The Capacitor-Start Motor

The capacitor-start motor has good starting torque. It has a wide range of applications. Its size ranges from fractional to 35 hp.

Components of the Capacitor-Start Motor

The main components of the capacitor-start motor are a stator (with a start winding and run winding), a low-slip rotor, a start-winding switch assembly, and an electrolytic capacitor (Fig. 4.6).

Operation of the Capacitor-Start Motor

Single-phase voltage is applied to both start and run windings. An electrolytic capacitor in the start-winding circuit creates a leading current flow. The startwinding current leads the run winding current by 90 ⁰(or 1/240 second). The result is a very efficient rotating magnetic field and high starting torque. When the rotor reaches 75 to 80 percent of synchronous speed, its centrifugal device releases the stationary switch contacts and disconnects the start winding. The rotor accelerates the load to nameplate RPM or above.

Troubleshooting the Capacitor-Start Motor

Following are the most frequent component breakdowns of the capacitorstart motor, in descending order:

• Start switch assembly (stationary switch and centrifugal device)

• Electrolytic capacitor

• Start winding

• Run winding

• Bearings

• Rotor

Start Switch Assembly The start switch assembly is responsible for connecting and disconnecting the start winding. The stationary switch contacts arc, vaporizing some of their material each time the motor starts. Over time, the contact area will no longer be large enough to make a good connection (resulting in dead spots, as described next). With the contact area

reduced, high starting current can melt the contacts, fusing them together (the start winding will then stay energized). The motor will be noisy and won’t get up to full RPM. If the high-current protection device doesn’t function, the start winding and capacitors overheat and fail.

Dead Spots Worn contacts, centrifugal device spool, or thrust washers can cause the motor to have dead spots. It won’t start unless the shaft is moved slightly. Moving the shaft changes the spool pressure point against the stationary switch, the contacts close and the motor starts.

To verify that the switch is the component that is intermittently faulty, disconnect T5 and T8. Connect an ohmmeter across these leads and turn the shaft. If no open circuit occurs, pull the shaft away from the switch end of the motor and, at the same time, turn the shaft. (The stationary switch is normally located on the end opposite the shaft.)

Magnetic Center and Thrust Washers Excessive shaft end play can cause intermittent starting problems. The rotor will try to seek its magnetic center. (The thrust washers should keep the rotor at this alignment.) If the rotor is not at magnetic center, the motor loses power and the magnetizing amperes are increased. This was discussed in Chapter 3 under “Misaligned Rotor Iron.’

If the start contacts aren’t closed when the rotor is aligned properly, the switch assembly should be adjusted accordingly.

Centrifugal Device Problems If the preceding tests don’t reveal an open circuit in the start winding, the problem may be in the rotating device or the capacitor (if it is in the capacitor, see “Open Capacitor” a bit later in the chapter). If the centrifugal device is dirty or worn, it can cause intermittent start problems. Cleaning or replacing it requires disassembling the motor or taking it to a repair center. When the centrifugal device is replaced, it must be positioned in exactly the same place as the old one.

The decision to replace the start switch assembly components depends on the overall condition of the motor and the repair cost. An electronic start switch may be the best option. Operation of the electronic start switch is explained later in the chapter under “Alternative Start-Winding Switches.’

The motor’s original stationary switch contacts and centrifugal device can be replaced with start switch devices, also described later under “Alternative Start-Winding Switches. ‘

Normal Braking Effect Caused by the Electrolytic Capacitor It’s normal for a brief braking action to take place as a capacitor-start motor coasts to a stop. As the motor slows, the centrifugal device closes the start contacts. The capacitor will now discharge into the closed circuit of the start and run windings. The power coming from the capacitor is DC (Fig. 4.7). (When direct current flows in the stator windings of an induction motor, there is a strong braking effect on the rotor.)

Some three-phase motor controls use DC braking to slow a coasting type load. DC power can’t, however, hold the motor’s shaft stationary.

Electrolytic Capacitor The following problems can occur with an electrolytic capacitor:

• Shorted

• Open

• Weak capacitor (loss of capacitance)

Shorted Capacitor A motor with a shorted capacitor has very little starting torque. The peak magnetism time of its two windings is too close together (see “Operation of the Capacitor-Start Motor” earlier).

An ohmmeter can be used to test the capacitor, but (because of possible internal parallel circuits) it’s necessary to disconnect one of the capacitor leads.

Short the capacitor terminals together and discharge it before using the ohmmeter. Meters are available that can measure the capacitance of capacitors.

A normal capacitor will make an analog ohmmeter needle peg as if there is a short; then it will drop back slowly and read the capacitor’s normal leakage. If the capacitor is shorted, the needle remains at the pegged position.

Open Capacitor If a motor has only one capacitor, the start winding is disconnected when the capacitor is open and the motor won’t start.

The capacitor can be tested for an open with an ohmmeter or a test light. One location in a capacitor where opens occur is under the lid. The connection straps for the plates are riveted to the terminals under the lid. These straps flex when the capacitor heats and cools, causing them to crack and break at the rivet. (The lid’s retaining ring can be pried out to check this problem.) An open capacitor is sometimes an intermittent problem. The problem can be mistaken for dead spots caused by faulty start contacts.

When the stationary switch is replaced because of faulty contacts, the capacitor should be checked for cracked or broken straps.

Some centrifugal devices don’t put even pressure on the stationary switch’s wear pads. The contacts open and close (flutter) when the rotor approaches disconnect speed. Contact flutter will decrease contact life. It also produces voltage spikes in the start winding that will destroy the capacitor. Excessive contact flutter can cause frequent capacitor breakdown.

Weak Capacitor Overheating is the main cause of a capacitor becoming weak. A capacitor is overheated if there are too many starts per hour or if the capacitor is mounted in direct contact with the motor shell. A weak capacitor reduces the motor’s starting torque.

Overheating dries out some of the capacitor’s electrolyte fluid. Loss of electrolyte fluid decreases the amperes available to the start winding and lowers the motor’s torque. Applying line voltage and using the formula explained under “Capacitor Test Formula” in Chapter 3 will determine if the capacitor is weak.

If the motor doesn’t have good starting torque, its capacitor may have been replaced with one that is too small. In this case use the

method explained in Chapter 3 under “Determining the Right Size Capacitor.”

Start Winding All four of the following problems require that the motor be rewound or replaced:

Burned

Open

Shorted coils

Shorted to the run winding

Burned Start Winding The start winding will overheat if it is energized for more than 3 seconds. An overload or a faulty start switch that doesn’t allow the start contacts to open is frequently the cause of burned start windings. (Burned windings can be seen and/or smelled.) A motor with burned or discolored start coils must be rewound or replaced.

Open Start Winding An open in the start winding can occur if it is shorted to the run winding. Because of the start winding’s smaller wire size, it will melt in two. Most motor manufacturers don’t put insulation between the start and run windings the way electric motor repair centers do. Rewound motors are less prone to start winding to run winding shorts.

It’s common for opens to occur in motors wound with aluminum magnet wire. If the environment is corrosive (for example, a washing machine), a small bare spot in the wire will corrode it in two. Electric motor repair centers never use aluminum wire.

If the start winding is not burned or discolored, the open can be located with a test light or an ohmmeter. For example, a faulty internal lead-to-coil or coil-to-coil connection.

The following procedure should be used: Disassemble the motor and remove the insulation from the lead-to-coil and coil-to-coil connections. Start at one end of the start winding and proceed toward the other end as shown in Fig. 4.8. An open connection can be scraped clean and resoldered.

Smaller capacitor-start motors with a faulty start winding should be replaced. The larger capacitor-start motors can be economically rebuilt if the cost is near that of a new motor. Service centers use a higher temperature class insulation than is in the original factory-installed winding. Most use class H (Fig. 4.9). Service centers also insulate between the start and run windings.

Shorted Coils—mal-Voltage Start Winding Some capacitor-start motors have dual-voltage start windings (Fig. 4.10). The dual-voltage start winding has two pairs of lead wires numbered T5 through T8. Half the start-winding circuit is between numbers T5 and T6, and the other half is between T7 and T8. Capacitor(s) are always located (as shown in Fig. 4.10) between the start winding and T6 and between the start winding and T8. The start switch contacts will be located as shown in Fig. 4.11 or 4.12.

The two circuits have identical data, so they can be comparison tested. It is unlikely that both circuits would have the same problem. A microhmmeter or limited AC current can be used for this test. The current shouldn’t be so high that it overheats the start-winding coils.

Disconnect the leads T5 through T8. Compare the reading between T5 and T6 to the reading between T7 and T8. (Do not include the capacitors.) It may be necessary to turn the shaft slowly while recording the high and low readings.

The results should be identical. If one circuit has lower resistance (with an ohmmeter) or higher amperes (with limited AC), it has shorted coils.

Each circuit should also be tested to the frame of the stator for an insulation breakdown to ground. A short to ground requires a complete rewind or replacing the motor.

Start Winding Shorted to Run Winding The start winding and run winding normally aren’t internally connected. The only exceptions are motors that have predetermined rotation, such as centrifugal pump motors. If there is continuity between the start and run windings, the motor will have to be rewound or replaced.

Connecting the Dual-Voltage Start Winding Figure 4.13 shows the startwinding lead number system. To connect a dual-voltage start winding, simply use the following phrase: in on the odds and out on the evens. Start-winding leads are always numbered T5 through T8.

Run Winding The run winding normally has fewer problems than the rest of the capacitor-start motor’s components. Overheating from overload, high ambient temperature, low voltage, and bearing failure cause the following problems:

Burned coils

Shorted turns

Shorted coils (dual voltage)

Short to the start winding

Short to the frame

Open winding (dual voltage)

Burned Coils Burned coils are always visible. They usually have a burned varnish smell. The motor may even run normally in this condition. It definitely should be replaced or rewound.

Shorted Turns Shorted turns lower the motor’s resistance. (Amperes will increase and cause the windings to overheat.) The stator’s magnetic balance becomes upset, making the motor noisy. A ringing or high-pitched sound sometimes indicates shorted turns.

A low power factor can be mistaken for shorted turns. If the no-load amps are 10 to 15 percent higher than nameplate amperes and the motor sounds normal, a low power factor may be the cause. Normally, most single-phase motors have a low power factor. With no load, it’s common for fractional horsepower motors to draw higher than their nameplate amperes. They often draw fewer amperes when fully loaded. The power factor of almost all AC induction motors improves when they are fully loaded.

Shorted Coils, Dual-Voltage Run Winding Many capacitor-start motors have dual-voltage run windings (Fig. 4.14). The dual-voltage run winding has two pairs of lead wires numbered Tl through T4. Half of the run-winding circuit is between Tl and T2; the other half is between T3 and T4.

The two circuits have identical data, so they can be comparison tested. It is highly unlikely that both circuits would have identical problems. Use a microhmmeter or limited AC current for this test.

Disconnect leads Tl through T4. Compare the readings between Tl and T2 to the readings between T3 and T4. It may be necessary to turn the shaft slowly while recording the high and low readings. The results should be identical. If one circuit has lower resistance (with an ohmmeter) or higher amperes (with limited current), it has shorted coils.

Each circuit should also be tested for a ground to the frame of the stator.

Both a short and a ground require that the motor be rewound or replaced.

Figure 4.15 shows the numbering system for run-winding leads. (In 1967 NEMA interchanged the numbers T2 and T3 on single-phase motors.) To connect a dual-voltage run winding, just remember the following phrase: in on the odds and out on the evens. Run windings are always numbered Tl through T4.

Run Winding Shorted to Start Winding Run and start windings normally aren’t internally connected. The only exceptions are motors that have predetermined rotation, such as centrifugal pump motors. If there is continuity between the start and run windings, the motor will have to be rewound or replaced.

Run Winding Shorted to the Frame Disconnect the power. With an ohmmeter, test from all run-winding circuits to a clean spot on the stator frame. If the reading indicates a solid ground, the motor must be rewound or replaced. If the

reading is between 2 and 50 megohms, clean and dry the motor. If this doesn’t restore the reading to infinity, the motor must

be rewound or replaced. The bottom coils are subject to water-related low ohmmeter readings.

Open Run Winding, Dual-Voltage, Low-Voltage Connection If one circuit of a dual-voltage run winding is open, the motor will lose slightly more than half its power. The motor will start slower than normal unloaded. Depending on its internal connection method, it will either run smoothly or be slightly noisy.

An ohmmeter or test light can be used to identify the open circuit. If the open is in a lead-to-coil or coil-to-coil connection, it can be soldered. If a coil group is open internally, the motor must be rewound or replaced.

Submerged Motor If a single-phase motor has been submerged in water, but not energized, it may not need rewinding or replacing. Cleaning and baking its windings might be all that’s needed. However, the electrolytic capacitor shouldn’t be baked in an oven.

The windings should be tested first with an ohmmeter. (A wet winding should never be subjected to a test voltage that could arc through the wet slot insulation.) The baking temperature should not exceed 200 F. The ohmmeter test should read infinity after baking. The windings should be given a coat of air-drying varnish after they have been cleaned, dried, and tested.

When water soaks the slot insulation, a battery action can sometimes be detected. As long as the slot insulation is wet, a small voltage can be read (with a millivoltmeter) between the winding and the frame. A zero reading would indicate that the motor has been baked long enough. If the ohmmeter test now shows infinity, other test instruments such as a megohmmeter or surge tester can be used.

If a motor has been submerged, it should be disassembled, cleaned, and dried as soon as possible. If it has sleeve bearings, the shaft’s bearing surface will soon become rust pitted. The sleeve bearing motor should have the oil wick material and oil replaced. (Ball bearings should be replaced.)

The capacitor and start switch should be cleaned and wiped dry, but if there was prolonged submersion, the capacitor should be replaced. (Contaminated water causes the aluminum components inside the capacitor to corrode.) Oil-filled capacitors can be cleaned and wiped dry because they are sealed.

Bearings A worn sleeve bearing will make sharp clattering or rumbling sounds, especially when the motor starts. Disconnect the belt and move the

shaft back and forth in line with the load. There should be no movement. Small motors are usually replaced because replacing sleeve bearings isn’t cost effective.

Ball bearings are very noisy when faulty. They can be replaced, but the repair cost must be compared with the cost of a new motor. The age and condition of the motor is part of this decision.

The stationary switch should also be replaced at this time.

Rotor Cast aluminum rotors are used in most single-phase motors. If they have open rotor bars or end rings, the motors are replaced because it isn’t cost effective to repair them.

Loss of torque is a sign of open rotor bars or end rings. The motor won’t draw many amperes with no load and will start a load more slowly than normal. Loaded RPM will be lower than the nameplate value. The start and run windings won’t show any problems.

Two-Value Capacitor Motor

(Capacitor-Start, Capacitor-Run Motor)

The two-value capacitor motor has the same components as the capacitorstart motor (Fig. 4.16). An oil-filled capacitor is added for power factor improvement. The function of the two-value capacitor motor is discussed in Chapter 3 under “Oil-Filled Capacitor Connection in a TWo-Value Capacitor Single-Phase Motor.’

Troubleshooting methods for the two-value capacitor motor are the same as those for the capacitor-start motor. They are discussed earlier in the chapter under “The Capacitor-Start Motor.”

Following are additional problems and troubleshooting methods for the oil-filled capacitor.