Inverter Overload

Available Power is No Longer Unlimited

One of the problems with alternative power is that you no longer have the virtually infinite power of the Eskom grid linked to your wall plug anymore. Most people have become so used to any appliances they plug into the wall simply working, they have trouble understanding that this may no longer be possible when alternative power sources are used.

This can sometimes lead to unexpected problems that can be disconcerting and difficult to properly identify.

In addition, it definitely leads to irritation, especially if the people involved refuse to acknowledge or accept this limitation.

Inductive Start-Up Current Spikes

One of these problems is the start-up power spikes of certain devices like fridges, motors and other inductive devices.

These devices can draw very high current for short periods of time, as they start up, after which they settle down to a much lower current draw. These very high, short duration current spikes can overload some inverters and perhaps cause damage to the inverter.  Or if the inverter has good protection circuitry, the current spike may cause the inverter to shut down unexpectedly, plunging the house into darkness.

Real Life Case Study

A 1200W UPS inverter was installed in a home as part of a backup power system to drive a medium size fridge, a few lights and the TV in a family home.

On the face of it there should have been no problems at all.  The fridge’s power consumption was measured at 120-130W and the UPS was rated at almost ten times that figure.  In addition, the UPS could withstand a maximum current of 10A (2200 VA) for short periods of time.

But after installation, an irritating problem arose in which the UPS would unexpectedly shut down at odd times while providing back-up power.

An investigation was undertaken in which a high resolution power quality meter was attached to the fridge and the mains power switched off to force the UPS into backup power mode.  And sure enough, it shut down after about 30 minutes.

The graphs below are the continuous voltage and current data plots from the high resolution power quality recorder.  They show what happened:

You can see the problem cause quite clearly.  As the fridge compressor turned on periodically, it caused a very high (11A), short duration (100ms) current spike which exceeded the UPS’ limitations (10A maximum), and which then resulted in the UPS protection circuitry shutting down the UPS. The problem was sporadic because the current spike did not always exceed 10A every time the fridge compressor started (as can be seen in the graph below), so some times it would run for fairly long periods without incident, but  other times it would shut the UPS down almost immediately.

The fridge in question was a modest medium sized, fairly modern fridge/freezer.  It was unexpected that its compressor start up would cause such high current spikes (conventional wisdom usually predicts a spike of 3-5 times the running current for an inductive device).

Conclusions

Given the price of alternative power solutions, people are naturally trying to go for the smallest systems that will potentially meet their needs.

But if certain highly inductive loads are included, such as fridges, motors, and other inductive devices, one should be aware that you may need a substantially bigger inverter than one would otherwise think necessary.

This problem is difficult to deal with because you need very specialised measurement equipment to properly measure these short duration current spikes.  Very few people have access to meters of this type.  And thus it becomes quite difficult to predict these problems before hand.  The appliance manufacturers do not usually specify information about start-up current spikes in their documentation unfortunately.