Where does "surge" capacity in Generators come from?

 

I've seen several posts that relate the ability of a generator to
handle a surge load to the stored rotational energy in the engine and
generator itself.

This has caused me to wonder about a few things.

How important is the stored rotational engergy compared to:
1) governor responsiveness
2) Torque rise of the engine.
3) Generator design

Specifically, how much energy IS stored in a typical generator running
at 1800 rpm versus how much is stored in a lighter generator running at
3600 rpm?

Does the energy stored go up with the square of the rotational speed?
Does the enegy stored go up linearly with the mass of the rotating
components?

Also, how important is constant voltage - versus constant cycles when
trying to start a reactive load?

Thanks
Phil

 

I think you are drawing some conclusions based on what inverters usually
market as "surge" capacity and 5-minute capacity.  Inverter 5-minute
capacity is based on heat inertia, and its "surge" capacity is only
milliseconds as the large capacitors in the high voltage DC section
discharge to supply a momentarily overload.

For generators, I can think of three ratings:  Prime (what can be supplied
indefinitely), Standby (what can be supplied for 1-hour), and motor
starting/fault clearing overloads.  You can imagine that the 1-hour rating
is a thermal limit.  Someone else may have a comment here about what I'm
about to say, but it appears that the 10% rating puts you closer to the full
rating of the conductors and protective devices.  That is, prime rating
would represent 80% of the circuit breaker rating, while the standby rating
would be closer to the service rating of the wire and CB which is acceptable
for short durations only.  These ratings are affected by the engine and
cooling system, as well as the thermal characteristics of the alternator
winding... for instance, do you want 105C rise, or 135C rise, type of
insulation, etc.

Regarding reactive loads, you can also add an optional Permanent Magnet
Generator (exciter) to the end of the alternator that will ensure you up to
300% rated current for a few seconds to start heavy inductive loads or clear
a fault.  The permanent magnets in the exciter will keep the field from
collapsing, and thus, loss of output.

You have to consider the type of load, crest factor and harmonics when
sizing a generator.  Remember that the engine supplies POWER or kW.  VARs
are supplied by the alternator.  A large amount of current to a poor PF load
doesn't necessarily tax the engine.  So it isn't uncommon to put a 50 kVA
alternator on a 30 kW engine if the VAR's are required.  Another reason for
oversizing the alternator is to keep the voltage THD under control with
non-linear loads.  The alternator and wiring must be rated to supply the
current.  The engine must be rated to supply the power.

Then you have manufacturer engine ranges.  For instance, a Wilsons generator
with a Caterpillar 4 cyl diesel, no turbocharger will have the same engine
in a 30kW or 40kW generator.  Only the amount of copper in the alternator
changes.  The 36 kW (Prime) 40 kW (standby) engine/alternator combo will
supply 44 kW for a while... make it 45 kW and the engine slows down and
stalls.  Go to 50 kW, the SAME engine gets a TURBOCHARGER...  go to 60 kW,
the same engine gets an oil cooler.   The 50 kW generator with PMG will
supply 60 kW until the CB opens because the engine is strong enough to turn
the alternator.


If you were asking this in relation to small consumer generators, things are
not so controlled.  Quality and design take a back seat to affordability.
Anything can happen.

 

"I think you are drawing some conclusions based on what inverters
usually
market ..."

Sorry that I left you with that impression.
That was not my intent.
I was trying to ask questions about generators - not voice conclusions.

"....For generators, I can think of three ratings:  Prime (what can be
supplied
indefinitely), Standby (what can be supplied for 1-hour), and motor
starting/fault clearing overloads. "

I was trying to refer to the 3rd item that you list. "...motor
starting/fault clearing..."

I've seen folks comment that the rotational mass of the unit affects
the ability to supply surge.

I was skeptical of this because I thought that the only way to "obtain"
that stored energy was for the RPMs to drop - which would affect
cycles.

Thus, I was lead to wondering:
How much rotational energy is stored in the unit?
How much of it can be used to start motors?
and,
Are these quantities significant when compared with other things like:
Normal load,
Torque rise,
Governor responsiveness and
Generator design.

"....Regarding reactive loads, you can also add an optional Permanent
Magnet
Generator (exciter) to the end of the alternator that will ensure you
up to
300% rated current for a few seconds to start heavy inductive loads
..."

This "seems" like it would be a "good thing" - what is the down side if
any?

 

I can't respond directly to the rotational aspects, but I don't think there
is much stored energy in the rotating machinery.  At least not much beyond a
few cycles at best.  Look at the published specifications.  One of them is
the allowable voltage and frequency dip for a "step-response".  I would
often just test this by turning on a switch that takes the machine from 0 to
100% load while capturing the sag on datalogging equipment.  You can hear
the machine dip and recover speed and it does so very quickly.  How much of
a dip you can accept depends on the load.  You get a surge when the load is
disconnected depending on how quickly the automatic voltage regulator can
back it off.

Have you every listened to the ice cream truck generator?  You hear that dip
whenever the dude starts to pour a cone.  Mechanical vs electronic governor,
and turbo lag tend to be the factors.

PMG?  Cost.  Some manufacturers offer it as an option.  Others (like Kohler)
build it into their alternators as standard.  For instance, the residential
Kohler (12RES) which is 10.5 kW claim they can start a 4 tonne air
conditioner (probably not much else can be on).  Don't know of any other
issues with the PMG.  Recall that a compressor is essentially a locked rotor
for a few cycles drawing 120A for a 4 tonne unit.  That 120A doesn't have
much to do with power.  The compressor is very inductive as it begins to
spin.

 

I cant answer your question but point out on my 7500exl at 7000w for 5
minutes load the exuast was glowing red about 4 inches out the head on a
50f day. After all its air cooled, I wonder what engine temps would be
on a 90f day 90% humidity.  And a test on a 4000exl for load and surge
failed on a Consumer Reports test, avalaible online. I personaly view
surge load as something I never want to use as I like to run equipment
conservativly. Ive seen other home gens just not start something near
surge, Im sure it stresses every component. It isnt like buying a
furnace where you have an independant  AFUE rating and can be reasonably
confident of performance.