What's the Difference between a Start and a Run Capacitor?


Capacitors in General


A capacitor is an energy storing device. It is a medium by which energy is stored to either be released suddenly or over a period of time. The energy or capacitance of an electric capacitor is measured in the form of micro-farads. Essentially, two plates are separated by a material known as a dielectric or insulator. These insulators can be mica, ceramic, porcelain, Mylar, Teflon, glass, or rubber. Capacitors will also limit the current. They can be used to store voltage or build it up until the call for it to be released is present.


Start Capacitors


A start capacitor is found in the circuit of start windings when the motor is starting. This capacitor contains a higher capacitance than a run capacitor. It varies, but a start capacitor will measure between 70 and 120 micro Farads. The start capacitor provides an immediate electrical push to get the motor rotation started. Without a start capacitor when the voltage is applied, the motor would just hum. The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current.


Run Capacitors


A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. It is used to maintain a charge. In AC units, there are dual run capacitors. One capacitor provides power to the fan motor. The other sends power to the compressor. Run capacitors measure in at approximately 7-9 micro-Farads. The value or rating of the run capacitor must be accurate. If the value is too high, the phase shift will be less than perfect and the winding current will be too high. If the capacitor value/rating is too low, the phase shift will be higher and the winding current will be too low. If run capacitors are not ideal, then the motor could overheat and the true torque will not be enough to drive current.


What is Equivalent Series Resistance?


The equivalent series resistance of a capacitor is the internal resistance that appears in series with the capacitance of the device. Almost all capacitors exhibit this property at varying degrees depending on the construction, dielectric materials, quality, and reliability of the capacitor. The equivalent series resistance (ESR) values range from a few milliohms to several ohms, and results into power losses, reduced efficiency, and instability of power supplies and regulators circuits.


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The aluminum electrolytic capacitors and tantalum ones, have higher ESRs than ceramic capacitors of the same capacitance and voltage rating. The polypropylene and polyester capacitors fall in between, but are not commonly used in the SMPSs due to their large physical sizes.


Main parts of an ESR


Metallic resistance


Electrolytic and paper resistance which is dependent on frequency and temperature


Dielectric which depends on frequency


Factors that increase the ESR value


Bad electrical connections; – The connection between the copper leads and the aluminum plates in the capacitor are usually welded or made using mechanical crimps. This type of connections introduces some series resistance, and is used because the aluminum cannot be soldered.


The drying of capacitor electrolyte solution. As the liquid component of the electrolyte dries out due to elevated temperatures, the electrical resistance increases.


ESR increases with increase in temperature and frequency. In power supplies with high currents, the power dissipation associated with the ESR may further increase the temperature and lead to capacitor failure.


Minimizing ESR in circuits


High performance applications use the low ESR capacitors such as the low ESR solid polymer capacitors, tantalum capacitors and the multilayer ceramic capacitors (MLCC).


Capacitors are connected in parallel in places such as the power supply smoothing circuits. Small value capacitors are connected in parallel as opposed to connecting a single large capacitor. This reduces the effective ESR in addition to reducing the ripple volatge, and allows the circuit to handle higher currents with less losses.


What is the Start Capacitor for a Bore Pump Motor?


A capacitor is a an electronic device that stores energy. In a single phase bore pump a start capacitor increases the starting torque for a short while and then brings the motor rotation up to a rate approaching the speed at which it will run the pump constantly. The bore start capacitor then drops out and a run capacitor takes over for energy efficient running of the pump.


In most submersible pumps the capacitor is a dual start/run capacitor that does both. A capacitor used with a bore pump motor is normally perfectly cylindrical and about half the diameter of a soft drink can. It should not have black marks or be ruptured.


With a typical Perth submersible bore pump the capacitor, or condenser as it is sometimes referred to, is normally in a bore start box often mounted near the homes meter box. Remember bore start boxes are 240 volt and should only be accessed by a licensed electrician. We provide a fast mobile service all over Perth to replace burnt or blown bore pump capacitors.


The submersible pumps with start boxes are known as “3 wire” motors (the wiring to the surface is 3 core and earth). Some submersible motors known as “2 wire” have the electronics and capacitor in the actual pump motor below ground and have 2 wires and an earth running up to the surface. 2 wire pump motors therefore do not have start boxes or easily replaceable start capacitors “3 wire” submersible motors are more common in Perth bores.


Why Are Capacitor Discharge Resistors Now Mandated As Essential Safety Device


In Electronics, Capacitor discharge resistors or Bleeder resistors are resistors connected in parallel with the output of a high voltage power supply circuit with the express purpose of discharging the residual electric charge stored in the filter capacitors of the power supply.


As an example, a switch mode power supply uses a bridge rectifier to convert AC Mains Power into DC at 320V (where the mains voltage is 220/240V) or 160V (where the mains voltage is 110/120V), before the voltage is reduced by the chopper. One or more Filter capacitors are incorporated to smooth the pulsating output voltage from the rectifier. Now these capacitors must necessarily store enough energy at this high voltage to power the load during the Zero crossings of the AC input.


Further, the capacitors chosen in many power supplies are designed to be large enough to supply the load during Power breakdowns lasting for a second or so.


This stored charge is often enough to deliver a lethal shock in devices such as Lasers, X-Ray machines, Radio transmitters and also the old style CRT screens.


It is most important that it must be recognized that this stored charge can remain in the capacitors for a significant period of time even after the unit has been turned off, and therefore poses a life threatening hazard for the user or maintenance personnel who may believe that because the device is turned off it is safe.


To meet the challenge of this safety hazard, it is necessary to discharge the capacitors after the power has been turned off and typically a large or high resistance value resistor is connected across the terminals of the capacitor.


So after the device is switched off, the charge on the capacitor will drain off through this Capacitor Discharge or Bleeder Resistor, thus rendering the device safe for servicing etc.
It is pertinent to note that when the device is ‘ON’ a small current flows through Capacitor Discharge/Bleeder resistor which results in the wastage of a small amount of power.


In order to reduce this power wastage it is necessary for the designer to select, an optimum resistance value due to the fact that there is a trade off between the speed at which the capacitor can be drained off and the wastage of power during normal operation.


The lower the resistance value of the Capacitor Discharge /Bleeder Resistor, the faster the bleed down rate but wastage of more power during normal operations and vice verse for a higher resistance value.


To meet this need, HTR provides a Capacitor Discharge/Bleeder Resistor which is so designed that the resistor can be mounted directly to the capacitors terminals and not through any connectors, thus eliminating the risk of the Bleeder resistor being disconnected accidentally.


Reactive power of capacitors


The current flowing through capacitors is leading the voltage by 90°. The corresponding current vector is then in opposition to the current vector of inductive loads. This why capacitors are commonly used in the electrical systems, in order to compensate the reactive power absorbed by inductive loads such as motors.


Inductive-reactive power is conventionally positive (absorbed by an inductive load), while capacitive-reactive power is negative (supplied by a capacitive load).


As reactive-inductive loads and line reactance are responsible for voltage drops, reactive-capacitive currents have the reverse effect on voltage levels and produce voltage-rises in power systems.