Safely replacing a 3-phase motor

Replacing a motor looks like a simple mechanical job, but most of the risk lies in what you don't immediately see: stored energy that's still present after switching off.

Step 1 — residual energy before you touch anything

• Frequency drive (VFD): the DC link (DC intermediate circuit) of a frequency drive contains capacitors that can still be charged after the supply has been switched off. Wait at least 5 minutes (check the VFD's manual for the exact discharge time) and measure the residual voltage across the DC link before working on the motor cables — never rely blindly on the waiting time alone.
• Mechanical energy: lock out hoisted loads, springs or other mechanically stored energy that could make the motor shaft turn unexpectedly as soon as the brake or holder is released.
• For the electrical side, simply follow the 5 steps of working de-energised (LOTO) — residual energy in the VFD comes on top of that procedure, not instead of it.

Step 2 — reading the nameplate

Before determining or connecting a replacement motor, read the nameplate of the old motor:

Step 3 — star or delta: which winding connection?

The nameplate usually shows two voltage values (e.g. 400 V Δ / 690 V Y). Which connection you choose depends on the actual mains voltage supplied to the motor:

• If the motor is connected directly to a 400 V mains (phase-to-phase), the winding is connected in delta (Δ) (each winding receives the full line voltage of 400 V).
• If the motor is intended for a 690 V mains (phase-to-phase), the winding is connected in star (Y) (each winding receives only 400 V, the phase voltage, because the star connection divides the voltage by √3).

A motor that should be delta-connected but is accidentally connected in star (or vice versa) on the same mains produces a strongly deviating torque and current — always check this before switching on, not afterwards.

Only disconnect the motor cables after you have marked the leads: U1/V1/W1 for the start of each winding, U2/V2/W2 for the end, plus PE. Without this marking there's a high chance the replacement motor comes back with a swapped connection, which unnoticed changes the rotation direction or the Y/Δ connection.

Re-setting the motor-protection switch (MBS)

A replacement motor almost never has exactly the same In as the old motor, even with an "equivalent" replacement. Does the circuit have a motor-protection switch (MBS) (see also fuse systems — MBS)? Then always check and re-set it to the nameplate of the new motor, before letting the motor run under load for the first time:

• Set the MBS dial exactly to the In (rated current) on the new nameplate — not to the old setting, and not rounded up "to be safe".
• Note the star/delta choice above: the current the MBS sees depends on where it sits in the circuit. If the MBS sits ahead of the windings in the supply line, that's the line current; if it sits per winding (for example in a star-delta starter), that's the phase current of the winding (line current / √3 in delta). Never mix these two up.
• An MBS still set for the old motor will, on the new motor, either trip too early (set too low, motor won't start) or no longer protect the winding against overload (set too high).

This check belongs to motor replacement just as firmly as reading the nameplate itself — a motor that's mechanically connected correctly but running with an incorrectly set MBS is still left unprotected against overload.

Step 4 — insulation resistance per winding

Before commissioning, measure the insulation resistance per phase combination, not just in general:

• U-V, V-W, W-U (between the windings themselves, if accessible)
• Each winding to PE (motor frame)

Important: never measure through a frequency drive — disconnect the motor cable from the VFD output before the insulation test, otherwise you'll damage the VFD's electronics with the test voltage.

Step 5 — after connecting: checking the current

Start the motor briefly unloaded (test start, see also phase-rotation check) and measure the current on all three phases with a clamp meter. The currents must be symmetrical (mutual deviation typically within a few percent); a large deviation points to a winding problem, an incorrect Y/Δ choice, or a fault in the supply (for example a missing phase).