Inverters are designed with self-testing circuits to prevent damage to the system components when switched on or operational for extended periods. The hardest working components in an inverter are the MOSFETs (Metal-Oxide-Semiconductor-Field-Effect-Transistor). These are the most likely cause of an inverter that stops functioning.
Inverters will signal that they are under strain and starting to fail employing beeping alarms, noisy cooling fans, failing to turn on, not charging the battery bank, error codes appearing on LCD, or only functioning in DC to AC inverter mode. These symptoms are all indicators of pending trouble.
The primary function that inverters are designed for is to rapidly switch and invert sections of direct current into a square-shaped standing wave and then smooth the wave function into a pure Sine-wave. The invertor is made up of the following components:
- Bipolar Junction Transistors
- Transistors (NPN and PNP type)
- Cooling fans
- Input and output terminals
The components that fail most often due to their high workload are the MOSFETs. Let’s look at potential failure modes and how these can be repaired.
What Does An Inverter Do And What Can Go Wrong?
Inverters are designed to convert a low voltage direct current into a high voltage (120V) and high frequency (60Hz) alternating current to power AC-powered devices in your home or workplace.
Inverters are designed with circuits and alarms to protect the system from catastrophic failure and emit warning alarms when they detect strained operation. The diagnosis of which component has failed in an inverter poses a significant risk of electrocution and should only be done by qualified technicians.
An inverter draws a direct current (DC – 12V, 24V, or 48V) from a battery bank into a collection of switches (MOSFETs) and filters that cut the direct current into sections and invert every alternative section. The rate of inversion of the clipped sections of current is the frequency of the alternating current.
Once the DC has been converted to AC, other components are used to smooth the alternating transitions from a standing wave to a modified sine wave and then to a pure sine wave. The degree of smoothing the wave into a pure sine wave is measured as Total Harmonic Distortion (THD).
Low-cost inverters have a THD of 80%, whereas more expensive models will have a THD between 90% and 95%. The higher the THD value of an inverter, the better the AC generated will AC power devices. Low-cost inverters cause AC devices to make a humming sound and create a lot of heat.
The conversion of DC to AC and subsequent smoothing of the AC wave to achieve a high THD implies that energy conversion is not 100% efficient. Inverters typically only convert 85% of the DC power to AC power, with the difference being lost to heat generation and powering the inverter components.
The MOSFETs are the hardest working components, and they have a limited lifetime. If the inverter shows a red indicator during the self-test phase and fails to function, it is most likely that one or more of the MOSFETs have died.
A qualified technician will open up and diagnose the failed components on an inverter and can replace such components making the inverter fully functional again. It is imperative that you read the installation instructions carefully when installing and engaging an inverter for the first time.
What Are The Symptoms Of A Failing Inverter?
Inverters are designed with built-in pre-operational test routines and alarms that will alert the user to abnormal conditions that may lead to catastrophic failure of the device.
Common symptoms of inverter failure are:
- The inverter does not turn on
- The inverter does not charge the battery bank when grid power is on
- The battery bank backup time decreases
- The inverter only operates in “inverter mode.”
- The inverter beeping alarms persist
- The inverter and cooling fans are noisy
When an inverter does not turn on, it could be an indication of several conditions:
- The inverter has tripped (reset the trip switch)
- The inverter is not connected to the battery bank, or connections are loose (repair connections)
- The battery bank does not have a sufficient state of charge (charge/service the battery bank)
The inverter should also be charging the battery bank when the AC grid power is on. The inverter’s function is not just to convert the DC battery power to AC when the grid supply goes down but also to maintain the battery charge when the grid power is on.
If the inverter is not charging the battery, it may be due to several conditions:
- The batteries are dead or need maintenance (replace or service batteries)
- The AC rectifier in the inverter is not working (test and replace rectifier)
- The wiring between the inverter and the battery bank is loose or poor (correct connections)
- A fuse in the inverter rectifier circuit is blown (replace the fuse)
A reduced battery backup time can indicate that the inverter is overloaded with AC devices. Note that inverters have a normal operating output Watt rating and a surge output Watt rating. The inverter can operate close to the surge rating for a couple of seconds when inductive loads are connected.
The normal operating output Watt rating should never be exceeded more than a few seconds. It is good practice to only load an inverter with 80% of its operational Watt rating to protect and prolong its life.
If the inverter fails to operate in battery charge mode, it may be that the input voltage protection fuse is blown. This input protection safeguards the inverter from damage by unstable AC input spikes from the grid supply. Replace the defective fuse and reset the input protection circuit.
If the inverter generates a beeping alarm and refuses to convert DC to AC, it may signify that the MOSFETs have blown. These components can be replaced by a qualified technician and will cost you a lot less than replacing your inverter.
Other beeping alarms may also indicate a low battery state of charge or when the too high current draw is placed on the inverter. Please do not ignore these alarms as they are intended to draw your attention to disconnect and investigate.
Inverters generate a lot of heat during operation. The inverter must be installed in a well-ventilated location out of direct sunlight. The cooling fans draw cold air through the body of the inverter and dissipate heat via the cooling fins on the device’s body.
Noisy fans may be due to the inverter working too hard, or the fan motor needs to be replaced.