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How to select MPCB?

Created by: PISCEAN 2010 at: 6/26/2013 8:49 AM (22 Replies)

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6/26/2013 8:49 AM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello Experts, 3 phase AC squirrel cage induction motor Kw=5.5, Volt = 415. RPM = 1400, I = 10.5A. Motor drives hydraulic pump. Motors running/normal load current is 4.2 A. Problem facing on this machine is, hydraulic pump fails, i.e. gets crack within 1 month. Hydraulic experts are working on this problem. Meanwhile it is suggested that motor should tripwhenever the motor drawn current exceeds normal working current i.e. 4.2. Since the motor rated current is 10.5 A, original MPCB was of range 8-13A. This MPCB replaced with lower capacity MPCB i.e. with range of 3.2-5A with current setting at 5A. But after replacingMPCB, the lower capacity MPCB trips during starting of motor, (because of inrush current) In this case, how protection to be provide, is my querry. Can any one help me? Thanks in advance.
PISCEAN 2010 Posts: 8092 Rating: (884)
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6/26/2013 11:42 AM	Rate (0)
Ste39 Posts: 4620 Rating: (710)	Hi PISCEAN 2010, The MPCB you use, are the series 3RV10?
Ste39 Posts: 4620 Rating: (710)
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6/26/2013 12:07 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello, Thanks for your attention. No presently we use 3VU series.
PISCEAN 2010 Posts: 8092 Rating: (884)
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6/26/2013 1:54 PM	Rate (0)
DS SURESH KUMAR Bronze Member Joined: 4/17/2012 Last visit: 11/21/2025 Posts: 474 Rating: (10)	PISCEAN 2010 Hello Experts, 3 phase AC squirrel cage induction motor Kw=5.5, Volt = 415. RPM = 1400, I = 10.5A. Motor drives hydraulic pump. Motors running/normal load current is 4.2 A. Problem facing on this machine is, hydraulic pump fails, i.e. gets crack within 1 month. Hydraulic experts are working on this problem. Meanwhile it is suggested that motor should tripwhenever the motor drawn current exceeds normal working current i.e. 4.2. Since the motor rated current is 10.5 A, original MPCB was of range 8-13A. This MPCB replaced with lower capacity MPCB i.e. with range of 3.2-5A with current setting at 5A. But after replacingMPCB, the lower capacity MPCB trips during starting of motor, (because of inrush current) In this case, how protection to be provide, is my querry. Can any one help me? Thanks in advance. Hi, i also faced same like problem in factory. My suggestion is the 8-13A MPCB is good because this hydraulic pump fails due to over heat of motor. 1.It may try to run at Full load without any good ventillation . 2.During the full load the RPM is may be less, because of this motor don't get sufficient air circulation for cooling . 3.even if it take 4A with low RPM motor coil start to produce heat so don't accept the problem because of electrical. It is fulll of mechanical problem. How i telling strong in your case means i am Mechanical Engineer 4. Another thing if heat increase the Terminal get loosen easily . So it is also one of the reason.
	Last edited by: DS SURESH KUMAR at: 6/26/2013 1:57 PM
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6/26/2013 2:16 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	DS SURESH KUMAR PISCEAN 2010 Hello Experts, ….Hi,i also faced same like problem in factory. My suggestion is the 8-13A MPCB is good because this hydraulic pump fails due to over heat of motor.1.It may try to run at Full load without any good ventillation .2.During the full load the RPM is may be less, because of this motor don't get sufficient air circulation for cooling .3.even if it take 4A with low RPM motor coil start to produce heat so don't accept the problem because of electrical. It is fulll of mechanical problem. How i telling strong in your case means i am Mechanical Engineer4. Another thing if heat increase the Terminal get loosen easily . So it is also one of the reason. Hello,Thanks for your reply.We replaced MPCB to trip the motor if it starts to draw more current than normal loading current, which is 4.2A. idea behind this is to ensure that pump should not be loaded because of any other reason. Original MPCB current setting is 8-13 & hence its of no use. With new MPCB motor can be set to trip at 5 A.From above data, it can be easily concluded that motor is never loaded to its rated power & as such never reached to 10 A & hence speed dropping is not the issue. From hydraulic pump chart (output lpm / pressure vs motor power) also it is confirmed. Motor temp & oil temp are monitored & found within permissible limits. Hope this is clear now. So how the motor can be protected?
PISCEAN 2010 Posts: 8092 Rating: (884)
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6/26/2013 2:24 PM	Rate (0)
Ste39 Posts: 4620 Rating: (710)	Hi PISCEAN 2010, If you have a 3VU you can try putting a 3RV14 the width is the same, this model was made for the transformers so it has a higher inrush current.
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6/26/2013 3:57 PM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello Ste39, Once again thanks for your interest. I think there is difference in charateristics im motor protection & transformer protection. Can you help to find out the difference between these two? May be, it may not trip during inrush current but at the same time, will it protect the motor during normal loading? ( as per regular MPCB charachterstic? just a thought)
PISCEAN 2010 Posts: 8092 Rating: (884)
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6/26/2013 4:19 PM	Rate (0)
Ste39 Posts: 4620 Rating: (710)	Hi PISCEAN 2010, Have a look at page 8: Display - Siemens Industry Online Support This version of motor starter protector is not necessary in the case of control-power transformers with low inrush currents, such as control transformers from Siemens. 3RV1 motor starter protectors for motor protection can be used in this case.
Ste39 Posts: 4620 Rating: (710)
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6/27/2013 9:06 AM	Rate (0)
PISCEAN 2010 Posts: 8092 Rating: (884)	Hello, Sorry, I think our discussion has deviated from main problem. Please read carefully.
PISCEAN 2010 Posts: 8092 Rating: (884)	Last edited by: PISCEAN 2010 at: 6/27/2013 9:08 AM
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6/27/2013 9:25 AM	Rate (0)
Ste39 Posts: 4620 Rating: (710)	Hi PISCEAN 2010, Sorry but I had understood that you needed something for the initial current peak, because a normal motor protector (3RV10 is 13 times) and instead (3RV14is20 times), I got it wrong?
Ste39 Posts: 4620 Rating: (710)
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