Voltage Drop During Starting

This is mainly depend on the local grid capbility to allow the voltage dip, not depend on the motor. The Y-delta start method can decrease the voltage dip when start.

Voltage drop on starting is a function of the supply cable sizing. Is British Standard 7671 relevant here?

I recall (not certain) that NEMA (U.S. National Electrical Manufacturers Association) specifies that motors shall be able to start at 85% of rated voltage. In places where the grid is less robust, 80% could make sense; if more robust, 90% could also make sense. It may depend on your locale. "Norsok" = "Norsk"?

Norsok, Norwegian petroleum industry, mentions only:

- Total voltage drop at motor terminals during start shall be maximum 20% (typical power factor 0.2)

It mentions also:

- Voltage drop, stationary and transient, according to IEC 61892-2, clause 4.1.4. Except that 10% is accepted for lighting at last fixture.

But this standard didn't mention about voltage drop during motor starting.

if can't find the standard of voltage drop, you can get the corresponding current standard.

Choosing y- △ start method to decrease start current.

The breaker is designed according to the current.

The cable current is bigger than the breaker calculating current.

Please refer to IEC 60034-Part 12 (Starting Performance of Three-phase and Single-phase cage induction motors) - which in its foot note to Clause 6.1 permits a voltage drop of 10% of the rated voltage at motor terminals during the acceleration period.

Thanks for the info - IEC 60034-12. I usually go with 20% as mentioned in the contract, that's why I think 10% is still too much. Do you know the IEC standard that specifically mentioned about the maximum drop voltage?

Yes it is 20% is the standard we follow in India .Most consultants specify this as a condition

Have you trying asking the motor manufacturer yet?

The reason this is not specified in national standards is because it must include time as a factor and also where in the system this is measured. A small household motor may cause a dip at the washing machine or hand tool terminals for a tenth of a second during startup.

For large machines this is more complex as the power output & torque will be less at reduced volts causing a further rise in current. So from an engineering design point of view it's important to know what is being driven and how long the voltage drop will last during starting. The worst case I have experienced was a piston pump which meets the full mechanical load at the first revolution, and also large fans with slow run up times.

The figures of 10% & 20% for transitory motor starting dips are usually used in general specifications from the consultants rather than in national standards, you can usually satisfy them with a few calculations showing the peak current from the makers charts without reference to the load.

The national standard is more concerned with the sustained voltage drop when running normally.

I don't think they have to include the time when they say 'during starting'

Time cannot be included as it depends on Motor + driven equipment Inertia.Accelerating time is not specified normally.The other factor is when voltage drops the torque reduces by a value which sq of voltage.So if voltage drops by 20% torques will be ).64 of rated torque.This reduced torque will further increase the acc.time.Voltage drop is important as the reduction of Pull out torque due do drop can stall the motor.

I agree with you.

"The national standard is more concerned with the sustained voltage drop when running normally".

The voltage drop caused by cable is concered, but the size of cable need to allow for the peak current.

Decreasing the starting current and adding the cable size are two methods to control the start voltage and let the motor start.

It's better to ask how much voltage drop will not impact the start of motor. During designing, can ask manufacture. First calculating the peak current and check at that time the voltage hasn't dorp than permitted level.If dip than manufacture's permitted level, then change start mode,using y-delter,or softer start (if the mechine over 50KW, nearly never using derect start). If after that the motor still can't start due to lower voltage, then change the cable size, increase the cable size.

The cable size used to check the voltage drop when the start peak current appear is firstly decided by the running normallly current, then being adjusted according the standard gird voltage drop , at last by motor properties.