--- title: "Capacitors in an injection molding machine servo drive" url: "https://www.imspares.com/injection-molding-machine-parts/electrical/capacitors/" --- # Capacitors in an injection molding machine servo drive What DC bus and film capacitors do in an injection molding machine servo drive, why electrolytic capacitors age, the signs of failure, and how to replace them. Servo drive capacitors come in two kinds. Electrolytic **DC bus capacitors** store energy between the rectifier and the power module and smooth the DC voltage that the drive switches to run the motor. **Film capacitors** across the incoming lines filter electrical noise. The DC bus capacitors are wear parts: their electrolyte slowly dries out with heat and age, so they are among the parts most often replaced when a drive is repaired. ## Where the capacitors sit in a servo drive The drawing follows the power path through the drive: 1. **Mains input terminals:** the three-phase supply enters the drive. 2. **Film capacitors:** filter capacitors across the lines suppress the electrical noise (EMI) that the drive would otherwise send into the supply. 3. **Rectifier:** turns the incoming AC into DC. 4. **DC bus capacitors:** a bank of electrolytic capacitors that store energy and smooth the DC voltage. 5. **Power module:** switches the DC bus voltage into the variable AC that drives the [servo motor](https://www.imspares.com/injection-molding-machine-parts/electrical/servo-motor/). The same kinds of capacitors are found in the [power supply board](https://www.imspares.com/injection-molding-machine-parts/electrical/power-supply-board/) and other power electronics in the cabinet. The [servo drive guide](https://www.imspares.com/injection-molding-machine-parts/electrical/servo-drive/) explains the rest of the drive. ## DC bus capacitors The enlarged capacitor in the drawing shows how they are built: - **Can and sleeve:** the aluminum can holds the wound foil and the electrolyte. The plastic sleeve insulates it and carries the rating and the polarity stripe. - **Safety vent:** opens under internal pressure when the capacitor fails, instead of letting it burst. A bulging vent means the capacitor needs replacing. - **Screw terminals:** bolt the capacitor to the DC bus bars. Electrolytic capacitors are polarized: fitted the wrong way round, they fail violently. On drives fed from a 380 V three-phase supply, the DC bus voltage is higher than the rating of a single capacitor, so the capacitors are usually connected in series pairs, with balancing resistors that share the voltage evenly between them. Our [electrolytic capacitors](https://www.imspares.com/parts/ec-ce-01/) are rated 400–450 V DC, in various capacitances. ## Film capacitors Film capacitors are not polarized and age much more slowly than electrolytic ones. In the input filter they are connected between the lines and from the lines to ground. These are safety-rated capacitors (class X across the lines, class Y to ground), so a replacement must have the same safety class, capacitance and voltage rating. See the [film and filter capacitors](https://www.imspares.com/parts/ec-ce-02/) we supply for line filtering and EMI suppression. ## Why DC bus capacitors fail - **Heat:** the life of an electrolytic capacitor falls sharply as its temperature rises, so blocked filters, failed fans and a hot cabinet shorten it. - **Age:** the electrolyte slowly escapes through the seal; the capacitance falls and the internal resistance (ESR) rises. - **Ripple current:** heavy, fast-changing loads heat the capacitors from inside. - **Overvoltage** on the DC bus. - **Long storage without power:** the oxide layer inside can degrade, so spare drives stored for a long time may need their capacitors reformed before use, as the drive maker's instructions describe. ## Signs of aged or failed capacitors - A bulging vent, a swollen or split sleeve, or electrolyte leaking at the base. - Drive alarms for DC bus overvoltage, undervoltage or ripple, especially under heavy load. - The drive trips during fast movements but runs at light load. - The drive does not power up. - A hum or a burnt smell from the drive. ## Testing and replacing capacitors Capacitor work is for qualified electricians and is usually done as a drive repair on the bench. The DC bus can hold a lethal charge long after the supply is switched off. 1. Switch the machine off at the main isolator, then lock out and tag it. 2. Wait at least the discharge time on the drive's warning label, then measure the DC bus voltage at the bus terminals and confirm it has fallen to a safe level before touching anything. 3. Inspect the capacitors for bulging, leaks and discoloration. 4. Remove a suspect capacitor and measure its capacitance and ESR with a suitable meter, and compare them with the rating on the sleeve. 5. Replace the capacitors of a bank together, with the same capacitance, an equal or higher voltage and temperature rating, and the same terminal type and can size so they fit the clamps and bus bars. 6. Fit each one with the polarity stripe the right way round, keep the balancing resistors, tighten the terminals to the torque in the drive manual, and test the drive at low load first. ## What to send for a replacement quote - A photo of the sleeve printing: capacitance (µF), voltage, temperature rating and the capacitor maker's series. - The can diameter and height, and the terminal type and spacing. - How many capacitors are in the bank. - The servo drive model from its nameplate, and the machine brand and model. The [electrical system guide](https://www.imspares.com/injection-molding-machine-parts/electrical/) shows where the drives and power supplies sit in the machine. ## Questions ### What does a DC bus capacitor do in a servo drive? It stores energy between the rectifier and the power module and smooths the DC voltage, so the drive can supply the motor with steady power through fast load changes. ### How do I know if servo drive capacitors are bad? Look for bulging vents, swollen sleeves or leaking electrolyte, and for drive alarms on the DC bus voltage, especially under heavy load. Measuring capacitance and ESR on the bench confirms it. ### Should all the DC bus capacitors be replaced together? As a rule, yes. Capacitors in a bank age together, and mixing new and old ones loads them unevenly, especially when they are connected in series. ### Can I use a capacitor with a higher voltage rating? Usually yes, if the capacitance is the same, the temperature and ripple current ratings are equal or higher, and it fits the space, clamps and terminals. A lower voltage rating must never be used. ## Replacement parts - [EC-CE-01 Electrolytic Capacitor](https://www.imspares.com/parts/ec-ce-01/): High-capacity electrolytic capacitor for injection molding machine servo drives and inverters. - [EC-CE-02 Film Capacitor / Filter Capacitor](https://www.imspares.com/parts/ec-ce-02/): Film capacitor for injection molding machine drive filtering and power conditioning.