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compressor start capacitor

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IamSmooth

Tue Jul 16 2013, 11:17PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Compressors use a start and run winding. The start winding has a run capacitor. We can say its 80uf. A start capacitor, in parallel with the run capacitor, helps phase shift the current in one winding, to reduce the net current. What I fail to understand is if one adds a start capacitor (say 200uf), the net Zc is reduced since the total capacitance increases and

Zc = 1/2piFC

If Zc is reduced, the start winding Zinductance - Zc results in a smaller phase shift when compared to the run winding, which is predominantly Zinductance

This seems opposite of what is intended.

I can only guess that with the lower Zc, the current through the start winding must be more, and offsets the current in the run winding. I'll have to make some measurements on my air conditioning unit.

...

Wed Jul 17 2013, 04:45AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I am pretty sure your second explanation is correct, the idea is to just give the motor a bit of a kick in the pants to get it turning from a standstill before it has time to properly sync with the line, by allowing more current to flow

radiotech

Wed Jul 17 2013, 06:48AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

In an squirrel cage ( SC) motor, without a capacitor, the start winding
is wound with smaller guage wire and is placed in the stator displaced
angularly from the start winding. At start, both winding may be in parallel.

The only reason torque is developed is because the phase angle of the
current in the start winding is displaced by the resistance in series with the
inductance of the coil This displacement is not equal to the run winding, because it has larger wire, and less resistance.

These motors never are in sync with supply line. The difference between
the line frequency, and the speed they run at is called the slip.

If a 60 Hz motor is labeled 1725 RPM, it is the full load speed. unloaded,
it may run just under 1800 PRM.

A compressor needs more start torque, so a heavier start winding is used,
and a capacitor provides more phase angle than resistance does. The displacement of the capacitor is the same direction as is resistance.

If you were to replace the capacitor with an external inductance of appropriate
reactance, the motor would start in the reverse direction to what the capacitor
did.

The motor designer chose that capacitor with care, and it should not be messed with.

When an SC induction motor starts it will run without an auxiliary phase.
Why, has been the subject of debate for 100 years, and the two models,
cross and rotating field come with ample existological data gained by
years of experience.

The run winding improves power factor and also changes the motor to
a two phase type. A two phase motor, when connected to a two phase
power supply does not need any starting auxiliary. (it starts by itself)

A two-value capacitor machine runs smoother and is less likely to growl at the
slip frequency, like common SC motors do, necessitating shock mounts to make then not vibrate their mountings. The run capacitor is oil continuous duty and the start capacitor is short cycle once every hour or so.

If you took a motor, and placed it with a torque arm stopping rotation,
different values of capacitors would produce different torques with the optimum
when main winding current is the same at run winding current (quadrature).
If you are so motivated to experiment with motors, you can make a stick
dynamometer like the one shown below.

The best reference I know is C.G. Vienott, Fractional--& Subfractional--Horsepower Electric Motors.

1374043629 2463 FT155867 Friction Dynamometer Inside

1374043690 2463 FT155867 Friction Dynamotor

Shrad

Wed Jul 17 2013, 08:00AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

my fridge capacitor runs with a PTC feeding the start winding

as an alternative, I have a 555 monostable triggering a relay which connects a power resistor briefly, in place of the PTC

maybe there are compressors using the same PTC design

if someone has a more convenient way of starting a fridge compressor I'm open to suggestions :)

radiotech

Sat Jul 20 2013, 10:41AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Zc = 1/2piFC--- Xc = 1/377C for 60 Hz. simpler.

Erlend^SE

Sat Jul 20 2013, 07:28PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Sharad: a current-relay is a very common way (or used to be common)

a stalled motor pulls more current -> activates start-windings.
The motor spins up and the current relay turns off.

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