Voltage drop caused by motor starting

[lume7006]

Hello,

Is there any standard or practical recommendation to limit the magnitude and duration of a voltage drop caused by a motor starting?

We have done some analysis and the voltage drop is 12%, however, our customer says it is not significative if not forbiden by a standard.

So we have explained some effects on motors, the limits given by IEEE Emerald book and so on, however, we are looking for something more "specific"

I thank you in advance

 

[ScottyUK]

Is the 12% drop at the motor terminals or at the point of common coupling? Normally the PCC would be at a busbar.


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

It depends on what your concern is. In terms of starting and accelerating the motor, NEMA MG-1 requires that the motor be able to start at 90% of NAMEPLATE voltage - the % drop is not an issue.

As for running motors on the same bus, again it is the actual voltage that is the main concern, not the % drop.

If you are concerned with voltage dip and flicker imposed on other utility customers or others, then there are different concerns.

You also have to consider how often the motor will be started.

For a typical industrial facility, I would not be too concerned about a temporary dip of 12% for infrequent starting of a large motor.

 

[lume7006]

Hello

I thank your answers.

My concern is related to the effect on the motors and other equipments.
11% is on the busbar but, at motor terminals it is higher, besides the starting time is around 15 s, because it is a full voltage starting (100% of nominal voltage).
During these 15 s, all the other motors and loads connected on the MCC's will experience a voltage drop (sag) that may affect them.

This is our concern!

Related to the starting frequency, at least once per day!

Best regards

 

[burnt2x]

Simply, an 11% drop in voltage means an 11% increase of current to those motors that are running at the time you start that big motor! Expect motors that are on-line to draw more current if the supply voltage drops: P = sqrt3 x voltage x amps x PF. If the voltage drops, the amps compensate to deliver the same shaft power! If you have a big load, that would mean some dollars down the drain. Your customer must have calculated it's no big deal or he/she has deep pockets to say he/she can live with that situation. It is best to speak to the owners in dollar terms than talk about regs. Been there, done that, and people listen well when you talk about money they will loose if they don't act.

 

[electricpete]

I'd think any increase in I^2^R due to voltage drop during motor start would be a minor consideration due to the short duration.

Some plants may have loads that are particularly sensitive to voltage droop. Perhaps contactors/relays may drop out or electronics may respond unexpectedly. Depends what those other loads are and whether sensitive loads have voltage regulating supply tranformer.

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(2B)+(2B)' ?

 

[7anoter4]

It depends which standard you have to keep.
For instance IEC 60034-1:
"A machine shall be capable of performing its primary function, as specified in Table 3,
continuously within zone A, but need not comply fully with its performance at rated voltage
and frequency (see rating point in Figures 11 and 12), and may exhibit some deviations".
Table 3 primary function for a.c. motor=rated torque[Nm]
Zone A [granted performances] frequency minimum=0.96 voltage minimum 0.95 from rated
Zone B [temporary admitted]] frequency minimum=0.95 voltage minimum 0.90 from rated
As a designer I should not dare to take more than 10% temporary deviation from rated motor voltage at any running motor terminals
[when any other motor will start].
If Utility will supply %5 less ,sometime, you'll get 15% temporarily.
But I saw 15% drop [at a running motor terminals from the beginning temporarily]-and all was o.k.

 

[jraef]

I read this as being that you already know the specifics of what would happen and are duly concerned, but you need ammunition in the form of industry or other standards that you can use to convince your customer that they need to be concerned. It's an age-old problem: people can be shown the right path but they refuse to walk it unless someone puts a gun to their head. That's why we end up with a lot of people putting guns at our heads...

I believe you may have an ally in your power utility (assuming you have one and are not generating on-site). Here in the US, Utility regulations, generally in the form of local Public Utility Commissions, call for THEM to deliver power +-5% to their customers. So if you are causing a sag that shows up as 5.1% at the NEXT DOOR NEIGHBOR's facility, the utility may / will force you, in the form of penalties, to limit your starting power draw. I would call them and elicit their help, I'm sure they can come up with a convincing argument, like a gun to his head...

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