Acceptable Voltage dip

[matthewm1965]

When doing calculations for voltage drop when motor starting, I have always used figures of 20% at motor terminals or 15% at the switchboards as maximum allowable.

Can anyone tell me if there is a British Standard that actually defines the allowable voltage drop and time periods when motor starting?

Many thanks
Matt

 

[ScottyUK]

I'm not aware of a limit, but it would likely be somewhere in the in BS EN 60034 set of standards for rotating electrical machines. I'm at home and don't have access to all the standards right now. A lot depends on the driven load - a centrifugal blower will be a lot more tolerant than a reciprocating compressor, for example. In some instances you could consider the cable as a crude 'soft starter'.

There are limits within which the machine should be capable of delivering its rated power output and rated starting torque, but that is a slightly different question. I think there's a voltage-frequency chart in 60034-1 and the machine should be capable of meeting its design spec over +/-5% voltage and in the range 49 - 51Hz although possibly not both extremes simultaneously: 95% voltage at 51Hz would result in moderate underfluxing and high slip, and 105% voltage at 49Hz would result in stator iron saturation and overheating.


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

 

[matthewm1965]

Thanks Scotty,
I was thinking more of the dip on the switchboards affecting other equipment when using a weak supply such as a generator. Contactor mfrs often quote figures of acceptable control voltage before they drop out.
Voltage dips can cause re-strikes on sodium lighting etc.
I was wondering if these limits were defined anywhere or just different mfrs recommendations?

 

[ScottyUK]

If it's a private network then it's largely down to your discretion. BS EN 60947-4-1 defines performance requirements for electro-mechanical contactors including pull-in, hold-in and drop-out voltages. There probably is a standard for discharge lighting although I don't know which one it is.

The Energy Networks Association publishes documents on behalf of the distribution companies: Engineering Reccomendation P28 details the planning limits for disturbances to the public system caused by consumers, P29 deals with planning limits for voltage imbalance caused by consumers, G5/4 deals with planning limits for voltage harmonic distortion caused by consumers equipment.


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