250 amp fuse blowing ( 480V 1800 rpm 150hp motor )

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Lost_RFTech

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IL., Ia., Mo.
Maybe the OP should consider a system design change to avoid the frequent starts. This motor is running a pump, pumping what I do not know. Can the system be configured to recirculate whatever is being pumped to divert flow rather than starting and stopping the motor?

Regardless of method, frequent starts on a 150 HP motor is not a good idea.
 
Besoeker

Besoeker

Senior Member
Location
UK
ptonsparky said:
One of the larger soft starts we installed would only allow so many starts per hour. I believe it was 4 for the motor we had. We may have been able to defeat that parameter, but did not. After 4, the wait time was considerable, more than an hour.
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Four starts per hour is something I've encountered a few times. The resistance starter I included a few posts back is usually rated for four starts per hour evenly spaced and two consecutive starts from cold.
Our starters are usually part of a large VSD system which includes a PLC and that takes care of the timing. It can be defeated by powering down the PLC but we don't normally divulge that to the customer/end user. But some are smart enough to work it out for themselves.......:blink:
 
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Haji

Banned
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Besoeker said:
Four starts per hour is something I've encountered a few times. The resistance starter I included a few posts back is usually rated for four starts per hour evenly spaced and two consecutive starts from cold.
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Large induction motors are not generally rated by starts per hour, but rather by the number of cold/hot starts.
 
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Haji

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Haji said:
You need to ask the manufacturer of the motor.
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Or you need to know
the starting current =I1
the full load current =I2
the acceleration time=t
the design constant=K and
the number of consecutive starts =s for your motor and substitute the same in
SQR[(I1/I2)-1]*t*s=K
Thus for a DOL starting current of 6 times full load current and acceleration time of 4 seconds and allowable starting duty of 2 consecutive starts, the value of K will be
SQR(6-1)*4*2=200.
For soft starter with a starting current of 3 times full load current and acceleration time of 10 seconds and design constant K at 200, the allowable starting duty will be
s=200/[10*SQR(3-1)]=5
 
Besoeker

Besoeker

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Location
UK
Haji said:
Or you need to know
the starting current =I1
the full load current =I2
the acceleration time=t
the design constant=K and
the number of consecutive starts =s for your motor and substitute the same in
SQR[(I1/I2)-1]*t*s=K
Thus for a DOL starting current of 6 times full load current and acceleration time of 4 seconds and allowable starting duty of 2 consecutive starts, the value of K will be
SQR(6-1)*4*2=200.
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You might want to re-visit that calculation.

Haji said:
For soft starter with a starting current of 3 times full load current and acceleration time of 10 seconds and design constant K at 200, the allowable starting duty will be
s=200/[10*SQR(3-1)]=5
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And hence, this one.
 
Besoeker

Besoeker

Senior Member
Location
UK
Haji said:
Is it perhaps because you deal with electronic soft starters and not with magnetic ones?
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All sorts of starters.

Here's a suite of starters we made earlier.
Some are resistance starters, some electronic soft starts, and a number of direct on line units. They range from a few kW to 1350 kW.

Suite01_zpsb0369623.jpg



Haji said:
Somewhere else you mentioned your age around 105.
Is it true? ( I am a bit worried: I plan to have a lot of discussion with you. ;))
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I'll live until I die....:blink:
 
G

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
Haji said:
Including FCMA motor starters? :eek:hmy:

BTW, I took the calculation from this paper

http://turbolab.tamu.edu/proc/pumpproc/P22/05.pdf
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Apologies in advance, but I could not help grinning at the following statement in the paper [the following is a brief quotation without specific authorization made under fair use exceptions as part of a critical review of the referenced document]:

Moreover, sudden uncontrolled acceleration of the pump from standstill may create large reverse thrusts on the mechanical and civil foundations as well as jerks on the drive elements.
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Where I work, the jerks are more likely to be at the controls than on the drive elements. :)
 
H

Haji

Banned
Location
India
Besoeker said:
It is usually forum etiquette here, and elsewhere, to cite the site that you are quoting from.
Otherwise it looks as you are passing off their work as yours.
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It is usually forum etiquette here, and elsewhere, to base statements on sources believed to be reliable . But it is impractical to quote the source every time. Since you said you did not see the calculation, I refereed you to the source subsequently.
 
H

Haji

Banned
Location
India
Besoeker said:
If you were able to do so in post #54, you presumably could have done so in post #48.
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Yes. You are correct. I could have done that.

I now realize my ulterior motive was to spark discussion about the validity of the calculation in post#48, since the derivation of the equation was not given in that paper.
 
Besoeker

Besoeker

Senior Member
Location
UK
Haji said:
Yes. You are correct. I could have done that.

I now realize my ulterior motive was to spark discussion about the validity of the calculation in post#48, since the derivation of the equation was not given in that paper.
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i2t is a common way of dealing with thermal calculations. It's routinely used for determining the fuses required to protect power semiconductors.

I'm a bit surprised that the paper doesn't use real motor curves. Maybe that tells you something about its provenance.
 
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