Gen Size For A 100 Hp motor 1

[mash98]

We have a 275kva stand by generator and it doesn't start a 100hp pump motor having DOL starter.Can I connect the motor directly to the generator and than start it? as I have read somewhere that this method can be successful if any member have an experience of such starting kindly let me know.

Thanks

 

[ScottyUK]

mash,

An induction motor at standstill has a really poor power factor of maybe 0.2 - 0.3. It consumes a lot of reactive power which causes high current to flow from the generator, but doesn't transfer much energy into the load. The engine only provides active power, not reactive power, so the high current drawn during the start which is maybe 6 - 8 times normal running current doesn't equate to a 6 - 8x overload on the engine.

I wonder if the AVR is behaving correctly during this start? Knowing the type of governor would be helpful too.

Does the engine shut down during the DOL start or does a breaker trip?


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty;
You are correct, except that I would expect the real power overload on this combination to be near or over the capability of the gen set. I have a customer who regularly starts a motor of about 80HP. on a 275kW set. This is a 3550 RPM motor with a relatively high starting current ratio. The motor starts but the lights dim and the gen-set leans into the motor mounts and blows black smoke until the turbo spools up.
Note, the lights dim a little when the motor is started on the grid also.
A naturally aspirated engine would handle the block loading a little better.
I have had other experiences with gen sets where the protection had to be "softened up" before large motors could be started.
It is not just the real power and the reactive power. It is also natural aspiration versus turbos, AVRs and some voltage collapse versus PMG excitation, gen set rotational inertia, governor response to block loading, and protection settings.
On this size of set, I would expect a droop governor with 3% proportional band. No integral or derivative. The electronic governors that I have seen have similar performance to the mechanical governors but I haven't seen a wide enough variety of electronic governors to be an expert on them.
Running an electronic governed set in parallel with a mechanically governed set worked quite well when the mechanical set was warmed up. When the sets were cold at first start in the morning, we couldn't get the large motors online due to governor lag on the mechanically governed set. After 5 or 10 minutes of warm-up, both the electronic governor and the mechanical tracked well together and shared the load well. The lag of a cold governor would be an additional factor adding to speed and voltage drop (UFRO).

Bill
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"Why not the best?"
Jimmy Carter

 

[JIMGEN]

I would expect the 275kva or 220kw genset to start the 100hp motor across the line. Starting DOL I would expect to see the AVR life span shortened significantly.

Electronic governors are superior to mechanical or hydraulic
mechanical. I can not say now many Woodwards we have tossed in the dumpster.

We just built a set using a 8.1 liter engine with 394hp rated at 275kw. We installed a 360kw alternator without pmg. This set will start a 150hp motor across the line. Then it starts a 75hp across the line which we did plan on soft starting. Then several small motors are started. This unit replaced a 350kw gen set with a 14 liter engine. Our customer saves significant fuel consumption.

 

[ScottyUK]

"Starting DOL I would expect to see the AVR life span shortened significantly."

Really? Can you explain why? I admit most of the AVRs I work with are considerably larger than this entire genset but I am surprised that small ones don't have limiters and protection to prevent life-shortening damage during routine operation. I think I've only seen one AVR blow on a rental set and that was when we had it loaded right up across two lines as a 1-phase primary injection source. We kept testing going with a DC power supply hardwired to the field, then reported the fault afterwards.


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If we learn from our mistakes I'm getting a great education!