A drive can pass a bench power-up and still be nearly finished. The checks that tell you which one you are looking at.
A variable frequency drive is one of the better used buys in a marine panel. It is expensive new, it is largely solid-state, and a healthy unit will run for years. It is also one of the easier things to be disappointed by, because the components that age are not the ones you can see.
What ages in a drive
The DC bus capacitors. These are electrolytic, and they dry out with time and temperature whether or not the drive is used. A drive stored for years without being energised can be in worse condition than one that ran continuously in a cool room. Their degradation is gradual: capacity falls, ripple rises, and the drive becomes intolerant of load steps long before it fails outright.
The cooling fans. Bearings are a wear item and a marginal fan pushes the whole drive into thermal derating. Cheap to replace, but only if you know to look.
The heatsink. In an engine room the fins fill with oil mist and dust, which turns into a felt that insulates rather than cools. A drive that trips on overtemperature under load often just needs the heatsink genuinely cleaned.
What a power-up test proves, and what it does not
Energising a drive on the bench proves the control board boots, the display works and the power supply is alive. That is worth confirming, and it rules out the units that are simply dead.
It does not prove the capacitors are healthy, because there is no load. It does not prove the output stage is balanced across all three phases. And it does not prove the drive will hold up under the current your application draws.
A test into a motor under some load tells you considerably more. Ask what was actually done rather than accepting "tested" as a category.
Check the rating against the application, not the old drive
Match on:
- Output current, which is the rating that matters, rather than the kW figure on the label. kW ratings assume a motor type and duty.
- Supply voltage and phases, including whether the vessel's supply is 380V, 400V or 440V, and at what frequency.
- Overload profile. A drive rated for 110% for 60 seconds and one rated for 150% is a different machine for a pump versus a winch.
- Enclosure rating, if it is not going into a sealed panel.
- Control interface. If the panel expects a fieldbus card, confirm the card is present. They are frequently removed before a unit is sold.
Parameters and passwords
This catches people out. Many drives can be parameter-locked, and a used unit may arrive configured for its previous application with a code nobody recorded. Ask whether the drive has been reset to factory defaults and whether any lock is in place. A drive you cannot reconfigure is not a spare.
Also ask for the parameter list from the previous installation if it exists. Even where it does not apply directly, it tells you what options are fitted.
A reasonable position on condition
Used power electronics carry real risk, and anyone claiming otherwise is selling. The sensible approach is to match carefully, ask exactly what testing was done, and treat the capacitors as consumables with unknown remaining life rather than as new.
For a standby unit or a like-for-like replacement of a discontinued drive, that trade is usually worth making. For a critical single point of failure with no spare, weigh it against new.
If you are matching a drive, send us the rating plate, or see what is listed.
Looking for a specific unit?
Send a model or part number. Most of what we ship never reaches the catalogue.