Start Verses Run Capacitor

What's difference between 'run' and 'start' capacitor - Practical ..

Completely different construction, and design limits.
Start caps are actually electrolytics which are designed for operation on ac. These have a limited number of starts, and are only allowed to be in the circuit for a few seconds.
Run caps are actually oil-filled caps. These have a virtually unlimited lifetime, and are allowed to be in the circuit continuously.
Start caps have a very high capacitance to volume ratio.
Run caps have a low capacitance to volume ratio.

Motor capacitor - Wikipedia, the free encyclopedia

1. A motor capacitor, such as a start capacitoror run capacitor (including a dual run capacitor) is an electrical capacitor that alters the current to one or more windings of a single phase AC induction motor to create a rotating magnetic field.

Hi lyn;

For small motors, Start capacitors are often non-polarized paper capacitors generally of 400V for 230C mains single phase power. Run capacitors are also paper capacitors but of 1/2 or 1/3 value. These often go bad in 2 years.

I think Polystyrene capacitors perhaps may be better. I have not seen these in Fan or water pump motor / starter circuits in India.

Some good reference to quality capacitors for power electronics will be ideal.

Relative to "power factor correction", I installed these in my welding power supply (Miller Synchrowave 250) to prevent circuit breaker tripping (purchased the kit from Miller). These were oil filled capacitors intended for continuous duty across the power line. I saw no difference in their metal can build method, relative to run capacitors.

Start capacitors with most fractional hp motors are start/run dual duty, they are used for pf correction....some motors have dedicated starting circuits that drop out at 3/4 speed, usually these are the round black higher value caps....Basically with a dedicated start circuit you get a higher starting torque generally used on pumps of all kinds...fan motors are usually the start/run capacitors for increased starting torque and pf correction....pumps with starting circuits are usually equipped with run capacitors for pf correction as well.....

A run and a start capacitor do the same thing electrically. They change the phase angle of the current running through another set of windings in the stator. This is the most common method of running a two or three phase motor with a single phase power source.

A start capacitor does this only during the starting cycle while a run capacitor will do this constantly while running. Most starting capacitor current paths flow through a centrifugal switch that opens when the rotor gets up to speed. As such the tolerance of a start capacitor is much wider because it is normal for a large and varying amount of slip to exist most of the time that current runs through a starting capacitor. Additionally this capacitor can have higher losses for it will not have current flowing through it for long. A starting capacitor and most run capacitors will shift the current by 90°. To run a three phase motor with a single phase power source requires two run capacitor paths where one path is 120° and the other is 240° out of phase with the power source. [I digress for a moment. Other phase angle and number combinations (5 phase 72°) motors have been used in the past to reduce cogging effects but are rarely seen today.]

What are cogging effects?

Cogging is the non-uniformity of torque that a motor will make depending on the position of the rotor. This effect should not be confused with the change in torque that happens with a change in slip. These changes in torque happen with a constant slip. Some will claim that cogging does not happen with an induction motor but it does. The rotational inertia of the rotor mass can make it difficult to detect cogging in a particular induction motor but it is still there.

thank you.

This is the most common method of running a two or three phase motor with a single phase power source.Light bulb moment~ Is this how my single phase lathe motor can run in reverse? If the answer is yes; does the start/run cap configuration also reverse? That is, does the start cap become the run cap when in reverse?Jim

No it doesn't...The start and run capacitors remain what they are.

In some configurations, one of the windings is reversed, this is normally done where both windings have differing inductances and resistances.

In other configurations where both windings are identical, the start and run windings are interchanged. The current through the caps is redirected so that they are now connected via the other winding. This reverses the phase rotation in the stator and is generally easier and cheaper to implement than the first method

Below is the directional part of a wiring diagram of a control system that I designed and build for a conveyor company, it uses the second configuration. You can see how the reversing switch alters the connections, thus making the start winding the run winding and vice versa.

Thank you so much. My lathe keeps 'blowing' start caps and now i can work towards a solution. From other posts ( Yours? ) i can approximate the 'should be' values and see if it has been bastardised in the past. If i was smarter i could get the exact values required, but i'm not.
Jim

First point is that it is important to use the correct size microfarad caps - close enough is not really good enough even though it may work. A local motor rewinder is probably the best place to get correct info on sizing for your particular motor. Go higher on the cap voltage if you can fit the larger can. A start cap should be at least equal to the RMS of the supply voltage, a run cap should be at least 1.56 times RMS.

Next point is when and why is the start cap failing?

If it is failing when the motor has been running for a little while, I would be looking at the start circuit as that cap should not be in circuit any more than a second or so.

If it has a centrifugal switch, is it activating to disconnect the cap? The weights could be jammed, the contacts could be fused etc.

If it has a voltage sense disconnect or a timer, is that working?

Is there a bleed resistor across the cap? If not then the cap may be failing because it is not able to discharge.

Thanks again. I understand most of what you said and can investigate further. The one bit i didn't get is the bleed resistor across the cap. There isn't one. I didn't know there should be one and i don't know what sort of value and power i should use. It is 240v maybe 1000w motor ( didn't know i would be doing this post ) that has never been able to get to top speed on first start up. As it is virtually new (old but still had packing grease on ) i thought it needed to be run in. I would run for a while at slow speed while a had a cuppa then start work. It would take at least 15 s to get to top speed. Took the caps off a year ago and took them to a rewinder and he just checked resistance and declared them both bad. I was dubious but as new ones were cheap i bought both. I fitted them and the lathe got to top speed as it should, for the first time ever. Happy me . But now it is back to the bad old way.
As i intimated, i am not prepared for posting as i would have all the required details BEFORE posting. I just jumped in on this thread.
That's not to take it away from you. I am very grateful for your help on my problem. I would give you several GA's if i could.
Jim

As the start capacitor is disconnected under power, there is a likelihood that residual voltage will be present and the capacitor charged, the bleed resistor is permanently connected across the terminals to remove any residual voltage from the start capacitor.

They are not always used but it is safer to do so, and essential if your motor uses a potential relay in the start circuit as the relay contacts can be damaged.

Not having one doesn't necessarily cause the capacitor to fail unless the voltage rating of the cap is too low. Caps are normally rated at peak voltage, but your household supply is rated at RMS which is a lesser amount. The import of this is that if you have a 240v RMS supply and use a 240v peak rated capacitor, then it will actually be subjected to around 340v DC for a prolonged period if the capacitor happens to be disconnected at the top of the sinewave. This can cause failure

Commonly a 20k 2w resistor would be adequate to provide timely discharge.

From what you say, it sounds like your original start cap was faulty, and leaving the start circuit energised for such a long period (normally 1 second is enough) may have stressed other parts of the start circuit as well, so it may not be the cap that is the problem this time.

Lathes don't normally require a lot of starting torque unless you are turning large diameter chucks, face plates, etc., so start cap value is not all that important. For a 1kW motor, I would think that something with a mid-range of 65 microfarad would be acceptable, but you should confirm this with your local motor rewinder as he would be better placed than I for this sort of information.

That was a very good and clear explanation of something I never really understood before.

Just a little correction to my post.

This sentence should read "the start capacitor is of too large value to remain in circuit continuously and would fail (see below), but it is an advantage to retain some capacitance in the start circuit during running to maintain torque, and so a smaller capacitor is connected in series with the start winding, and the start capacitor is then connected in parallel with them the run capacitor and switched out when a set speed (or time etc.) has been reached.

And for the pedantic among us, the word momentum would have been a better choice than inertia for that explanation.

Most things are covered in the answers above. I like to add that start capacitors have larger farad value and short time rated. While run capacitors may have smaller values of farad in same rating motor and can run continuously. Time for start capacitor is decided by designer of winding. And being dis connected with the help of centrifugal switch in or outside the motor. Or even with electronic switch which requires less of mechanical alignment to be observed while manufacturing. Combination of the two types of capcitors gives better power factor and efficiency in single phase motors. Though it is lesser than 3 phase versions.

These are mainly AC rated capacitors that are used to drive AC motors. The construction is similar in both types. The difference is in the role of the capacitor in the process of driving the motor.

A start capacitor (as its named implies) is used to generate enough current for the start of the motor. This requires high current. These start capacitors should have a high capacitance (normally in the order of 400 to 500 microfarads). Start capacitors only work (fired) when the motor starts. Once the motor is in stable rotation the capacitor ceases to operate.

A run capacitor is used to produce enough current to maintain the rotation of the motor. For this operation it is required much less current than at the start of the operation of the motor. These capacitors have a capacitance - in general - in the order of 5 to 20 microfarads. These capacitors are in continuous operation for as long as the motor is running.

Here you have an interesting youtube video explaining the difference between these two capacitors: http://www.youtube.com/watch?v=OMd9QkinXz4

Abe

The capacitors have nothing to do with producing high currents to start or run the motor, they are there simply to alter the phase relationship between the start and run windings in order to generate the rotating magnetic field that causes the rotor to spin. The start capacitor is larger in order to produce a larger phase shift, and due to its construction type, it cannot be left in circuit for more than a couple of seconds.

Their only construction similarities are that the plates are spiral wound and are in a can, other than that their makeup is entirely different.

While the start capacitor itself does not allow for more current to be drawn, the whole start circuit and the high slip relationship during starting will draw a lot of current.

So this is an understandable "slip of the tongue".

[Sometimes I just can't stop myself from making a bad pun.]

I agree that they will draw high current on starting, but my point was that it is not the capacitor causing this.

I offer a comparison of split phase versus capacitor start motor starting currents as an example:- A split phase motor (no capacitor in the starting circuit) draws typically up to 10 times its run current during starting yet produces relatively low torque (up to 1.75 times its run torque). Start circuit current will be considerably less than the run circuit current due to the higher resistance of that winding.

The same horsepower capacitor start motor will draw only about 6 times its run current for a much greater starting torque (up to 4 times its run torque). Start circuit current will be a little less than the run circuit current due to the higher impedance caused by the addition of capacitive reactance.

So the addition of the capacitor actually reduces the starting current.

One reason for this is, as you stated, the issue of slip... Slip, torque and current are all greatest before the rotor begins to turn, and reduces as the motor gains speed - the faster that happens, the lower the starting current - and it gets to speed much more quickly with the capacitor in circuit.

Another reason is that a run winding typically has a 70° current lag, the addition of a correctly sized start capacitor will create a 20° leading current in the start winding thereby giving a full 90° phase shift. To achieve this, the capacitive reactance is greater than the original inductive reactance of the start winding circuit and so its impedance increases, and start winding current decreases.

I think we've blown NotUrOrdinaryJoe's capacitors.