we have 2 machines with a motor on each which has a star delta starter.they are identical motors, driving an identical load(variable and constant hydraulic pump) but machine A pulls 15 amps at idle wher machine B pulls 25 Amps. under full load this reverses so machine A pulls 65 amps and machine b pulls only 35! these machines run an identical product with an identical program.
there is no hydraulic leakage .
could there be a problem with the way the motors are wired.ie incorect phase rotation?....they r both changing from star to delta....please help

 

Resonding to Pat Hutchinson's query:

1) What are the corresponding ph-to-ph voltages?

2) Are they VFD driven?

3) Are the pumps constant displacement?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected]) {[email protected]}

 

If these are servo-valve controlled diplacement pumps (swash-plate) I would check the analog signal going to the servo. If it is incorrect the pump stroke will not return neutral, therefore causing the motor to continue to 'work'. Check your system pressure gauges, and pilot pressure supply for the servo valves. If the analog signal is wrong that would explain why under load the amps on A & B reverse also being that the pump B never achieves full stroke.

 

1) it would depend on wheather it was hooked up in a Y or a Delta connection.

2) In a purely induction motor that has its stator winding in parallel with its rotor, increasing current through the stator winding will only increase counter emf towards the rotor winding. The hole purpose of VFD's if to increase frequency with voltage. Also know as your voltage to hertz ratio.

 

Replying to Anonymous' Apr 21, 11:42 pm comments:

1) Your statement is unclear. Are you talking about the connection following the start-to-run transition? Or, that the transfer is unsuccessful, inadvertently leaving the stator wye connected? The original post mentioned the "strange" Amps when running at full load... not during runup.

2) What does "stator winding in parallel with its rotor" mean?

To Pat H:
Have you compared the two motor currents when disconnected from their driven machines?

Regards,
Phil Corso, PE