FAQ
TL;DR: A 5.5 kW, 1455 rpm induction motor can deliver roughly 2–4 kW off-grid when driven above 1500 rpm and excited with ~150 µF 400 V capacitors per phase [Elektroda, marcin55246, post #10487630] “Connect three capacitors and spin it” [Elektroda, ZGG, post #10473064]
Why it matters: Knowing the limits and setup steps prevents burnt loads, wasted fuel, and frustration.
Quick Facts
• Self-excitation capacitance: 27–35 µF per kW (≈150 µF for 5.5 kW) [Elektroda, marcin55246, post #10487630]
• Required overspeed: ≥3 % above synchronous (≈1500 rpm for 4-pole motor) [Elektroda, Aleksander_01, post #10467530]
• Typical stand-alone output: 35–70 % of motor rating (efficiency 70–80 %) [KOMEL, 2008].
• Voltage drift: ±20 % between no-load and full-load without AVR [Elektroda, czesiu, post #10466747]
• Edge-case surge: over-excited units can spike >500 V on sudden load loss [Elektroda, Strumien…, #16974061].
1. Can any three-phase induction motor act as a generator?
Yes. Reversible electromagnetic principles let a squirrel-cage motor produce power once you supply shaft torque and reactive excitation. Grid-tied units borrow Vars from the mains; stand-alone sets need capacitors [Elektroda, opto17, post #16966914]
2. What speed must I reach for a 1455 rpm (4-pole) motor?
Drive it slightly over synchronous—about 1500 rpm. A 3 % overspeed lets it begin generating; higher speed raises voltage and frequency linearly [Elektroda, Aleksander_01, post #10467530]
3. How much capacitance should I use for a 5.5 kW machine?
Start with 137–187 µF total (≈45–60 µF per phase, 400 V AC). This range produced full brightness on three 100 W lamps in tests [Elektroda, marcin55246, post #10487630]
4. Why did my bulbs stay dark when I spun the shaft?
The rotor probably lacked residual magnetism. Without an initial flux, the stator cannot build voltage, so nothing lights [Elektroda, pjetrekk15, post #10466578]
5. How can I re-magnetize the rotor?
Briefly apply 12 V DC between any two phase leads for 1–2 s, then spin again; the residual field returns [Elektroda, marcin55246, post #10487630]
6. How stable are voltage and frequency under load?
They vary. Expect voltage to sag up to 20 % and frequency to fall 1 Hz per 2 % speed drop when you add load [Elektroda, czesiu, post #10466747]
7. Can a simple 230 V regulator fix the fluctuations?
No. Standard triac regulators need a fixed frequency. Use an electronic AVR or inverter stage after rectifying the three-phase output [Elektroda, Aleksander_01, post #10467883]
8. How much power can I safely draw?
Plan for 40–70 % of motor nameplate, so 2–4 kW here. Above that, voltage collapses and the caps overheat [KOMEL, 2008].
9. Is grid connection possible and legal?
If you leave the capacitors out and sync by overspeeding the motor, it will feed energy into the mains, but local regulations demand anti-islanding protection and utility permission [Elektroda, retrofood, post #11936376]
10. Three-step stand-alone build guide
- Delta-wire stator, attach 50 µF 400 V AC capacitor to each phase.
- Spin above synchronous; flash 12 V DC if no voltage appears.
- Adjust engine throttle until output reaches 230 V at 50 Hz, then connect loads gradually.
11. What if my capacitance is wrong?
Too low and voltage never rises; too high and open-circuit voltage can spike beyond 500 V, damaging insulation [Elektroda, Strumien…, #16974061].
12. How does fuel cost compare to utility power?
A 3 kW diesel generator burns about 0.8 L h⁻¹; at €1.60 L⁻¹ that is €0.43 kWh⁻¹, triple Poland’s retail tariff of ~€0.14 kWh⁻¹ [IEA, 2023].
13. Should I swap the rotor for permanent magnets?
Yes if you need tighter voltage and frequency. A milled rotor with neodymium magnets can supply 3×220 V at 45–55 Hz without capacitors [Elektroda, Marek_Pe, post #10467200]
14. What safety steps are essential?
Install an RCD, overspeed governor, and 400 V class fuses. Add a voltmeter to avoid over-voltage situations and ground the frame [Elektroda, Jerzy Bartnicki, #10466714].
15. How does an induction generator compare to a synchronous alternator?
Induction units are cheaper and brushless but need reactive power and give poorer voltage regulation; synchronous or PM alternators deliver stable output but cost more and need field control [KOMEL, 2008].
How many times have I written about this.
We are constantly talking about an over-synchronous generator because that is what the squirrel cage motor is.