By William Hirsch
Updated Mar 24, 2022
Choosing the right capacitor for an electric motor can be the difference between a smooth start and a stalled motor. A capacitor provides the initial burst of energy that spins the motor’s shaft. It stores charge and releases it at the moment the motor needs it. The required capacitor size depends on the motor’s starting energy demand and the voltage supplied to it.
Power on a digital multimeter and set the dial to the DC voltage setting—denoted by a capital “V” with straight lines above it.
Touch the red (positive) probe to the battery’s positive terminal and the black (negative) probe to the negative terminal. Record the voltage reading. For example, the battery might read 11.5 V.
Calculate 0.5 × (V2). In our example, 0.5 × 11.5 V × 11.5 V = 66.1 V². This value is labeled “x.”
Divide the motor’s start‑up energy requirement (in joules) by “x” to obtain the necessary capacitance in farads. The start‑up energy can usually be found in the motor’s documentation or on the motor itself. If the motor requires 0.00033 J, then 0.00033 J ÷ 66.1 V² = 5.0 × 10‑6 F.
Convert farads to microfarads by multiplying by 1 × 106 (since 1 µF = 10‑6 F). Thus, 5.0 × 10‑6 F = 5.0 µF—a common unit for capacitors.
