SERVICE

What are the causes of excessive vibration and loud, harsh noise during motor operation, and how can these issues be addressed?

There may be several possible causes of the malfunction, such as, Misalignment: The coupling connecting the motor to the pump, fan, or other load is off-center; in belt drives, the belt tension is uneven. Mechanical faults: bearing wear, oil starvation, bearing spalling; rotor shaft bending, rotor imbalance; Anchor bolts are loose, and the base foundation is unstable. Electromagnetic noise: three-phase voltage imbalance, loose iron core, and broken rotor cage bars. Stator rubbing (bearing failure causing the rotor to rub against the stator core). The handling method is as follows: Identifying abnormal noises: a steady hum indicates an electromagnetic issue; metallic friction suggests bearing wear or stator‑rotor rubbing—replace the bearings promptly. Re‑align the coupling to ensure coaxiality and adjust the belt tension. Tighten all anchor bolts and reinforce the vibration-isolating base. Balance the three-phase voltage and inspect for broken rotor bars. Bearing clearance inspection requires shutdown for maintenance, including core repair and bearing replacement.

How can you determine whether the stator windings are shorted?

Winding short-circuit faults can be classified into two types: phase-to-phase short circuits and turn-to-turn short circuits. A phase-to-phase short circuit occurs when the insulation between two windings of different phases fails, resulting in a short circuit. A turn-to-turn short circuit involves a short between some turns within a single coil or between coils of the same phase. The phase-to-phase insulation of the windings can be measured using a megohmmeter. If the insulation resistance between phases is close to zero, it indicates that the insulation between these two phases has been damaged, resulting in an interphase short circuit. Otherwise, it is likely an interturn short circuit.

How can you verify that the motor’s ground insulation is safe and reliable?

When an electric motor is left unused for extended periods in a humid environment, or exposed to rain and sunlight, or operated under prolonged overload conditions, it may be subjected to corrosive gases, leading to a decline or aging of its insulation performance. Additionally, damage to slot insulation can result in breakdown between the motor windings and ground, causing high‑current faults that energize the motor housing and pose a serious safety hazard. Ground fault detection can be performed using a megohmmeter, which measures the insulation resistance of each phase winding to ground. If the insulation resistance of all three phases to ground is low but not zero, it indicates that the motor has become damp or contaminated with oil. However, after cleaning and drying, as long as the winding-to-ground resistance is restored to at least 0.5 MΩ, the motor may continue to be used.

As soon as the motor is started, the circuit breaker trips immediately, or the fuse blows.

Possible causes include interphase short circuits in the motor windings, ground faults, seized motor bearings leading to excessive starting current, and undersized circuit breakers or contactors. In the event of a power outage, use a megohmmeter to measure the insulation resistance between motor windings and between windings and ground. Rotate the motor shaft to verify that the bearings turn freely and smoothly, and confirm that the motor current rating and the selected circuit breaker/contactors are appropriately sized.

After starting the motor, it was observed that the no-load currents of all three phases are balanced, but the current values are relatively high, and the motor heats up rapidly.

This may be due to an excessively high supply voltage, or because a Y‑connected motor has been mistakenly wired in a Δ configuration. Check the power supply and restore the rated voltage, or, following the correct wiring method, rewire the Δ connection as a Y connection. It could also be due to an excessively low supply voltage, combined with the motor operating under its rated load, causing excessive current and resulting in overheating of the windings. If abnormal noises can be heard, they can be used to diagnose the issue, or the motor can be disassembled to check for any rubbing between the stator and rotor laminations. Alternatively, check the motor’s nameplate rating to determine whether the motor is overloaded (i.e., a small motor driving a heavy load) or experiencing overheating due to frequent starting.

When the motor is difficult to start, or when it drives its rated load, does the motor speed drop significantly below its rated speed?

The power supply voltage may be too low; measure the supply voltage immediately and address the issue. Alternatively, the motor may have been incorrectly selected—trying to drive a heavy load with an undersized motor—which can make it difficult to handle the current load. In such cases, consider choosing a motor with appropriate power rating. If abnormal vibration and noise are accompanied by high current, it may indicate a phase loss in the power supply, an open circuit in one phase, or a single-phase open or short circuit in the motor. Immediately disconnect the power and stop the machine; inspect the power supply and the wiring in the junction box, or disassemble the motor to examine the winding condition.

When power is applied, the motor fails to rotate, yet there are no unusual noises, odors, or smoke.

It may be due to the power supply being disconnected (with at least two phases unpowered) or to a blown circuit breaker fuse (with at least two phases blown). You should immediately inspect the circuit breaker and fuses, verify the fuse rating and the cause of the blowout, and then replace the fuse.  Alternatively, check whether the cable connections at the junction box are secure and free of breaks.  If all of the above conditions are normal, it’s still possible that the motor has failed; in that case, the motor should be inspected and repaired immediately.

What are the common cooling methods used for electric motors?

When an electric motor converts energy, some of the losses are dissipated as heat. This heat must be continuously removed through the motor’s housing and the surrounding medium; this heat‑dissipation process is referred to as cooling. The designation for an electric motor’s cooling method consists primarily of the cooling method code (IC), the circuit arrangement code for the cooling medium, the cooling medium code, and the drive method code for the cooling medium’s circulation. “IC + Circuit Arrangement Code + Cooling Medium Code + Drive Method Code” IC – The cooling method code is an abbreviation derived from the English term “International Cooling.” Cooling medium circuit arrangement code – Expressed by a characteristic numeral, with commonly used codes including 0, 4, 6, and 8. Specific meanings are provided in the table below.

What are the commonly used protection classes for electric motors?

The motor’s protection class conforms to the IPXX rating standard recommended by the International Electrotechnical Commission (IEC). The IP protection rating consists of two digits: The first digit indicates the level of protection against dust and solid object intrusion; the second digit indicates the degree of sealing against water and liquid ingress. Higher numbers correspond to higher protection levels; common protection ratings are summarized in the table below.

What are the installation methods for electric motors, and how do you determine the appropriate one?

Motor mounting methods: Motor mounting types are typically designated by a code. This code uses the international abbreviation “IM,” where the first letter indicates the mounting type: “B” denotes horizontal mounting, and “V” denotes vertical mounting. The second digit represents a characteristic code, expressed as an Arabic numeral. For example, “IMB5” indicates a motor with no base, a large flange on the end cover, and a shaft extension at the flange end.

How to Select the Motor Power

The power rating of an electric motor is the mechanical power it delivers at its rated operating conditions. Motor power indicates the size of the load it can drive; therefore, selecting the appropriate power rating requires matching it to the actual characteristics and magnitude of the load. This ensures that the motor is neither undersized—leading to insufficient torque and excessive temperature rise—nor oversized—resulting in low efficiency and unnecessary waste of electrical energy and capital costs.

How do you select the number of poles for a motor?

Select a motor with an appropriate number of poles based on the speed requirements of the load. Motor pole numbers always occur in pairs, so motors are available with 2, 4, 6, 8, and so on poles. The number of poles determines the synchronous speed of the motor’s magnetic field. The typical synchronous speeds of commonly used motors at 50 Hz and 60 Hz are listed in the table below. Since induction motors exhibit slip, their actual operating speed will be slightly lower than the synchronous speed shown in the table.

How should the voltage and frequency of an electric motor be selected?

Each country or region uses a different power supply standard, with variations in voltage and frequency. For example, mainland China employs a voltage of 380 V and a frequency of 50 Hz. When purchasing, select an electric motor whose voltage and frequency match those of the intended operating region. The table below lists the common voltages and frequencies used in several major countries and regions for reference.

What are the main application areas of electric motors?

1 Industrial Production Machine tools, water pumps, fans, compressors, conveyor belts, cranes, metallurgical equipment, chemical processing machinery, and more serve as the core power sources in factories. 2 Transportation Electric vehicles, high-speed trains, subways, electric bicycles, marine propulsion systems, elevators, escalators, and other applications. 3 Household Appliances Refrigerators, air conditioners, washing machines, electric fans, hair dryers, range hoods, soy milk makers, vacuum cleaners, and similar devices. 4 Smart Systems and Automation Robots, CNC machine tools, servo control systems, automated assembly lines, 3D printers, and other precision applications requiring speed regulation and positioning control.

What are the different types of electric motors?

(See the figure below.) Among these, AC induction motors are the most widely used; depending on the power supply, they are further classified into three-phase AC induction motors and single-phase AC induction motors.

What is an electric motor?

An electric motor is a type of power device that converts electrical energy into mechanical energy.