start up current
start up current
(OP)
the system has one generator. How much is reserve power so that I can start up a large motor? Ex: generator: 480kW, motor: 110kW.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: start up current
RE: start up current
JRaef.com
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RE: start up current
you have a motor: P=110kW -> S=138 KVA at nominal running.
when starting : S=(5to7)x 138 = 690 to 966 KVA needed !it but yoy have just a generator about 480 KW-> 600 KVA.
i thing you need to change the starting mode or buying a larger generator.
RE: start up current
If the generator has other loads the minimum sizing is generally 3:1
However, each instance should be evaluated by someone familiar with generator applications.
respectfully
RE: start up current
RE: start up current
I agree with you.
I have never started one on a generator, but a high slip motor such as are used for shears may use less power starting than full load power.
My experience with motors and generators has been mostly with air conditioners, which unfortunately do draw more power (real power) starting than running.
When we try to start a motor on an undersized generator, it usually follows the following sequence:
1> The motor load overloads the engine and pulls the frequency down.
2> As the frequency drops below about 58Hz. the Under Frequency Roll Off feature of the voltage regulator starts to lower the voltage setpoint proportionally.
3> The voltage and frequency drop to a level that allows the contactor in the automatic transfer switch to drop out. It does this with considerable current flowing and creates a substantial arc.
Note: As long as the frequency is below 59 or 60 Hz. the governor on the engine will be wide open. The engine will be producing maximum power throughout this sequence.
4> With no load, the engine accelerates and the voltage and frequency build up.
5> When the voltage is sufficient the contactor closes. (Note, it may be the contactor or the contactor control relay cycling on low voltage. It doesn't matter, the effect is the same.)
6> The cycle repeats until the contactor is destroyed.
Note: This sequence applies to a small set with a relay controlled transfer switch with no time delays. With a more sophisticated transfer switch, there are some time delays added to the sequence. Also the engine alternator typically reaches 60Hz. and the governor closes to the high idle position before the contactor closes.
The end result is that it takes a little longer to destroy the contactor with a more sophisticated transfer switch.
I have found by trial and error that the minimum reserve capacity to start an A/C unit is about 2:1 There will be excessive voltage drop and some frequency drop but it will start without causing damage.
At 3:1 there is some voltage drop and little frequency drop.
Some years ago there were a number of gensets sold in Central America that were too small for the number of connected A/C units.
When word spread that I knew something about gen-sets, I got to see most of them. It was a process of:
1> Experiment to see how many A/Cs the set would start before it overloaded.
2> Timer circuits and electrically latched circuits to prevent the A/Cs from starting simultaneously, and in some cases to prevent automatic restarting. (This was a challenge, because the customers often wanted the sets in the sleeping areas to restart automatically, but were willing to manually restart A/Cs in other areas.)(I think you would have enjoyed this one Keith.)
3> Customer education.
It is on this adventure with about 12 sets that I base my recommendation:
Less than 2:1 capacity and you are in for problems.
More than 3:1 capacity and you are safe.
Between 2:1 and 3:1, it depends.
I have used these guidelines for sizing many more sets with no problems whatsoever.
respectfully
RE: start up current
>>The power factor of a starting motor is very low, lots of reactive power, but not a great increase in real power above running power. Depending on the load type the starting (real) power could be well below the running (real) power even though the starting current is well above the running current.<<
What could be a typical PF of 4-ton central A/C at startup?
Another question is: could a genset's circuit breaker trip by instanteneous startup surge current even if average real power is not exceeding its rating?
RE: start up current
RE: start up current