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Old 03-08-2012, 12:25 PM   #31
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help needed with relay switch


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Originally Posted by curiousB View Post
....10 times it's operating CURRENT....
What?

Do you not know that amperage is a measurement of current?..and since this is a discussion about motors....tell me where is says "current" on a motor nameplate.

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Last edited by Missouri Bound; 03-08-2012 at 12:33 PM.
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Old 03-08-2012, 12:35 PM   #32
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Originally Posted by Missouri Bound View Post
What?

Do you not know that amperage is a measurement of current?..and since this is a discussion about motors....tell me where is says "current" on a motor nameplate.

I think he was referring to the voltage statement.

"The motor will draw more on start up...but it won't draw 10 times it's operating voltage. Motors rarely draw more than 2 to 3 times their operating amperage on startup..and that's a 10 amp relay."
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Old 03-08-2012, 12:48 PM   #33
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Oh ...I didn't catch that.

Well thanks for clearing it up, and I humbly apologize for my rush to judgement.
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Old 03-08-2012, 01:38 PM   #34
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help needed with relay switch


I want to buy a 5 current motor and 10 feet of 83700 circular mill wire.
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Old 03-08-2012, 05:43 PM   #35
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help needed with relay switch


Its not just a matter of the running/start up current !
Its also a matter of the large spikes and back EMFs
because motors are a highly reactive load.

How much current they pull on start up depends on many factors,
such as load on motor, and position of rotor reletive to the field,
the design of the motor, etc etc etc.
but suffice to say it is much larger then its normal running current.

If the contacts of the relay are too small,
they will soon burn out, due to the many spikes.
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Old 03-09-2012, 11:23 AM   #36
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help needed with relay switch


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Originally Posted by M Engineer View Post
The pump is only one amp. I would bet that the relay would handle that but he need to check to be sure.
Looking at the relay again I concur it could handle it. But this is a motor load and a small contactor with OL protection (the OP has not told us if the pump has internal OL protection or not) is what he should use. The relay should be used to switch the load, not start and carry the load. Just my opinion and how I would proceed.

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Originally Posted by pondo View Post
thank you so much zappa - you made my day. @dmxtothemax - thanks to you too. i will get a bigger relay
No. Keep the relay and get a small contactor with OL protection. The pump will be in the well. The OL protection should be where you can access it.

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Originally Posted by Missouri Bound View Post
The motor will draw more on start up...but it won't draw 10 times it's operating voltage. Motors rarely draw more than 2 to 3 times their operating amperage on startup..and that's a 10 amp relay.
Motors can draw up to 8-10 times their full load current on start. This depends on the motor design and the load. This is why I suggest the contactor using the relay to control it. Not to use the relay to carry the starting and running current of the pump motor.
While this relay could do this, it is not advised. I would not count on the relay to last very long at all. We also do not know the amount of cycles. How many times a day or how many times an hour the relay will be relied upon to do something it is not intended to do. A contactor is intended for this operation and will have electrical and mechanical operations for the lifetime of the contactor available from the manufacturer.
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Old 03-09-2012, 08:32 PM   #37
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help needed with relay switch


Quote:
Originally Posted by dmxtothemax View Post
Its not just a matter of the running/start up current !
Its also a matter of the large spikes and back EMFs
because motors are a highly reactive load.

How much current they pull on start up depends on many factors,
such as load on motor, and position of rotor reletive to the field,
the design of the motor, etc etc etc.
but suffice to say it is much larger then its normal running current.

If the contacts of the relay are too small,
they will soon burn out, due to the many spikes.
This is wrong. This spike theory is mis information. A motors peak current is when it starts up. Most commercial grade motors specify a locked rotor current which is meant to provide guidance on disconnect switch rating and protection sizing for the circuit. It isn't about so called mystery pulses.

Contacts need to be rated for the peak inrush current of startup. They also take some beating (arcing) at disconnect due to the di/dt impact of interupting current but this is the lesser of the two challenges.
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Old 03-09-2012, 08:53 PM   #38
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help needed with relay switch


PONDO

Can't quite read the label on the motor, any clue on the MFG and model number? It might be helpful to determine the motor operating characteristics.

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