Inverter Generators and Transfer Switches

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RCinFLA

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Who out there has been requested to install backup power transfer switches in residences with the customer desiring to use an inverter generator?

The attraction is quieter generator and better fuel economy.

There are a couple of technical issues on inverter generators. Honda, Yamaha, and Robin inverter generators do not have a groundable common on their A.C. output. This is because of the H-bridge Pulse Width Modulator that produces the 60 Hz output. Second issue is they are only 120 vac output.

I see two possible solutions, both require an isolation transformer to allow common-ground connection at breaker box.

First is 120 v to 240/120 v isolation transformer to provide L1,L2 sides for auxilary breaker box.

Second is 120 v to 120 v isolation transformer having transfer switch that connects L1,L2 sides of auxilary breaker box together when on backup generator. This would have to be restricted to no 240v service breaker ports in auxilary box but can still maintain L1-L2 load balancing when on mains. Not sure what code says about this approach.
 
Re: Inverter Generators and Transfer Switches

If the neutral is unswitched at the transfer switch, then you wouldn't want the neutral and ground bonded in the generator anyway.

[ January 11, 2006, 07:40 PM: Message edited by: ron ]
 
Re: Inverter Generators and Transfer Switches

At the generator is not the problem until neutral is grounded when connecting to house breaker systems unless you float neutral in auxilary box. I am pretty sure that won't pass code not to mention is messy requiring switching neutral in and out when going between normal power mains and generator backup.
 
Re: Inverter Generators and Transfer Switches

There are a couple of technical issues on inverter generators. Honda, Yamaha, and Robin inverter generators do not have a groundable common on their A.C. output.
Click to expand...
Why can't it be grounded? You should be able to ground a single conductor of any system.
Don
 
Re: Inverter Generators and Transfer Switches

This situation is similar to many of the DC-AC inverters out there.

What these inverters and inverter-generators do is produce between 180 and 250 vdc. (modified-sinewave inverters produce about 155 vdc) A full wave MOSFET H-bridge then chops the DC into PWM pulses such that the instantaneous duty cycle produces the instantaneous value of the sine wave. A final high frequency L-C filter is used to average out the pulse duty cycle leaving a nice clean sinewave.

The H-bridge flips the polarity when producing the negative half cycle of the sinewave therefore the conventional neutral side of the plug has half the 120 vac with respect to the actual ground pin on the AC plug. The capacitors of the AC filter are usually common connected to the AC socket ground pin.

You can directly connect the neutral side of the generator plug to breaker box neutral (which is ground bonded at box) but case of generator cannot be grounded then and anyone that touches the generator housing (gen ground) and any exposed conduit or house ground common metal appliance is going to get zapped.
 
Re: Inverter Generators and Transfer Switches

The code requiers that you use a grounded system (250.20), and a grounded system requires a system bonding jumper. Given the defintion of premises wiring in Article 100, I don't see how this unit can be used for any application.
Don
 
Re: Inverter Generators and Transfer Switches

They're great for portable use with cord connected loads and I know the Hondas can be paralleled if you need to run your window AC unit.

As Don said, not going to work in any approved manner with a 120/240 panel.
 
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