What happens if a capacitor of different value is replaced in a motor?

There is often a perception that replacing a defective capacitor in fans with one with higher value may help in getting better speed. What is the truth?

Old fans and motors, using MPP capacitors, sometimes lose speed as their capacitors “go weak”, and their value goes down over time. While replacing these, it is thought that if they are replaced with a higher value capacitor, speed may be better, and that the capacitor may last longer. To understand the truth, one has to understand the functioning of motor.

Rotating torque, responsible for spinning of its shaft and the load, is given by a rotating magnetic field, which in turn is generated by interaction of magnetic fields of two windings placed in suitable sequential positions in stator of motor. A capacitor placed in one winding, called auxiliary winding, creates an electrical phase difference in the current through the two windings. This in turn creates a phase difference between the magnetic fields of the two windings. The rotor experiences the resultant effect of these two magnetic fields, which is in the form of a rotating field. This rotating field exerts a torque of the rotor, making it to spin, along with any load coupled to it. The capacity to rotate and support the rotation of load is designed into the motor by careful design of windings, the capacitor and overall configuration of slots in stator and rotor.

All motors (including fan motors) are designed for optimum performance for their applications, power supply conditions and variations. Any deviations will cause a change in deliverable torque, a degradation in performance, depending upon how much the deviation is. This factor includes any deviation from design phase angle difference between main and auxiliary windings. Engineering tolerances are included in design and ratings on all components or winding turns etc., and a motor will behave as per its intended performance within these tolerances. Any appreciable change in capacitor value beyond these limits will degrade the performance, and may even damage the motor.

Generally, engineering tolerance on capacitor value is specified as 5% on design value, meaning it must be within 5% in value on higher or lower side (some fan manufacturers even specify closer tolerance of just 2%). What this means is the motor gets its optimum running torque when capacitor value is within this tolerance limit, and any deviation beyond this will degrade its performance.

Let us consider what happens if a 2.25 µF capacitor in ceiling fan is replaced by 2.5 µF capacitor.  2.25 µF capacitor, taken with tolerance limits of 5%, comes closer to lower limit of 2.5 µF, but if this 2.5 µF has actual value on higher side, it will cause a deterioration in performance. A 2.5 µF capacitor may give a higher fan speed (at least at the time it is connected), but the torque on motor, and current in windings will be different from their design parameters.

In old days, going back over 40-50 years, fans and motors used oil filled paper / foil capacitors, which were extremely stable in values. Their mode of failure was as a short-circuit and they had a lifespan of over twenty years. So they never faced a replacement issue within this long period. In fact, it was common to find capacitors intact after over such a long service period. Metallized capacitors came in the world around 1980s, and got popular because of their costs, lower size and ability to self-heal (clear any defective spot developed by itself). However, the self-healing property has a downside that capacitor value goes down by a very small extent every time self-healing takes place. It depends on capacitor design and manufacturing as to by how much value its value goes down over a given time. A good capacitor remains within its parameters over major part of design life of fan / motor.

Most capacitors in fans today use metallized polypropylene (MPP) capacitors, which have a tendency to reduce in value over time (due to voltage fluctuations or other reasons) – though it takes years for a good product. When this happens, fan speed goes down. A well designed capacitor will last quite long, and it is important to choose a good product from a known brand for replacement, whenever such need arises.

Electricians, sometimes even engineers, often develop a habit of using 2.5 µF to replace 2.25 µF, the logic being its value will come within limits of 2.25 µF after reducing, and will last longer. Similar thing is done in some motors up to 5 HP, which is again not advisable for proper performance. In large single phase motors having capacitor from 3 µF to 10 µF or higher, any change in value of capacitor may actually harm the motor.

Motor speed does not necessarily go up with capacitor value (it may, in fact, start dropping after a limit), its auxiliary and main currents get affected, and motor performance is affected. The result could be early failure of winding, lower torque or some other issue which is not easily discernible. The load / torque characteristics does get affected, and such deviations must be avoided. Windings of motors are made from optimally selected sizes of wires, and it may not be possible to rewind them to original design if a need arises anytime. It is therefore best to stick to original design parameters of motors, and try to select any replaced capacitor as per original design. The only care needed is to select a good reliable make of capacitor.