I have a 3000 rpm 11.8 KVA Fisher Panda generator that has both 1 and
3 phase outputs. I am looking to use the 3 phase output to power a 3
phase motor of around 10kW to drive a bow thruster in a barge at
approx 1500rpm. A colleagu has a Northern Lights 10KVA genset and
successfully powers a 9kW 3 phase motor in a similar set up. Would
appreciate any pointers - such as need for a capcitor bank to meet the
surge on starting.
Does the bow thruster actually pull rated power or not. Your 11.8KVA gen
equates to only 9.44 Kw, which if the motor it is supplying is over specced
for the job you should be ok. There will be quite a surge on switch on if
there is no speed control type apparatus.
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It would be 'problematical'. A cap bank won't really help with starting of
I *have* seen a 2400 hp electric motor started on a 3000 hp diesel-gen set.
Engine running at about 60.5 hz, closed in the breaker. Engine dogged down
like anything and the motor started turning. The two met about 40 hz and
the governor of the engine was full open. Then both came up to full speed
and things ran fine from there. Had special overcurrent trips that were
blocked for 5 seconds right after shutting breaker. Quite a sight to
Starting such a large motor on a generator that is closely rated, well, it
may work, it may not. The voltage and frequency dip will be substantial.
But if its just dedicated to that one load, hey it may work. It would be
*nice* if you could start the load unloaded, but I don't know how you could
'unload' a bow thruster.
Since it will take longer than usual to come up to speed if the engine is
only just big enough, you may have to look at the overload protection. And
you will definitely want to limit the number of starts you do in a given
time frame. Obviously, being a bow thruster you may have to start/stop it
more than you would like, just to keep the barge from going where it
You don't mention the type of thruster, but I guess it would be water-cooled
so probably not much problem there. The genny may need some extra cooling
if you plan on lots of start/stops.
And obviously you will want an engine that's well tuned and not going to
stall/sputter when you hit it with the load. My boating experiences on the
GLakes has taught me when you need maneuvering power, you *need* maneuvering
power. Not the time to have to nurse a sickly engine.
Why not use the boat engine - or a small car engine - to charge a bank
of car batteries and use a DC motor instead?
10kw is not hard to do - a big truck starter motor would do that I reckon.
You could use the rest of the truck motor and alternator to power the
I have no direct experience of this. However the power requirements of
the bow thruster won't approach anything like the rated 10KW until the
speed of the propellor is fairly close to the 1500 rpm.
IIRC the power required is proportional to the cube of the prop speed.
The power factor of the motor will be poor on starting but improve as
the prop comes up to speed and a pf of 0.8 is plausible giving 11.8 kVA
* 0.8 = 8.9 kW.
Does your colleague have capacitors? What sort of over current
The fact that he/she has it working means it's possible in principle and
there is nothing that makes you setup different.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
But there is another factor, that motor start current is many times
rated run current. So the only thing that'll keep it going thru the
start period is heavy gen speed drop reducing motor current. Thre's
only one way to find out if your gen will do this reliably without
There is a way round it tho, and that's to electrically reduce the
motor power during start. Several methods possible:
1 electronic start control unit
2 massive series resistor and switch
3 switching down the output V on the gen, if it has this option
4 starting the motor coils in series then switching to parallel
2 is the one you can use with any setup, the rest depend on equipment
It should be possible, and some gens will manage it with no start-up
provisions, but you certainly cant count on that without trying it.
Good points. Another one that comes to mind is a star-delta motor starter.
If all six leads of the motor can be taken out to the motor control, this
*may* be a possibility. Depends on the various connections and voltage
What size of barge? The power requirement of the bow thruster seems to
be out of all proportion ot those normally fitted. Are you using a
waterjet (horribly inefficient) rather than a ducted propellor?
I'm guessing it has to be a waterjet to permit you to control
port/starboard thrust since the motor won't be reversible.
What about a hydaulic driven bow thruster. Needs a hydraulic pump off
the main engine. Does not have the same duty cycle restrictions as the
electric ones do.
On 2 Oct 2003 11:35:48 -0700, email@example.com (Rob) wrote:
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