compressorelectric motorhvac
I understand the purpose of a run capacitor. And I understand the purpose of a start capacitor. I understand the concept of a dual capacitor.
With respect to HVAC condenser unit dual capacitors which are labeled "HERM" "COM" and "FAN": Is the purpose of the HERM capacitor for run or for start? Also, is the purpose of the FAN capacitor for run or for start?
I had a capacitor replaced once. Everything seemed to work fine, until 5 minutes after the a/c guy left, then the fan overheated and stopped. I couldn't get him to come back, so I investigated and found that he hooked up the wires to the capacitor backwards; the fan was connected to the "run" prong on the capacitor and the motor to the "fan" prong. I switched it back and it's been running fine ever since.
Most of the motors associated with fans and such, utilize an induced magnetic field, and don't have a permanent magnet. So as the capacitor releases its charge (quickly), a magnetic field would be induced in the motor and cause the motor to turn maybe once or twice, and that should spend most, if not all, of the the stored charge. So the capacitor will release the charge to turn the motor... or more technically, the motor will draw the stored current out of the capacitor (quickly).
With respect to the kind of motor that is turning, for most any AC fan motor, it is highly doubtful that a dynamo-style (using a permanent magnet) motor is being used (except on wind mills). I mention this because the efficiency of the discharging effect could be different compared with a dynamo-style motor... but I also think it should be kind of the same (roughly). The difference being that inertia of the spinning dynamo, will continue to generate electricity, until it nearly stops- at which point it would draw the stored current from the capacitor.
But something else to be aware of is that, typically, large capacitors do not perfectly insulate or isolate or hold charges... they naturally discharge within themselves after a while. Furthermore, large capacitors usually have a bleeder resistor attached to the capacitors terminals. Discharging through motors and other electrical items where a circuit exists is (probably) the primary way, though.
Best Answer
A dual run capacitor is always a run capacitor. HERM is the run capacitor for the compressor and fan is the run capacitor for the fan. Common completes the circuit to both. They are essentially 2 separate capacitors and could be split into 2 capacitors at a later date if you couldn’t find the right dual capacitor. Dual capacitors are usually used to save space and cost of manufacturing. Not all air conditioners have start capacitors. Depending on whether or not the engineer feels a start capacitors necessary will determine if yours has a start capacitor or not. Since a start capacitor can only stay in the circuit for a split second without burning out the motor it will always be attached to a device to take it out of the circuit like a potential relay or a solid state relay. The only time I ever add a start capacitor to a system is if the bearings are tight and the start capacitor is a stop gap measure before getting a new compressor but I let the owner know there are no guarantees. I have had some last years this way and others fail within a day. I have also installed a few when they were dimming the lights significantly on startup but this was only after conferring with the manufacturer first.