Starting Current

[magneticted]

HV Motor Starting current or locked rotor current, in the project i'm involved with the current has to be limited to a certain value when machine is started by DOL. However the utiliy company is stating no "flicker" and is talking about peak starting current. But i'm am write in assumming the peak current cannot not really be measured, and we can only measure the starting current by locking the rotor and measuring the current at reduced voltage, and by exploration to rated voltage we can then calculate the starting current.

 

[electricpete]

I think you need to clarify with your utility.

The ratio of peak to rms for sinusoidal current is sqrt(2) as you know.

A worst-case current starting waveform can include a decaying offset portion with same magnitude as the pk/0 of the locked rotor.

i.e. a current waveform something like
i(t)=sqrt(2)*ILRCRMS * [exp(-sigmt*t)+cos(w*t)]

For this worst-case waveform, the ratio of peak to rms locked rotor current is 2*sqrt(2).

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[weh3]

What is DOL? Direct on line?

The utility should not see any flicker or sag unless they are small time or have extremely long transmissions at relatively low voltage.

The flicker depends on utility impedance, transformer impedance, and load impedance at starting. The larger the transformer, the lower real impedance for a given percentage impedance, and the lower your flicker. Using a soft start is usually preferred.

I imagine that by "peak starting current" is meant "peak RMS starting current", since metering measures the RMS anyway.

Regards,
William

 

[jraef]

Generally, the utility is not directly concerned with peak currents, starting or instantaneous, other than as a component of starting kVA. It is kVA that they usually require limits on in order to avoid flicker, yet sometimes they will equate that to a current value out of convenience since not all motor nameplates give starting kVA information. Find out what their peak starting kVA requirements are, and see if your motor manufacturer can provide that information.

And by the way, you can get soft starters for medium voltage motors now (assuming you meant MV when you posted HV, since there are few motors at >15kV around).

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

 

[Marke]

Hello magneticted

When you start a motor DOL, you wil get an initial inrush current into the motor until the magnetic field is established. This is like the inrush curent into a transformer and lasts a very short time. The current will then drop to the Locked rotor current and slowly drop as the motor accelerates to full speed. Typically, the current will not drop significanlty from the LRC until the motor reaches around 80% of full speed.

The peak current the the utility company is concerned about is probably the LRC, but could be the inrush current as well. Flicker is related to magnitude, rate of change and frequency of occurence.

If this is an issue, consider the use of an MV soft starter.

Best regards,
Mark.

Mark Empson

http://www.lmphotonics.com

 

[dpc]

Utilities often try a "one size fits all" approach to voltage dip and flicker issues, especially when it comes to motor starting. In reality, each situation is different.

Flicker is a valid concern if motor is started fairly frequently and there are other customers who would be impacted.

And the peak current can certainly be measured, with the proper equipment.

 

[electricpete]

Good comments. I agree with those who said there doesn't seem to be much reason for utility would be interested in peak current... nevertheless I don't see any other reason they would use the term peak. Certainly requires clarification.

Under the assumption that you are talking about peak starting current, the terminology is a little tricky and I personally would avoid ever trying to represent the true peak with anything resembling an rms number. The relationship between rms and peak before in presence of the exponentially decaying offset transient is not just the normal sqrt(2) conversion that we associate with sinusoidal current.

One comment on attempting to measure peak you should remember it would vary between trials depending on phase at time of closing.

I don't see any reason to attempt to measure it given you can estimate the upper bound as above.

Also the original poster talked about measuring LRC.. no need to do that either - it can be determined pretty closely from the nameplate KVA/HP letter. It of course has to be known for the relay settings.

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[rbulsara]

While a clarification may be in order, here the term "peak " may have been used to mean maximum, just like a peak demand, a common term for utilities.

It is not unusual for utilities to ask for minimizing peak starting currents to minimize flicker. I have personally run into such problem, even after the utility reviewed the drawings and we had a autotransformer type reduced voltage starter on a 600HP, 5kV motor. It did cause some flicker in a plant about a mile or two away. Not all utility grids are that strong, at least not all sections of the grid.

As for the maximum starting current, it should be known by the Code letter or the manufacturer’s data of the motor which should give the starting KVA and hence the maximum starting current. The starting current will be reduced in proportion to the square of the reduction in voltage and so will the starting torque. So if you choose a reduced voltage starter, also evaluate if the motor will produce adequate starting torque.

 

[electricpete]

Good comments.

A minor correction - starting current goes in proportion to voltage to the first power. You're right about the torque.

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[rbulsara]

electricpete:

Yes, thank you for pointing that out and you are correct.

 

[magneticted]

Thanks to you all, I now have a better understanding, will talk to the utility company for clarifications.

 

[Achor123]

The Utility is likely concerned about other customers or clients who would be impacted with the initial starting current of your motor.It is usually the case like others have written that there is an inrush current called starting current when you start a motor and this is usually much higher that the running current.I believe the peak curent can be measured if you have an idea of the running currnet which you can easily measure or look up on the motor rating .To be on the safe side,you could assume that the peak current(starting current)is 10 times the running current. say the running current is 5Amps,you could then set your ammeter to measure a current of 50Amps(to avoid blowing the ammeter).Start the motor and keep your eyes glued to the meter.The meter dial will shoot up to a maximum and drop.This maximum is the starting current.So you could then get a suitable MV soft starter as others suggested.

 

[GGOSS]

One of the statements in the original post conserns me a little. "In the project i'm involved with the current has to be limited to a certain value when machine is started by DOL".

Assuming a stiff supply, the DOL starting current will be equal to motor LRC irrespective of motor loading. It cannot be controlled/limited in any way! If the utility is saying to you that you must limit the starting current to a level below motor LRC, then some form of reduced voltage starting strategy will need to be employed. The type of RVS used will of-course be influenced by a number of technical and commercial considerations. Electronic soft starters are always a good choice but not necessarily the best choice in every application.

Regards,
GGOSS