Subsection 4.4.6 Motor Controllers
Motor controllers electromechanical devices that control the operation of electric motors, and allow them to be operated manually, by automation or both.
Motor controllers come in various types and forms, ranging from simple relay-based control systems to complex microcontroller-based systems. The choice of motor controller depends on factors such as the specific application requirements, the types of motors being used, and the desired levels of control sophistication.
Motor controllers usually provide three basic functions: starting, stopping, and motor protection. The controller monitors the motor circuit and will secure the motor if it detects common faults such as overload, short circuits, or low voltage conditions.
Additional functions such as direction control, speed control, regenerative braking, and reduced voltage (soft) starting, can be added to the control circuitry when required by the application. Many modern motor controllers also provide diagnostic information and feedback about motor performances, temperatures, and statuses. This information helps with troubleshooting and preventive maintenance.
Next, we will describe the operation of one simple and common motor controller: an across-the-line starter. It is given this name because the controller connects the motor’s stator windings directly across the power lines and starts it using full line voltage. This type of motor controller is very common, and is worth studying as its basic control circuit is the foundation of many other devices.
The power and control circuits for an across-the-line starter are shown in Figure 4.4.12. The circuit consists of two parts: the three-phase power circuit (blue) , which connects the power supply, entering from the left, to the motor, and the single-phase control circuit (black) which is responsible for the control functions.
The power circuit consists of a circuit breaker (1), three main contacts (2), three overload sensors (3), and connections to the motor (4). The control circuit consists of two control fuses (5), start (6) and stop (7) buttons, an auxiliary relay, (8), the contactor operating coil (9), and normally closed overload relay contacts (10).
The operation is as follows:
- The start button is a momentary push-to-close switch. When it is closed, it completes the control circuit, and energizes the main operating coil.
- The main operating coil is part of the contactor assembly 1 . When the main operating coil energizes it closes three main contacts in the power circuit and the auxiliary contact in the control circuit.
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Closing the main contacts completes the circuit between the power supply and the motor, allowing current to flow directly to the motor’s terminals and start the motor.Since the auxiliary contact is wired in parallel with the start button, after it closes the start button may be released and the motor will continue to operate. For this reason, the auxiliary contact is sometimes called the latch contact.
- The motor will continue to run until one of the following happens:
- The stop button is pressed. This breaks the control circuit and de-energizes the main coil. This causes the main and auxiliary contacts to open and stop the motor.
- One of the three overload sensors detects the motor drawing high current. This opens the corresponding overload contact in the control circuit and stops the motor.
- A short circuit is detected by the circuit breaker, tripping it. This secures power to both the power and control circuits and stops the motor. Fuses will de-energize the control circuit if a short occurs there.
- A low voltage condition occurs when the supply voltage drops below about 90% of its nominal value. If this happens, the current through the operating coil decreases, causing its electromagnet to weaken enough to release the main contacts, which stops the motor. If the voltage returns to normal, the motor will remain stopped until the start button is pressed again.
A motor which does not restart after a power failure is described as having Low Voltage Protection (LVP). The alternate behavior, where the motor automatically restarts after a power failure is called Low Voltage Release (LVR). Critical machinery such as the steering gear and lube oil pumps have LVR, while other machines have LVP.
Because across-the-line starters provide full voltage to the motor from the start, the motor experiences a high starting current. This initial surge of current is called the inrush current, and it can be several times the motor’s full-load operating current. This problem is more acute for large motors.
Large starting current has two negative effects. First, the high current in the windings can generate enough heat to damage the motor over time, especially if the driven load accelerates slowly. Second, very high starting current can draw down line voltage sufficiently to cause lights to dim and other equipment being supplied from the same line to malfunction.
To reduce the starting currents of large motors, several methods are employed.
- Wye-delta starters initially connect the motor to the power supply using a wye connection, which provides only 57% of normal voltage. After a configurable time delay, the connection switches to delta, and full line voltage is available for normal operation.
- Autotransformer starters operate similarly but can supply more intermediate voltage steps.
- Soft starters and variable frequency drives, which are electronic devices, gradually increase the voltage, effectively limiting the starting current.
