A motor control circuit works by using a control circuit to energize the magnetic motor starter, which in turn activates or deactivates the motor's power contacts, controlling whether or not power is supplied to the motor; overload protection is provided by heater elements connected to an overload relay.
Understanding the Components
To understand how a motor control circuit works, consider the key components:
- Control Circuit: This is typically a low-voltage circuit (e.g., 120V AC or 24V DC) that controls the operation of the motor starter. It includes components like pushbuttons (start, stop), selector switches, and relays.
- Motor Starter (Magnetic Contactor): This is an electrically operated switch that connects the motor to the main power supply. It has a coil that, when energized by the control circuit, pulls in the armature. The armature closes the power contacts, allowing current to flow to the motor. When the coil is de-energized, the armature drops out, and the power contacts open, stopping the motor.
- Overload Relay: This protects the motor from damage due to excessive current draw. It contains heater elements that are wired in series with the motor windings. If the motor draws too much current for too long, the heater elements heat up and trip a set of contacts in the control circuit, de-energizing the motor starter coil and stopping the motor.
Operational Sequence
Here's how the circuit operates during a typical start/stop sequence:
- Start: When the "Start" button is pressed, it completes the control circuit, energizing the motor starter coil.
- Energizing the Motor Starter: The energized coil pulls in the armature of the motor starter.
- Closing Power Contacts: The armature closes the power contacts, connecting the motor to the main power supply and starting the motor.
- Holding Circuit: A holding contact (also called a seal-in contact) on the motor starter, connected in parallel with the "Start" button, keeps the motor starter coil energized even after the "Start" button is released.
- Normal Operation: The motor runs until a "Stop" command is initiated or an overload condition occurs.
- Stop: When the "Stop" button is pressed, it breaks the control circuit, de-energizing the motor starter coil.
- De-energizing the Motor Starter: The de-energized coil releases the armature.
- Opening Power Contacts: The armature opens the power contacts, disconnecting the motor from the main power supply and stopping the motor.
- Overload Protection: If the motor draws excessive current, the overload relay's heater elements heat up and trip the overload contacts, which are wired in series with the motor starter coil in the control circuit. This de-energizes the motor starter coil and stops the motor, preventing damage.
Example Circuit (Simplified)
Imagine a simple circuit with a Start button (momentary contact), a Stop button (normally closed), a motor starter coil, a holding contact on the motor starter, and an overload relay contact (normally closed).
| Component | Function |
|---|---|
| Start Button | Momentarily completes the control circuit to energize the motor starter coil. |
| Stop Button | Normally closed; opens the control circuit when pressed, de-energizing the motor starter coil. |
| Motor Starter Coil | When energized, pulls in the armature and closes the power contacts to supply power to the motor. |
| Holding Contact | Maintains the control circuit after the Start button is released, keeping the motor running. |
| Overload Relay | Protects the motor from overcurrent conditions by opening the control circuit and de-energizing the coil. |
In this simplified example, pressing Start energizes the coil, closing the holding contacts. Pressing Stop breaks the circuit, or an overload condition opens the overload relay contacts, both de-energizing the coil.
Conclusion
In summary, a motor control circuit regulates the start, stop, and protection of a motor by using a control circuit to energize or de-energize a motor starter. The motor starter then manages the power supply to the motor, while overload protection ensures the motor is protected from damage due to overcurrent conditions.
