Motor Starting

[Rzn]

Dear All,

 

I want to know any rule of thumb for selecting contactor and cable rating for ant motor.

(1) My normal practice is that I check the full load current of motor and select the both contactors and cables 125%. Is it ok?

(2) Are there any de-rating factor for contactors and cables?

(3) A three phase contactor's rating is 50A. Is it means that its each contact can pass 50A? Can we use this contactor for switching DC supply?

 

Please explain in an easy way.

 

[marke]

Hello Rzn

 

These questions should be answered by the suppliers of the equipment that you are using. Different manufacturers have different rating/derating information.

 

Best regards,

[jraef]

1) Yes, a 125% FLA rating of the conductors is the place to start, but you still need to factor in voltage drop, starting issues and any other mitigating circumstances of each application. For instance if your load is a centrifuge that takes 5 minutes at elevated current to finish accelerating, the conductors must be sized for the starting current, not the running current. If your motor is 2000m away from the starter, the conductors will need to be over-sized to prevent voltage drop. Everything must be considered on a case-by-case basis.

 

2) Derating factors have to do with other environmental issues that need addressing, i.e. voltage drop over distance, high ambient temperature etc. for the cables, duty cycle, altitude ambient temperature etc. for the contactors. Most contactor manufacturers will provide you with the necessary information if you ask.

 

3) Yes, if a 3 phase contactor is rated 50A, that means that each set of contacts can handle 50A continuously. DC current ratings of contactors are usually lower. Each contactor manufacturer will be different, there is no universal formula to apply.

[Rzn]

1) Yes, a 125% FLA rating of the conductors is the place to start, but you still need to factor in voltage drop, starting issues and any other mitigating circumstances of each application. For instance if your load is a centrifuge that takes 5 minutes at elevated current to finish accelerating, the conductors must be sized for the starting current, not the running current. If your motor is 2000m away from the starter, the conductors will need to be over-sized to prevent voltage drop. Everything must be considered on a case-by-case basis.

 

2) Derating factors have to do with other environmental issues that need addressing, i.e. voltage drop over distance, high ambient temperature etc. for the cables, duty cycle, altitude ambient temperature etc. for the contactors. Most contactor manufacturers will provide you with the necessary information if you ask.

 

3) Yes, if a 3 phase contactor is rated 50A, that means that each set of contacts can handle 50A continuously. DC current ratings of contactors are usually lower. Each contactor manufacturer will be different, there is no universal formula to apply.

 

Dear Jraef

 

Thanks a lot for replying.

 

[jOmega]

JRAEF,

 

With regard to your answer to 3), what will you say when the AC contactor, used in an inductive DC circuit, blows up .... because it failed to quench the arc upon opening..... in time ..... ????? (i.e., DC motor Armature Ckt ..... DC Motor Field Ckt, etc. )

 

Is it reasonable to expect that if an AC contactor were to be rated for DC duty, its manufacturer would rate it as such, and would so state the dual service in its literature.

 

Perhaps that's why the poles of DC contactors have Blow-Out Magnets and Arc-Shutes (Arc-Chutes).

 

Could be that the reason you don't see any AC-DC load rated contactors is that in an AC application, the Alternating Current would de-magnetize the blowout magnets.

 

Perhaps the ONE exception to the above would be the use of an AC Contactor in a DC Circuit where the load is purely RESISTIVE; in which case, the manufacturer's literature for such a contactor will state that the contactor can be used in a DC Resistive Circuit, and will list the voltage and current rating of the poles. As you said, you can expect their rating to be appropriately derated. An example would be an AC contactor that is approved for use in non-inductive heater control.

 

With regard to the wiring consideration, a few additional points to ponder ......

 

In specifying wire cables for a given application, no mention was made of the need to consult national and local electrical codes that may be in force in Rzn's piece of the world.

 

Should they just be ignored ? Hard to craft a "Rule of Thumb" that would accommodate pages of written code requirements.

 

Should the effect of enclosed or open wiring be ignored ? What about bundling ? Such issues are likely to be addressed by the 'Codes' and / or the guidelines of the wire cable manufacturer/supplier. This too isn't likely to find its way into a "Rule of Thumb" ...

 

Shortcuts often lead to unforseen disaster, which is why good engineering doesn't stand on "Rules of Thumb".

 

 

 

[marke]

The DC rating on AC contactors can be very low except at low voltages. You must refer to the manufacturers.

 

Best regards,