Autotransformer starting current

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mull982

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I was hoping that someone could explain to me why the line current when using an Autotransformer is 25% of the locked rotor current when the autotransformer is tapped at 50%.

I understand that with a 50% voltage tap the current seen by the motor will be 50%. I have also read that the line current (I'm assuing on the line side of the contactor) will be equal to the square of the voltage which in this case will be .5^2 = .25 or 25%. I follow this calculation to result in these numbers but wanted to see if I could get a theoretical answer as to why the lilne current would only be 25%.

Does the line only see the current going through the autotransformer?
 
mull982 said:
I was hoping that someone could explain to me why the line current when using an Autotransformer is 25% of the locked rotor current when the autotransformer is tapped at 50%.

I understand that with a 50% voltage tap the current seen by the motor will be 50%. I have also read that the line current (I'm assuing on the line side of the contactor) will be equal to the square of the voltage which in this case will be .5^2 = .25 or 25%. I follow this calculation to result in these numbers but wanted to see if I could get a theoretical answer as to why the lilne current would only be 25%.

Does the line only see the current going through the autotransformer?
Click to expand...
At 50% volts the motor, on locked rotor would take 50% current (all other things being equal).
Because of the step down ratio of the transformer, the supply would see 50%of that 50%. Hence 25% of the locked rotor current would be when supplied directly.

Here's one we supplied that went a bit wrong because of incorrect installation.
http://uk.youtube.com/watch?v=nibB2c9djCo
 
Besoeker said:
At 50% volts the motor, on locked rotor would take 50% current (all other things being equal).
Because of the step down ratio of the transformer, the supply would see 50%of that 50%. Hence 25% of the locked rotor current would be when supplied directly.

Here's one we supplied that went a bit wrong because of incorrect installation.
http://uk.youtube.com/watch?v=nibB2c9djCo
Click to expand...

Ouch! That was a nasty surprise. What was determined to be the cause?
 
Jraef said:
Ouch! That was a nasty surprise. What was determined to be the cause?
Click to expand...
A surprise, rather expensive, and quite amazing that someone had set up a phone camera that got footage of it.
The cause was probably a sequence of events.
Initially, the cable contractor got input and output the wrong way round so the 11kV supply was connected to the 8kV tap.
Naturally, it didn't quite work as a "reduced" voltage starter....:D
That error was discovered and the cabling was corrected.
Unfortunately, then the bolts used were a bit long, pointing the wrong way, and not sleeved. Clearance to ground was just inadequate. Flash bang wallop........as you see.
 
Besoeker said:
That error was discovered and the cabling was corrected.
Unfortunately, then the bolts used were a bit long, pointing the wrong way, and not sleeved. Clearance to ground was just inadequate. Flash bang wallop........as you see.
Click to expand...
Did the arc blow itself open without tripping anything (other than the fire system)?
 
LarryFine said:
Did the arc blow itself open without tripping anything (other than the fire system)?
Click to expand...
When the transformer was wrongly connected it took out the the external source and much of the Clyde valley region. Probably fewer than half a million people were affected.
The arc took out the local protection and just shut down the plant.
Still telephone numbers of digits for loss of revenue.
It was a messy problem to sort out from a contractual perspective.
 
Ok I see why this is now thanks for the explanation.


The other thing I was trying to figure out, was what the ideal time was for switching from autotransormer to full voltage. I have heard that this switch should be made when the motor is at or above 85% speed, and I have also heard that is should be switched at 125% of FLA. I'm guessing that maybe these two parameters are related or a result of one another.

I'm curious as to why this is the ideal switching point(s). I have heard that anything thing prior to these points would'nt reduce the current by much or would be like having a full voltage start. I have seen in other posts that it has something to do with fulx penetration into the rotor but was looking to get a more complete explanation.

Can anybody explain why this is the ideal switching point or point me to references?
 
mull982 said:
Ok I see why this is now thanks for the explanation.


The other thing I was trying to figure out, was what the ideal time was for switching from autotransormer to full voltage. I have heard that this switch should be made when the motor is at or above 85% speed, and I have also heard that is should be switched at 125% of FLA. I'm guessing that maybe these two parameters are related or a result of one another.
Click to expand...
The 85% is essentially correct, but is actually a middle ground between 80% and 90% speed. The 125% FLA is not the best measure. It would need to be qualified to what taps you were using. Relate everything to LRC, or 600% in most cases. If you are at 50% taps, you apply the ratio discussed above to that as well, so the starting current, as related to LRC, is 25% of 600%, or 150% FLC. So if your current drops to 125% FLC, that would be a decent indicator that the motor accelerated to at least 85% speed. But if you were at the 80% taps, current is 64% so 64% of 600% FLA is 384%. In that case, if the motor current has already dropped down to 125%, it has already pretty much fully accelerated, meaning that you could have probably used a lower tap or switched earlier to accelerate sooner.


I'm curious as to why this is the ideal switching point(s). I have heard that anything thing prior to these points would'nt reduce the current by much or would be like having a full voltage start. I have seen in other posts that it has something to do with fulx penetration into the rotor but was looking to get a more complete explanation.

Can anybody explain why this is the ideal switching point or point me to references?
Click to expand...

The deep explanation has a lot to do with flux propagation in iron cores, magnetic fields etc., pretty much EE stuff that would be a long discussion in here. The lay version is, a motor is like a transformer, but instead of just converting one voltage for another, it also converts power into motion. The motion is reacting with the power as that happens, so the model is more like a variable transformer with the conditions changing based on the number of lines of force being cut by the rotating rotor cage. So when the motor is not turning, the current is at maximum. But as it increases speed, the various interactions of impedance, reactance, reluctance and inductance change and combine to stabilize the current is it performs the work of creating the motion. If you switch too soon, the combined interactions don't work to reduce the current yet.

Here is a decent description;
http://www.lmphotonics.com/m_control.htm
 
What would the current look like if you waited until the motor was completely up to speed at 80% before switching? Would this have any negative effect opposed to switching at 85-90%
 
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