Guides · How it works
How a Generator Holds 230 Volts at 50 Hertz
A healthy generator in South Africa delivers about 230 V from each live to neutral (400 V between phases on a three-phase set) at 50 Hz, the same as the mains. Frequency comes from engine speed, held by the governor; voltage comes from the alternator’s excitation, held by the AVR. When either drifts, motors, UPS units and solar inverters react first, so these two readings are the quickest check of a generator’s output.
Technical guide · Last reviewed 16 October 2026
What 230 V and 50 Hz mean
Voltage is the electrical pressure that pushes current through a circuit; frequency is how many complete cycles alternating current goes through each second, so at 50 Hz it changes direction 100 times a second. South Africa’s standard supply is 230 V at 50 Hz, with the voltage allowed to vary by about 10% either way, roughly 207 V to 253 V. Appliances sold here are designed for that window.
A generator should land inside the same window: about 230 V phase to neutral, and 400 V between phases on a three-phase set (see single vs three phase). The national grid holds its frequency within a fraction of a hertz; a generator drifts a little more, which most equipment tolerates.
Engine speed sets the frequency
An alternator’s frequency is fixed by its speed and the number of magnetic poles on its rotor: frequency (Hz) = rpm × poles ÷ 120. Most diesel sets use a four-pole alternator at 1,500 rpm to give 50 Hz; many small portable units use a two-pole alternator at 3,000 rpm. The relationship is direct: an engine running 4% slow gives 48 Hz.
The governor holds that speed. A mechanical governor allows droop: speed falls slightly as load rises, so the set is often adjusted to run a little above 50 Hz with no load and settle near 50 Hz at full load. An electronic governor reads speed from a magnetic pickup on the flywheel and drives a fuel actuator, and on a set running alone it is usually set to hold the speed constant (isochronous). Inverter generators are different again: their engine speed follows the load, and electronics produce a steady 50 Hz.
The AVR holds the voltage
The voltage an alternator produces depends on its speed and on how strongly its rotor is magnetised. The automatic voltage regulator measures the output and adjusts that magnetism, the excitation, many times a second to hold the set voltage as the load changes. What an AVR does explains the loop in detail.
The two readings are linked by design. Most AVRs include under-frequency roll-off: if the engine slows below a set point, the AVR lowers the voltage in step, protecting the exciter and easing the load so the engine can recover. Low voltage together with low frequency therefore usually points to the engine, not the AVR. Small sets regulated by a capacitor rather than an AVR let the voltage wander more with load.
Load steps
What happens when a big load switches on
- 01
Load arrives
A pump, compressor or the whole building switches onto the set through the changeover.
- 02
Speed and voltage dip
The engine slows under the sudden torque, and the voltage sags as the current rises.
- 03
Governor and AVR respond
The governor adds fuel and the AVR boosts the excitation. A turbocharged engine needs a moment for boost to build.
- 04
Output settles
A healthy, well-sized set recovers within a few seconds. Deep dips or slow recovery point to overload, the governor or the AVR.
What wrong readings do to equipment
| Reading | Effect on equipment | Where to look |
|---|---|---|
| Voltage too high | Effect on equipmentCan damage electronics, power supplies and lights | Where to lookAVR, sensing wiring, AVR settings |
| Voltage too low | Effect on equipmentMotors overheat and struggle to start; equipment resets | Where to lookOverload, AVR, excitation, engine speed |
| Voltage hunting | Effect on equipmentFlickering lights; equipment trips or reboots | Where to lookAVR stability, governor, loose connections |
| Phase voltages uneven | Effect on equipmentThree-phase motors overheat; equipment on the high phase is stressed | Where to lookLoad balance across phases, a loose neutral |
| Frequency too low | Effect on equipmentMotors run slow; clocks and some electronics misbehave; UPS rejects the supply | Where to lookOverload, governor, fuel supply |
| Frequency too high | Effect on equipmentMotors run fast; clocks gain | Where to lookGovernor setting, very light load on a droop governor |
| Frequency unstable | Effect on equipmentUPS and solar inverters disconnect; sensitive equipment trips | Where to lookGovernor or actuator, fuel supply |
Why UPS units and solar inverters reject generator power
During load shedding many homes and offices pair a generator with a UPS or hybrid solar inverter. These check the incoming voltage and frequency, and sometimes the waveform, before accepting it, and drop off if the readings leave their window. A set that runs lights and kettles happily can still be rejected.
A common loop: the inverter accepts the generator and starts charging its batteries, the extra load pulls the frequency down, the inverter disconnects, the frequency recovers, and the cycle repeats. Limiting the inverter’s charge current from the generator, having its installer check the generator-input settings, and having the governor and AVR checked usually ends it. If the speed itself is unstable, see frequency and speed problems.
Owner checks
Reading voltage and frequency safely
Most standby sets show voltage and frequency on the controller screen, often behind a window in the canopy so it can be read with the doors shut, and many portable units have a panel meter. Note both readings with nothing connected and again under your normal load, and photograph the screen: those numbers tell a technician a great deal.
A plug-in power meter in an ordinary socket on a generator-fed circuit is another safe way to see the voltage, and many show frequency too. Measuring at the terminals, inside the canopy or in the distribution board means working near live conductors at up to 400 V, so leave it to a technician.
- Read the controller or panel meter
- Compare no-load and loaded readings
- Never open live panels or terminal boxes
FAQ
Frequently asked questions
Why does my generator’s frequency drop when the load comes on?
A small drop is normal with a droop governor. A large drop, or one that does not recover, points to overload, a governor fault or a fuel supply restriction. See frequency and speed problems.
Why does my generator show 52 Hz with nothing connected?
With a droop governor that can be normal: the set runs slightly fast at no load so it settles near 50 Hz at full load. If it stays well above 50 Hz under load, the governor needs adjusting.
Is 240 V from my generator a problem?
It is within the usual tolerance of 230 V, but a set that runs high may be adjusted wrongly or have an AVR or sensing fault. Have it checked if it is noticeably higher than usual.
Can I use a 60 Hz generator in South Africa?
Equipment here is designed for 50 Hz. Some sets can be set up for either frequency by changing the engine speed and AVR settings, but others cannot, so have a technician check before connecting one.
Related pages
Related services
Related generator problems
Generator types
Related brands
Need a generator technician?
- 24/7 operations
- We Arrive Within 1–2 Hours (usually within 1 hour)
- Free callouts and quotations