Motor Starting

Dear All,

I want to know any rule of thumb for selecting contactor and cable rating for ant motor.
(1) My normal practice is that I check the full load current of motor and select the both contactors and cables 125%. Is it ok?
(2) Are there any de-rating factor for contactors and cables?
(3) A three phase contactor's rating is 50A. Is it means that its each contact can pass 50A? Can we use this contactor for switching DC supply?

Please explain in an easy way.

Power Factor

Dear All,

There are many kind of single phase motors available in market. Some motors have capacitors which remains connect with supply after motor starting like ceiling fan etc. Some motors capacitors disconnect through centrifugal switch after start up. We have such kind of motors in our plant. I want to know what is the power factor of such single phase motors? I have no data sheet for these motors as they are too old.

Dc Motor

Dear All,

We have some data of a dc shunt motor as following:

Vdc = 440
Ia = 92
Rpm = 1750
Feild = 200V,2.5A

How can we calculate its input power "P"?

Profile Options

Dear Marke,

When we check our profile, there is a tab "Profile options" in which we can view the topics/answers which we have posted. My suggestion is that there should be another option that such answers should also be displayed if any member posts against our any question (which we have posted ever before). Is it possible? If you design that option, this will be very helpful for members for checking the answers against their posts.

Star Delta Starting

Dear All,

As we know that any induction motor draws a peak current from supply when it starts. So the contactors should be able to pass on such a large starting current. But we have Japanese machines which star contactors have less rating (<30 - 40%) as compare with delta contactors in their star delta starters. some one can explain it why star contactors have less rating?
If we want to start, for example, a 37KW, 4poles, 400VAC, 50HZ motor on star delta mode, what should be the rating of star delta contactor?