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.

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).