Electric Motor Commissioning: Applying First Power After Installation in the Right Order

A centrifugal fan running backwards still moves air. Flow drops, current sits well below nameplate, and nobody notices. In a ventilation plant this can go undetected for months, until someone investigates why the hall never cools down in summer. Commissioning exists to catch exactly this kind of "running, but not running right" problem before the motor is handed over to production.

This article covers the day an electric motor, already mounted on its machine and wired to its panel, is started for the first time. Incoming inspection when the motor is delivered is a separate step, described in incoming acceptance inspection. Here the motor is in place, and the question is in what order to apply power and what to confirm at each stage.

Before power: the mechanical side

Most vibration and bearing problems at first start come from installation. Before anyone touches the motor terminals, go through the following with the team that did the mechanical work.

The motor should sit on all four feet. Loosen one foot bolt with a dial indicator in place: any lift shows a soft foot, corrected with thin shims. Coupling offset and angular misalignment must be within the coupling maker's limits, allowing for the small rise of the shaft centre as the motor warms. On belt drives, pulleys must be in line and belt tension at the manufacturer's value, since an over-tight belt wears the drive-end bearing early. Turning the machine by hand should reveal no binding or rubbing. Finally, the fan cowl needs free space behind it to draw air, and no hot exhaust should be directed at the motor. Alignment and coupling choice are covered in flexible or rigid coupling and shaft alignment.

Before power: the electrical side

On the panel side, the point is to confirm that the chain meant to protect the motor is really in place.

• Nameplate versus supply: on a 400 V supply, a 230/400 V motor runs in star and a 400/690 V motor in delta. A motor fed from a star-delta starter must be 400/690 V. Check by eye that the terminal links are arranged accordingly.
• Cable and earthing: cable size must suit current and length, and the protective conductor must be connected to the earth terminal on the frame. Glands should be tight and unused entries closed with blanking plugs, otherwise IP55 protection is lost.
• Protection setting: the thermal overload or motor protection circuit breaker is set to nameplate current. Setting it high "so it won't trip" is the first commissioning mistake. If the motor has PTC or PT100 sensors, check they are actually wired to a relay.
• Insulation resistance: if the motor stood for a long time, was installed somewhere damp or may have got wet during installation, test it with a megger before power. Disconnect the motor cables from any drive first.

Reading the setting from the nameplate is shown with examples in MPCB selection and setting.

First power: a short bump for direction

Where possible, first power is applied with the motor uncoupled. If the coupling cannot be separated, be sure reverse rotation will not harm the machine: some pumps, screw compressors and conveyors with backstops do not tolerate it.

Give the motor a one or two second bump and watch the shaft or fan. If direction is wrong, swap any two of the three phases, and record whether the swap was made at the motor terminals or at the panel outgoing side; otherwise someone may "correct" the cables at the next service and reverse it again. On fans and pumps, compare direction with the arrow on the casing. As the opening example shows, checking that "air is coming out" is not enough.

No-load run: the first reference values

Once direction is confirmed, run the motor uncoupled or lightly loaded for 15 to 30 minutes. During this run:

• measure current in all three phases; no-load current is below nameplate, and a clear difference between phases calls for a supply or connection check;
• listen at the bearings: a smooth hum is fine, grinding or clicking is not;
• measure and log vibration at the bearing positions;
• watch the frame temperature rise slowly and level off.

These values become the reference for the motor's healthy state. If they are not recorded, there is nothing to compare against when a symptom appears six months later.

Under load, together with the process

Checks under load depend on the application, but some are common. The motor should reach rated speed in the expected time; on high-inertia fans, mills and crushers that can take several seconds, so confirm the protection allows for it. Current at normal process conditions should be at or below nameplate; a motor that continuously exceeds it is either wrongly sized or the process is heavier than designed. Watch the current step at star-delta changeover, the soft starter ramp, or the drive's acceleration setting. On motors that will run at low speed on a variable frequency drive for long periods, check the frame temperature at minimum speed, because the shaft-mounted fan may not move enough air there.

For the relationship between current and torque during start, see the speed-torque curve and breakdown torque.

The first days: rechecking a warmed-up installation

Commissioning does not end when the motor first turns under load. After a few hours of running, check the tightness of foot and flange bolts, since heat and vibration can relax them on the first day. New belts stretch during their first hours and need re-tensioning. Measure current, temperature and vibration once more within the first week and compare them with the first record; together they become the starting point of the motor's routine maintenance plan.

The commissioning record

A good commissioning record fits on one page and is often the most searched-for document later. It should contain the nameplate data and serial number, connection type, protection setting, insulation test result, how direction was corrected, no-load and load currents, vibration and temperature values, and who carried out the checks and when. If the motor fails later or a warranty assessment is needed, this record is the only evidence that it was installed correctly.

Conclusion

Commissioning is not the moment the motor first turns, but the process that proves it turns correctly: mechanical and electrical preparation, a short bump for direction, no-load reference values, process checks under load and repeat measurements in the first days. If you are planning a new installation or a motor replacement, you can review our 3-phase electric motors and send application-specific questions to our technical team.

Frequently Asked Questions

What is wrong with connecting a new motor and simply switching it on?

A wrong connection, reverse rotation, a loose phase or an over-set protection can all go unnoticed while the motor is put under load. Some of these show up at once, others weeks later as winding or bearing failure.

How do I check rotation before coupling the motor to the machine?

Separate the coupling, give the motor a one or two second bump and watch the direction of the shaft end or fan. If it is wrong, swap any two of the three phases and record the change.

Does a new motor need an insulation test as well?

Yes, if it has been stored for a long time, installed in a damp place or may have been exposed to water during installation. On drive-fed motors, disconnect the cables from the drive before testing.

Why should I record the values measured at commissioning?

They are the reference for the motor's healthy condition. When current, temperature or vibration change months later, the commissioning record is the only way to make a meaningful comparison.