Identifying Winding Pairs

[Grubbs1960]

I guess the problem you are having is trying to control an induction motor, during the starting process, by using frequency, without reducing the voltage to limit the reactive current. The reactive magnetising current that must flow, is limited by the inductance of the motor windings, and the higher supply frequency.

Applied motor voltage, V, volts. Motor inductance, Lm, henry. Supply frequency, f, Hz.
Motor reactance, XL = 2⋅π⋅f⋅Lm
Reactive current, Ix = V / XL
Keep f high, and V low, to limit the reactive current. You should not be operating it at low frequencies on full voltage, because then the reactive current will be high, so the power factor will be horrible for the VFD.

If I understand this correctly, you are suggesting that I can reduce the amp draw at the lower frequencies by lowering the output voltage from the VFD. I will give this a go.

Select your speed range by choice of Dahlander winding and connection. Then to get closer to the wanted operating speed, use VFD frequency, but stick to say the 50Hz to 100Hz window. Any Dahlander connection switching should be selected before starting, perhaps changing the connection while running, should automatically drop a latched contactor to, or from, the VFD.

If I lower the voltage, do I need to keep the frequency above 60 Hz or does the lower voltage allow even lower frequency to be used?

It is an induction motor, not a synchronous motor, so slip during start is not a problem, it is a blessing. When starting, there will be no cutter load, so you might hit it with 60 Hz immediately. If the start current is too high while it initially slips, then lower the starting voltage, not the frequency.

When running, keep the operating voltage below the plate specification, stick to the 200v to 220v range.

I did lower the output voltage to 220v. The amp draw did decrease slightly to 2.3 amps. At 205 volts the amp draw was 2.1 amps. These tests were both conducted under no cutting load.

 

[Grubbs1960]

I agree, set the motor rated voltage to what it says on the plate.

Done, see my reply above to Baluncore.

For the 600 rpm mode problem (and maybe the others), It might be worth trying simple V/f mode instead of sensorless vector control. During ramp-up to speed, it will keep the V/f ratio (flux) constant and not try to be too clever. I have had nuisance tripping in SVC mode on an old, heavy motor. Set P-51 to 1, if I remember rightly.

During discussion with the VFD tech, he suggested the same thing, so it was in that mode when I was reporting the results in my post #58.

You have to have some ramp-up time (say, 3-5 seconds to avoid impatience) or the drive will trip on overcurrent. Ditto ramp-down, or you’ll get an overvoltage trip.

I have had set the ramp up time to 5 seconds for all of the tests previously discussed.

 

[Baluncore]

If I understand this correctly, you are suggesting that I can reduce the amp draw at the lower frequencies by lowering the output voltage from the VFD. I will give this a go.

I see no reason to ever use frequencies below 50 Hz, as they will always result in unnecessary reactive currents.

If I lower the voltage, do I need to keep the frequency above 60 Hz or does the lower voltage allow even lower frequency to be used?

Lowering the voltage will allow lower frequencies without over-current. But you do not need to use lower frequencies during the start, the rotor will slip more, so there will simply be a higher frequency induced in the surface of the rotor.

I did lower the output voltage to 220v. The amp draw did decrease slightly to 2.3 amps. At 205 volts the amp draw was 2.1 amps. These tests were both conducted under no cutting load.

Does the current fall, under no load, as you increase the running frequency from 50 Hz to 100 Hz ?

 

[SongDog]

Without a current-limited supply you’d risk overheating the windings, so perhaps careful experimentation with a 12V automotive bulb in series?

Sorry, I can’t find the video. This chap was configuring a 415V Dahlander to accept 240V by changing the series delta to parallel. He used the compass and DC to ensure, for example, that the 1U and 2U pair seen in post #21 were oriented correctly after separating out all 12 winding ends. The stator was placed upright on the bench, and the compass on a wooden block in the middle. As he energised each of the six windings around the clock, the compass needle followed.

An elegant experiment,

 

[Guineafowl]

If I understand this correctly, you are suggesting that I can reduce the amp draw at the lower frequencies by lowering the output voltage from the VFD. I will give this a go.

If in V/f mode, as you state, the VFD will automatically lower the output voltage as you drop the frequency.

As far as I can see, we’re back to gently bumping the overcurrent setting up until the motor/VFD combo performs as expected.

 

[Grubbs1960]

I see no reason to ever use frequencies below 50 Hz, as they will always result in unnecessary reactive currents.


Lowering the voltage will allow lower frequencies without over-current. But you do not need to use lower frequencies during the start, the rotor will slip more, so there will simply be a higher frequency induced in the surface of the rotor.


Does the current fall, under no load, as you increase the running frequency from 50 Hz to 100 Hz ?

 

[Grubbs1960]

Baluncore said:
Does the current fall, under no load, as you increase the running frequency from 50 Hz to 100 Hz ?

Yes. At 50 Hz, the amperage is 2.4A, and remains at 2.4A at 60 Hz. It drops to 1.8A at 75 Hz and to 1.3A at 100Hz.