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Our house air conditioner went out last Saturday afternoon on a +100 F day. Was able to diagnose the problem itself as a problem with the starting circuit of the big compressor fan of the outside compressor unit. If I gave the fan a strong push with a stick to start it rotating, then the fan motor would start up and the air conditioning would work fine. An AC technician later determined that the compressor fan needed a new start-up capacitor, and now the AC works fine.

My question is this: Why are large 2-ton house air conditioners designed so that their compressor fans require start-up capacitors to turn on? The diameter of the compressor fan on our house AC appears to be roughly the same as that of an inexpensive box fan that I can get at Walmart, and the ac motors on those simple box fans don't require a start-up capacitor to turn on, do they? You just plug them in and they smoothly start revving up to speed all by themselves. So why do house air conditioners use compressor fans that need these trouble prone start-up capacitors, and why can't a simple self-starting ac motor like that of a box fan be used instead for the compressor fan?

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The simple answer is that for a box fan the load on the motor is proportional to its speed so its starting torque is very low. In the case of a compressor the motor is working against significant resistance just to turn over.

Simple single phase AC motors in high starting torque applications are often configured so that they are wired differently for starting to provide more starting torque, in this case the capacitor is essentially part of an automatic switching circuit which alters the motor wiring from staring to running configuration.

There are other ways to achieve this such as star/delta configuration or by driving the motor by a digitally controlled inverter. But the capacitor method is a fairly cheap and simple way to drive high powered single phase AC motors.

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  • $\begingroup$ But I'm not talking about the compressor itself here. I understand how the compressor motor used to pump refrigerant fluid would be starting under high-torque conditions and how it might need a starter capacitor to get going. No argument there. But I'm talking about the big air fan that you see on top of most common house ac compressor units, the big air fan which is used to blow away the heat that is pumped out of the house. That fan does not start under high torque conditions. In fact, it seems that, in principle, a Walmart box fan could do the same function. $\endgroup$
    – Barney Cowell
    Jun 20 '17 at 20:42
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    $\begingroup$ Air conditioning units often have a 'dual run' capacitor which supports both the fan and compressor motors. Equally the fan for the heat exchanger may well be under more load than a simple box fan as it needs to develp higher static pressure to push air th rough the radiator and as such may have more blades thus more inertia. $\endgroup$
    – Chris Johns
    Jun 20 '17 at 20:56
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An induction motor works by trying to sync up the rotating squirrel cage to the sine wave of the mains voltage.

With a single phase supply there is no difference between going forwards or backwards so you need some way of setting the direction on startup.

For low torque applications a shaded pole is enough. That is a small loop of copper on the core placed in such a way that the magnetic field gives a push in the right direction.

For higher torques they instead have a secondary winding perpendicular to the main winding. A capacitor is used to create a LC circuit to run the secondary winding slightly out of phase with the main winding. Once the motor is up to speed a centrifugal switch may be used to disconnect the secondary winding and starter capacitor which are often not built for continuous use.

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