Power Distribution and Excessive Voltage Drops 3

[noel0589]

Hi all,

I was wondering if anyone could enlighten on the following.
Why do some building power supplies have more voltage drop during motor starting than others? Lets say one 50HP motor causes momentary 5% voltage drop due to transient current surge during y-d transfer. Then lets say in another building the same 50HP motor causes momentary 15% voltage drop during y-d transfer.
What are some things that would lead to a weaker voltage supply in a building. Does it have to do with the transformer? Does it have to do with certain utility grids?
Thanks!!

 

[itsmoked]

Hi noel0589; It could definitely have something to do with transformers! Not specific transformers but the fact that in each building the loads are likely different. One building may have a bunch of PCs in it which loads the building's transformer more than the one next door. Also the motors may have different loads. If one has a larger inertial load the transients it causes will be greater even if they are both 50hp.

 

[itsmoked]

It also depends on where you are measuring the building voltages. Just a different location of measurement could make a large difference.

 

[noel0589]

So does it just mean that power can be "weak" (meaning voltage can drop more during motor start) not just between buildings but even within a building? Are there any remedies to this or is it just a "fact of life" type deal?

 

[itsmoked]

Sure there are remedies of all sorts! and... costs.

It is not possible really to tell you what remedy would be required in specific case with out a WHOLE bunch more info.

You need to focus on the building with the problem and tell us as much as you can about the general layout(office? manufacturing?) and the specific complaint(lights go out?),
for us to help you.

 

[jraef]

Noel0589,
Voltage drop from motor starting in a facility is caused by a combination of many factors, none of which can be boiled down to being called the "main" reason. Transformer size vs loading, transformer impedance, circuit impedance due to cable length, cable and/or bus size, types of loads, utility source impedance, short circuit capacity, transient reactance (if it is a site generator supply), connection topology of loads, motor starting methodologies, accelerating torque requirements of the connected load etc. etc.

There are very complex and highly accurate software programs available that take everything into account to analyze your system, such as SKM's Transient Motor Starting program, but there are other less complex (and less expensive) packages available as well. Here is one I use frequently, from a well respected contributor to this forum. Busbar.exe link. It is very inexpensive and covers most of the common issues. It doesn't necessarily analyze what is causing the problem, but it will work to see what your system is doing, and I have used it in "reverse engineering mode" to find a solution by changing system parameters until I get what I need, then recommending that change to the user (i.e. upsize transformer or reduce starting current etc.).

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Barry1961]

Un-balanced voltage can cause high current and more voltage drop also.

Barry1961

 

[Marke]

Hello Noel0589

In addition to all the good advice above, there can also be a difference in the starting current due to the way the starter is set up and the actual motor curves. When you mention the same motor, do you mean an identical motor or one of a similar size?
There are big differences between motors in terms of their starting characteristics and this can result in very different results in apparently identical situations.

The voltage drop is going to be a function of the supply impedance (as covered above) and the starting current.

The start current is determined by the minimum start torque requried by the driven load, the current required by the motor to develop that start torque, and the ability of the starter to control the current to that level.
In the case of the star/delta starter, you have two settings, star and delta. If the motor does not develop enough torque to get the motor up to full speed in star, then is will start the motor in Delta and the current will increase by a factor of three.
ASdditionally, if the timer is set for too short a start time, the motor will be switched to delta before it has had time to reach full speed and that will also cause a full voltage start and three times the start current.

Areas to look at are the starting conditions, i.e. a compressor should be offloaded at start to reduce the start torque, and the timer setting. The change over should not occur until the motor has almost reached full speed. If the motor can not reach full speed in star, then you either have to use a motor that provides a higher torque in star, or reduce the start torque required by the driven load, or change to a starter that can provide a higher start current for this installation. The soft starter is able to supply any start voltage less than line voltage and to stepplessly change as well so it is becoming a popular option.

Best regards,


Mark Empson

http://www.lmphotonics.com