Impact of Motor Starting on Transformer Primary 2

[buddy91082]

Hi all.

This is a general question - when a transformer feeds a motor, does the primary side also see a voltage dip during startup, in ratio with what the secondary experiences? We have a light flicker problem when a motor starts and one solution is to install a transformer ahead of the motor so all other loads are not directly connected to the same bus as the motor.

Thoughts?

thanks.
c

 

[RRaghunath]

Yes, the voltage dip will appear on primary as well. When you introduce a transformer in between the bus and the motor, what actually you are doing is introducing additional impedance in the motor circuit.
This results in lower voltage at motor terminals while starting and it is possible your motor may not be able to pick up full speed due to lower available torque. Torque is proportional to square of the motor terminal voltage.
This needs a detailed study with the connected load, motor torque characteristic as well as to fix the %Z of the captive transformer.

 

[unclebob]

Depends of the application, but a soft-start or a drive would help.

 

[waross]

When a motor starts there may be a voltage drop all the way back to the source.
It depends on the source impedance, the impedance of the primary and secondary conductors, the impedance of any conductors and the starting current.
It depends.
If the source is the grid the source impedance and the source voltage drop will be insignificant.
If the source is a diesel generator that has three times the rating of the motor the source impedance will be considerable.
You may want to consider a reduced current starting arrangement.
A starting impedance is one way of reducing the starting current.
Either a series reactor or a series resistance will reduce the starting current.
A transformer will act as a series reactor.
A reactor may be cheaper than a transformer. I don't know, I would have to check prices of actual equipment.
But,
A reactor may be short time rated and shunted out once the motor starts.
That would reduce the cost of the reactor.
A series impedance will not always allow the motor to start.
It depends on the load characteristics.
That is a general answer to a general question.
If you have a specific issue, then you need to provide specifics.
You need to provide more specific information before we can give you an answer that will justifying spending any money.


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

 

[edison123]

Excellent suggestions from Bill. Follow his advice.

I would look into soft starter or VFD at start (or even run). Adding a trafo won't solve your flicker issue.

Muthu

http://www.edison.co.in

 

[itsmoked]

A transformer is larger than probably any other solution and a transformer will have continuous losses likely even when the motor is not running.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A quick question;
Do you notice the lights going a little brighter when the motor stops?
If so, you will net be able to get rid of all of the light flicker problem.


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

 

[jraef]

Adding a transformer can only make it worse, not better. Look into soft starting or some other form of reduced voltage starting, if your load profile will allow it. If not, you may need to use a VFD.


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

 

[aussiejohn2]

Hi Buddy91082,
What exactly do you mean by "install a transformer ahead of the motor"? Everyone seems to be interpreting it as the motor being fed from the same bus as the rest of the load, the voltage drop being caused by the motor current flowing through impedances upstream of this point, and the transformer (presumably 1:1) added in between. They are right that this will not help, and will make things worse for the started motor.

However there is another possibility: the upstream impedances include the transformer that feeds the problematic bus, and the intent is to add a new transformer to feed only the motor load. This *will* reduce the voltage dip seen by the other loads (for both motor starting and running). The imdpedance of the new transformer must be small enough that the voltage dip at the motor remains acceptable, which may require it to be similar rating to the existing one.


HTH

John.

 

[waross]

Actually, a transformer between the bus and the motor will act as a series impedance and will reduce the starting current a little.
That will reduce the voltage drop a little.
Not worth the money.
There are better solutions.
A dedicated transformer that takes the motor off of the bus completely as suggested by aussiejohn2 is probably a good solution in many cases.
However, if the greatest voltage drop is on the incoming lines this will also be a waste of money.
We don't know what the load is.
A series impedance may not allow the motor to start.

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

 

[HamburgerHelper]

You could install motor starting cap banks that drop out when the motor gets up to speed.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[jraef]

You could install motor starting cap banks that drop out when the motor gets up to speed.

True, but that's one of those things that looks OK on paper but is difficult to properly implement in the field, compared to other technologies that do a better job with less risk.


" 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]

Hi HH; Yes technically that may work.
I first read about that technique about 40 years ago.
I have never seen it in use.
Given the cost of monitoring relays and the contactors as well as the cost of the capacitors, there are probably cheaper ways to get better results.
But, I once worked at an old mill where they had most of the electrics from a decommissioned warship stored in the warehouses.
If you have lots of bits and pieces already on hand, the capacitor method may become cost effective.

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