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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Although the application is to phase shift a synchronous motor, the question is not specific to Tesla coils so I hope no one minds it being here.
I have constructed a simple current phase-shift circuit as described by John Freau. It is a variable inductor (variac) L1 and capacitor C1 in series connected to a power source (variac). The capacitor output connects to one end of a synchronous motor; the other end (grnd) goes to the other lead of the motor. The motor has a running capacitor, C2.
IamSmooth wrote ...
phase shift circuit and synchronous motor
When I view the shifter circuit alone on a scope and monitor the source current and capacitor current I can shift 180deg. I connect the circuit to the motor and begin increasing the variac's (L1) inductance and I observe the current to the motor shift close to 90degrees. Once I hit 90degrees of shift the motor loses synchronization and the voltage and current to the motor (from C1) drops to near zero.
So, I connected my scope to the two magnetic coils on the motor to see what happens. As I start shifting the current to the motor I observed the coil voltages fall until the synch is lost. The variac/capacitor act as a voltage divider.
So, the problem is that current shifter is not an isolated system. At a certain point the currents in the parallel C2-L2||L3 circuit are in phase (and thus the electromagnetic poles are all exactly 180 out of phase) and the rotating magnetic field is lost. The inductor/capacitor of the phase-shifter integrates with the inductors/capacitor of the motor to form a new circuit that doesn't let me get past 90 shift. Is there a simple solution to the problem?
EDIT: I have added two pictures showing the scope tracings. The first is the basic current waveforms (almost 90deg apart) to the pair of motor windings. The second are the currents right before the synchronization is lost. Notice the phase difference is almost gone and the currents are losing their sinusoidal tracing, almost having a double-hump.
IamSmooth wrote ...
currents to motor windings 1 and 2 and right before synch is lost
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The problem with this phase shifter solution is that it is already too simple. Im afraid i dont see any easier way of phase shifting without some inherent drawback of the output voltage falling off (or as you discovered, and unwanted relationship between the motor windings). You can try using a larger capacitor across the motor inputs, but watch out for the extra voltage you will see at the mid range setting of the variac. It may exceed what is safe for the motor.
The most stable solution is to build a solid state motor drive, where phase shift is taken care of in logic, and any phase shift works equally well.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
The problem with the 1800 rpm (30 rps) synch motor is that 90deg of current shift translates to 45deg of flywheel shift. For a 3600 rpm (60 rps) each degree of current shift equals 1 degree of physical shift.
I am probably going to convert an 1800 rpm synchronous motor to a 3600 rpm motor by removing a pair of poles. Has anyone done this?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think that conversion would involve a complete rewind of the stator. It doesn't make any difference anyway, as you need to use half as many flying electrodes on a 3600rpm motor, so the range of timing adjustment ends up the same: 90 of what motor textbooks call "electrical degrees".
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Yup. I'm resigned to doing a complete rewind. I'm acquiring a spare 1800rpm motor since I am having trouble locating a 3600. This should be interesting.
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Phase shifting current problem
