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relays, contactors, and motor starters

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labster

Civil/Environmental
Sep 24, 2003
4
Forgive my ignorance, but new to the motor controls game. I need a contactor for a 1 hp sump pump that is being connected to a level control circuit. The pump data indicates that it is 115 volt, 1 hp, and draws 10 amps. does not indicate if this is running or starting current.

Trying to discern the difference between relays, contactors, and motor starters. I understand relays. Contacotrs appear to be a relay that may be same primary and secondary coil voltage, just diffeent draws for current on the control and dry contact side.

And a motor starter appears to be a contactor with overload protection....Is this correct?

 
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yes, correct. the contactor is a relay that is made for carry a lot of amperes on it's contacts. The starter is a contactor with overloads on it- used for motor applications. the contactors are also used for lighting circuits.
 
The 10A is the running amps under normal conditions (voltage and frequency at nameplate). The water level will hardly affect the amps unless there is no water.
 
Suggestion to labster (Civil/Environme) Oct 1, 2003 marked ///\\Forgive my ignorance, but new to the motor controls game. I need a contactor for a 1 hp sump pump that is being connected to a level control circuit. The pump data indicates that it is 115 volt, 1 hp, and draws 10 amps. does not indicate if this is running or starting current.
///The full load amps (FLA) of 1HP, 115V, 60Hz, motors are listed in NFPA 70 NEC to be 16A. Therefore, 10A is the motor continuous load ampacity draw. The motor may be drawing less amps than 16Amps if the motor shaft is less than rated shaft load.\\Trying to discern the difference between relays, contactors, and motor starters. I understand relays. Contacotrs appear to be a relay that may be same primary and secondary coil voltage,
///The contactors normally have one coil and one voltage applied to it. Some specialty contactors may have two coils, one for a swing to one position and another for swing the other position.\\ just diffeent draws for current on the control and dry contact side.
///The main contacts can be rated in 1000s of amps, if required. The auxiliary contacts are fairly standardized, about 10A of resistive load and low voltage, e.g. from about 24VAC to about 600VAC.\\And a motor starter appears to be a contactor with overload protection....Is this correct?
///Yes, as a minimum. It may also have a disconnect switch with fuses or circuit breaker, control push buttons, indicating lights, communication module, etc.\\
 
Hello Labster,

In very simple terms, a contactor is just a relay on steroids!

As per a conventional relay, a contactor has a coil and a set of contacts. Because contactors are designed for use on 3-phase networks, they generally have 3 x notrmally open power contacts and 1 or 2 auxiliary contacts in either N/O, N/C or a combination.

Contactors are generally used in motor circuits, although it is also possible to find them in lighting and distribution controls.

In your application (as advised by Buzzp), the 10 Amps to which you refer would be the current drawn by the motor when operating at rated load and under normal supply conditions.

You should note however that there are 3 different levels of current that come into play in any industrial application.

1. Motor nameplate amps (as described above).

2. Motor starting amps. This is generally in the order of 6 to 8 times the motor nameplate amps. However as contactors are designed for AC3 operation, you don't need to take starting current into account when selecting one.

3. Motor running amps. This is defined by the application and is generally lower than motor nameplate amps.

A motor starter comprises a contactor and an overload relay. The function of the overload relay is to protect the motor against overload (over-current) conditions.

Contactors and relays are manufactured by many companies including; Rockwell Automation/Allen Bradley, Asea Brown Boveri/ABB, Sprecher + Schuh, Cutler Hammer, Moeller, Siemens, Schneider Electric etc, just to name a few. All have web sites that provide detail of their products and network of distributors.

Good luck,
GGOSS

 
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