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Runnning Generators in Parallel

Two generators running in parallel or synchronized must be at the same speed and also the same phase to be paralleled. If the neutrals are connected together and then the voltage between the hot legs of the two generators is measured (hot leg to hot leg and not hot to ground), the following conditions will be noted:  In phase:  zero volts 180 degrees out of phase: the sum of the two voltages.    Since the voltages must be the same, this will be 2X the nameplate voltage.  The generators can only be connected together when the voltage between the hots is zero, e.g. in phase.  This suggests a phasing method.  The standard power plant method of synchronizing two generators is to use a very expensive instrument called a synchroscope.  This instrument indicates whether the incoming generator is faster, slower or in phase with the bus.  Since the rotors of power plant generators weigh many tons, the phases must be extremely accurately matched or else the rotor will be forcibly yanked into phase, possibly wrecking the generator.  For small units, we don't need to be so precise.  We can use a pair of lamps.  What you'll need is a couple of lamps hooked in series and connected between the hot lead of the running generator and the hot lead of the newly cranked generator.  You'll also need a switch of some sorts to parallel the units.  If the generators are 180 deg out of phase, the voltage across the lamps will be 2X the nameplate voltage (240 volts in the case of two 120 volt generators.) and the lamps will burn full brilliance.  If the generators are nearly in phase, the lamps will be out because there will be no voltage on them.  If one generator is faster than the other, the lamps will flicker on and off as the gens are in phase one moment and out the next.  The procedure is as follows.  Start the second generator.  The lights will be flickering or slowly coming on and off.  Manipulate the throttle of the incoming generator which ever direction is necessary to slow the flickering.  As the speeds become almost equal, the lamps will stay off for a long period of time and then slowly start lighting, slowly get fully bright and then slowly dim again.  You want to manipulate the throttle until the lamps are off for as long as possible.  You want to close the breaker when the voltage between the generator is the least.  Since the lamps will go out before the voltage reaches zero, you'll want to mentally time the period between going out and coming back on again and close the switch about in the middle.  Once the breaker is closed, the generators are locked together.  Indeed, you could close the throttle of one engine and the coupled generator will motor the engine at precisely the sync speed (3600 RPM for small gens).  If the generators are just a little bit out of phase, then they will be yanked into phase as momentary heavy current flows between them.  And if you close it out of phase, then you have a double voltage short circuit.  Usually there is severe mechanical and electrical damage.  (I heard and saw the results of a 50 MW diesel genset being synched 180 degrees out as the result of reversed leads on the synchroscope.  Literally ripped the stator out of the foundation and twisted the shaft.)  Once the generators are in parallel, the load accepted by each generator is governed by the governor setting.  The generator with the most throttle will accept the most load.  On larger generators, the field excitation is manipulated to control VARs but you don't have that control and so you have to accept what you get.  Actually, the idea of using more than one generator is very good if you only occasionally have a heavy load to drive.  If you bought a generator large enough to run this occasional load, then most of the time it would be running very lightly loaded and thus very inefficiently.  Cranking the second generator for only those occasions when the large load is needed is a good solution.


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