Last updated B.Hornby

This program has been written as a quick way to determine if motors can be started sucessfully without causing "flicker" or excessive voltage dip to other customers.

Voltage Drop
Motor starting results in a voltage dip due to the inrush current drawn by the locked rotor. This voltage dip must not exceed 20% of the nominal voltage or there is a risk that the motor will not start. At the point of Common Coupling (PCC) the limit is 5% relative voltage change.

When analysing these problems, the worst case for relative voltage change at PCC occurs at no load. The worst case for voltage dip at the motor terminals happens when no other load is connected.

It is considered acceptable for motors starting up to 4 times per hour to have a voltage drop of up to 5% at the point of common coupling (PCC). For motors starting more than 4 times per hour this percentage change is less. See Figure 7A.1 (Perception of Relative Step Change Voltage Changes) in the Distribution Planning Manual for acceptable voltage drops.

Point of Common Coupling
The Point of Common Coupling is the last point in the distribution network that is common with the customer being investigated and any other customer.

This point could be:

• where the service connects to the LV mains
• the transformer LV (secondary) terminals
• a point between the transformer LV (secondary) terminals and where the service connects to the mains
• the HV (primary) terminals of the transformer
• the 11kV spur connection point

Fault Level (MV.A)
The fault level determines the source impedance at the 11kV/433V transformer supplying the motor. The following table should be used to determine the fault level.

Area	Distance	Fault Level (MV.A)
Rural	> 10 km from Substation	<=20
Outer Urban	> 5 - 10 km from Substation	20 - 40
Urban	< 5 km from Substation	>= 40

Single Phase Calculations
When calculating voltage drop for single phase motors the program assumes that the neutral conductor is the same as the phase conductor.

Examples

If you have questions or comments please contact Brian Hornby