Common Motor Starting Methods

Overview

Electrical personnel are most familiar with motors. There are several ways to start a motor, including direct-on-line (DOL) starting, autotransformer reduced-voltage starting, star-delta (Y-Δ) reduced-voltage starting, soft starter starting, and variable frequency drive (VFD) starting. The following summarizes the differences and applications of these methods.

1. Direct-on-line (DOL) starting

When the power network capacity and the load permit full-voltage starting, direct-on-line starting can be used.

Advantages are simple control, easy maintenance, and low cost. It is mainly used for small motors; from an energy-saving perspective, motors larger than 11 kW are generally not suitable for this method.

2. Autotransformer reduced-voltage starting

Autotransformer reduced-voltage starting uses the multiple taps of an autotransformer to reduce voltage. It can adapt to different load starting requirements and provide larger starting torque, so it is often used for starting larger-capacity motors.

The main advantage is higher starting torque. With the tap at 80%, the starting torque can reach 64% of the DOL value, and the starting torque can be adjusted by selecting different taps. This method is still widely applied.

3. Star-delta (Y-Δ) starting

For a squirrel-cage induction motor whose stator winding is normally connected in delta during operation, connecting the stator winding in star at startup and switching to delta after starting reduces the starting current and lessens the impact on the power network. This method is called star-delta reduced-voltage starting.

When using star-delta starting, the starting current is only one third of the direct delta start. If the direct-start current is 6–7 Ie, the star-delta starting current is about 2–2.3 times Ie. Accordingly, the starting torque is reduced to one third of the direct delta-start torque.

Star-delta starting is suitable for applications with no-load or light-load starts. Compared with other reduced-voltage starters, it has the simplest structure and lowest cost. Another advantage is that when the load is light, the motor can run in star connection, matching rated torque to load and improving efficiency, which saves power.

4. Soft starter

Soft starters use the phase-angle control principle of thyristors to achieve voltage-controlled starting. They are mainly used for motor start control, with good starting performance but relatively high cost. Because thyristor devices generate harmonics during operation, they can affect the power network. Power network fluctuations can also affect thyristor conduction, especially when multiple thyristor-based devices are connected to the same network. Thyristor device failure rates can be higher, and maintenance requires knowledge of power electronics, increasing the demands on maintenance personnel.

5. Variable frequency drive (VFD)

VFDs are the most advanced and feature-rich motor control devices in modern motor control. They regulate motor speed and torque by varying the supply frequency. Because VFDs involve power electronics and microcomputer technology, they are costly and require skilled maintenance. Therefore, they are mainly used in applications that require speed control and precise speed regulation.

Comparison: Reduced-voltage, Soft Starter, and VFD

Reduced-voltage starting, commonly star-delta, has the disadvantage of low starting torque and is only suitable for no-load or light-load starts. Its advantage is low cost.

Soft starters can set start time and initial torque to provide soft start and soft stop, and they can limit starting current. Their price is moderate.

VFDs can provide smooth start according to a set ramp time and run the equipment at a set frequency. Their price is relatively high.

Performance and Principle Comparisons

Comprehensive analysis of soft starters, VFDs, and reduced-voltage starters:

1. Price

   VFDs are the most expensive, while Y-Δ and autotransformer reduced-voltage starters are relatively cheap. For projects with limited investment, economy is often the primary consideration.

2. Controllability

   Y-Δ and autotransformer reduced-voltage starters are simple and only perform starting. In highly automated environments, they are used less. VFDs can control motor parameters including speed and voltage, offering control capabilities far beyond reduced-voltage or soft starters, making VFDs the preferred option for large or highly automated production lines.

3. Network communications

   VFDs can integrate or expand communication ports for network monitoring. Soft starters can provide some monitoring, but achieving real-time motor monitoring is difficult for reduced-voltage and soft starter solutions compared with VFDs.

4. Maintenance

   Y-Δ and autotransformer reduced-voltage starters are relatively simple and therefore easiest to maintain. The author prefers Y-Δ or autotransformer reduced-voltage starting over soft starters if a VFD is not chosen.

5. Load capability

   VFDs can implement soft start and soft stop and are superior in relatively high-load scenarios; Y-Δ, autotransformer, or soft starters cannot match VFDs in these cases.

Supplementary Comparisons

1. Soft starter vs VFD

Both VFDs and soft starters belong to the reduced-voltage starting category. VFDs reduce voltage by changing frequency. Soft starters change the thyristor conduction angle to achieve a start from zero voltage to full voltage.

VFDs provide continuous control and can be controlled by instrument signals to set motor speed at any time. Soft starters only reduce voltage during motor start and stop.

VFDs include all the functions of soft starters, but VFDs are much more expensive and structurally more complex.

2. Common motor starting methods summary

Common starting methods: direct-on-line, autotransformer reduced-voltage starting, star-delta starting, soft starting, and VFD starting.

When both the power network and the load allow, direct-on-line starting is preferable because it is easy to operate and economical.

Autotransformer reduced-voltage starting is often used for larger-capacity squirrel-cage induction motors. Although it is an older method, the autotransformer's multiple taps accommodate various load start needs and can provide higher starting torque. Fitted with thermal relays and undervoltage trip devices, autotransformer starting provides comprehensive overload and undervoltage protection and remains widely used.

Star-delta starting has favorable current characteristics but poor torque characteristics, so it is only suitable for no-load or light-load starts. It is the simplest and cheapest method and can save power during light-load operation.

These stepped reduced-voltage starting methods share a notable drawback: they can produce secondary impact currents during the starting process.

3. Soft starter vs traditional reduced-voltage methods

Differences of soft starters compared with traditional reduced-voltage methods include:

1. No impact current

   Soft starters gradually increase the thyristor conduction angle during startup, so the motor starting current rises linearly from zero to the set value. This avoids impact currents, improves supply reliability, provides smooth starting, reduces impact torque on the driven equipment, and extends machine life.

2. Soft stop

   They provide smooth deceleration and controlled stopping, which overcomes the drawbacks of instantaneous power loss, reduces impact on heavy-load machinery, avoids water hammer effects in pumping systems, and reduces equipment damage.

3. Adjustable start parameters

   Start parameters can be adjusted according to load conditions and protective relay characteristics to set the optimal starting current.

Soft starters and VFDs are different products for different purposes. VFDs are used where speed regulation is required, changing both voltage and frequency. Soft starters are essentially voltage controllers used only during motor start, without changing frequency. VFDs can perform all functions of soft starters, but are more expensive and more complex.