Jump to content

Induction Motors


shermaine

Recommended Posts

Any one can advise me on that.

(1) Why is the no-load input current and power as a percentage of their full(rated)values recorded are considerably

higher than those of the transformer?

 

(2) Estimate the starting current of the motor(as a percentage of its rated value) when it is started at rated voltage.

How can the starting current be reduced to an acceptable value in practice.

Link to comment
Share on other sites

Hello Shermaine

 

Although there are similarities between the equivilent circuit and a motor, the two are not the same.

1) The induction motor behaves a little bit like a transformer, but the secondary is short circuited. The frequency of the current flowing in the rotor is proportional to the rotor slip. This greatly influences tha current and power flowing into the motor under all conditions. An induction motor, even under no load conditions, has rotational losses, frictional and windage as well as rotor losses. These losses do not occur in a transformer.

 

2) When an induction motor is started at full voltage, it will initially draw locked rotor current. This high current will slowly reduce as the motor accelerates, but will only reduce significantly when the motor is approaching full speed. The actual current and the speed/current curve of the motor are a function of the motor design.

The Locked rotor current is typically between 550% and 900% of the rated cuirrent of the motor.

 

Best regards,

Link to comment
Share on other sites

For No-Load Testing

 

Any one knows why the no-load current and power of the motor expressed as percentage of their full-load values is considerably higher than those of a transformer?

 

 

 

For Blocked Rotor Test

 

Can the starting current be reduced to an acceptable value in practice?

 

Are there many methods?

 

Pls advise.

Thanks

Link to comment
Share on other sites

Hello shermaine

As referred to previously, the induction motor is like a transformer with a shorted circuited secondary. I also has additional losses that do not occur with a transformer, hence the current is higher.

 

2 The Locked rotor current can be reduced by reducing the applied voltage. This is done in industry and is referred to as reduced voltage starting. If you look on the web pages for this site you will find examples of reduced voltage starters and their characteristics.

 

Best regards,

Link to comment
Share on other sites

Hello Shermaine

 

You could try this website for a start. There is a page on motor starters showin how they operate and reduce start current. http://www.lmphotonics.com/m_start.htmThere is also a page on soft starters.

 

Best regards,

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...