Hello

If we lost battery power or self starting motor of a 300KVA diesel generator but we have utility power. Is it possible to start the DG set by switching on the main breaker of alternator to allow utility power in to the alternator keeping excitation manually very low, to behave like synchronous AC motor. If somebody want to share experience. Expert opinion of Phil Corso is also requested please.

Regards
Shaheed

 

Shaheed... the first of many problems to overcome is the fact that the typical alternator has insufficient starting torque!

Do you need additional information?

Phil

 

Shaheed... correction:

The first of many problems to overcome is the fact that the typical alternator, used as a synchronous motor, has insufficient starting torque!

 

Shaheed,

There are "alternators" and there are "alternators."

Most reciprocating engines with battery-operated starting motors have a means of charging the battery when the motor is running, and these days those are called alternators, because they actually produce AC that is rectified into DC and used to charge the battery while the reciprocating engine is running. This "alternator" would not have enough torque to serve as a starting motor. These "alternators" have permanent magnets for rotors (many do, anyway) and do not require a separate source of excitation.

A synchronous AC generator is more properly called an "alternator" because it produces AC. It does require a source of power for the generator rotor (excitation), and it would likely have sufficient starting torque.

However, if you were to just close the generator breaker and apply AC from the bus to a stopped alternator, there would probably be a very large noise (explosion-like) coming from the generator set. First, this is called across-the-line starting for an AC motor and it draws a LOT of current from the mains (grid). Second, because the rotor was not spinning there would be a very large induced current on the generator stator components--which would develop a lot of heat very quickly. Third, the sudden jump in speed caused by the starting torque would likely damage the coupling between the reciprocating engine and the "alternator."

Lastly, the engine would be spinning so fast--and possibly even in the wrong direction!--that it would not be controllable by the engine governor.

For such a scheme to work, you would need what's referred to as a "soft starter", or a "static frequency converter" or similar named device, that would reduce the AC mains frequency to a very low frequency to accelerate the "alternator" to the proper starting speed for the governor to then admit fuel and be capable of controlling the acceleration and speed. It would be a very delicate control scheme, and it would also likely require some kind of motor to very slowly start the alternator rotor spinning in the proper direction for the soft starter to sense the proper rotation and apply the low-frequency AC to accelerate the unit to starting speed.

This would require a separate set of breakers for opening the "alternator" ground (they are usually wye-connected with the center-tap grounded through a neutral grounding resistor, and that would have to be opened during "soft starting"), and another breaker for applying the variable frequency AC from the "soft starter". There would have be a control system capable of switching all of the breakers appropriately, and, of course, the soft starter itself.

So, what you are proposing would not be possible without a LOT of additional equipment, and engineering.

Even if the reciprocating engine at your site does not have a battery-charging "alternator", since so many do it was probably confused with the synchronous AC generator, which is more correctly called an "alternator", also, but most people don't refer to the as "alternators", just generators.

And older reciprocating engines did use DC generators for charging the starting battery.

Hope this helps!

 

Thank you for your helpful information. Yes I was asking about starting of reciprocating engine by mean of his connected main generator and as per your clarification it is not an easy task without additional equipment ie breakers, soft starter.

Thank you Mr. Phil C these information were sufficient.

Regards
Shaheed

 

Dear Mr. Phil

In the case of battery backup low to run the starter motor for the Genset, can this self starter motor be powered to run the Genset through an adapter from the source of AC power. This is to recharge the battery by starting the Genset. If possible, what capacity of of DC adapter required equivalent to 12V 100 AH automotive battery, to connect temporarily to starter motor. Please advise.

Thanks & Brgds,
Venkat

 

Venkat... is your query similar to Shaheed's? That is do you want to power the Generator with an AC source so that it, the generator, can be used to charge the battery?

Regards, Phil Corso

 

Venkat,

What you are looking for is what's commonly called a "trickle charger" to keep the 12 VDC battery charged between operation of the Genset.

They are very commonly available for many different types of applications, from automotive batteries to motorcycle batteries. They operate on regular mains (110 or 220 VAC) and put out anywhere from 0.5 to as much as 5.0 amps and reduce their output when the battery is "topped up" to prevent overcharging. The better models go to absolute zero output, while the less expensive models go to some minimum value which can still overcharge the battery over time. The price differential is negligible, so go for the better models.

If the 12 VDC battery is not a "deep cycle" battery (marine batteries are typically deep cycle type batteries) you should consider this type of battery. They hold a charge longer between charges and can recover faster from a low charge than automotive batteries. (Automotive batteries are cheaper, hence they are usually supplied with the equipment or purchased by people who don't understand their purpose--and long-term value....

 

Dear Phil

Yes that is what the requirement.

Brgds,
Venkat

 

VenKat... my response is the same as the one given to Shaheed on Jun 2nd!

If you have an AC supply available to "Start the generator" then why isn't it enough to "Charge the battery?"

Phil

 

Dear Phil

Thanks for your valuable input. Will do
accordingly.

Best Regards,
Venkat