staging RTU's to downsizes a generator

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dannyrzk

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More than often we oversize emergency generators in our calculation, base specifically on the inrush current tripping the generator by turning all RTU's at he same time. A way of solving that problem I believe can be using a time delay relay on each HVAC, that way they won't start up all at the same time. This solution came after Value engineering the project.
now here is my question:
As the RTU's are starting up in sequence, how much should I consider the inrush current of that last RTU in the sequence? do I have to take in account the LRA of the unit for my sizing?
after a regular breaker is design to sustain that inrush current, I'm not sure the generator is design the same way.

Thanks
 
The Locked Rotor Amps of a motor begin to fall off as soon as the shaft begins to turn, at which point 'accelerating amps' may be a better term. In any case, they are completely gone once the motor reaches normal speed and loading. Inrush current is a common phrase for the current needed to set up the magnetic field of the motor, it very quickly drops off to become the LRA.

Assuming your staggered timing allows each unit to 'get up to speed' before the next one starts, your idea should work.
 
Are the RTUs centrally controlled by an EMS or individually controlled directly by thermostats?

In the work I do RTUs are usually centrally controlled so it is a simple programming entry to have them delay and start in sequence after a power down. We could even make them start just the fan before starting the compressors.
 
those are interesting comments, about the RTU controls. On this project (is small) we are using an EMS but wall mounted thermostat. I think I would like he mechanical designer to explore using "time Delaying" thermostat. Which bring me to another question.

if the RTU's are operating under the emergency generator, then the thermostat will control the units, completely separating it of my initial idea of using relays with time delay. Can that be a recipe for disaster or do Thermostat are smart enough to sense the load and not start all at the same time, or even a couple of them at the same time???:(


Also I'm printing that article from EC&M, it appear it have a lot good stuff on it!! :D

Thanks
 
Dannyrzk,

What you have described is often done to reaccelerate very large industrial plant loads following an interruption of supply. In most cases the effect of accelerating a 'new' load having a significant inrush, while a 'running-load' is on-line, nil! However, the situation can't ignored! Success is quite dependent on the capacity of the source, and the current-path capability associated with the new load!

If you are interested in an Excelsheet to carry-out the calculation, contact me!

Regards, Phil Corso
 
Concurrent Starts

Concurrent Starts

How many RTUs could possibly start simultaneously
after the initial time delay. I have had a pumping station
(with staggered start upon utility failure) almost take down
the emergency generator when there was a call to run for
both pumps. This was supposedly a "statistically impossible"
scenario. What is your contingency for several starts after
the time delay on transfer to emergency? It is highly unlikely
to have call to run of multiple RTUs, but certainly possible.

Our solution was delay on make combined with delay on break
relays to solve start up, and possible simultaneous calls to run.
These relays were interlocked also to prevent accidental double
start by the operators in the control room since the pumps were
approxamately a mile away.

Your plan of delays is the right way to go about doing this without
the need for a generator that can start everything at once.

So it can be done, and good luck

JR
 
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