1. Why Reduced-Voltage Starting? (The Problem with Direct-On-Line Starting)
During direct-on-line starting, motor starting current is4–7 timesthe rated current, causing:
- Grid voltage drop: Direct-on-line starting of a large motor pulls busbar voltage down by 10%–15%, affecting other equipment on the same network.
- Mechanical shock: A sudden change in starting torque damages couplings, belts, gears, and other drivetrain components.
- Motor heating: High starting current over a long duration drives winding temperature up quickly, shortening insulation life.
Reduced-voltage starting brings starting current down to2–3 times rated current, protecting both the motor and the grid.
2. How a Thyristor Soft Starter Works (Phase-Angle Firing → Continuous Voltage Regulation)
A thyristor soft starter connectsa pair of anti-parallel thyristors in each of the motor's three phases, and throughphase-angle firing control of the conduction angleStepless voltage regulation:
| Startup phase | Firing angle α | Output voltage | Motor status |
|---|---|---|---|
| Initial | 120° | ~30%Ue | Rotor begins to turn |
| Acceleration | 120°→30° | 30%→90%Ue | Speed rises gradually |
| Full voltage | 0° | 100%Ue | Reaches rated speed |
| Bypass | — | — | Bypass contactor closes, thyristors drop out |
3. How the autotransformer reduced-voltage starter works (taps → stepped voltage regulation)
The autotransformer reduced-voltage starter uses anautotransformerto reduce the voltage at the motor terminals, typically with 65% and 80% taps:
- Startup phase: power → autotransformer 65% tap → motor. The motor starts at 65% Ue, and the starting current drops to 65%² ≈ 42% of direct-on-line starting current.
- Transition phase: as the motor approaches rated speed, the autotransformer is cut out and the motor runs directly at full voltage.
- Transition method: open transition (break-before-make, with a brief power interruption) or closed transition (make-before-break, no power interruption).
Thyristor soft starter: stepless voltage regulation, smooth starting, adjustable start time, but higher cost (about ¥8,000–15,000 for 75 kW).Autotransformer reduced-voltage: simple construction, low cost (about ¥3,000–5,000 for 75 kW), but stepped voltage regulation causes transition shock and the unit is bulkier. For new projects, a thyristor soft starter is recommended; for retrofit projects, autotransformer reduced-voltage starting is still workable.
4. Bypass contactor in the soft starter cabinet (bypass after startup · reduces losses)
After the thyristor soft starter completes the startup sequence,the bypass contactor closes, and the main circuit current flows through the bypass contactor instead of the thyristors. The benefits are:
| Comparison | Startup phase (through thyristors) | Run phase (through bypass contactor) |
|---|---|---|
| On-state voltage drop | ~1.5V per phase (thyristor drop) | <0.1V per phase (contact resistance) |
| Power loss | Yes (heat generation) | Very low |
| Harmonics | Yes (generated by phase-angle triggering) | |
| None | Heat dissipation requirement | Low |
5. Soft Starter Cabinet Selection (3 parameters + 1 protection)
Selection depends on3 parameters + 1 protection:
- Rated current: Soft starter rated current ≥ motor rated current. For example, a 75 kW motor draws ~142 A rated current; select a Schneider ATS480N4 (rated current 210 A).
- Starting current limit: Set to 2.5–3.5 times rated current, adjusted according to load inertia. 2.5 times for pumps, 3–4 times for fans.
- Starting time: Adjustable from 5–30 s; set according to the time the motor takes to reach rated speed.
- Protection: Overload protection (electronic thermal relay), phase loss protection, overheat protection, thyristor overheat protection.
Zhejiang Golden Unicorn soft starter cabinets integrate Schneider ATS480, Renle JJR, Chint NJR2 and other brands. MOQ 1 unit, lead time 15-25 days.
FAQ FAQ
What is the difference between a soft starter and a VFD?
Soft starterReduced-voltage starting only (bypassed after startup; no speed control in run mode);VFDAdjustable-speed operation (0-50Hz stepless speed control). Soft starters cost less (1/3 to 1/2 of a VFD) and suit high-power motors that do not need speed control; choose a VFD when speed control is required.
Why must a soft starter cabinet be bypassed after startup is complete?
Thyristor conduction has avoltage drop (~1.5V per phase), which generates power loss and heat during continuous operation. After bypass, current flows through the contactor (drop <0.1V), reducing losses, lowering temperature rise and eliminating harmonics. Bypassing also takes the thyristors out of the circuit, extending their service life.
Are autotransformer reduced-voltage starters still useful?
Yes. Although new projects mostly use thyristor soft starters, autotransformer reduced-voltage starting is simple in structure, low in cost and highly reliable, and is still widely used in retrofits of old equipment and small projects with limited budgets. GB/T 14048.6 still lists it as a standard starting method.
Can a soft starter replace a thermal overload relay for overload protection?
Yes. Thyristor soft starters have built-in electronic thermal protection with selectable overload curves (Class 10/20/30). However, we recommend keeping an independent thermal overload relay fordual protection, in case the soft starter's protection function fails.
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