What Is a Capacitor Discharge Unit? Types and Selection Criteria
A capacitor discharge unit is a discharge coil or resistor that quickly drains the voltage left in a power factor correction capacitor after it is switched off.
A capacitor discharge unit is a discharge coil or discharge resistor connected to the capacitor terminals that quickly brings the voltage left in a capacitor down to a safe level when a power factor correction step is switched off. A capacitor stays charged for some time after it is disconnected; if it is reconnected before discharging, it draws a high inrush current and stresses the contactor and the capacitor. A discharge unit lets the step be reconnected sooner and reduces the risk of shock during maintenance.
Types
| Type | How it works | Example in the catalog |
|---|---|---|
| Discharge coil | A low-resistance winding discharges the capacitor quickly; it presents a high impedance at mains frequency, so losses are low in operation | Siemens 4AJ9903-2YC |
| Discharge resistor | A resistor across the capacitor terminals turns the energy into heat and discharges more slowly | Siemens 4RB9810-0AA00 (EW22, 1 step) |
The Entes DU-3 is a three-phase discharge unit; check its internal design and the step power it can be connected to in the datasheet.
Selection criteria
- Discharge time: The product data gives a discharge time of under 10 seconds for the Siemens 4AJ9903-2YC. The relay's reconnection delay must not be shorter than the capacitor's discharge time.
- Step power: For the Siemens 4AJ9903-2YC, the manufacturer recommends 1 discharge coil for steps up to 50 kVAr and 2 for 50–100 kVAr steps. Determine the number from the datasheet according to the step power.
- Voltage and phases: The Entes DU-3 is for three-phase, 200–480 V AC and 50 Hz; its product data gives a power loss below 1 W and a maximum ambient temperature of 40 °C.
- Connection: The unit is connected after the contactor, directly to the capacitor terminals, so it stays with the capacitor when the contactor opens and discharges it.
Common mistakes
- Connecting the discharge unit before the contactor: When the contactor opens, the unit is cut off from the capacitor and does not discharge it.
- Shortening the reconnection delay: A capacitor that has not discharged causes a high inrush current when switched in.
- Touching without measuring because a discharge unit is fitted: Before maintenance, isolate the step, wait for the discharge time and measure the voltage at the terminals.
Capacitor discharge units at Welibaba
The Capacitor Discharge Units category has 3 active products: the Entes DU-3, and the Siemens 4AJ9903-2YC discharge coil and 4RB9810-0AA00 discharge resistor. The Siemens 4AJ9903-2YC is listed with a price, the others are sold on quotation. Capacitors are in the Power Capacitors category, and the other parts of a correction panel are explained in the power factor correction guide.
Frequently asked questions
Why is a capacitor discharge unit needed?
A capacitor that is switched off stays charged. The unit discharges it quickly, so the step can be reconnected sooner and the risk of shock during maintenance is reduced.
Should I choose a discharge coil or a discharge resistor?
A fast-discharging coil for steps that switch frequently and need a short reconnection time; a discharge resistor can be used where slower discharge is not a problem. Follow the capacitor manufacturer's recommendation.
How many discharge units does a step need?
It depends on the step power and the unit's rating. For the Siemens 4AJ9903-2YC, 1 unit is recommended up to 50 kVAr and 2 units for 50–100 kVAr.