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How does an electric motor capacitor work? This article gives a short simple explanation of how a motor starting capacitor or motor run capacitor actually works to get a motor spinning or to keep it spinning efficiently.
Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the most common symptom is that the motor won't start.
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The starting capacitor, used on motors such as air conditioning compressors that require a high starting torquie, helps a motor start spinning by creating a high-torque, rotating, electrical field in the motor.
To deliver this starting kick or torque boost, a start capacitor will always have a high microfarad (MFD or µF) rating compared to the run capacitor used for the same motor. Electrolytic type starting capacitors are designed only for intermittent use (each time the motor starts).
Starting capacitors extend electric motor or HVACR compressor motor life, and surprising to non-engineers, a hard start compressor actually reduces the operating cost of the equipment it is serving by reducing the in-rush of current during the start-up phase of motor operation.
But starting capacitors themselves can fail for any of several reasons including exposure to harsh conditions, weather, corrosion, loose connections, or improper control, selection, or use.
If the start capacitor has failed the symptom is that the motor won't start. If either or both start and run capacitors are defective the motor may try to start but will hum and won't keep running.
Watch out: If you observe a humming electric motor that is not starting we suggest that to avoid damage you turn off the system while waiting for repairs.
The starting capacitor works by "accumulating" a large electrical charge inside the capacitor. During compressor or other motor startup, the start capacitor releases its charge to give a voltage "boost" to get the electric motor spinning.
During an electric motor start-up (such as an air conditioner compressor motor and some fan motors) where a starting capacitor is included in the circuit, with the added charge stored in the capacitor, run-start and start-common voltages increase to a maximum value to start the motor spinning.
The total supply subsequently current drops back to normal run conditions when the start device is dropped from the circuit - the motor continues to run.
This "electrical starting booster charge" can be particularly needed if an air conditioner is suddenly switched off and back on when it has been operating. Suddenly switching off an air conditioning compressor leaves a high "head pressure" inside the compressor which can provide extra mechanical resistance when the motor is attempting to re-start.
Traditionally electrical capacitors were also called "accumulators" for this reason. The capacitor's electrical charge is released at motor start-up time, gives the compressor motor or other electrical motor a boost for starting.
As we explain at HOW to CHOOSE a START / RUN CAPACITOR, once a starting capacitor has provided the necessary boost to get the electric motor spinning, to avoid possible motor damage the capacitor has to drop from the electrical circuit, leaving the motor to run on normal operating current.
Typically a centrifugal switch or possibly a PTC or PRD drops the start capacitor from the motor's electrical circuit once the motor has spun up to operating speed.
We illustrate and explain these special controls or features below at ELECTRIC MOTOR CAPACITOR SPECIAL FEATURES
To avoid burning out the motor windings or the capacitor itself, the huge current delivered by the start capacitor has to be turned off once the electric motor is running.
There are two reasons the start capacitor can’t stay in the circuit full time.
1. The start winding of the compressor can’t carry such a heavy current continuously without overheating and burning out.
2. The start capacitors are made very compact and would overheat in a short while because they aren’t big enough to dissipate heat as rapidly as it’s generated.
The plastic casing on the start capacitor also plays a role in its tendency to overheat.
When a start capacitor does overheat, the little putty filled hole in the top of the capacitor blows and all the fluid inside runs out, causing the capacitor to fail. - SUPCO Hard Starts booklet cited below.
Some starting capacitors such as those designed by and used in Copeland HVACR compressor motors, include a bleed-through resistor wired between the capacitor's two terminals. Typically a 15K-18Kohm two-watt resistor is used for this purpose
Copeland describes the bleed-through resistor as follows:
The use of capacitors without these resistors will result in sticking relay contacts and/or erratic relay operation especially where short cycling is likely to occur. This is due to the start capacitor discharging through the relay contacts as they close, following a very short running cycle.
The resistor permits the capacitor charge to bleed down at a much faster rate, preventing arcing and over-heating of the relay contacts.
... If sticking contacts are encountered on any start relay, the ﬁrst item to check is the start capacitor resistor. If damaged, or not provided, install a new resistor, and clean the relay contacts or replace the relay.
Suitable resistors can be obtained from any electronic parts wholesaler or retailer. - Copeland Electrical Handbook p.A5- A6.
A pack of five 15K ohm [15K Ω 2W], 2 Watt Resistors from Electron Components, an online vendor in July 2018, sells for as little as less than $1.00 USD.
A mechanical centrifugal switch may be used to disconnect the starting capacitor when the motor speed has reached a critical rpm, typically about half of the motor's run speed.
Centrifugal switches for electric motors are sold with two to 6 poles and are specified as having specific-range of switch CUT-OUT (open or off) and CUT-IN (close or on) speeds.
For example a centrifugal switch for a modern "high speed" electric motor rated at a running speed of 3450 rpm under full load might cut in or close from 1000-2000 rpm, and might open at 2600-3000 rpm.
Positive Temperature Coefficient devices are the traditional means of dropping the capacitor from the motor circuit once the motor has successfully started.
PTC devices are basically a thermistor-type device, using a tiny electric heater that use the change in electrical resistance of the heating element to open an electrical switch that removes the start winding in the motor from the run circuit. The switch opens in less than a second after the motor has started.
PTC devices have the advantage of being simple and avoiding the need for more complex electrical wiring of a motor starting system.
Supco explains that this device is unable to sense whether or not the motor has successfully started, and if the motor does not start, several minutes are needed to let the heater cool down before the motor restart can be attempted again.
This cool-down time provides a safety margin that helps protect against burning out the windings of a hard-starting motor.
Potential Relay Devices use voltage sensing (The Supco method) or current sensing devices (two different approaches) to determine when to release the starting capacitor from the motor run circuit.
Supco points out that "The electronic potential relay is inherently more reliable and precise than the older type mechanical potential relay." 
Both of these start capacitor control approaches work fine, and typical HVAC or residential appliance motor repairs the technician won't need to consider which method is being used to control the capacitor as long as she/he follows the manufacturer's recommendations on the product for its selection and use.
SUPCO offers this explanation of why a potential relay hard start kit would be used:
Most single phase air conditioners and heat pumps use non-bleed thermostatic expansion valves (TXV’s or TEV's) to control refrigerant.
A problem with TXV’s occurs when a reciprocating compressor shuts off, refrigerant pressures don’t fully equalize. Pressures do equalize in scroll compressors; which, typically do not need hard start kits for this condition.
In a reciprocating compressor, the discharge pressure will drop to about 150 psig and the suction pressure will rise only to about 100 psig.
When the compressor tries to start, there’s too much load for the starting motor torque to overcome. This is especially true if the supply voltage is low. To increase starting torque, a start assist device can be used.
When a TXV is used, a potential relay hard start kit is employed. This will increase starting torque by a minimum of 300 % over using just a run capacitor.
... Systems with capillary tubes or fixed restrictors usually don’t need a full hard start kit, unless the compressor bearings are tight. In such cases, the compressor is probably near the end of its useful life anyway. - SUPCO Hard Starts booklet cited below.
The SUPCO booklet we cite below provides wiring connection details for 3-wire and 2-wire connected potential relay / hard start kits.
In this case, the device can add as much starting torque as a 3-wire hard start, but installation is made simpler and cost is usually lower.
The Figure [shown here] illustrates the wiring diagram for a 2-wire hard start.
Two wire hard start kits are connected in parallel with the run capacitor for Permanent Split Capacitor (PSC) type compressors; and connected to the Start and Run terminals on the compressor for Capacitor Start Induction Run(CSIR) type compressors.
There is no need for a third connection to common wire.
The start relay has normally closed contacts, so when the compressor starts, both the run and start capacitors are connected to the start terminal.
This causes a very high current to go through the start winding when power is first applied. This high start current increases the starting torque of the compressor motor enough that the motor will start even though the refrigerant pressures haven’t equalized, or in an “under-voltage” condition.
Once the compressor begins running, the voltage across the start winding increases. This occurs because the motor acts partly like a generator and partly like a transformer.
The start relay coil is connected in parallel with the start winding. When the voltage across the start winding increases above the pick-up rating on the start relay coil, the start relay contacts open. The start capacitor is then out of the circuit. - SUPCO Hard Starts booklet cited below.
Continue reading at TEST a MOTOR START or RUN CAPACITOR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Feb 21, 2014) Anonymous said:
is it possible to rebuild a submersible well pump
Yes ... maybe; it depends on the condition of the pump casing, parts, an in my opinion, an accurate diagnosis of the trouble; at some point rebuilding is not cost effective. Indeed there are specialists (H Shreck in Poughkeepsie used to be one of them) who rebuild electric motors of all kinds.
There are also many water pumps whose design makes it quite reasonable to replace pump impellers.
So to answer your question, I dunno - it depends on what's broken.
(June 1, 2014) Anonymous said:
The fan on my Conquest 80 will not stay on (interior fan). It starts and then stops after a few seconds. It was installed in 2005. Help.
The start capacitor is for getting a motor started, not keeping it running.
Often motors have two windings, a start winding and a run winding. Your motor's run winding may be damaged.
Or your fan motor may require a dual capacitor (start and run) or a separate run capacitor to keep it spinning.
Or your system may have a faulty control.
(June 7, 2014) Joel said:
I have a commercial hood & exhaust fan (120v) running in a food truck that is used for catering. We occasionally need to run off a generator (3000w) and we've found that the fan cause the generator overload to trip. The fan has been tested and observed to draw 8 amps running at full speed. Given this, would it be possible to add a hard start kit, such as SUPCO SPP4E? Will need to confirm the motor size, just wondering if this would help.
The total draw of all items is less than 20 amps and the generator is capable of supplying 25 amps constant.
Thanks for the advice.
Joel I think the problem lies elsewhere and needs some further diagnostics. I suspect that your total current draw is exceeding the ability of the generator - you may be running more than the fan, such as lighting, a cooler, toasters, other electrical appliances. If it were just the fan, drawing 8A, it has no business tripping the breaker.
Put another way if the problem is the fan and there are NO other appliances running, then there is a failing fan motor drawing high current, or an electrical short circuit or other unsafe condition to find and fix.
A start capacitor or a run capacitor won't change the current drawn by the motor.
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The SUPCO E-Class Series comprise the most advanced developments in start device technology:
1. Voltage sensing technology that monitors for motor start (current sensing devices require internal fuse protection).
2. A 2-wire connection that simplifies installation
3. A secondary timing circuit that ensures that the capacitor is not permanently left in the start winding circuit
4. A fully electronic device - minimizing the limitations of mechanical devices and secondary fusing associated with triac devices
5. A start device matched with an appropriately sized capacitor to cover the range of compressors for the intended application (one size does not fit all)
The use of compressor start devices results from a need to ensure that a compressor (usually air conditioning) will start under voltage conditions that are less than ideal. As discussed, several options exist in the market to address compressor start concerns. Start devices exist in many forms for specific applications. SUPCO provides a full range of products in all relevant technologies to effectively match the proper start device to the application. Care should be taken to utilize a device that meets the requirements of the job. Extra caution should be observed when employing the "one-size-fits-all" and "a bigger capacitor is better" approach to applying a start device. Consult SUPCO, a manufacturer with a complete product range, to ensure the greatest success in the start device application.