Diagnosing a Compressor That Fails to Start
Why won’t the compressor start?
When an air compressor refuses to start, it’s frustrating – but it isn’t always a fault.
On modern machines, including the Atlas Copco range, intelligent controllers are always monitoring operating conditions. If something isn’t right – high temperatures, an electrical issue – the system will prevent the compressor from starting to protect it from damage.
Is it a mechanical fault or a safety stop?
Before you look for broken parts, make sure the machine is just protecting itself.
Large industrial compressors are very powerful, and safety rules say they have to stop if they go over their operating limits.
The controller frequently displays a message indicating the specific reason for its failing to start. It may be awaiting the activation of a timer, or it may have detected an issue with the safety valve.
Recognising this distinction can facilitate time savings – as well as reduce stress.
What safety precautions should I take?
Any work on industrial equipment needs to start with safety.
Power down the machine at the main supply, then check it’s completely off before opening anything up.
Variable Speed Drive (VSD) air compressors retain electrical energy even in an inactive state. Prior to interacting with any items within the cabinet, wait for the discharge light to go out. This usually takes around ten minutes to complete.
Remember, don’t try to release pressure by cracking open fittings or pipework – always use the proper drain valves instead.
Diagnosing a Compressor That Fails to Start
Could it be an electrical supply issue?
One of the most frequent causes of a compressor not starting is problems with the electrical supply. Modern controllers are very sensitive to power quality.
The system may stop the motor from starting to protect it if the voltage falls too low, even for a brief period of time.
Another frequent cause is blown fuses. The motor may hum but not turn on if a fuse has blown on one of the three phases. This is often caused by older fuses getting tired over time, rather than a major fault.
Why does the phase sequence matter?
Rotary screw compressors are designed to spin in one specific direction. If they run backwards, even for a second, it can cause damage to the internal screw element.
To prevent this, most units have a “phase sequence relay.”
If you have recently had electrical work done in your factory and the wiring was reconnected in a different order, this relay will spot the change and stop the machine from starting.
What if the overload relay has tripped?
An overload relay is a safety feature on the motor. It works like a circuit breaker for the motor and will trip if the current gets too high.
If the motor is working too hard, it could be because of a mechanical problem or a stiff drive train. Most of the time, the relay inside the cabinet will have a reset button.
You can try to reset it once. If it trips again straight away, stop there. Repeated resets usually point to a real fault, not a nuisance issue, and it’s something a professional needs to investigate.
Is the controller stopping the machine?
Sometimes the machine is fine, but the controller logic is telling it to wait.
- Timers: To protect the motor, the system limits how many times it can start per hour. It might just be counting down a “rest” timer.
- Remote Mode: If the compressor is tied into a central control system, it may simply be waiting for permission to start. In remote mode, the local start button won’t do anything on its own.
- Communication: The same applies if the controller loses contact with the drive or starter. As a safety measure, the machine won’t run. The display will usually show a message that hints at where the link has dropped.
How do VSD faults affect startup?
Variable Speed Drive (VSD) compressors are great for efficiency, but they do have their own internal checks.
The drive keeps an eye on its own temperature and power levels. The drive can get too hot and stop the machine from starting if the electrical cabinet’s cooling filters are blocked with dust.
Cleaning these cabinet filters is an easy way to stop these kinds of problems.
Can a stuck valve stop the motor?
The “unloader valve” controls how much air comes in. This valve should be closed when the compressor starts so that the motor can speed up with less load.
If this valve stays open, the compressor is trying to push air from the moment it starts. The motor can’t generate enough torque, so it trips out – much like trying to pull away in fourth gear.
You’ll see the same problem if pressure from the previous run hasn’t been fully released. Residual pressure makes the restart far harder than it should be. The motor struggles to push against the trapped air and gives up.
Is air leaking back into the compressor?
Your system should have a “minimum pressure valve” that stops air from your factory pipework flowing back into the compressor when it stops.
If this valve leaks, your factory air pressure can push back into the compressor element. That added load makes the motor struggle to turn.
A quick check of the tank pressure gauge before starting will tell you whether pressure has been properly released. It should read zero. If it shows pressure, you may have a backflow issue. Correct efficient compressed air system installation helps prevent this.
Could it be a pneumatic or filter issue?
Sometimes the problem is a blocked pipe or filter.
- Frozen Lines: Cold weather can cause its own set of problems. Moisture sitting in small control lines can freeze, blocking the signals that tell valves what to do and when to do it.
- Blocked Filters: Airflow can be just as much of an issue. If the intake filter is heavily clogged with dust, the compressor effectively can’t breathe, which makes starting a real struggle.
Routine air leak detection and maintenance checks can spot these issues early.
How does the UK weather affect startups?
On cold mornings in the UK, compressors in unheated plant rooms can struggle. Once temperatures drop below about 5°C, the oil thickens up – more like treacle than oil – and that makes starting much harder, which can sometimes set off the overload safety.
How can I prevent these problems?
Regular checks are the best way to prevent the “fails to start” scenario. Simple habits can save a lot of downtime:
- Daily: Look for any warning messages on the screen and check the oil levels.
- Weekly: Check the air intake filter isn’t blocked.
- Monthly: Make sure that the electrical connections are tight (loose wires can cause voltage drops). Search Air offers full-service agreements to take care of these important checks for you, so your machine will be ready to go when you need it.
When should I call for help?
If it keeps tripping while you’re going through the usual checks, that’s the point to call a professional. Resetting a safety device again and again without understanding why often leads to bigger, more costly problems.
And don’t try to force the issue. If the machine won’t turn by hand, or you notice burning smells or unusual noises, stop there and get it looked at properly.
Search Air can fix broken air compressors in Yorkshire and the East Midlands seven days a week in case of an emergency. Our helpful staff is here to help you get back up and running safely.
Summary
If a compressor won’t start, it’s rarely random.
It’s usually pointing you towards something – a blown fuse, a cold start, a sensor that isn’t reading correctly. Take it steadily and work through the checks one by one.
Put safety first throughout. And if you’re unsure at any point, the Search Air team is on hand to help.