HOW DOES CAPACITOR TYPE FAN REGULATOR WORK?

Old fan regulators for all types of fans used a set of resistors in combination with a selector switch (rotary or slider). The regulator introduced a resistance in series with fan, whose value was selected by switch position. Supply voltage gets dropped in this resistance, and a lower voltage appears across the fan, which causes a reduction in fan speed. Depending on fan type and requirement, the regulator could be 3-way 4 position or 5-way 6 position including an off position. Wire wound resistors, carrying the fan current, would consume significant amount of I²R power, thereby heating the resistance and the regulator. Heat dissipation had to be taken care by designing efficient heat sink, and providing ventilation in regulator housing.  The regulator had to be mounted such that whole regulator is open to atmosphere to allow heat dissipation. Elaborate ventilation and heat sinks had to be provided to dissipate the considerable heat produced in regulator.

Capacitor fan regulator essentially replaced the resistances with capacitors. This eliminated heat loss in regulator, allowing it to be mounted inside a panel or flush with switch board, with only the knob (and a cover plate) being visible from outside. Since power loss is eliminated, these regulators are energy efficient in nature, and also aesthetically pleasant.

However, the capacitors pose a problem- they can remain charged when switched off, and have inrush current when switched on. Therefore, switching can damage the switch as well as capacitors. The circuit of the regulator must be able to discharge the capacitors safely when switched out, and also limit their inrush current to acceptable level. The switch also has to be suitably designed for this duty. Rotary type switches are most common for these regulators.

The circuit of a 5-way regulator is shown above. The switch is a 2-pole 5-way 6 position switch to enable combination of capacitors in different positions to adjust capacitance value. This reduces number of capacitors used, and also saves on space and cost. Position 0 is the off position, while fan is directly connected to supply in position 6, which is the full speed position.

Capacitor values most commonly used are C1-2.4 μF, C2 and C3- 3.3 μF each. This gives capacitor values of 2.4, 3.3, 5.7 and 6.6 μF for four speed steps, before the fan is connected to supply directly at step 6. The parallel resistor Rp allows a capacitor to discharge to safe level before it gets switched on again by the operation of switch. The low value resistance Rs (of a few ohms) in series with capacitor limits the inrush current. It may be noted that when making combinations of capacitors, a capacitor previously charged in one step gets discharged into the new capacitor being added, which is a short-circuit path for the charged capacitor. Hence its discharge current needs to be controlled for the safety of both capacitors.

Often, a 4-speed regulator is used. It is more common in table fans and other applications. The circuit is given below.

Most prevalent values for C1 and C2 in these regulators for ceiling fans are 2.4 μF and 4.2 μF. The combinations give values of 2.4, 4.2 and 6.6 μF for 3 speeds, before going to direct fan connection. Table fans use different set of values. Cooler fans, air circulator fans and others also are mostly 4 speed, and have different capacitors specific to their motor ratings.

Capacitors for fan regulators thus have an onerous duty, and subject to frequent surges. They are usually made from Metallized Polypropylene, and with liberal design to account for its duty conditions. The series and parallel resistors also have to be meticulously designed to discharge them safely within short time, and to enable safe inrush currents without heating up appreciably. The series resistance particularly adds to heat loss, which must be low enough not to affect the temperature of regulator or surrounding in a closed environment. The 2-pole switches rotary are designed specifically for the purpose, which must withstand frequent switching in and out of capacitors.

Capacitor type fan regulator offer the following advantages over conventional resistance type, or later inductor (choke) type regulators:

1. Much smaller size
2. Amenable to front mounting / panel mounting
3. Fit snugly in switchboard panel wiring
4. More aesthetic appearance
5. No watt loss in regulator
6. No heating of regulator