Motor Starting Methods: Direct Online, Reduced Voltage, Electronic

Direct Online (DOL) Motor Starting

Direct Online (DOL) starting involves applying the full rated voltage directly to the motor's terminals. This method is simple and cost-effective but has significant characteristics:

• High Starting Torque: Produces a high initial torque, typically 1.5 times the nominal torque.
• High Power Consumption: Power consumption is much higher than nominal values during startup.
• High Starting Current: Draws a very high starting current, typically 6 to 8 times the rated current.

Reduced Voltage Motor Starting

Reduced voltage starting methods aim to lower the initial voltage applied to the motor during startup. This reduces the nominal current, magnetic field intensity, and motor torque, with the voltage gradually increasing to its full value. Common methods include:

Star-Delta (Wye-Delta) Starting

This method offers simple construction, low cost, and good reliability. It involves connecting the three-phase motor windings, which are designed to work with a delta-connected network (e.g., 380V). Initially, the windings are connected in a star (wye) configuration, receiving a reduced phase voltage (e.g., 220V). After a set time, they are reconnected in a delta configuration to the full line voltage (380V).

During star connection, the voltage applied to each winding is reduced by a factor of 1.73 (line voltage / √3). Since the influence of voltage on power and torque is quadratic, both the starting current and the starting torque are reduced by three times compared to DOL starting.

Requirements:

• The motor must be built to operate in a delta configuration.
• Six accessible stator terminals are required for the star-delta switch.

Electronic Motor Starters

Electronic starters consist of static power converters that allow for the gradual application of voltage to AC motors, effectively limiting both starting current and torque. These devices offer advanced control over the motor's startup characteristics.

Soft Starters

A soft starter gradually increases the voltage supplied to the motor at startup, according to a pre-programmed ramp. This restricts the initial current flow, allowing the motor to smoothly accelerate to its operating voltage. Soft starters also offer the possibility of gradually stopping the motor (soft stop).

Benefits of Soft Starters:

• Reduced voltage and current amplitudes during startup.
• Prevents tripping of electrical protections during startup.
• Reduced peak load demand on the power supply.
• Reduction of mechanical stress on the motor and connected equipment due to sudden torque changes.
• Smooth acceleration of mechanical elements.

Disadvantage: The acceleration torque provided by a soft starter is not constant. Acceleration torque is the difference between the torque produced by the motor and the load torque required to maintain constant speed. With soft starters, the acceleration torque is lower at the beginning of the start cycle and increases as the motor approaches approximately 85-90% of its full speed.

Variable Frequency Drives (VFDs)

VFDs, also known as inverters, convert fixed-frequency electrical energy into variable-frequency electrical energy for precise control of squirrel cage induction motors. This frequency variation is achieved through three main stages:

1. Rectifier Stage: Converts incoming AC power to DC power.
2. Filter Stage: Stabilizes the DC signal.
3. Inverter Stage: Transforms the DC current back into variable-frequency AC power, which is then supplied to the motor.

VFDs offer the most comprehensive control over motor speed, torque, and acceleration, making them suitable for applications requiring precise process control and energy efficiency.