You may be looking at the problem backwards, the freezing lines could be a symptom of the problem not the cause. Start by looking for blockages in the system that would cause lower air flow / heat exchange.
- Dirty filters.
- Clogged ducts.
- Closed dampers.
- Closed/Blocked vent covers.
- Dirty coils.
Basically if the system cannot exchange the heat/cold, it will not function properly and could lead to over heating/cooling.
Frozen lines could also be caused by high or low levels of refrigerant, so you'll want to get a service person out to check the refrigerant levels.
Here is a really good image to visualize how it works.
Depending on your specific equipment setup, you may have a variable speed condenser fan, variable speed compressor, or both. With a variable speed compressor and condenser, the system would ideally maintain stable superheat and suction pressure (effectively controlled by the Siemens superheat controller) and would maintain a stable temperature differential across the condenser. If the compressor modulates to meet the demand on the coil, the discharge pressure should remain reasonably stable.
I have seen in refrigeration and HVAC that a technician will often override the compressor and/or condenser modulation (either VFD or ECM controller) and put them "in hand" or into fixed speed mode. The first thing you should check is to see if the controllers are able to modulate speeds; if they have been put into override, set them back into auto control.
Whether the compressor is variable speed or not, if the compressor is being cycled on/off too rapidly and the condenser controller settings are off, it may be driving up and down rapidly to accommodate the fluctuating load from the compressor. It may also be tripping on current overload, as already mentioned in another answer. The following assumes that the condenser fan is not having electrical issues.
Observe the compressor behavior in conjunction with the condenser's cycling: is the compressor cycling off and on with the condenser? If so, verify that your compressor controller settings are not trying to adjust to changing suction pressure too rapidly (slow down the ramp time, increase the proportional band, etc.) Regardless of whether the compressor is fixed or variable speed, it should be running for 5-10 minutes at a time (at a minimum) in cold weather; any more frequently and the compressor is short cycling.
If the compressor is short cycling, correct this and verify that your condenser controls are no longer having issues.
If the compressor is not short cycling (or if you have fixed the issue and are still having condenser fan problems), take a look at the condenser controller settings. Verify that the set points are reasonable (i.e. a 20-30F, sometimes 40F temperature differential over your condensing medium is typical for HVAC systems; refrigeration typically runs between 10-20F). Also check the controller settings (proportional band, ramp time) for the condenser cycling. Like the compressor, the condenser should be running for longer periods of time, but it will not be running during extended periods when the compressor is off.
Verify your evaporator settings, too. The Siemens superheat controller should be trying to maintain a specific leaving air temperature temperature (typically 55F) with a temperature differential across the coil (anywhere from 5-30F) with 10-20F superheat at the outlet. When the thermostat sends a cooling signal to the unit, the compressor and condenser should cycle on, and the superheat controller should maintain a stable evaporating temperature.
If you are still having issues after verifying that the controllers are set correctly, begin checking the temperatures and pressures. Verify that the condenser pressure is somewhere between 20-40 degrees F over your ambient outdoor temperature (assuming the condenser is outdoors; if it is condensing in another conditioned space for some reason or using a fluid cooler, substitute that temperature for outdoor air temperature). Use a refrigerant pressure-temperature chart (US, SI) to check your discharge pressure versus the condensing temperature. If your ambient temperature is 90F and you have a 10F temperature differential (TD), your discharge pressure should be around 225 psig from the compressor (which corresponds to 100F). Do the same for the evaporator: the evaporator inlet pressure should be approximately equal to the condensing pressure and the evaporator outlet pressure should correspond to the evaporating temperature. With a 10F TD across the evaporator (i.e. 45-55F), the evaporating pressure should be around 70psig. The evaporator outlet temperature should be in excess of this value; the difference between the temperature that corresponds to the evaporator outlet pressure (measured using a transducer or pressure gauge) and the actual evaporator outlet temperature (measured using a temperature sensor) is known as the superheat. The superheat should match the commanded superheat on the controller.
If you are still having issues after verifying all of the above, post back with additional information about the system (model numbers) and the pressure/temperature readings from the controllers.
One final thing to consider: R-407C, being a blend of different refrigerants, will exhibit what is called "glide." What this means for you is that at a given pressure, you can have two observe two different temperatures (dew and bubble) depending on how much of the refrigerant is liquid and how much is vapor. This can cause problems with your controllers if they are expecting a specific temperature/pressure setting, e.g. you have P=20psig at the bubble temperature (when the liquid begins to vaporize) instead of the dew temperature (when the vapor begins to condense to liquid). The P-T chart I linked to above indicates whether the number is a dew or bubble temperature. The concept makes more sense when you look at a pressure-enthalpy diagram (I can't post a third link; Google "Dupont R-407C pressure enthalpy diagram"). Note that inside the saturation dome, the isotherms (lines of constant temperature) have a slope between the left and right sides of the dome. For single-component refrigerants, the line is flat/there is no glide.
Best Answer
The fan is not running at high speed to prevent icing, but for maximum cooling of the space. The cause of icing is that the system is undercharged with refrigerant or the throttling valve (expansion valve) is malfunctioning so that the low side pressure is so low that the temperature of the evaporator coil is below the freezing point of water.
I have always been told that an a/c system is supposed to be operated so that the internal temperature in the evaporator coil is 38 to 40 F. I have always supposed that this allows a safety factor in the case where the expansion valve closes too far.
My understanding of the dehumidify mode is that the cooling cycle is run continuously, but at a lower fan speed. The lower fan speed means less cold air is circulated, but the automatically controlled expansion valve opens to maintain the evaporator coils at the set temperature (of say 40 F). By running continuously at a lower flow rate all the water vapor is removed from the room but the room is not overcooled.
Our 26 year old Carrier is a base model 12 SEER which (at 42 kBTU/h or 3-1/2 ton) is undersized for our house in Dallas summer heat. On very hot days (~> 100 F) the compressor will run continuously for 6 to 8 hours in the middle of the day. The result is very dry air inside and I have to use lip balm in the middle of the summer or my lips get very chapped.
I have not tried to measure the humidity inside, but I may do that. We keep the thermostat at 74 or 75 F. I sit under a vent at the kitchen table and like to have cold air from the vent bathing me. This air would be very dry so perhaps this is why I get the chapped lips.