Motors are integral to industrial manufacturing, playing a critical role in advancing processes and transforming raw materials into finished goods or services.
Selecting the appropriate starting method minimizes motor stress and promotes effective start-up. This article, the second of a four-part series, provides a detailed examination of the Across the Line Motor Starting method and its associated schemes.
Understanding Across-the-Line Motor Starting
In this approach, motor windings connect directly to the power supply at full voltage during start-up
The "Across the Line" method is a widely used technique for initiating induction motors. In this approach, motor windings connect directly to the power supply at full voltage during start-up.
While straightforward and budget-friendly, this method may lead to a significant inrush of current and exert mechanical stress on the motor. Two specific schemes within this method are known as the Full Voltage Non-Reversing and Full Voltage Reversing schemes.
Full Voltage Non-Reversing Motor Starter
This straightforward method involves controlling the motor via a forward contactor, complemented by safety mechanisms like a Short Circuit Protection Device (SCPD) and an overload relay.
The full line voltage applied to the motor terminals causes unidirectional rotation. Initially, the starting current may surge to 5-6 times the Full Load Ampere (FLA) but decreases as the motor stabilizes at its operational speed.
Here, the starting torque rises until it reaches a peak, then settles to full-load torque levels. Notably, this method demands increased reactive power from utility networks, potentially incurring penalty costs. It suits smaller motors better due to the minimal magnitude of inrush current and reduced reactive power demands.
Full Voltage Reversing Motor Starter
One contactor remains engaged at any time, thanks to a mechanical and electrical interlock
Similar to the non-reversing method, the full voltage reversing method applies full line voltage across motor terminals. This method employs two mechanically interlocked contactors, controlling the motor’s forward and reverse functions. One contactor remains engaged at any time, thanks to a mechanical and electrical interlock.
Upon activating the reverse contactor, two phases of the supply line switch, altering the electromagnetic field and rotation direction. The inrush current impacts this scheme similarly to its non-reversing counterpart, requiring components sized to accommodate the motor’s high starting current.
across-line motor starting technique
Favored for its simplicity and minimal cost, the across-line motor starting technique is a prevalent choice. For insights into its advantages and drawbacks, consult available resources. NOARK Electric is dedicated to supplying dependable products tailored to starting motors effectively.
Their team can advise on selecting the appropriate motor starter components to ensure efficient and safe machine start-up. Additionally, subscribing to the NOARK Newsletter offers monthly updates, product news, and downloadable content.
Throughout industrial manufacturing, motors play a very important role in the advancement of processes and the handling or transformation of materials from raw materials to finished products or services. To ensure a reliable and efficient start-up of a machine, the correct starting method should be chosen to minimize stress on the motor and ensure a smooth start-up.
This article is the second installment of a four-part series designed to educate readers about the different motor starting methods and will take a comprehensive look at the various schemes associated with Across the Line Motor Starting.
What is the Across the Line Motor Starting Method?
As a common technique for starting induction motors, the “Across the Line” method is one of the most popular methods for starting motors. The windings of the motor are directly connected to the power supply at full voltage during the starting phase of the motor. While this method is straightforward and cost-effective, it may result in a high inrush current and mechanical stress on the motor.
The following is an explanation of two starting schemes associated with “Across the Line” motor starting method, namely Full Voltage Non-Reversing and Full Voltage Reversing schemes.
Full Voltage Non-Reversing Motor Starter
This is the most common and simple way to start the motor. The motor control in this case is achieved through the forward contactor, as shown in the figure, while overcurrent protection and isolation means are achieved by the SCPD (Short circuit protection device) and overload relay. The full line voltage is applied across the motor terminals to the stator, which gets the motor to rotate unidirectionally.
To overcome the inertia at the start, the locked rotor current or starting current goes up to 5-6 times the FLA (full load ampere) of the motor and gradually reduces as the motor reaches its run speed. Similarly, the starting torque of the motor increases rapidly till it gets to the maximum possible torque (breakdown torque) before settling at the full-load torque.
This method of starting has the most amount of starting current compared to other starting methods and therefore demands more reactive power from the utility network. It is important to note that across the line method increases the reactive power demand in the network and the Utility provider would typically apply a penalty cost to such a consumer. This starting method is particularly suitable for small and medium-sized motors where the inrush current is of small order magnitude, as the reactive power effect is reduced.
Full Voltage Reversing Motor Starter
The full voltage reversing motor starting is similar to the full voltage non-reversing method, as the full line voltage is also applied across the motor terminals. However, in this situation, it uses two contactors which are mechanically interlocked – one applied in the forward direction of the motor and the other in the reverse direction. As shown in the figure below, the motor starts in the forward direction when the full line voltage is applied to the motor terminals and continues running while the reverse contactor is on time-delay.
The mechanical and electrical interlock between the contactors ensures that only one contactor is on at any given time. Once the reverse contactor is pulled in, it changes the direction of the rotation. By switching two of the phases in the supply line, the direction of the rotating magnetic field changes and hence the rotation in the opposite direction.
The effect of inrush or starting current on the full voltage reversing starter is the same as previously stated for the full voltage non-reversing method. The components for this starting method are sized taking into account the high starting current of the motor.
Implement the Across the Line Motor Starting Method with NOARK Electric
Traditionally, the across-line motor starting method is the most commonly used method because of its cost and simplicity. They can learn more about the pros and cons of this starting method by link.
At NOARK, they are committed to delivering the best and most reliable products for motor starting needs. To ensure that the motor is starting safely and efficiently, contact the NOARK Electric team to find the right motor starter components for the job.
Lastly, sign up for the NOARK Newsletter to receive free monthly product updates, news, and downloadables.
