Extending Power Inverter life-span – Squashing Back EMF (Counter EMF)

For users of battery-based power inverters there are some stated important environmental conditions that preserve the life of the inverter.

Typically they are: mounting in a cool dry place, away from battery fumes, dust, and combustible fumes from fuels and lubricants. Also, feeding power into the AC output of an inverter voids the warranty. For most installations, following the above is sufficient for most inverter applications.

If an inverter is serving as a backup to commercial power, it is recommended that a full second of time elapses before the inverter takes over powering the AC load. So why is this delay recommended?

The first reason for the delay is that commercial power may have a momentary dip in voltage and then quickly recover. Many power supplies in electronic equipment can ride through this momentary drop in voltage. Certainly, fans, drills and other rotating equipment may not even slow operation during a momentary drop in AC voltage. Momentary dips usually do not warrant switching to backup power when commercial power quickly resumes full voltage output.

The second reason for a several second delay before the inverter takes over powering the AC load has to do with minimizing the negative effects of “Back-EMF”.

EMF is an old term for “voltage”. EMF stands for “Electro Motive Force”. Back-EMF is unwanted voltage that is produced when power to inductive loads is switched off. Whenever you power a fan, compressor, electric drill, a Back-EMF is generated that results in a spark across switch contacts.

By waiting a few seconds before the inverter connects to the AC load, the Back-EMF dissipates.

Without the delay, some of the spark energy feeds into the output of the inverter. Technically, this violates the principle of not feeding voltage into the AC output of the inverter. However, most power inverters can easily handle momentary Back EMF events. But, if the inductive load is large enough and Back-EMF occurs often enough, the inverter’s output transistors can be degraded and early failure can occur. Multiple inductive power tools as AC load can contribute to early inverter failure.

So, how can a user protect the inverter from Back EMF? It’s actually quite simple.

The easiest way is to have a surge protecting power strip connected an inverter outlet. Ordinary use for surge protecting outlet strips is to protect appliances from AC spikes that occur from lightning and other power surges.

Surge protectors are inexpensive – some sell for as little $12 as of this writing. So, what’s in a surge protecting power strip? There is a 15 amp pop out circuit breaker, outlets, an on/off switch and a device that soaks up voltage spikes. The device is often an MOV – Metal Oxide Varistor. A varistor is wired across the AC power and is invisible to ordinary working voltages. Once a momentary power spike occurs, the MOV turns on and instantly squashes the power spike. The spike energy is turned into heat by the MOV. If the MOV fails by shorting, the circuit breaker in the outlet strip pops open.

Does that mean that all inverter outlets should have a surge protecting outlet strip connected? The answer is NO. Any one inverter outlet with a surge protecting outlet strip protects the entire inverter AC output because all outlets and high output terminals are wired in parallel.

There are other ways to soak up voltage spikes from Back EMF. If an inverter also powers resistive loads, such as incandescent lamps or heating devices, these can help dissipate damaging Back-EMF voltage spikes produced by inductive loads.

The Wrap-up

While ordinary use for surge protecting outlet strips is to protect appliances from AC spikes, surge protecting outlet strips can also protect inverters from spikes produced from Back-EMF.

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Best Regards,

Jack Colton
Product Specialist for Wagan Tech