Can an electromagnet be powerful with a low voltage?

Everything is a trade-off. To get high current with low voltage, you'll need large-gauge wire, which will limit the number of turns.

It happens that I was looking at specs for electromagnetic door locks yesterday. I recall seeing 0.5 Amps at 12 Volts (that's only 6 Watts) on an electromagnet with a 600 pound holding force. So, that's something for you to shoot for.

(I'll admit to being very surprised at how little power it took.)

Henry answered this question about 1800 in NY. Ampere turns is what makes the magnet. Wire size limits the ability to be wound tightly. Depending on your power supply, (high voltage, or low voltage) you have to wind your magnet with an optimum wire size, an optimun connection (parallel windings or one long continuous wire. Round or square wire. Look VERY carefully at the alloy materials for making the magnet. Your magnet design may not give the fields you want or expect. The "standard" double 'e' pattern may give you a higher field than expected. Experts in the field told me that you have to use a 'fudge' factor with the calculations. Be aware, Magnetics is still very much an art. Generally, full wave rectification of 3 phase power gives the best DC for large magnets.

Those are good answers. If you can use super conducting wire, much less than one volt is possible. Otherwise a few hundred volts dc will be optimum for large electro magnets to lift a few tons. I suppose 12 volts will lift several tons if you can supply hundreds of amps to the magnet, but over heating and reduced efficiency are likely.

Possibly you can use a powerful permanent magnet and release the load with a pulse from your coil that approximately equals the magnetic field but reversed magnetic polarity. If that works you will only need electricity for a few milliseconds to DROP the load. Not practical for setting down the load gently. Neil

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It depends on the resistance of the wire and the current required to magnetize the core to the maximum flux density.

LOWER VOLTAGE MEANS MORE CURRENT IS NEEDED TO PRODUCE THE SAME MAGNETIC FORCE.