I have an application for a diesel
I have an application for a diesel
(OP)
I have an application for a diesel generator which calls for electric heating as a means of heat treatment on welded joints. The job in question is a large pressure vessel which requires about 75 kW in local heating. The heater elements are ni-chrome wires wound in-situ with suitable glass wool insulation and the resistance ( hence kW )may vary from joint to joint. Each heater element with a rating varying from 1 kW to 6 kW is controlled by a 'simmerstat' ( % ON time control ) and a contactor. Since the heaters are all single phase ( 240V ) and are randomly on & off, it appears to me a nightmare as to how to connect the load so as to achieve phase balancing. Further the application requires non-stop heating for 72 hours so that a continuous power source in the form of a diesel genset is envisaged.
Finally my question ! :
What would be the safe rating for a dg set for a 100% resistive load with a totally unbalanced load ?
Finally my question ! :
What would be the safe rating for a dg set for a 100% resistive load with a totally unbalanced load ?






RE: I have an application for a diesel
RE: I have an application for a diesel
RE: I have an application for a diesel
RE: I have an application for a diesel
As a practical experince , you can use a 3 phase generator with 75% derating.
But please take care of starting currents
rgds
RE: I have an application for a diesel
RE: I have an application for a diesel
you said that the heaters are single phase..then you should be able to rearrange them as balance as possible and form a delta configuration..
dydt
RE: I have an application for a diesel
75000W/480V/√3 ≈ 90 amperes
{The labeled stator current will be higher, based on kVA, but in this case the load is purely resistive.}
So then, the 1ø heaters, in whatever combination, load one phase to 90 amps, and, say, only 50 amps on the other two phases. Is this risking thermal or any other damage to the stator?
RE: I have an application for a diesel
Graeme38, yes, the distribution will be designed to form equal kW load on each phase as far as possible. But the nature of individual load control ( simmerstat )is such that it may be on or off for intervals of upto 5-10 minutes
dydt, No. it is not possible to connect a heater in a delta configuration as the individual heaters are wound in situ over the welded joints and only 2 lead wires are brought out. To make a delta connection, at least three heaters will have to linked and they may not be close by.
RE: I have an application for a diesel
RE: I have an application for a diesel
The best way to handle this is to go for single phase power generation by either a single phase generator (probably hard to get) or a three phase generator with a power transformer (3 phase primary, 1 phase secondary).
I realize this is not the cheapest option, but the advantage is that you don't have to select an oversized generator in order to withstand the expected unbalanced loads.
Hope this helps.
RE: I have an application for a diesel
As we are dealing with resistance heating elements, how about connecting the generator to a 3-phase rectifier bridge and then connecting the load to the DC output? Can this work with the existing controls? All AC load would be balanced for this configuration.
RE: I have an application for a diesel
Great thinking, and in my opinion this is the ultimate solution!!! My compliments for it.
Note:
Don't forget to calculate the power loss (heat dissipation)of the rectifiers.