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Regarding relay ratings

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Embedded_Geek

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Hi Members,

I want to turn On a water pump (induction motor) of 2hp using mechanical relay. Can any one guide me to select the relay with correct rating keeping in mind the inrush current involved during the turn On phase.

I have been struggling for this quite a while. So please help if possible.

Thanks in advance
 

The relay should be rated for mains voltage of course.

As for Ampere rating, on the chance that simple calculations will give a realistic answer...

2 HP equals 1440 W. (Based on 720W per HP.)

1440 W / 240 V = 6 A.

Multiply this by 4 or 5, because of startup surge (as you expect)...
as well as wear on the contacts if there is arcing at disconnect, etc.

By the way, aren't motors supposed to have their Ampere draw printed on the label in India?
 
Embedded_Geek said:
Hi Members,

I want to turn On a water pump (induction motor) of 2hp using mechanical relay. Can any one guide me to select the relay with correct rating keeping in mind the inrush current involved during the turn On phase.

I have been struggling for this quite a while. So please help if possible.

Thanks in advance
Click to expand...
IS it three phase or single phase motor ?

By the way, aren't motors supposed to have their Ampere draw printed on the label in India?
Click to expand...
yes, it is.
 
The relay should be rated for mains voltage of course.

As for Ampere rating, on the chance that simple calculations will give a realistic answer...

2 HP equals 1440 W. (Based on 720W per HP.)

1440 W / 240 V = 6 A.

Multiply this by 4 or 5, because of startup surge (as you expect)...
as well as wear on the contacts if there is arcing at disconnect, etc.

By the way, aren't motors supposed to have their Ampere draw printed on the label in India?
Click to expand...

I had a glance on the water pump. The following were printed on it:

HP = 0.5HP
RPM = 2800
Power Input = 0.65KW
Maximum Current = 3.25A

Now my question is is maximum current equal to surge current?

or is surge current = Maximum Current * 4
= 3.25A * 4

Also if I calculate current using the input power I do not get 3.25A, so please guide me find out the exact calculations involved in this.


Thanks in advance,
 

It seems you are using single phase motor.

Answer for second question:

in ac power = V * I * PF

so that means PF = 650 / 230 * 3.25 = 0.87 now you can calculate the power this way

First Question:

No, the maximum and surge currents are not same. each motor will take 10 times rated maximum current in starting

so the contactor (we will not say relay) should be able to make and break connections in that much range.

But their continuous current value can be as it as the maximum motor current. they will be designed to tackle the surge current.
 
It seems you are using single phase motor.

Answer for second question:

in ac power = V * I * PF

so that means PF = 650 / 230 * 3.25 = 0.87 now you can calculate the power this way

First Question:

No, the maximum and surge currents are not same. each motor will take 10 times rated maximum current in starting

so the contactor (we will not say relay) should be able to make and break connections in that much range.

But their continuous current value can be as it as the maximum motor current. they will be designed to tackle the surge current.
Click to expand...


Thanks Venkadesh_M!!!

For 1HP motor(single phase) the maximum current mentioned is 5.6A.

I have a relay with a rating (output side) equal to 50A (two contacts with 25A each, so I am planning to use the contacts in parallel to get 50A rating).

Since, you said that starting current is 10 times the maximum current, so if calculate it comes to around 56A. Some say that starting current is 4 or 5 times the maximum current.

If at all it is 10 times then it seems my relay (50A) will not work for 1HP motor.

Waiting for a reply.

Thanks in advance,
 

That 10 times current will be only for one electric cycle so you can leave it.

can you post a part number of your "contactor/relay"? Might be it was 50 A but we will normaly use the maximum current rating as the contactor current rating. because the contactor will have the power to make and break circuits of 10 times rated current. so you can just select a contactor of 8 or 10A max current, after looking in its datasheet.

using a 50A relay is totally waste of cost.
 

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