip in voltage, hence suitable only for small motors.
- DOL Starter reduces the lifespan of the machine.
- Mechanically tough.
- Unnecessary high starting torque
Applications of DOL Starter
We use the DOL starter where high inrush currents do not cause any harm, like to start small water pumps, compressors, fans and conveyor belts etc.