Motors - Operation and Issues - 3

Reduced Voltage Starting in Motors

Reduced Voltage Starting in Motors refers to the practice of reducing the motor at a reduced voltage. This is done to limit the heavy inrush current during starting. The voltage is reduced by connecting resistors or reactors in series.

Soft Starters can also be used to reduce the voltage during starting.

The disadvantage of using reduced voltage starting in motors is that as the voltage is low during startup, the torque is also low at starting. This method of starting may not be suitable for applications such as conveyors, lifts, etc which require the motor to start with the load.

Types of Reduced Voltage Starters are

Resistance Starters
Reactance Starters
Autotransformer Starters
Soft Starters
Star Delta Starters


Soft foot in Electrical equipment

Soft Foot in rotating electrical equipment such as motors and generators is condition in which all the four feet are not in equal contact with the surface. This leads to a distortion of the frame once the bolts in the feet are tightened. This, in turn, affects the shaft alignment.

Soft foot can also result in vibration when the machine rotates.

Soft foot can be caused by


  • manufacturing defects
  • improper shim fitment
  • presence of foreign material such as dirt or trash
  • Soft Foot can be minimized by selecting proper shims and ensuring the the surface of the foot does not have any foreign materials.


http://thealignmentblog.com/blog/2010/06/04/soft-foot-what-it-is-and-how-to-minimize-it/


Noise in Electric Motors

Noise in Motors is a nuisance and a waste of energy. The more efficient a motor, the lesser the noise it produces.  However, noise is also an important parameter for diagnosing the condition of a motor. However, a careful analysis of the noise of a motor can given an idea of the condition of the different components of a motor.

Noise analysis can tell about the condition of the bearings. Misalignment of the motor coupling can also given a distinct noise signal

A choked filter for cooling will produce a distinct noise caused by the altered airflow.

Powerful tools such as FFT (Fast Fourier Transforms) help analyze specific frequencies and localize abnormalities, if any.

Technicians and Engineers should be trained to detect abnormality in sound. Noise Analysis of motors in an industry can be carried out at periodic schedules in consultation with the motor manufacturer.



Reduced Voltage Starting in Motors
  
Reduced Voltage Starting in Motors refers to the practice of reducing the motor at a reduced voltage. This is done to limit the heavy inrush current during starting. The voltage is reduced by connecting resistors or reactors in series.

Soft Starters can also be used to reduce the voltage during starting.

The disadvantage of using reduced voltage starting in motors is that as the voltage is low during startup, the torque is also low at starting. This method of starting may not be suitable for applications such as conveyors, lifts, etc which require the motor to start with the load.

Types of Reduced Voltage Starters are

Resistance Starters
Reactance Starters
Autotransformer Starters
Soft Starters
Star Delta Starters


Soft foot in Electrical equipment


Soft Foot in rotating electrical equipment such as motors and generators is condition in which all the four feet are not in equal contact with the surface. This leads to a distortion of the frame once the bolts in the feet are tightened. This, in turn, affects the shaft alignment.

Soft foot can also result in vibration when the machine rotates. 

Soft foot can be caused by

manufacturing defects
improper shim fitment
presence of foreign material such as dirt or trash
Soft Foot can be minimized by selecting proper shims and ensuring the the surface of the foot does not have any foreign materials.

http://thealignmentblog.com/blog/2010/06/04/soft-foot-what-it-is-and-how-to-minimize-it/


Noise in Electric Motors

Noise in Motors is a nuisance and a waste of energy. The more efficient a motor, the lesser the noise it produces.  However, noise is also an important parameter for diagnosing the condition of a motor. However, a careful analysis of the noise of a motor can given an idea of the condition of the different components of a motor.

Noise analysis can tell about the condition of the bearings. Misalignment of the motor coupling can also given a distinct noise signal

A choked filter for cooling will produce a distinct noise caused by the altered airflow. 

Powerful tools such as FFT (Fast Fourier Transforms) help analyze specific frequencies and localize abnormalities, if any. 

Technicians and Engineers should be trained to detect abnormality in sound. Noise Analysis of motors in an industry can be carried out at periodic schedules in consultation with the motor manufacturer.


Harmonic Torques in the Induction Motor

If the supply to the induction motor contains harmonics, each individual harmonic will generate a torque depending on its frequency at a specific speed. Thus the 7th harmonic generates its torque at 1/7th of the rated speed. The 5th harmonic produces its torque at 1/5th of the rated speed. The 11th harmonic and the 13th harmonic generate the torque at 1/11th and 1/13th of the rated speed.

The 3rd harmonic is absent in a 3 phase supply. Harmonics whose order is greater than the 7th harmonic are usually neglected.

Of the harmonics, the 5th harmonic and the 11th harmonic rotate in a direction opposite to the direction or rotation of the motor. Thus the torque produced by the motor is the sum of the torque of the fundamental frequency and that of the 7th harmonic.

If the 7th harmonic is present in the supply, this can result in crawling.


Cogging in induction motors

Cogging or magnetic locking refers to the phenomenon in which the rotor of the induction motor gets magnetically locked to the stator and the motor refuses to start. Cogging is caused by a wrong choice of rotor and stator slots. If the number of stator slots is equal to the number of rotor slots, the reluctance is minimum when the stator and rotor teeth face each other and the motor refuses to start. This is overcome by making the number of rotor slots unequal to the number or stator slots. 

If certain harmonics are present in the supply, the slot frequencies can coincide with the harmonic frequencies and cogging can result.

To prevent cogging, the rotors are skewed with respect to the stator such that each rotor slots is facing more than one stator slot at any given time. 


Power Flow in an induction motor

In the induction motor, as in all motors, electrical energy is converted into mechanical energy. 

Some of the electrical energy supplied to the stator is lost as copper and iron loss in the stator. The remainder of the energy is given as input power to the rotor through induction between the stator and the rotor. The electrical power supplied to the rotor is used to produce the mechanical power in the shaft. In the rotor, some of the power is lost as heat. The electric power in the rotor is converted into mechanical power.

This power is available as the rotor output excluding the losses in the rotor (friction and the windage losses) 


The Power Flow Diagram of the induction motor is as follows

If the supply to the induction motor contains harmonics, each individual harmonic will generate a torque depending on its frequency at a specific speed. Thus the 7th harmonic generates its torque at 1/7th of the rated speed. The 5th harmonic produces its torque at 1/5th of the rated speed. The 11th harmonic and the 13th harmonic generate the torque at 1/11th and 1/13th of the rated speed.

The 3rd harmonic is absent in a 3 phase supply. Harmonics whose order is greater than the 7th harmonic are usually neglected.

Of the harmonics, the 5th harmonic and the 11th harmonic rotate in a direction opposite to the direction or rotation of the motor. Thus the torque produced by the motor is the sum of the torque of the fundamental frequency and that of the 7th harmonic.

If the 7th harmonic is present in the supply, this can result in crawling.


Cogging in induction motors

Cogging or magnetic locking refers to the phenomenon in which the rotor of the induction motor gets magnetically locked to the stator and the motor refuses to start. Cogging is caused by a wrong choice of rotor and stator slots. If the number of stator slots is equal to the number of rotor slots, the reluctance is minimum when the stator and rotor teeth face each other and the motor refuses to start. This is overcome by making the number of rotor slots unequal to the number or stator slots.

If certain harmonics are present in the supply, the slot frequencies can coincide with the harmonic frequencies and cogging can result.

To prevent cogging, the rotors are skewed with respect to the stator such that each rotor slots is facing more than one stator slot at any given time.


Power Flow in an induction motor

In the induction motor, as in all motors, electrical energy is converted into mechanical energy.

Some of the electrical energy supplied to the stator is lost as copper and iron loss in the stator. The remainder of the energy is given as input power to the rotor through induction between the stator and the rotor. The electrical power supplied to the rotor is used to produce the mechanical power in the shaft. In the rotor, some of the power is lost as heat. The electric power in the rotor is converted into mechanical power.

This power is available as the rotor output excluding the losses in the rotor (friction and the windage losses)

The Power Flow Diagram of the induction motor is as follows