Buying a Second-Hand Electric Motor: Bench Checks and When to Buy New Instead

A faded 11 kW motor sits on a dealer's bench at an industrial estate. The seller plugs it in, it spins up quietly, and the asking price is less than half of a new one. Many used-motor purchases are decided on the strength of that short demonstration. Yet a motor running with no load tells you almost nothing about how it will behave once it heats up under load, or on the first cold start after a damp night.

Buying a second-hand electric motor is not automatically a mistake. Getting it right, though, means doing a few measurements before money changes hands and being clear about what job the motor will actually do for you. Those are the two halves of this article.

Decide what the motor is for

The same used motor can be a sensible buy for one plant and a poor gamble for another. What changes is the role it will play. Three roles are worth separating:

• Stopgap: a motor that keeps a machine turning for a few weeks until the proper replacement arrives. Short-term reliability is all that matters; remaining life is secondary.
• Non-critical duty: a ventilation fan or workshop machine that runs a few hundred hours a year and does not stop production when it fails. A used motor can make sense here.
• Critical or long-running drive: a motor that stops the line when it fails, or runs more than 4,000 hours a year. In this role, the unknowns of a used motor can easily cost more than the purchase saving.

Writing the role down before you start haggling also keeps the price from doing the thinking for you. A cheap motor for a critical line still has to answer every question on the list.

What to check at the bench

If the seller agrees, the whole inspection takes about half an hour. You need a clamp meter, a multimeter and a 500 V insulation tester (megger).

Nameplate and housing

A legible nameplate comes first: power, voltage, current, speed and, ideally, the efficiency class. You can estimate the rating of a motor with a missing plate from its frame size, but the same frame is used for several power and pole combinations, so the estimate can be wrong. Weld repairs on the housing, a cracked foot, a broken end shield or re-drilled mounting holes all suggest the motor had a hard life.

Shaft and bearings

Turn the shaft by hand. It should rotate smoothly and quietly with no tight spot. Push it radially and axially to feel for play. A deformed keyway, rub marks from a coupling or a slightly bent shaft point to previous operation on a misaligned drive. Replacing bearings is cheap; a worn bearing seat in the end shield is not.

Winding resistance and insulation

Remove the links in the terminal box and measure the resistance of each phase winding. The three readings should be very close; a clear difference points to shorted turns or a loose connection. Then test each phase to earth at 500 V. A low reading does not always mean a dead winding: motors that have sat in damp storage often recover after drying. How to judge the readings is covered in our article on insulation resistance and megger testing.

No-load run

Connect the motor for its nameplate voltage, run it without load and clamp each phase. The three currents should be similar. After a few minutes, listen for bearing noise, feel for vibration and check how warm the bearing housings get. An unusually high no-load current compared with the nameplate can indicate a motor that was rewound carelessly at some point.

What no inspection will reveal

Even a thorough check leaves questions open. Winding insulation ages a little every time it overheats, and that ageing cannot be measured from outside. The motor may have tripped on overload several times or run for years with dust-clogged fins. None of that has to show up in the megger reading you take at the bench.

Rewind history is the second unknown. Depending on the quality of the work, a rewound motor can end up less efficient than it was originally. The technical side of that decision is discussed in rewind the motor or buy a new one. If the seller cannot say whether the motor has been rewound, it is safer to assume it has.

The third is efficiency class. Many older motors carry no IE code at all. Running a used motor inside a plant usually raises no legal issue, but cases such as used motors brought in from outside the market for the first time can be treated differently. That distinction is explained in how IE3 and IE4 rules affect existing plants.

Put the price gap next to the energy bill

On a motor that runs long hours, the purchase price is a small part of what it costs to own. Take a 7.5 kW, 4-pole motor: the IE3 minimum efficiency under IEC 60034-30-1 is 90.4%. An old motor without a class marking may sit a few points below that. If it delivers about 6 kW to the shaft for 4,000 hours a year, each percentage point of efficiency is worth roughly 300 kWh a year. A gap of several points starts to claw back the purchase saving through the electricity bill within a few years.

Put the same motor on a standby fan that runs 300 hours a year and the calculation hardly matters. So the energy comparison, too, depends on the motor's role.

Used or new: the dividing line

A used motor is reasonable when

• It will drive a machine that runs little and does not stop production.
• The bench checks came out clean, the nameplate is legible and the motor came from a known plant.
• You need a temporary fit for a motor with a special flange or shaft that would take a long production lead time new.
• The saving still stands after adding the cost of new bearings and drying.

Buy new instead when

• Phase resistances differ clearly, or insulation resistance does not recover after drying.
• There is no nameplate, the rewind history is unknown and the motor is going onto a critical line.
• The motor will run thousands of hours a year and its efficiency class is unclear.
• The seller will not allow any measurements.

A new motor removes most of these unknowns: the efficiency class is on the nameplate and in the declaration of conformity, the unit has passed routine tests and the supplier stands behind it. A simple acceptance check on delivery is still good practice; the steps are listed in our incoming acceptance inspection guide.

Making the call

A second-hand motor works when it is bought for the right role and checked properly. For a critical, long-running drive, the purchase saving usually looks small once unknown winding life and uncertain efficiency are added in. HEM Motor's three-phase electric motors from 0.25 to 355 kW are shipped from stock in Türkiye, and stock status for the power and speed you need comes back with the quotation.

Frequently Asked Questions

How can I tell whether a used motor has been rewound?

Lead wires in the terminal box with a different colour or insulation type from the original, or a different varnish colour on the winding ends, are common clues. There is no definitive test, so asking the seller for the rewind invoice or workshop details is the most reliable route.

Is a motor with a low megger reading scrap?

Not necessarily. Motors that have stood in damp conditions often recover acceptable insulation resistance once the winding is dried out. If the reading stays low after drying, the insulation is damaged and the motor needs a rewind.

Is replacing the bearings of a used motor enough?

New bearings usually cure mechanical noise and vibration, but they do nothing for the winding. If the bearing seat or shaft journal is worn, new bearings will fail quickly as well, so check the seats before fitting them.

How do I estimate the power of a motor without a nameplate?

Frame size and shaft diameter give a rough idea, but one frame can carry several ratings, so the estimate is uncertain. Measuring current and speed under load gives a better picture; for a critical application it is wiser not to use an unmarked motor at all.