Portable generator floating neutral

[kwired]

I dont think there is any. It is an ungrounded isolated power source. A fault should not shock you. Two faults maybe, but you or the dirt need to be involved. If the genny case is plastic, that further isolates you from earth making a shock less likely. GFCI is a secondary failsafe.

A negative for bonding it at the factory is that is technically not allowed with a solid neutral transfer switch. Mist people dont have transfer switches that switch the neutral.

I am in agreement with you, this is nothing more than an ungrounded system. If you connect it to a premises wiring system that is grounded it then becomes a grounded system, and technically you should have a separate EGC back to the generator frame to bond the frame.

Otherwise if there is no ground reference, a ground fault is not a hazard it simply becomes a grounded system, but a second fault can become a hazard.

The problem with electronics is mostly because the small size of the source in comparison to utility sources. Sudden change in load on the small flywheel will either drag voltage and frequency down or allow increase, usually temporary but if the item is sensitive to such fluctuation it will have issues. A 5kW load will have a noticeable effect even on a 25kW portable when the load switches on/off but this kind of load is negligible to effecting volts/frequency to a megawatts range generator the grid is using as primary source.

 

[synchro]

Nothing happens to you on a single fault on an ungrounded system. If there is a second fault that doesnt cause the breaker to trip yet creates a shock hazard, the gfci is your defence.

If the neutral conductor on the input side of the generator's GFCI is not bonded to its EGC, then a N-EGC or L-EGC fault on the load side of the GFCI should not trip the GFCI. However, if there's a load side N-EGC fault, then a person touching both the "hot" and EGC will not be protected by the GFCI because there is not a closed circuit around the CT of the generator's GFCI that would be conducting this leakage current.

A L-EGC fault should also not cause GFCI tripping in a generator without a bond. In this case a person touching both the neutral and EGC will also not be protected by the GFCI.

There could be line-to-EGC filter capacitor(s) in the generator on the input side of its GFCI. If so, then the generator's case and EGC terminal would not be completely "floating" relative to its line and neutral. It's very likely that there would be such filter capacitor(s) in an inverter generator to reduce common-mode switching noise. These capacitors would not draw a substantial leakage current, but it could be more than 5 mA when the generator is bonded and still not trip the GFCI because such capacitors would be on the input side of the GFCI.
When the generator is not bonded, equal L-EGC and N-EGC capacitors would tend to make the voltages across them 60V as well as balance the currents through them, and that would reduce the likelihood of noticeable L-EGC or N-EGC shocks from capacitive leakage.

 

[suemarkp]

You are correct. I need to take my own advice and think differently about how GFCIs could behave in ungrounded systems. GFCI fault handling will be different than anticipated during many fault conditions.

 

[PaulMmn]

As long as it's safe I only use in power outage I'm saving for a whole house generator.

And yes I don't trust my cheap 5000w generator from what I read you can't trust any portable generator that's not a inverter. Even tho my parents been doing it for years now with out issue.

According to Consumer Reports, the main difference between 'cheap' and 'inverter' generators is that the cheap ones run at full throttle all the time, and inverters are able to throttle down to just handle the load. Easier on the gas tank.

 

[qcroanoke]

I have a 5000w normal generator I use to back feed house it's floating neutral but my house has a bonding point so it's good I just don't trust to run my stuff with electronics.

I deal with more industrial electric and just wanted opinions who know code and residential better than me. Thanks

I have a 7500 watt cheap Troybilt generator that I use to back up my house thru a MTS that does not switch the neutral and I have used my smart tv's and computer with no problems.

 

[synchro]

According to Consumer Reports, the main difference between 'cheap' and 'inverter' generators is that the cheap ones run at full throttle all the time, and inverters are able to throttle down to just handle the load. Easier on the gas tank.

I wouldn't say that a cheap generator runs at full throttle all the time. It runs at a constant synchronous speed, which is 3600 RPM in cheaper generators (and in many of those that aren't so cheap). Full throttle operation (or close to it) would be necessary with a full electrical load in order to develop the higher torque required under such conditions to maintain a synchronous speed.

 

[kwired]

I wouldn't say that a cheap generator runs at full throttle all the time. It runs at a constant synchronous speed, which is 3600 RPM in cheaper generators (and in many of those that aren't so cheap). Full throttle operation (or close to it) would be necessary with a full electrical load in order to develop the higher torque required under such conditions to maintain a synchronous speed.

Does an inverter generator need to maintain prime mover speed to maintain the output frequency though?

 

[zbang]

No, that's the main point of them- generate DC/invert to 60Hz. At least every one I've seen varies the PM speed.

 

[synchro]

The output circuitry of an inverter generator is very much like that of a VFD, except that it has additional filtering to reduce the switching noise.
There are output transistors that pull up to the positive DC rail, or pull down to the negative DC rail. This is done with a pulse width modulated waveform to approximate a sine wave whose frequency is determined in the dugital electronics. In a VFD this frequency can be varied around, but that is not necessary in an inverter generator.

The alternator in the inverter generator is rectified to provide DC power to the DC rails. The frequency of this internal alternator is therefore not constrained to be any specific frequency. And therefore when the load is relatively low, the engine speed can be reduced because there is less power needed to maintain the DC bus voltage. Typically you can also set inverter generators such that the engine maintains full speed. This way it will be more responsive when larger loads are applied.