ACTUATOR POWER SUPPLY FAILURES
The power supply is a critical part of a DC actuator system and we often have customers tell us their actuator isn’t working only to find a power supply problem. We have a test rig for actuators that can assess the load vs current performance which you can see here – but this adds time and expense. A better option is to understand how power supplies effect actuator performance and how to test in the field.
Because magnetic flux is directly proportional to current the torque of an electric motor is proportional to current which in turns means the thrust output of a linear actuator is also directly proportional to current. Actuator load vs current ratings are commonly available and show a straight line from a starting current to peak rated output. The slope of the line is dependent of the efficiency and gearing of the actuator. A typical current vs load performance is shown below.
Most actuators do not have built in overload or overcurrent protection. In an overload situation they will continue to draw more current until mechanically or electrically damaged.
Use of a suitably rated power supply is critical to producing rated load of an actuator and can often be used to provide protection for an actuator. However power supply performance varies greatly. Types of overload handling methods in power supplies include thermal, fuse, electronic and current limiting or even a combination of these. To make it even more difficult only the more expensive types of power supplies have indicator functionality of a fault state to aid in diagnosing. Depending on the type of protection the power supply may be unserviceable after an overload. A thermal protection may require a time cool off before the power supply will run again and some electronic protections require the power to be cycled to reset.
Power supplies also make extensive use of capacitors and the service life of these components can vary greatly. Some power supplies have a power on LED which only indicates main supply power. These LED’s can be on when the power supply has actually failed. It is also not uncommon for some power supplies to show voltage on the output terminals but not produce any current.
Most multi meters can be used to check current output of power supply and a 10A rating is common for an ammeter. This is perfectly adequate for 24V DC actuators with load ratings up to 5000N. Clamp meters with AC and DC current functionality are also useful but just be careful with their accuracy and use. When faced with an actuator that does not seem to be driving it is always best to test voltage and current output of your power supply before racing to replace the actuator.