Testing portable generator output

[ADWSystems]

Out of curiosity, I wanted to check the output of of new portable generator. I have a meter than reads VAC and kHz. The house reading was 124.6VAC and 60-61Hz. The generator was 120.9-121.1VAC but the frequency was reading 180-200Hz. It is not an inverter style, so (the thinking is) there should not be much in the way of "digital" effects and harmonics for the meter to lock on to, but it still may be a possibility. I would like to check it with a scope. My father has a VERY old scope that I should be to borrow. If the scope is plugged in to the house/utility supply, can I place the probe ground on the AC neutral and the probe tip on AC Line (or hot) and see the power waveform? I don't often try to look at the incoming power as a signal. I did once up on a time and ran into some issues, but don't recall what the issue was.

 

[KeepItSimpleStupid]

The generator is isolated at this point. You said specifically that it was portable. There is a right way and a wrong way to connect to a house electrical system for power.

For this test, I would ground the generator to the house circuit and put the generator on something insulating.

The real way to do the grounding properly is to break the outlet ground and frame bond. Tie the frame to a ground rod and the output ground to the ground of the house system. You don't have to do that for this test.

You will run into a phasing issue. The generator and the house will not be in phase.

Therefor the SAFEEST way to make the measurement is to put the scope in the A-B mode. So, you will have ground conneted to the house ground and A connected to phase generator and B connected to gnd generator.

Hopefully the scope probes are X10 and you may or may not have to take that into account. Remember, your going to be measuring ABOUT +- 160 Volts. I'd also suggest that the generator have a load. Some generators will idle down if no current is being drawn.

A single channel scope should work, BUT ground the generator to the house ground. Check voltages with a DVM before using the scope.

 

[alec_t]

If the scope is plugged in to the house/utility supply, can I place the probe ground on the AC neutral and the probe tip on AC Line (or hot) and see the power waveform?

Depending how the scope is wired internally there's a good chance that if you do that you will blow a fuse or operate a mains breaker. There's also the risk of electrocution. Much safer just to plug in a transfomer and measure the waveform at the transformer secondary.

 

[Reloadron]

Generally using a small AC generator the generator shaft is direct coupled to the engine driving it. The frequency is a direct function of the rotational speed. The relationship is as follows:

P = 120*f/N where generator Frequency (f) = Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120 Conversely, P = 120*f/N. So for 60 Hz a two pole generator would be rotating at 3600 RPM and that same generator in 4 poles (like a small diesel engine) would be rotating at 1800 RPM. For our UK friends, 50 Hz would be 3,000 RPM and 1500 RPM respectively.

My best guess seeing your numbers is that if using a frequency counter to measure the frequency or a small DMM with a frequency function you are either seeing noise or triple triggering the counter. If the counter allows input amplitude ranging (like for example most Fluke hand held DMMs) try increasing the range. To see numbers like 180 Hz (60 * 3 the shaft rotation would have to be 10,000 plus RPM which I seriously doubt.

If you have an old AC powered 60 Hz wall clock (the type motor driven and not a digital clock) connect clock to generator and set it. Walk away for about an hour. How well did the clock keep time? Using a scope do as KISS suggest if the scope has the features. If not, connect scope ground to generator neutral as suggested.

Ron

 

[ADWSystems]

It's an old old scope. My father thinks there's only one channel. The probes are X10. I like (love) the transformer idea.

 

[ADWSystems]

Generally using a small AC generator the generator shaft is direct coupled to the engine driving it. The frequency is a direct function of the rotational speed. The relationship is as follows:

P = 120*f/N where generator Frequency (f) = Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120 Conversely, P = 120*f/N. So for 60 Hz a two pole generator would be rotating at 3600 RPM and that same generator in 4 poles (like a small diesel engine) would be rotating at 1800 RPM. For our UK friends, 50 Hz would be 3,000 RPM and 1500 RPM respectively.

My best guess seeing your numbers is that if using a frequency counter to measure the frequency or a small DMM with a frequency function you are either seeing noise or triple triggering the counter. If the counter allows input amplitude ranging (like for example most Fluke hand held DMMs) try increasing the range. To see numbers like 180 Hz (60 * 3 the shaft rotation would have to be 10,000 plus RPM which I seriously doubt.

If you have an old AC powered 60 Hz wall clock (the type motor driven and not a digital clock) connect clock to generator and set it. Walk away for about an hour. How well did the clock keep time? Using a scope do as KISS suggest if the scope has the features. If not, connect scope ground to generator neutral as suggested.

Ron

Someone suggested the clock idea on another forum as a joke. I wasn't convinced either way. Unfortunately I don't have a AC analog clock. All mine are battery powered. But I do have a AC hour meter with the similar motor/mechanical drive that I think would work as well.

If I connect the generator to the house but do not transfer the load to the generator then the generator ground and neutral will be tied to the house ground and neutral, in accordance with the installation instructions and the NEC.

 

[Reloadron]

What is the generator output as in 120 or 240 VAC? Before I went whole house I was using a little 4 KW 240 volt unit to power the house when we would lose power. Still have that thing out in the shed. An hour meter would work but most only resolve .1 hour ( 6 min). The best solution is monitor a frequency counter for a period of time followed by the scope. Remember, based on the formula, when the generator is loaded the RPM will likely drop then return so the frequency will vary. Also, yes, using a transformer is a nice added safety touch with isolation.

Also, yes, an old analog clock would work just fine since their accuracy is a function of line frequency.

Oh well, time to drag my butt out the door to work.

Ron

 

[ADWSystems]

The generator is 7kW able to produce 30A at 240VAC. I put two space heaters on it, each rated for 12A (doesn't mean they were drawing that much) and it didn't make a hiccup in the RPM. It certainly isn't running 10,000 RPM either.

If I go with the transformer idea. I obviously connect the generator line and neutral to the primary. The output of the transformer I'm thinking of using (if I can find it) is 24VAC. I was thinking of putting a 5K resistor across the secondary to give me a 10mA/0.5W load. Where am I hooking the scope? My first intuition would be across the resistor, my second would say to connect the scope ground to outlet ground.

 

[Reloadron]

Nice choice in a generator. 7 KW ain't all that shabby when power is needed. Also nice to have the 240 split phase just as power company mains power is delivered. Makes for easier transfer.

You can hang a resistor out there but it isn't necessary. You would measure per the attached. I assumed a 10:1 probe since you mentioned it. Really matters not. Set the scope for AC coupling using an appropriate vertical input range. The sine wave should be 16.666 mS. for 60 Hz.

Ron

 

[ADWSystems]

I am not really following your drawing, but it looks like you saying just to grab the two transformer leads with the probe tip and ground. Or if using the resistor, just hook across the resistor.

 

[Reloadron]

That's it!

Ron

 

[ADWSystems]

Thank you all for helping me not blow up my new generator, may father's old scope, or me. It appears that there's a higher frequency signal riding on top of the 60 Hz signal. I had to be outside to run the generator and by the time I got home it was dark. The camera wanted to use the flash even when I said no. I had to hold my finger to prevent the flash from washing out the trace. It is very close to 16ms cycles both no load and with the space heater running.



The trace is 2ms/dev and 5v/div on the load side of a 12V transformer. I tried my meter on the 12V side and it would pick up the power cycle and then momentarily pick up on the rider. It was measuring 63Hz. Close enough for me.

 

[RCinFLA]

I was going to say that reading a higher frequency on a simple DVM that has freq counter usually means the generator has a 'ratty' output waveform.

Looking at the waveform I would say the ripple is caused by the way the generator regulator is pulsing the field winding (rotor). For newer generators with electronic regulators this is a common way of doing it. The difference between good quality and the ripple you are seeing is the chop rate they are pulsing current to the field winding.

Sometimes there are adjustments on the electronic regulator module. My 15KVA Generac has a 'stability' adjustment.

You might want to look at waveform as you put some electrical loads on the generator. If you are lucky the ripple may smooth out as you put a little load on generator.

 

[ADWSystems]

I put a 12.5A rated space heater on it while I had the scope hooked up. You're correct, the ripple does die down and the sinusoidal shape improved too.

It was a little spooky to hook up the DVM and have it show 200Hz.

 

[Reloadron]

Glad you got it. Nice going.

My 15KVA Generac has a 'stability' adjustment.

Hmm, lives in Florida and has a generator. How unusual and must be an exception to the rule.

Ron