how to eliminate the voltage drop in the supply line which supplies an aircon.?

Question

it is single phase line to neutral supply line. the load is a split type airconditioner. supply voltage is 240 but after switch is on it drops to 175 volts. this caused the compressor not to run.

Answer 1

Voltage droop as you are experiencing is a sympton of leakage current to ground somewhere in the circuit. There are a couple of things that can cause this voltage droop.

If the compressor on the AC unit is old, it could be leaking current to ground through the windings on the compressor motor. Or you may have a failed starting capacitor on the compressor motor as well.

A second problem could be if you have very old wiring supplying the AC unit. Given that you have only a hot lead and a neutral going to your ac unit, I would guess the house is more than 20 years old, maybe even more. So you could have too small a guage of wire, and a degraded insulation on the wire going to your ac unit. I can give you an example of my parents home that is 40 years old....the wiring in that house has cloth insulation, and is not suitable for a modern ac unit.

In either case the problem must be addressed.

Even if your compressor runs. With a large voltage droop, it will further degrade the insulation in the motor windings, and the motor will fail if not already. And also the insulation on the line going to the compressor motor will continue to degrade. And the worst case could be an electrical fire.


There is a simple way to check both possibilities, if you can find the correct tool to test with. The device is commonly called a "megger." It is a special type of ohm-meter or resistance checker. Unlike a conventional volt-ohm-meter that you buy at radio shack. The old style device used a hand crank to generate a small current and measures the resistance. You de-energize the AC unit and and disconned the unit at the breaker or fuse panel. Then working on only the downstream side you attach one lead of the megger to the load side and the other to ground...not the common lead, but ground. On a new system, with new wire, the dial will read more than 10 megohms to infinite. If yours reads less than 1 megohm, you have a current leak. Given the large magnitude of your voltage droop, I would suspect that you have less than 500kohms. And again, the leakage is either in your hot lead or in the compressor.

Repeat the measurement at the compressor motor itself. And the same criteria apply to this reading.

You may be able to find a megger that you can rent from a tool supply place or you can buy one from a commercial electrical supply house. I don't know what they cost, but here is a link that describes them

http://www.biddlemegger.com/cgi-bin/webshop.cgi?config=ent-1kv


One other thing to check. Be sure that all connections are tight, and have no corrosion.

Answer 2

Well, if the AC is running in parallel to everything else in the line, and there's still a voltage drop, then that means that it's putting too large a drain on the power source. This would require upgrading the source. If the AC was hooked up in series, then a voltage drop would be expected regardless. If the power supply can supply enough current, you could try switching the AC to parallel.

Answer 3

The starting current of a large electric motor is very high. You could consider re-wiring with beefier wiring. But you should also consider the possibility that something is wrong with motor or compressor mechanics.

Let's say the wiring is OK, and the motor is OK. But the compressor bearings are shot and are sticking. If this is enough to prevent the motor from spinning, then huge currents will flow and the voltage will drop.

Answer 4

You need to run a larger supply wire size to the air conditioner.