Overview
Electrical workers are typically most familiar with electric motors. There are several motor starting methods, including direct-on-line (DOL), autotransformer reduced-voltage start, Y-Δ (star-delta) reduced-voltage start, soft starters, and variable-frequency drives (VFDs). The following summarizes their principles, advantages, disadvantages, and typical applications.
Common Starting Methods
1. Direct-on-line (DOL) start
If the power network capacity and the driven load permit, direct-on-line start can be used. Advantages include simple control and low maintenance, and it is economical for small motors. From an energy-saving perspective, motors larger than 11 kW are generally not suitable for this method.
2. Autotransformer reduced-voltage start
An autotransformer with multiple taps provides reduced voltage for starting, accommodating different load start requirements while yielding relatively high starting torque. Its main advantage is higher starting torque; for example, when the autotransformer tap supplies 80% of rated voltage, the starting torque can reach about 64% of the DOL value. The starting torque can be adjusted by selecting different taps, and this method remains widely used for larger-capacity motors.
3. Y-Δ (star-delta) start
For squirrel-cage induction motors whose stator is normally connected in delta for running, connecting the stator in star during start and switching to delta after starting reduces the starting current and the impact on the supply. This method is called star-delta or Y-Δ start. The starting current in star connection is approximately one third of the delta direct-on-line starting current. If direct start current is about 6–7 Ie, the star-delta starting current will be about 2–2.3 times Ie. Correspondingly, the starting torque also drops to about one third of the delta direct-start torque. Star-delta start is suitable for no-load or light-load starts. Compared with other reduced-voltage starters, its structure is simplest and its cost is lowest. When the load is light, operating in star can match the rated torque to the load, improving efficiency and reducing power consumption.
4. Soft starter
Soft starters use thyristor phase-angle control to achieve voltage-controlled starting. They provide good starting performance but have higher cost. Thyristor switching produces significant harmonic distortion and makes the device sensitive to supply voltage fluctuations; a power system with multiple thyristor-based devices can further affect conduction performance. Because soft starters involve power-electronic technology, they require higher maintenance expertise.
5. Variable-frequency drive (VFD)
Variable-frequency drives are the most feature-rich motor control devices, allowing speed and torque control by changing supply frequency. VFDs involve power electronics and microprocessor control, so they are more expensive and require skilled maintenance. VFDs are typically used where speed control and precise performance are required.
Comparing Reduced-Voltage, Soft Start, and VFD
- Reduced-voltage start (common example: star-delta): disadvantage is low starting torque, so it is suitable only for no-load or light-load starts; advantage is low cost.
- Soft starter: allows setting start time and initial torque for smooth start and stop, limits inrush current, and has moderate cost.
- VFD: enables smooth start according to a set ramp and allows the motor to run at a set frequency; cost is relatively high.
Performance and Application Considerations
1. Cost
VFDs are generally the most expensive. Y-Δ and autotransformer reduced-voltage starters are relatively inexpensive. For projects with limited investment, economy is often the decisive factor.
2. Controllability
Y-Δ and autotransformer starters are simple and only provide starting functionality. For high levels of automation, VFDs are preferred because they can continuously control motor speed and voltage, functions that reduced-voltage starters and soft starters cannot match. Therefore, VFDs are commonly used in large or highly automated production lines.
3. Networking and monitoring
VFDs typically include integrated or expandable communication ports for network monitoring. Soft starters can offer limited monitoring, but real-time motor monitoring capabilities are generally superior with VFDs.
4. Maintenance
Y-Δ and autotransformer starters are mechanically and electrically simple and are therefore the easiest to maintain. Soft starters and VFDs require more specialized maintenance skills due to their power-electronic components.
5. Load suitability
Because VFDs can implement smooth start and stop and provide continuous speed control, they are superior for relatively heavy or variable loads compared with Y-Δ, autotransformer, or soft-start solutions.
Supplementary Comparisons
Soft starter vs VFD
Both soft starters and VFDs can be used for reduced-voltage starting, but they operate differently. A VFD reduces voltage by lowering frequency, providing full control throughout operation. A soft starter changes the thyristor conduction angle to ramp motor voltage from zero to full, affecting only start and stop phases. VFDs can be controlled by instrument signals to set motor speed at any time, while soft starters only serve to reduce voltage during starting and stopping.
VFDs include the functionality of soft starters but are significantly more expensive and more complex.
Classification and general guidance
Common starting methods are DOL, autotransformer reduced-voltage, Y-Δ, soft start, and VFD. If the supply network and load allow, DOL start is preferable for its simplicity and economy. Autotransformer reduced-voltage start is frequently used for large-capacity squirrel-cage induction motors because its multiple taps accommodate different loads and provide higher starting torque. Star-delta start offers good current characteristics but poor torque characteristics, so it is suitable only for no-load or light-load starts; however, it is the simplest and cheapest option and can save energy during light-load operation.
Stepped reduced-voltage starting methods share a common drawback: they can produce secondary impact currents during the transition stages.
