Gen Size For A 100 Hp motor
Gen Size For A 100 Hp motor
(OP)
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
Thanks





RE: Gen Size For A 100 Hp motor
The alternative would be a soft starter.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Gen Size For A 100 Hp motor
RE: Gen Size For A 100 Hp motor
The issue you are having with your generator/motor is that the motor is trying to draw about 600kVA when you DOL. Your generator protection is being triggered.
A soft starter reduces current by effectively reducing the starting voltage dramatically so the motor's starting current is reduced also.
By starting your generator with the motor contactor closed you are doing something similar to a VFD. You are ramping the voltage and frequency slowly. This definitely limits the starting current.
Another alternative you might try first. If this is a centrifugal pump; Restrict its outlet for the start. This greatly reduces the load on the motor. Do not leave the pump restricted for very long or you will overheat the seals.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Gen Size For A 100 Hp motor
thread237-220645: 2200 HP soft start for fan ? crisis
Keith Cress
kcress - http://www.flaminsystems.com
RE: Gen Size For A 100 Hp motor
RE: Gen Size For A 100 Hp motor
A soft starter is still probably a better idea. Cheap compared to replacing that generator.
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RE: Gen Size For A 100 Hp motor
A 275 KVA set should start a 100 KVA or 100 Hp motor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Gen Size For A 100 Hp motor
Is your DG tripping on overcurrent or undervoltage ?
RE: Gen Size For A 100 Hp motor
Bill
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"Why not the best?"
Jimmy Carter
RE: Gen Size For A 100 Hp motor
RE: Gen Size For A 100 Hp motor
RE: Gen Size For A 100 Hp motor
If you do that and it still won't start the motor, you have other problems that the EE's need to advise on.
rmw
RE: Gen Size For A 100 Hp motor
- If this starting method would have any negative/positve effect on the health of our engine( both long term or short terms) specially with the above provided reduced starting voltage and current?
RE: Gen Size For A 100 Hp motor
A lot of an internal combustion engine's wear occurs during starting when the oil films are not properly set up, etc.
If you are starting and stopping your generator a bunch of times a day, you will be shortening its life due to the many starts.
I'd probably make sure your generator was warmed up fully by starting and running it for a while. Then drag it down to idle close the breaker and bring it back up again.
However waross may have a different take on it.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Gen Size For A 100 Hp motor
RE: Gen Size For A 100 Hp motor
Engine; you're OK Keith. If this is a standby situation and you must start and go without first warming up the set as Keith suggests then I strongly suggest installing block heaters set to keep the engine at 70 or 80 degrees F. If the engine does not have an oil cooler, investigate the possibility of retrofitting one. With the coolant heater keeping the coolant warm, the oil cooler will help to warm the oil on the initial cold start.
If you can't you can't. Life isn't perfect!
Many standby sets have to start cold or cool and imediately withstand an overload. After the initial abuse the set may run at a light load that is below the recommended load for the engine.
Bottom line: A diesel engine on a standby set has a hard life. We do the best we can and then accept the results.
I'm glad to hear that your motor is starting.
For your mechanical guy; Ramping up is probably a lot easier on the engine than DOL starting. When ramping up, I assume that the engine is able to accelerate up to speed. When going DOL I would expect the sudden overload on the engine to pull down the RPMs briefly before the engine is able to recover. Ramp-up is a lot easier on the engine. (It must be. Ramp-up will start the motor and DOL won't!!)
Bill
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"Why not the best?"
Jimmy Carter
RE: Gen Size For A 100 Hp motor
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If we learn from our mistakes I'm getting a great education!
RE: Gen Size For A 100 Hp motor
while the sme dol errticly reduced the engine rpm,overlood the genset und unble to strt the motor.
Still hving the keybord problem probbly will hve the new one tomorrow sorry for inconvinience guys.
RE: Gen Size For A 100 Hp motor
The UFRO (under frequency roll off) feature of the AVR (automatic voltage regulator) will also contribute to the light dimming. As the frequency decreases the UFRO reduces the voltage in an attempt to reduce the kW load and allow the engine to recover. The UFRO also protects inductive components by preventing saturation at lower frequencies and prevents AVR burnout from saturation and over load at lower frequencies.
Bill
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"Why not the best?"
Jimmy Carter
RE: Gen Size For A 100 Hp motor
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!
RE: Gen Size For A 100 Hp motor
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
RE: Gen Size For A 100 Hp motor
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.
RE: Gen Size For A 100 Hp motor
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!