Repair of induction motors

Induction motors are among the most common types of electric machines . Because of the simplicity of the device , high reliability, good performance and relatively low cost , represented by electric motors are used in all sectors of the economy : construction, industry, agriculture , transport , etc. However, as with any electrical machines, they tend to break and require periodic maintenance.

 Repair induction motors produce highly professional and skilled craftsmen , given all the necessary requirements and respecting quality standards . To perform the repair of any motor model , you need to begin to determine the amount of repair work by evaluating the nature of his faults. The most common failures of electric motors are: overload or overheating of the stator circuit between turns , various types of damage to the bearings , windings, stator , etc.
Current and capital repairs - are two main types of repair work with electric motors . Thanks to the current repair is possible to maintain the durability and reliability of the motor .
Overhaul requires more complex types of work aimed at making technical characteristics are refurbished machines were not inferior to the latest models of cars.

Typically , the reason for the failure of this item is in violation of the overall condition of the windings , so it becomes necessary rewind induction motor. Failure causes the motor windings can be different: from overheating working parts to large engine life . Thus, the engine malfunction , causing an urgent need to repair induction motors , is always manifested in different ways. Before performing repairs to motors , our experts will make a detailed diagnosis of all the elements and reveal the fault. Determine the reasons for what it needs repair induction motors , will try to identify how much whether to rewind the induction motor and find other reasons for which the device could be damaged.
Repair induction motors may be necessary when a decision of both mechanical and electrical problems . Before the repair work with induction motors (fast asynchronous motor) , the master of our company can significantly reduce the time required for the repair of electric motors , increasing the quality of the work.

 

Three-Phase Wound Rotor Induction Motor

Wound Rotor (Slip Ring) Induction Motors

 
This type of motor is identical to a squirrel-cage induction motor except for the rotor, which consists of a symmetrical, three-phase winding. The three terminals are connected to three slip rings. Three external variable resistors are connected to the three slip rings via carbon brushes. Motor and resistor connections are shown 

 

Carbon brush wear is considerably higher in a humid environment or when the current density is lower than the recommended value. The carbon dust deposits on the slip ring and adjacent metal parts can cause a ground fault that may initiate a flash over and fire.

 A slip-ring flash over protection that will open the motor controller
upon detecting a ground fault is required to prevent a fire in the housing. For large motors, a liquid rheostat is used to dissipate the generated heat more efficiently.

Wound-rotor or slip-ring induction motors are used where high starting torque or small variation in speed is required. The most common applications are:

• Conveyor drives: Higher starting torque is required for loaded systems.

• Cranes: For lifting heavy loads and changing speeds.

• Crushers, ball and sag mills: High starting torque is required.

Single Phase Induction Motor

Basic Induction Motor

Sectional view of a typical squirrel cage (induction) motor.

Field removal of rotor for winding cleaning and inspection of a large pipe ventilated induction motor.

1500 horsepower,1800 rpm , 4160 volt TEFC induction motor ready to have bearing rebuilt and rotor balanced.

Wound-rotor induction motor

Three Phase Induction Motor..

Three Phase Induction Motor

Squirrel-cage induction motor

The squirrel-cage induction motor is the most commonly used type in the industry. The rotor (rotating part of the motor) has copper or aluminum bars that are welded to the end rings. This configuration is similar to that of a squirrel cage; hence it has cquired the name “squirrel cage” induction motor. The NEMA standard [S1] has defined a set of motor characteristics, dimensions, and performance for motors rated up to 500 hp (375 kW).
 

Torque Characteristics

A typical speed–torque curve during starting (rest to full-load speed). Different types of torques developed by the motor are defined as follows: 

Locked-rotor or breakaway torque (point “a”): Locked-rotor torque of a motor is the minimum torque that it develops at rest for all angular positions of the rotor with rated voltage applied at rated frequency.

Speed–torque characteristic of an induction motor

Pull-up torque (point “b”): The pull-up torque of an AC motor is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs.

 
Breakdown torque (point “c”): The breakdown torque of a motor is the maximum torque that it develops with a rated voltage applied at a rated frequency, without an abrupt drop in speed.
Full-load torque (point “d”): The full-load torque of a motor is the torque necessary to produce its rated power at full-load speed.

The operating point is the intersection of the motor and load torque curves. Slip is the difference between the motor synchronous speed and the operating speed.

NEMA MG-1 [S1] has defined the minimum torques (a, b, c) and maximum locked-rotor currents for motors rated 1–500 hp (375 kW). Minimum torques developed by motors rated >500 hp (375 kW) are 60% locked rotor, 60% pull-up, and 175% breakdown.

NEMA Design Letters

NEMA has assigned design letters for motors rated up to and including 500 hp (375 kW) that define the torque characteristics. NEMA design letters shown in fig. 9.6 do not apply to motors rated above 500 hp (375 kW). The design letters and hp ranges are:

Designs A, B, and E: for motors rated 1–500 hp (375 kW)
Design C: for motors rated 1–200 hp (150 kW)
Design D: for motors rated 1–500 hp (375 kW)
Design E was introduced to accommodate the demand for a better than high-efficiency design, which resulted in a higher locked-rotor current and lower torques compared with design B motors.