Lights dim when a/c kicks on

[Luv2Q]

When our a/c kicks on, any lights which are on always seem to dim very briefly then return to normal intensity. Is this a "normal" condition, or should I be looking for a problem? House is about 15 yrs old, BTY, and has always done this.

Thanks in advance
Q

[HayZee518]

no need to worry. the a/c has two motors that start when the a/c turns on. this presents a direct short to the electrical system when the motors start and "draws" on the system. once the motors are up to speed everything returns to normal.

[DKAudio]

Like Tom said, when the motors start there is an incredible amount of in-rush current. It is so great for a small amount of time that your voltage in the house may actually decrease and available current for your other branch circuits will go down (hence the dimming lights) until everything reaches operating speed.

Many AC motors have start capacitors that give it a boost to start. This cap may be getting old and can't hold the charge it once used to. You can also verify that your conductors going out to the unit are of suffecient size.

Keep in mind though that motors are the heavest draw on an electrical system.

[davidg73]

mine used to do the same thing, if both the front and rear AC units kicked on at the same time it wouldactually kill the clocks and we would have to reset them all! when we replaced the front unit, the new tstat had a cooling droop option that they enabled. this starts the compressor 30 seconds before starting the indoor fan. since then we rarely ever see any lights dim. the original tstat they installed did not have this option, so the unit caused the same problem with dimming/flickering. they changed the tstat out because it was not the one i was quoted, and they enabled the CD for some reason. i like it though, the unit seems to cool better since they did it.

[HayZee518]

any motor except the real cheap ones fractional horsepower induction type will dim lights on a circuit. the dimming is less evident on three phase circuits. 120 volt and 240 volt single phase will dim lights for a split second. after all in a motor you have a center spinning object [armature - rotor] and an outside coil or bunch of coils connected to the lines [ mains] when power is turned on the coils act like a short circuit, but resistance [of the magnet wire] is taken into consideration. it builds up a magnetic field which acts upon the [rotor] to start spinning. the [rotor] is actually trying to catch up to the rotating magnetic field in the stator coils.

[Psycho0124]

Prolonged dimming (more than a quarter-half second) is almost a sure sign that the capacitor or compressor motor is beginning to fail. Be sure and investigate the cause if you see any extreme current draws from your a/c units. Near the end of it's life, I had a rather large window unit completely kill the breaker occasionally as it started to take large amounts raw current to get it to start up. It even managed to melt one of the internal wires (thermostat to selector switch I believe) as its last act of defiance before it was replaced. Good riddens to the ole fire hazard.

Split-second heavy draw on the line is perfectly normal for a big motor like an a/c compressor. Big induction motors are serious electron hogs on cold starts..