Excitation capacitors for induction generators

[ThomasS]

The 3 phase induction motor as generator IMAG has 

- a minimum capacitance to start self-excitation, and

- a maximum capacitance, beyond which the power output decreases again, because the power is turned into heat inside the generator.

The below calculations assume a resistive user load!

With inductive loads, capacitance needs to be higher. Or you compensate the inductive loads with their own capacitors, so they appear as resistive loads.

The 3 capacitors are connected in delta wiring, from line to line.

Pay attention to the voltage rating of the capacitors.

One source suggests

power          total capacitance (each capacitor is one 3rd of the total)

2,2kW           47 - 100uF total capacitance (15 - 33uF per phase)       //this is a bit more than the next row, because small motors have less efficiency

35 kw            300 - 500 uF

70kW            600 - 1000uF

100kW          850 - 1500uF

150kW          1200 - 2000uF

 

Approximating, the factor in the following equation for total capacitance is between 9 and 15, depending on construction and power factor of the motor.

total C(uF) =  power(kW) * factor 9 to 15

individual Capacitor C(uF) = power(kW) * factor 3 - 5

Power factor

Another way to estimate the required capacitance for a generator:

You have to bring the power factor above 1. Now the motor will be self-exciting.

Most motors have a power factor of 0.7 - 0.8, depending on construction and load.

reactive power KVAR = (1 - 0.7) x kVA

Example calculation:

A generator of 15kW ( = 20KVA) needs a minimum KVAR of 20 kVA x 0.3 =  6 KVAR

 6 KVAR = 120 uF (for 50Hz) -> so you need more than 120uF ( 3 x 40 uF)

With this calculation method, the factor would be minimum 8

total C(uF) =  power(kW) * factor minimum 8

On Site

The main problem with calculating the exact optimum capacitance in advance is, nobody can tell you the exact relevant properties for your motor as generator, because the manufacturer normally does not test and specify these.

So it boils down to some tests with different capacities on site. You can connect capacitors in parallel, to add their capacitance.

Delta Connection

Delta connection of capacitors has some advantages over star connection:

- star connection needs 3 times as much capacitance, because in effect, looking from line to line, the capacitors are connected in series, and go to 2 lines.

- some of the harmonics in the wave form will rotate in the delta circuit, and won't be transmitted over the power line

- if one capacitor fails, you still have 2 thirds of the total capacitance

However, in delta connection, the voltage across the capacitors is higher - > the capacitors need to be rated accordingly.