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<< Click to Display Table of Contents >> Analyzer > Appendix > Backup feeding routes > Motor start |
One or several motors can be tested at the same time, as the test of motor start is done for each motor separately.
•ID of the motor (otherwise it cannot be identified at the start of the calculation)
•Rated power (kW)
•Rated power factor (cosfi)
•Rated voltage (kW)
•Efficiency factor (for instance 0.9)
•Inrush current factor for motor start (normally 5 – 6)
Start a network calculation trace and select Advanced settings in the Network calculation form.
Check mark the motors to be part of the calculation (under Motor start objects).
The result is obtained as an extra report in the result archive.
Motor start
When starting an asynchronous motor a voltage drop obtained at the connection point of to the network. This voltage drop can be calculated as follows.
Given data:
Pm |
= Rated power (MW) |
cosφm |
= Rated power factor |
Vm |
= Rated voltage (kV) |
hm |
= Efficiency |
Sm |
= Pm / cosφm = Apparent rated power (MVA) |
Im |
= Sm / Vm = Rated power (kA) |
Ist_vm |
= Start-current (kA) at rated voltage, given as Stfac * Im / hm (where Stfac is a given factor) |
The start current is assumed to be proportional towards the voltage, i.e.
(5.60)
Ist_v / Is_vm = Vm
The relative change of voltage at the motor start can be calculated as:
(5.61)
DV% = Ist_v * Zk / V * 100
where
Zk = Short circuit impedance of the network as seen from the connection point of the motor.
Insertion of (5.60) into (5.61) and extension with Sm / (Vm * Im) gives:
(5.62)
DV% = Sm / V²m / Zk * Ist_vm / Im * 100
…or if the short-circuit power of the network is implemented at the rated voltage of the motor:
Sk_m = V²m / Zk, as seen from the connection point of the motor.
(5.63)
DV% = Sm / Sk_m * Ist_vm / Im * 100
The absolute change of voltage is:
(5.64)
DVkV = DV% * V / 100
…and the resulting voltage will be:
(5.65)
Vres = V (1 - DV% / 100) (kV)