Lights Dim When the Compressor Starts

It’s a bit disconcerting when you try to start your air compressor and the lights dim when the compressor starts. Why does that happen?

If we think a bit about what the sequence of events is when the air compressor starts, this may help.

When the pressure in your compressor tank drops below the normal cut in pressure for that air compressor, the pressure switch trips to passing, and power flows to the motor circuit. The compressor motor tries to start.

And the lights dim when the compressor starts! Or they flicker and dim.

To help reduce load on the motor, as most compressor motors use pretty much all the power a typical 120 VAC circuit can supply when they start, an unloader valve on the air compressor should have opened when the air compressor last reached cut out pressure, and stopped. The image below is of a typical unloader valve on the side of a compressor pressure switch. The “arm” sticking out of the side of the pressure switch depresses or releases the pin on the unloader valve as the pressure switch cycles, opening or allowing the unloader to close.

This unloader valve opening dumps all the air that may have been trapped over the compressor piston, reducing that load from the motor as it tries to cycle the pump. If there is air left in the cylinder over the piston, the motor has a substantially increased load that it may not be able to overcome. If that happens, the lights will dim, and quite likely the breaker will pop as well.

Unloader devices might be a valve working off the pressure switch (as shown above) on some air compressors, or it may simply be a small hole in the head of the pump in other, fractional or smaller air compressors. Just make sure yours is working. If not, expect the lights to dim when the air compressor tries to start.

As we can see, the more difficulty the compressor has to start, the more likely it is that the lights will dim.

If the motor pump has a mechanical failure, and the crankshaft is being impeded by the mechanical issue, then the motor shaft will have increased load on it, and that means that the motor will have a harder time trying to start, pull more amps than normal, and that will drain the circuit and cause any other electrical devices on the same circuit to have a reduction in power. If those devices include lights, then the lights will dim. It is likely, too, that the circuit breaker will pop or the fuse blow if the pump mechanical issue overloads the motor completely.

The usual reason why the lights dim when the compressor starts

Electric motors consume a lot more power when they are starting than when they are running. The increase in demand is called inrush, and this happens every time the compressor motor tries to start. In fact, the inrush demands of a motor typically exceed the power supply to the motor.

In order to prevent the breaker popping or fuse blowing every time the compressor motor tries to start, compressor motors often come equipped with a device called a capacitor, or start capacitor, a typical one of which is pictured below.

The compressor motor start capacitor is sort of a battery. Unlike the typical battery however, this one discharges it’s power very quickly. The start capacitor boosts the power supply to the air compressor motor, filling much of the power draw demanded by the inrush of the motor, and allows the motor to start without overloading the circuit.

If the start compressor is starting to fail, it is possible that the power it supplies is no longer enough to get the compressor motor running, the motor pulls more power from the circuit instead, and the lights dim.

If the start capacitor has failed completely, the circuit breaker or fuse will likely let go as the motor tries to pull more power than the circuit is rated for.

Checking the motor capacitor

There are lots of resources on line to enable you to check your own capacitor.

Copy and past this like into your browser for a pictorial method: https://www.electricaltechnology.org/2013/06/how-to-check-capacitor-with-digital.html#

Or, browse for videos about testing your capacitor. There are many.

As noted, earlier, a failing or failed compressor motor start capacitor is a common reason why your lights dim when the air compressor tries to start. Test yours would be worthwhile.