The plate area of the electrolytic capacitor is small compared to the oil-filled capacitor. The aluminum plates are thick because the electrolytic capacitors are designed for high current. Paper impregnated with a solution of water, borate, and glycol separates the plates. The liquid solution is an electrolyte which conducts electrons to one of the plates during each half cycle.
The dielectric of the electrolytic capacitor is the very thin oxidized coating on the aluminum plates. The extremely thin dielectric gives this type of capacitor a very high mfd rating for its size.
The components are encased in a plastic container with the terminals mounted in the lid. Straps of aluminum connected to the plates are riveted to the terminals. A small hole (sealed with rubber) is also in the lid. The rubber seal will rupture and release the pressure (when the capacitor’s water boils from overheating).
Prolonged current flow (more than 3 seconds) will make the water solution boil and can cause the electrolytic capacitor to explode.
Electrolytic Capacitor and Start-Winding Connections
Electrolytic capacitors are connected in series with the start switch and the start winding as shown in Fig. 3.35. There is no standard sequence for connecting the components of this circuit.
Multiple electrolytic capacitors are usually connected as a single unit in the connection sequence. They are connected either parallel or in series with each other when more than one capacitor is used.
Parallel Electrolytic Capacitor Connection
The parallel connection is shown in Fig. 3.36. The total capacitor plate area is increased with this connection. The mfd value of the capacitors is added to get the total capacitance of the circuit. The mfd rating of the capacitors connected in parallel can be different.
The capacitor’s voltage rating must be as high or higher than the voltage applied to the start-winding circuit.
Capacitors are connected in parallel for two reasons: If a large mfd value is needed, two or more capacitors are easier to mount on the motor than one large one, and it enlarges the capacitor’s cooling area.
Heat develops in a capacitor each time the motor starts. Frequent starting may prevent the capacitor from cooling sufficiently. All capacitors have a breakdown voltage value. When voltage reaches this value, it will puncture the capacitor’s dielectric and destroy it. As the temperature of the capacitor increases, it takes less voltage to break down the dielectric.
If frequent starting is necessary, a single capacitor can be replaced with two or more capacitors in parallel. (The total value of the parallel capacitors must be the same as the single capacitor’s value.) Two or more capacitors in parallel increase the cooling area, allowing more starts per hour.
The highest recommended temperature for capacitors is 150 a F. High temperatures dry out the electrolyte fluid. Loss of electrolyte fluid derates the mfd value of a capacitor.
Series Electrolytic Capacitor Connection
The series capacitor connection is used on high-voltage (above 200 volts) motors. When two low-voltage capacitors are connected in series, their dielectric is doubled, allowing the voltage to be doubled. The cooling area of two low-voltage capacitors is larger than that of one high-voltage capacitor. There are two rules for connecting electrolytic capacitors in series:
• Never have more than two capacitors connected in series with each other.
• When two capacitors are connected in series, they must be labeled the same mfd value.
The reason capacitors must be of the same mfd value is that the voltage divides across them in inverse proportion to their rating. A lower mfd capacitor would have voltage across it that is too high.
Example:
Volts x c + c + c — volts across C2
Line voltage — 240 volts
Capacitor #1 (Cl) = 200 mfd
Capacitor #2 (CD = 100 mfd
240v x c + c + c – 240V x 200V + (CD 200 + (CD 100 or,
48,000 + 300 — 160 volts across c02 (the smaller capacitor rated for 120V)
Under normal starting conditions, the voltage developed across the capacitor- and start-winding circuit will be 140 percent of the applied voltage for less than one-half of a cycle. Capacitors of equal mfd rating would have (half of 240 volts) 120 volts x 140 percent = 168 volts across each of them. The voltage across C in the above example would be 160 x 140 percent = 224 volts.
The actual mfd rating of the electrolytic starting capacitor is within 10 percent of its stamped rating. If there are two ratings, the actual mfd value will be somewhere in between. Such a small mfd difference isn’t critical because it’s normally in the circuit for 1 second or less.
Testing the Electrolytic Capacitor
A capacitor’s rating can be checked with an electronic test instrument like the one shown in Fig. 3.37. This instrument can quickly give a capacitor’s value in mfd. The test uses low energy and is not destructive.
An instrument for checking a capacitor. EXTECH Instruments.
An ohmmeter can be used to test a capacitor. When testing a good capacitor, it will show a low-resistance reading, which rises slowly until the capacitor is charged.
This test indicates the capacitor has capacitance but doesn’t give the amount of capacitance. No reading means the capacitor is open; a steady low-resistance reading indicates it’s shorted.
Some internal problems in the electrolytic capacitor aren’t revealed by using a low-energy test method. A test method (formula to follow) using line voltage will stress the internal components and determine the capacitor’s mfd value. This test will also cause a weak part to break down. (A capacitor has nearly this same amount of stress each time the motor starts.)
One of a capacitor’s weak spots is under the lid where the lead straps of the plates are riveted to the connection terminals. These straps flex when the capacitor heats and cools, causing them to crack or break at the rivet. The motor occasionally fails to start when this happens. The high amperes of this test method will blow the weak spot open.
Excessive heat dries out the water-based electrolyte in the capacitor. (Too many start cycles per hour is a common cause of overheating.) A thick piece of insulation is needed between the capacitor and the hot motor frame to limit the heat transfer.
A capacitor’s mfd rating is lowered when some of the electrolyte evaporates. This condition lowers the capacitor’s amperes.