2 speed transition

[jimmy2times]

Im familiar with issues of 2speed motor and changing from high to low speed. But is also changing from low to high speed also problematic?

Fan starter has recently had impeller change (blade change and material change)but now protection trips on switch from low to high speed. Not sure if issues existed previously. Now some starts ok but others trip on high speed instantaneously.

Upped instantaneous as a temp arrangement to allow fan to run. Motor running current in high speed is below rated.

Put analyser on current and sees x15 inrush where motor nameplate says 7 x flc.

Could this be similar issue to open transition starter problem issues when motor briefly open circuit? I read a post referring to Westinghouse advice to include timer to allow magnetic flux to decay (1-2 secs) we don’t have this presently

 

[waross]

If it worked before, it is probable that the deceleration rate of the fan has changed.
The different material may have a different inertia.
The blade change may be affecting the fan drag.
If the motor was disconnected, the connections between low speed and high speed may have been rolled.
When a motor is energized with no flux present you will see an inrush that may be as much as 7 times.
If the motor is energized with magnetic flux present and the residual flux is out of phase with the energization flux you will see an extremely high inrush. There may also be a severe torque transient that may be increasing the inrush.
15 inrush times is not unreasonable if the existing magnetic field has not decayed.
First, check that the winding connections have not been rolled.
Second try an adjustable time delay and adjust the delay for the most consistent starting.
There are two target times.
1> The time it takes to the remnant magnetic flux to come into sync with the grid.
2> Enough time for the residual flux to decay.
Option two is easier to hit and is safer. The timing is not as critical.

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

 

[che12345]

Dear Mr jimmy2times

Q1. Im familiar with issues of 2speed motor and changing from high to low speed. But is also changing from low to high speed also problematic?
A1. Please advise the 2-speed motor is with [Dahlander connection] or with [two separate windings]. Also the RPM1 kW1 and RPM2 kW2 ratings. Better still, if you can take a photo of the name-plate. Manufacturer/brand is immaterial [may be blanked up].

Q2. Put analyser on current and sees x15 inrush where motor nameplate says 7 x flc.
A2. For 2-speed motors, there are two values of Ist/I. One for low speed and the other for high speed, respectively. These values are (usually) [not] shown on the name-plate.
FYI: These values are based on the [average starting value with the motor at stationary].

Humble pledge: I will follow up if you are willing to furnish more details as stated above.

Che Kuan Yau (Singapore)

 

[jraef]

I agree with everything Bill said. This sounds like an inertia vs decay issue and you have changed the parameters with the blade change. I would not even bother trying option one though, that’s like trying to predict your spouse’s mood before coming in the door... if you get it right, you were lucky but the consequences of getting it wrong make it not worth attempting.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

And I agree with everything that Jeff said, particularly;
"I would not even bother trying option one though, that’s like trying to predict your spouse’s mood before coming in the door."

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

 

[ScottyUK]

Inrush and locked rotor current are different - inrush is a transient which occurs until a stable rotating magnetic field is established in the stator iron. Locked rotor current is just that - a stable rotating magnetic field in the stator, with a locked rotor. Inrush can be much higher than the 6x FLC / 7x FLC rule-of-thumb value normally applied. Inrush is normally only present for a cycle or two, so you don't see it unless you have a fast analyser or a 'scope.

Last year I took some scope traces of a slow-fast and fast-slow transition, and as I recall the fast-slow transition wasn't particularly aggressive for the fans I was looking at - 30kW 4-pole motor, variable torque Dahlander-connection, belt-driven low speed fan.

 

[jimmy2times]

Yes thanks for correction ScottyUK and other welcome responses.

I was of similar thinking that higher inertia of new impeller may be at play. I guess my comment in relation to Locked rotor current was in concern that to me it seems the motor/shaft line are now possibly experiencing higher mechanical forces than if the motor/fan were allowed to start to the high speed mode directly from standstill. (Note: Direct high speed mode starting is not permitted by control logic)

 

[jraef]

You can't physically get that high (1500%) of a current spike from a mechanical torque transient, it can't be higher than the LRC of the motor, around 600%. It has to be a magnetic field issue, like connecting a generator out of sync with a grid.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr jimmy2times

Q1. Fan starter has recently had impeller change (blade change and material change)but now protection trips on switch from low to high speed. Not sure if issues existed previously. Now some starts ok but others trip on high speed instantaneously.
A1. This is a typical case where the [starting in-rush] current value is [too close] to the instantaneous magnetic trip of the circuit breaker which causes the "nuisance" tripping occasionally.

Q2. Put analyser on current and sees x15 inrush where motor nameplate says 7 x flc.
A2. In this case the tripping is due to the [starting in-rush] current. The name-plate "Code" does [not] come into play nor the blade or material change.

FYI: 1. analyser on current and sees x15 inrush is normal for [in-rush] current value.
2. possible solution is to select a CB with [magnetic trip] that is 6-12 times the CB rating. In case this does not cover the in-rush current, select the next higher current breaker.
3. Take NEC regulation into consideration.

Che Kuan Yau (Singapore)

 

[jimmy2times]

So update on above. Introduced dwell timer. Incremental in 0.1secs steps. Residual motor component found to decay to almost zero in 1,25secs. During tests Set timer from anywhere from o.1 up to 2secs. On this particular fan we did not trip circuit breaker in any transitions to high speed (and quite a few). However don’t know prior history of this particular fan starter if was ever issue beforehand. Circuit breaker happens to be a frame size higher though.

Transition current found to be approx 12-16 times rated current

Identical(ish) to problem starter in OP it has also had some modifications over years not quite sure of full history. Motor data sheet says hispeeed in rush is approx x7. Low speed start inrush also approx x7 and was found to be about 7-8 in reality.

Basically dwell timer made no significant difference to the transition current was our conclusion.

Can’t make same tests on OP motor yet as it is undergoing other maintenance