Displaced Phase Excitation Collapsing Under Load

[JohnMcNutt]

I had a case recently where a customer called me out for what sounded like a control problem.

It turned out they had a 3 hp motor with a very heavy blower impeller they were attempting to run off an 8 kW no-name genset wound and regulated in the Honda style. That is DPE to power the AVR and direct field out through brushes.

When the motor was connected, the output voltage of the generator would collapse to almost nil and the blower would turn very slowly. The prime mover continued to run as if nothing had happened. I surmise the power to the field was being dragged down such that it was losing excitation with this load connected.

I tested the generator with a load bank and it easily powered well above its rating on a resistive load. The blower was moved to a 20 kw diesel generator for power and it functioned normally there though the customer did comment on how hard it lugged the diesel down when starting DOL.

My question is this: I thought the purpose of DPE was to continue to provide power to the field even if the main windings had been temporarily overloaded. Why the total collapse? Does the DPE have enough interaction with the main windings that hitting it like this will pull it down too? Or is it possible this no name set is not really using DPE but just powering the AVR direct off the main windings which I did not check for?

 

[wayne440]

I claim no specific expertise, but do have have some experience with small generators. The reason for the "total collapse" was probably because the set was overloaded by the far too large DOL motor starting load. The DPE winding would be affected by said overload, since it shares the stator with the output windings. Situations like yours are one reason why PMG exciters are useful.

 

[waross]

AVR and brushes sounds like the generator is using main power to power the AVR and through that the field. Somw 8 kW sets will start a 3 HP motor but some won't.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

Try to connect motor to genset and then star-up genset; even it's not a normal procedure, may avoid AVR/genset colapse.