How to control a three-phase motor?

Why and how to inspect a three-phase motor?

Three-phase electric motors are used in many industrial and agricultural applications. Their operation is simple: they convert electrical energy into mechanical energy. However, even though three-phase motors are extremely reliable, they sometimes fail, bringing your production line or your client's to a halt. That's why it's important to know how to inspect a three-phase motor.

Whether you're a maintenance technician, a farm manager, an electrician, or a machine builder, do you have a three-phase motor on a piece of equipment that's causing you problems? Are you wondering how to inspect a three-phase motor? We'll explain how to do it and what equipment to use to test an electric motor.

Inspect the physical condition of your three-phase electric motor.

Before any electrical inspection, it's essential to first check the general condition of your three-phase asynchronous motor. Your motor might have been stored for many years before being put into service, water might have come into contact with the winding, or condensation might have caused an insulation fault.

You can check its general condition. Are the mounting feet or flange in good condition? Is the paint not chipped or rusted, and does the shaft have no defects or rust? Does the rotor turn normally? Is the fan cover not dented or damaged, is the cooling fan not broken, and is the terminal box in good condition? If everything is compliant, that's already a first step passed.

This simple test helps determine if your motor is new or if it has been reconditioned, rewound, or overhauled.

Inspect the bearings of your three-phase motor.

One of the main causes of failure in a three-phase electric motor is a mechanical problem, particularly a bearing defect. Before putting it into service, you can check the rotation of your motor's rotor. Pay close attention and listen carefully to the sound. The rotation should be fluid. You shouldn't feel any friction, and no noise should be present during the rotor's rotation. If you feel friction or hear an abnormal noise, you absolutely must replace the bearings of your three-phase motor.

Remember to check the end shields and bearing journals on the rotor before replacing the bearings. The outer cage of the bearing must not rotate in the end shields, and the bearings must be mounted using an induction heating device on the rotor.

Check the mechanical play using a caliper. Also, be careful to select C3 type bearings with increased clearance. This characteristic is essential for a three-phase motor.

Test the windings of your three-phase electric motor.

Several symptoms can guide you on the proper functioning of your motor. Has it been making an abnormal noise for some time? Does it trip your installation as soon as it starts or after a few minutes or hours of operation? Your asynchronous motor has a fault.

Before any manipulation or inspection of your electric motor, it is necessary to cut off and lock out the circuit breaker of your electric motor and perform a voltage absence verification (VAT). This operation must be done by an electrician or by the maintenance team authorized to perform this operation. Remember that your life is at stake!

Once this operation is complete, you can open the terminal box to access the power cable. Disassemble the coupling bars from the terminal plate to inspect the three-phase motor windings.

First, inspect the windings. To perform this test, an ohmmeter or a multimeter set to resistance measurement is necessary.

It is imperative that the coupling bars and power cables are disconnected to perform the tests.

Your electric motor's terminal plate consists of six terminals: U1, V1, W1 and U2, V2, and W2.

The motor's winding must have identical resistance across all windings. To inspect the windings, you need to measure the resistance between U1 and U2, V1 and V2, and W1 and W2.

If you have resistance on each winding, the winding is intact. Otherwise, if one of the windings has no resistance, replacement or rewinding of your electric motor will be necessary.

Inspect the insulation of your three-phase asynchronous motor.

Another operation is necessary to know how to inspect a three-phase motor. After checking the windings, it is necessary to check if none of the windings are grounded. This means that no winding wires should be in contact with the electric motor casing.

It is possible that after many years or after an overload, the insulation of your motor may be worn or melted. This phenomenon causes a fault in your electric motor. Your circuit breaker will trip as soon as your motor starts, or your frequency converter will indicate a fault.

To check if your motor's winding has an insulation fault, you need to use a megohmmeter. This measuring device allows you to take measurements in kilohms and megohms.

You must place one test lead on the motor casing's ground terminal and start by testing each terminal on the motor's terminal plate (U1, V1, W1, U2, V2, W2).

Each measurement should indicate infinite resistance; if you measure resistance on one of the terminals between ground and one of the terminal plate terminals, you will first need to clean and bake the motor's stator. If the problem persists after this operation, rewinding or replacing the motor is necessary.

Commissioning your three-phase motor.

Now that you know how to inspect a three-phase motor, you will put your application into service or back into service. Reconnect your three-phase electric motor. Be careful to correctly tighten the wires to your terminal block to avoid overheating, close the terminal box cover, and un-lockout your installation.

Once your installation is in service, we advise you to check the voltage and current under load on each phase. This operation will allow you to know if your electric motor is working correctly.

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