How to Learn DOL Electric Motor Control – A Basic Motor Controller Guide for Direct On Line Electrical Motor Controls

The electric motor is by far one of the best known invention ever developed by the ingenuity of man. It is the most innovative piece of electrical device that converts electrical energy to mechanical energy, and conversely, it is also capable of generating electrical energy from mechanical energy or more commonly known as generators, which produces electricity while the motor is coupled to and driven by a gas or a diesel engine.

Electric motors are widely used in various applications as prime movers of machines utilized in diverse applications and industry such as electric cars, the electric locomotive, elevators, escalators, water pumps, air compressors, electric fans, hand drills, sewing machine, washing machine, microwave ovens, alternators that recharges a car battery, CD and DVD drives, or even the smallest device such as battery operated wrist watches, and with a whole lot more of other uses and specific applications which we may not even be aware of, but which are just common things within our reach in our everyday life.

Depending on the types of application governed by the manner of operation and the motor capacity involved for a specific load to be driven, an electric motor is most certainly operated by means of an electrical activation and control system known as a motor controller. Motor controllers are basically the most indispensable component of electric motors which plays a significant part in essentially providing a suitable operation required with the many methods involved in running and stopping an electric motor.

Although there are a variety of complex methods which can be implemented, this article aims to cover some of the most fundamental electric motor control applications with alternating current (AC) induction motors commonly used in every industry, and the most basic type of motor controller is the Direct On Line (DOL) motor controller.

The simplest method of motor control is the Direct On Line (DOL) motor controller, which directly supplies the line voltage to the motor via a switch or by a single unit magnetic contactor. This type of motor controller is applied mostly to small size motors because small motors does not cause too much load burden that can adversely affect the supply source voltage coming from the electric power grid.

The CAD drawing on the right shows an electrical diagram of a typical method of 3 phase DOL motor controller.

The main circuit breaker acts as the main switch supplying power to the system. It is also equipped with over current and short circuit protection that automatically trips-off to disconnect the supply of electricity to disable the load upon detection of fault in the load circuit.

The main magnetic contactor acts as the motor operation switch which connects and disconnects the supply voltage coming from the main circuit breaker to the motor. When the main contactor is closed, the supply voltage proceeds to the motor terminal which runs the motor.

The thermal overload relay serves to detect motor overload current which when detected will instantaneously disconnect the control circuit of the motor controller to stop the operation to prevent burning of the motor.

The control circuit diagram shown on the right illustrates a typical switching system for a DOL motor controller. When the human operator pushes the RUN push button switch, the control circuit becomes complete which permits power to travel down to the main contactor coil which energizes the main contactor. Once the main contactor is powered up, its internal three pole mechanical contacts (refer to the motor control diagram) closes which connects the source voltage to proceed to the motor terminal, hence running the motor.

Going back to the control circuit diagram, since the auxiliary normally open contact of the main contactor which is connected in parallel across the RUN push button switch was already at closed state following the activation of the main contactor coil, electricity continues to flow down to the main contactor coil even after the operator releases his finger from the RUN push button switch, this serves as the holding contact switch which maintains a complete circuit to run the motor continuously without further human intervention, providing ease of use for operator convenience to switch ON and switch OFF the motor only once via the RUN and STOP push button swithces.

There are two circuit disconnection in the control system. Aside from the OFF push button switch, the thermal overload relay also serves as a disconnecting switch that renders an open or incomplete control circuit which deactivates the main contactor to stop the motor upon detection of motor overload current.

Electric motors can run in both forward and reverse direction depending on the requirement of the application where it is to be installed, for example, like in a conveyor system which would require moving on both direction of items contained in the conveyor table. When such arrangement is necessary for a certain type of application, then a forward reverse motor controller is applied to the control circuit of the electric motor to achieve this purpose. Again the device required for the possibility of this operation is the magnetic contactor.

Reversing motor rotation of 3 phase AC induction motors can be achieved by means of interchanging the configuration of any two of the three motor terminals U1, V1, W1 with respect to the reference supply voltage L1, L2, L3. The CAD drawing below provides a visual explanation for this method of operation.

You will notice from the motor control diagram above that there are two units of magnetic contactors (forward contactor and reverse contactor) connected in parallel across each other. Notice that the line side terminals of these two contactors follows a common connection configuration for the supply voltage L1, L2, L3, while the load side terminals of these two contactors have different configuration for the motor terminals U1, V1, W1. The forward contactor is connected to make L1 connect to U1, L2 to V1, L3 to W1 which makes the motor run in the forward direction. While the reverse contactor is configured with two terminals in the reverse order, with L1 connected to W1 instead of U1, then L3 connected to U1 instead of W1, while maintaining only L2 connected to V1.

The forward reverse control circuit diagram illustrated above shows two DOL control circuit with two magnetic contactors to accommodate both the forward and reverse motor rotation, but with the inclusion of an additional interlocking normally close contacts each inserted for each contactor coils. These interlocking contacts are intended as a safety precautionary feature to prevent simultaneous activation of both contactor coils at the same time, which if not prevented can destroy the electric motor.

When the forward contactor coil is activated, its auxiliary normally close contact connected before the reverse contactor coil will be opened, thereby preventing any power to flow to the reverse contactor coil in case of accidental pressing of the reverse push button switch while the motor is running in the forward direction with the forward contactor coil energized.

Similarly, when the motor is running in the reverse direction with the reverse contactor coil energized, it is not also possible to power up the forward contactor coil because of the presence of an open reverse contact switch provided by the energized reverse contactor coil, thus, preventing the motor to run in forward direction while it is actively running in the reverse direction.

Another indispensable electrical control method for AC induction motors is the star delta motor controller.

Motor Controllers are also an indispensable part of Industrial Process Automation Technology.

Bonus Tip - Want to learn how to become a Skilled Process Automation Electrical Technician with the most simplified way of learning possible? Check out my Industrial Process Automation Guide which will help you gain better understanding about the many aspects comprising Electrical process automation control systems.

How to Learn Electric Motor Control – A Basic Motor Controller Guide for Electrical Motor Controls by ianjonas is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.