Furnace Motor can use higher MFD cap.?

[G2Jim]

I'm replacing the electric motor in my gas furnace, and the manufacturer parts supplier said to use a 15 MFD capacitor instead of the 10 MFD capacitor listed on the 1/2 HP motor itself. The motor (Protech) even says "CAUTION - Use only the capacitor ratings shown."

Should I go ahead and try it with the higher MFD capacitor, or wait and try to find one tomorrow at the noted rating? Might I damage the motor by using the 15 instead of 10?

Jim

[LazyPup]

Normally increasing the capacitor size up to about 25% is okay, but using a 15mf capacitor in place of a 10mfd capacitor is a 50% increase.

Capcitors can be easily located at all electricl supply houses and HVAC or Appliance parts suppliers as well as any supply houses that sell electric motors such as Graingers or Johnstone.

To explain the actual purpose of the capacitor would require a long and drawn out discussion that would be a bit much for this forum, but in a nutshell, the motor has two sets of electrical windings, the run winding and the start winding. The start winding is physically positioned a few degrees out of alignment with the run winding in the direction of rotation.

In the USA alternating current is constantly changing polarity 60 times a second, hence we call it 60HZ current (50 hz in Europe). As the power is fed into the motor circuit the run winding is fed directly, while the power to the start winding passes through the capacitor which causes a minor delay in the time the peak current arrives at the start winding. The end result is the start winding is nearly at full power and the same instant that the run winding is at minimal power. The cycle continually repeating itself 60 times a second. If the capacitor is properly sized the amount of delay is equal to the amount of physical rotation angle between the run winding and the start winding. Increasing or decreasing the microfarad rating of a capacitor alters the time delay which then introduces internal stresses in the motor as one winding is trying to pull against the other, which then causes a minor increase or decrease in the speed of the motor, and in a worst case scenario a drastic increase in the actual operating temperature of the motor windings. The end result is, the motor may appear to run okay, but it will overheat the windings and dramatically shorten the service life of the motor.

It is always best when changing a motor to select a capacitor matched to the new motor and change the capacitor as well.

Also, when selecting a capacitor, pay particular attention to the voltage rating on the capacitor. The voltage rating on the capacitor is typically 75-100 percent higher than the supply voltage to the motor. When a motor is running the voltage being supplied to the motor is called EMF (electromotive force). During operation the motor winding is also generating CEMF (counter electromotive force). Both the EMF and the CEMF are present on the capacitor at the same time which explains why the capacitor must be rated higher.

A WORD OF CAUTION WHEN HANDLING OR CHANGING CAPACITORS- Not only is the operating voltage on a capacitor much higher than the supply voltage, but a capacitor has the ability to store an electrical charge for days after the power has been turned off.

Before attempting to change a capactor make absolutely sure the source of voltage is turned off, then using a well insulated pair of needle nose pliers remove the wires from the capactior and discharge the capacitor by using a well insulated screw driver and touch it across both terminals of the capacitor to discharge its stored charge. Failure to do so can result in a serious electrical shock.

Look in the illustrations forum for an illustration that i prepared on handling & testing motor capacitors...listed under "motor capacitors"

[G2Jim]

Thank you very much! I did install the higher capacitor simply because my family wanted heat, but I will pick up the appropriate size capacitor ASAP and change it. Also great tip on the potential accidental discharge, since I forgot all about that and am just lucky when I removed the old one!

Jim