PTC Relay and compressor cycling

[nevinjohn]

When using PTC relay along with start capacitor, after the start up of the compressor wont the relay engage soon after cooling down? How is this prevented? To put in other words, lets say the compressor kicked in and its required to run for 10min. In the mean time the PTC cools down in 1-2 min. Wont this affect the operation of the compressor?

[seanf]

does it not draw a small current while the compressors running, keeping the ptc warm.

[Brian_UK]

To reinforce seanf reply here is the description of operation by Danfoss...

https://www.google.com/url?sa=t&source=web&rct=j&url=https://assets.danfoss.com/documents/DOC000086435243/DOC000086435243.pdf&ved=2ahUKEwigu4K4uYTqAhVyoFwKHaW9A8QQFjAWegQIAhAB&usg=AOvVaw2RQGazR7rovaBc0D2ifGcK

[Glenn Moore]

The Standard PTC relay device has a PTC chip in series with the start winding. PTC or Positive Temperature Coefficient devices when cold, have a very low resistance, but when heated their resistance will go very high, a thousand times higher than when there cold. So when the compressor is ready to start, the PTC relay must be cold .so when power is sent to the compressor the power goes directly to the run winding and also to the start winding via the PTC device. Within a second or so of the compressor starting the power to the start winding is virtually cut out due to the PTC device rapidly being heated by the power drawn by the start winding. The power to the start winding is now virtually nothing ,simply a small current to keep the PTC device warm and the start winding out of circuit while the compressor is running.
When the compressor cuts out the PTC cools down and it’s resistance drops to around 25ohms ready to restart. The PTC requires about 5 minutes to fully cool down so that it’s resistance is low enough to energise the start winding due to cooling demand.
Low start Torque compressors also need this time delay for pressure equalisation ( suction /discharge ) to help achieve a good clean start up

This cooling down time of approx 5 minutes often causes compressors to stall if for instance the compressor is manually stopped and a restart is attempted before the 5 minutes has elapsed. As the PTCs temperature and it’s resistance will be too high and only the run winding will be energised and the compressor will trip on internal overload as the compressor fails to start and spin.
PTCs are an improvement on the old potential relays as there is no moving contacts which often caused radio and tv interference during the fridges starting times.

The latest PTCe device has an electronic circuit inbuilt to reduce the wasted energy that’s used to keep the standard style of PTC device warm during the compressors run times.(Energy savings)

Often on fridges and freezer thermostats there is a piece of plastic sleeving on the last 200mm of the stats capillary tube that is clipped to the evaporator. This plastic tubing helps to give a longer off cycle time to the thermostat to allow the PTC device to cool down sufficiently for a perfect restart. It gives a thermal delay to the thermostat.

If a decent off cycle time cannot be guaranteed then the PTC should be changed to a potential relay to prevent start winding damage , or to the new PTCe relay which due to its reduced current draw during the compressor run times the PTCe only needs a few seconds to cool to reset the relay for restart.
So the PTC device during compressor run has a small current running through it keeping it warm and it’s resistance high to prevent the start winding re-engaging

Washing machines use the same PTC device on their door locking mechanism .while the machine is in operation a small current runs through the PTC in the door lock preventing the door being opened during operation.
Once the complete cycle has finished this small current is removed from the door lock and after about a minute the PTC has cooled down enough to allow the door to be unlocked and opened safely

PTC devices should be checked every few years as they can crack with being heated and cooled on a regular basis. When they crack they lose there resistance qualities and the granular structure can give off hot glowing chip embers. These have caused fires in kitchens in scandinavia with wooden floors where during trying to start the compressor molten particles get thrown from the compressor terminal area, and the encapsulated versions now have a fireproof material covering for safety .

[nevinjohn]

Within a second or so of the compressor starting the power to the start winding is virtually cut out due to the PTC device rapidly being heated by the power drawn by the start winding. The power to the start winding is now virtually nothing ,simply a small current to keep the PTC device warm and the start winding out of circuit while the compressor is running.

The latest PTCe device has an electronic circuit inbuilt to reduce the wasted energy that’s used to keep the standard style of PTC device warm during the compressors run times.(Energy savings).

Thanks a lot guys! But one doubt still remains. How is the 'small current' ensured, if it was not for the PTCe types.
For the current to flow, we need a closed loop. Now the supply voltage (230V) remains constant, and the PTC is in series with the start winding and start capacitor. So what causes this small current? Is it the electromagnetic induction within the compressor caused by the run wingdings or something else?

[seanf]

Thanks a lot guys! But one doubt still remains. How is the 'small current' ensured, if it was not for the PTCe types.
For the current to flow, we need a closed loop. Now the supply voltage (230V) remains constant, and the PTC is in series with the start winding and start capacitor. So what causes this small current? Is it the electromagnetic induction within the compressor caused by the run wingdings or something else?

The ptc dosnt open circuit, it resistance value just goes very high. So youd still have a small current able to flow through the ptc to the start winding.

[Glenn Moore]

When the compressor is off there is no power to the compressor windings. When the compressor starts, power is sent directly to the run winding , and to the PTC device and onto the start winding .
At start up the start winding has a small resistance ( say 10 ohm ) the PTC when cold has a resistance of approx 25 ohm. As the compressor starts the resistance of the PTC goes up to thousands of ohms (K) which reduces the current draw in the start windings to less than a watt , this small current flow keeps the PTC device hot ,which keeps its resistance very high (k ohms) which in effect keeps the start winding de energised although the start winding is still in circuit .
So to put it simply the PTC device acts like a thermal gate .when the PTC device is cold the gate is wide open allowing full current flow . When power is connected to the PTC it becomes hot and the gate closes with a small current leakage.
So the start and run windings are always in circuit when the compressors running , but the PTC resistance is so high that the start winding and start capacitor are no longer able to function ,allowing the compressor to operate purely on the run winding as design.