An Excitation Capacitor is a three terminal gismo that is used as part of a 3 phase Asynchronous Generator System. This type of Generator consists of three integral components, namely a Stator (a type of Transformer), a Rotor (a driven shaft with magnetic properties), and a number of Excitation Capacitors. The Excitation capacitors are used to store the resultant vigor (Charge) that is produced from the Stator Windings (in a Star or 'Y' configuration) as the Rotor turns.
Internally, the Excitation Capacitor consists of three individual capacitors that are connected in such a way that two are in series with a third capacitor in parallel. Due to this arrangement, when measuring the capacitance in the middle of each terminal, the measured value will always be 50% higher than the stated Capacitance of each individual component. For example, a 50uF Excitation Capacitor will read 75uF when a Capacitance Meter is connected in the middle of each pair of terminals.
Basic Microcontroller
Before Testing Excitation Capacitors it is very leading that they are fully discharged. A fully expensed Excitation Capacitor would commonly have hundreds of Volts applied to it, and are capable of storing an large number of Energy. This vigor sits there for a long duration of time, just seeing for a place to go (a extraction path). Failing to safely extraction these Capacitors straight through a favorable Bleed Resistor can cause an galvanic shock hazard, or cause damage to your Test Equipment. For your safety, please do not ignore this Warning!
There are three base methods of testing Excitation Capacitors. They are:
1. What I will call, 'The Poor Mans' method. By performing this type of Functional Test, all that will be determined is either the Capacitors are capable of taking charge. It is not a definitive Test method, but is an very base practice for evaluating functionality of these components. This method should only be used when no other Test equipment is ready apart from a Dvm (Digital Volt Meter) or Vom (Volt Ohm Meter). Assuming that you are using a Dvm.
The Dvm is setup for taking a Resistance measurement. Apply the Test Leads to each Capacitor. The Dvm will initially show a low Ohms reading (towards Zero Ohms). After a small duration of time, the Resistance reading will increase. This is the general characteristic of a charging Capacitor. Perform this test for all three Capacitors. I.e. Place the Test Leads over Terminals 1 - 2, 2 - 3, and 1 - 3. When you do this, what you are seeing is the Capacitor taking charge from the internal battery inside the Dvm. This method relies on the fact that when a voltage is first applied to a Capacitor (at time=0), a Capacitor will appear as a short circuit. Hence, the zero Ohm reading. As time goes on (constrained by the formula: T=Rc), the Capacitor will finally charge to the applied Voltage. For example, if 10Vdc is applied to a Capacitor, after 1 Time Constant the Capacitor would have expensed to roughly 6.3Vdc.
2. Using a Capacitance Meter, apply the Test Leads to each Capacitor. Confirm that the Capacitors value is 50% greater than the specified Capacitance for each individual Capacitor. This Test method will confirm the Capacitance rating of each component (within the tolerance limitations of the component).
3. Using an Esr (Equivalent Series Resistance) Meter or Esr Tester, apply the Test Leads to each Capacitor. If using an Esr Meter, confirm that the Esr is low in association to the type and value of Capacitor being tested. This is commonly correlated from a lookup chart. For a good Capacitor the Esr will generally be less than 3 Ohms (and preferably less than 1 Ohm). If using an Esr Tester, a Microprocessor / Microcontroller shows the status of the component (Good, Bad, or Suspect), thus simplifying Fault Analysis. Capacitor Esr measurements show the internal Resistance of the Capacitor utilizing an Ac Test method. The higher the Esr the greater the internal losses inside the component.
How to Test Asynchronous Generator Excitation Capacitors