Capacitors and motors

[mark32]

I understand that a capacitor helps improve a motor's PF, by getting the current and voltage closer to unity.(?) I also know that many single phase motors are wired so they need a capacitor to start and/or run. Now my question is, aside from the PF correction, what does a cap do do a circuit that helps a motor to start and/or run? How does a motor "See" a cap? When a cap discharges, how does that help a motor operate?

 

[72.5kv]

Single phase motors need to have rotating magnetic field inorder run. When AC is applied to the motor it also charges the cap which is part of the start winding of the motor. The cap provides enough of a phase shift to get the rotor turning. Once the motor is up to speed the starting circuit drops out.

 

[rattus]

The cap provides enough of a phase shift to get the rotor turning. Once the motor is up to speed the starting circuit drops out.

True for a split-phase motor, but some motors leave the cap in the circuit after starting.

Residential AC compressors typically use a combination of start and run caps; a start relay causes the start cap to drop out after starting. This is done apparently to obtain extra torque when starting the compressor.

AC fans and blowers use a single cap for start and run--no relay. Not much torque needed to start a fan.

Of course, there are many variations to this basic scheme.

 

[Jraef]

To expand on both of those posts, in a general sense an AC motor feed from a 1 phase source has no way of "knowing" which way to spin; the magnetic fields in the windings are essentially diametrically opposed so it would just sit there humming. One way to get it spinning is to create a false "phase shift" by having a start winding and a capacitor in the circuit that creates a delay in the voltage in that part of the circuit reaching it's peak (capacitive time constant, the caps have to charge up). That little bit of delay, compared to the field established by the other set of windings, starts the motor rotating just a hair, but that's enough. Some then switch that cap out of the circuit, others leave it in and the design inherently makes it insignificant, others yet have no capacitors at all and use a "shading coil" to create that pseudo-shift.

Like he said, there are lots of ways to skin that cat.

Examples:

 

[rattus]

Two phase?

Two phase?

One could treat these cap start motors as two-phase motors. Right? Not very good two-phase, but good enough to be powered from a single phase source? Right?

 

[mark32]

Thank you all for the replies. So, the sole purpose of a cap is to cause a phase shift in the circuit, which in turn helps PF and create a magnetically revolving field to start a single ph motor, correct?

 

[rattus]

Thank you all for the replies. So, the sole purpose of a cap is to cause a phase shift in the circuit, which in turn helps PF and create a magnetically revolving field to start a single ph motor, correct?

It is doubtful that PF is significantly affected. The cap would be wired in parallel with the motor windings to correct PF.

 

[LarryFine]

One way to get it spinning is to create a false "phase shift" by having a start winding and a capacitor in the circuit that creates a delay in the voltage in that part of the circuit reaching it's peak (capacitive time constant, the caps have to charge up). That little bit of delay, compared to the field established by the other set of windings, starts the motor rotating just a hair, but that's enough.

Actually, a cap's charging current is the greatest when the voltage is changing at its maximum rate, which is when it's crossing the zero point. Therefore, the current peak precedes the voltage peak by 1/4 cycle.

That 90-degree-leading current runs through the start windings, generating the quasi-rotating magnetic field you described, and quits when the motor speeds up, usually through a centrifugally-activated switch.

 

[mark32]

You just blew my mind

You just blew my mind

It is doubtful that PF is significantly affected. The cap would be wired in parallel with the motor windings to correct PF.

Ok, would it be correct to say that when a cap is wired in series it's purpose is to help start and/ or run a motor and if a cap is wired in parallel it's purpose would then be to help improve a motor circuit's pf?

 

[TxEngr]

On 3 phase systems, capacitors are placed in the starter for both power factor correction and to prevent sagging of voltage on start. This is particularly helpful when the starter is a long distance from the power source. When the motor starts, the voltage will sag due to cable impedences and voltage drop on the cable requiring additional current to meet horsepower requirements. A set of capacitors will help reduce this sag. I'm most familair with this application of medium voltage well pumps for industrial plants.

 

[rattus]

Ok, would it be correct to say that when a cap is wired in series it's purpose is to help start and/ or run a motor and if a cap is wired in parallel it's purpose would then be to help improve a motor circuit's pf?

A cap in series with a start winding results in a current phase shift which lets the motor start and perhaps run as a 2-ph motor. It will provide some PF correction as well, but its size will be dictated by the motor requirements.

A cap in parallel with the line results in a leading current phase angle which compensates for the lagging phase angle of the load. Its size will be dictated by the phase angle of the load.

 

[rattus]

Imho:

Imho:

On 3 phase systems, capacitors are placed in the starter for both power factor correction and to prevent sagging of voltage on start. This is particularly helpful when the starter is a long distance from the power source. When the motor starts, the voltage will sag due to cable impedences and voltage drop on the cable requiring additional current to meet horsepower requirements. A set of capacitors will help reduce this sag. I'm most familair with this application of medium voltage well pumps for industrial plants.

I would think that these caps improve the PF, and that in itself reduces the starting current. It is possible that two sets of caps are used, one for starting and one for running.

 

[Hameedulla-Ekhlas]

Some information:

Capacitor Start Motors:

The capacitor start motor is essentially the same as a split phase motor where the two windings are out of phase with each other, providing the necessary torgue for starting the motor to rotate. The addition of the capcitor in series with the start winding adds to the initial torque and causes the current in that winding to lead the running winding phase voltage, obtaining a greater angle of displacement between the start and running windings. In a capacitor start motor the starting winding and the capacitor are disconnected by centrifugal switch.

Capacitor Run Motors:

The capacitor run mtor design is ver similar to the capacitor start motor. A capacitor run motor has the starting winding and the capacitor in series at all times. Because the capcitor remains in the circuit at all times, a lower value of capcitor is used. The capacitor run mtor has a higher running torque than a capacitor start motor. In a capacitor run motor, the capacitor remains in the circuit while the motor is running.

Capacitor Start and Run Motors:

Capacitor start and run motors combine the advantage of the capcitor start motor which has high starting torque with that of the capacitor run motor which has high running torque. the capacitor start and run motor starts with a high value and a low value capacitor connected in parallel with each other but in series with the starting winding. This provides a very high starting torque. The centrifugal switch disconnects the high value capacitor at eighty percent speed, but the lower value capacitor remains in the cricuit.
In a capcaitor start and run motor the staring capacitor is disconnected when the motor reaches speed but the running capacitor remains in the circuit.

 

[mark32]

A cap in series with a start winding results in a current phase shift which lets the motor start and perhaps run as a 2-ph motor. It will provide some PF correction as well, but its size will be dictated by the motor requirements.

A cap in parallel with the line results in a leading current phase angle which compensates for the lagging phase angle of the load. Its size will be dictated by the phase angle of the load.

Thank you Rattus, so wouldn't the resulting "current phase shift" from a series wired cap be the same as from a parallel wired cap, except to a lesser degree? In other words, would the current phase shift be 45 degrees from the supply and the parallel cap be 90 degrees from the supply? Am I looking at these two scenarios incorrectly?

Thank you Hameedulla-Ekhlas for the post as well. I see you are currently in Kiev, what's it like there? I had plans of visiting the place a few years ago.

 

[rattus]

Thank you Rattus, so wouldn't the resulting "current phase shift" from a series wired cap be the same as from a parallel wired cap, except to a lesser degree? In other words, would the current phase shift be 45 degrees from the supply and the parallel cap be 90 degrees from the supply? Am I looking at these two scenarios incorrectly?

The current through any cap leads the cap voltage by 90 degrees. The net phase shift in a start winding is determined by the cap itself and the effective impedance of the start winding. Ideally, this net phase shift is 90 degrees ahead of that of the run winding.

A PF correction cap is sized to cancel the inductive current of the load. In practice however, full compensation is not done.

Help Winnie, I am digging a hole for myself!

 

[mark32]

Rattus, I am guessing all caps are not created equal, for example, a start cap maybe be different in construction from a run cap and likewise, a cap used to correct a low PF will be constructed differently as well?

 

[rattus]

Rattus, I am guessing all caps are not created equal, for example, a start cap maybe be different in construction from a run cap and likewise, a cap used to correct a low PF will be constructed differently as well?

That is true, but some expert will have to provide the fine details.

 

[Hameedulla-Ekhlas]

?

Thank you Hameedulla-Ekhlas for the post as well. I see you are currently in Kiev, what's it like there? I had plans of visiting the place a few years ago.

It is a very nice as usual and it is a good place too. Hope to plan for visiting in the future.