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Signode
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Thermal Overload or Circuit Breaker for Single Phase Motor

05/01/2016 1:30 PM

Dear Friends; For a 220V, 1hp 8A blower motors application, we want to add safety to stop the operation of blowers in case of overload. As normal circuit breaker also has thermal characteristics, so can we use a 9/10A single pole circuit breaker instead of a thermal overload type breaker 6.5-9.5A?

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TonyS
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#1

Re: Thermal Overload or circuit breaker for single phase motor

05/01/2016 1:51 PM

You could use a three phase contactor/overload unit, relatively cheap and easy to come by.

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kvsridhar
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#2

Re: Thermal Overload or circuit breaker for single phase motor

05/01/2016 9:15 PM

In my experience, a thermal overload relay is more accurate than an MCB. It is also usually ambient-compensated (which is not the case in an MCB). It is also phase-loss-sensitive (which is unnecessary for you here). A contactor+overload relay is good for you here. Please note that there is no SC protection though.

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Signode
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#3
In reply to #2

Re: Thermal Overload or circuit breaker for single phase motor

05/01/2016 9:51 PM

Please note that in my case, the motor is single phase. As there are many blowers, so want to install cheaper items but should be appropriate. That's why want to discuss that if we can use a simple circuit breaker 9/10A instead of a 3pole overload type circuit breaker or a contactor overload package. A 10A breaker costs $3 but a contactor overload package costs around $30 and same the overload type circuit breaker.

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kvsridhar
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#4
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Re: Thermal Overload or circuit breaker for single phase motor

05/01/2016 11:28 PM

Yes of course, it makes sense for you to use a single-pole MCB. However, please note : a) Overload protection is coarser than what an overload relay will provide.

b) If used for On-Off switching, life will be less than that of a contactor.

And c) there will be a magnetic trip unit in the MCB - you need to ensure that it won't trip due to the starting current.

Good luck.

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TonyS
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#8
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Re: Thermal Overload or circuit breaker for single phase motor

05/02/2016 5:22 AM

OK an overload unit is $30, how much is a motor? How much downtime does changing a motor cause?

It sounds to me as if you've had a motor burn out and you're being asked for a solution. MCB's aren't the solution.

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Tornado
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#5

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 12:27 AM

Many single-phase motors have thermal protection cutouts built in, in which case a circuit breaker of 15A can suffice. Be cautious about whether the thermal protection is auto versus manual reset, and whether it is acceptable for the motor to restart on its own. This will depend both on the application and local regulations.

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JIMRAT
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#6

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 4:40 AM

220v, 1ph, 1 HP (740w) 8A?

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Ed Watts
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#10
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Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 10:04 AM

42% efficiency...

"Yeah, sounds good to me," said the pointy-haired boss.
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HiTekRedNek
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#7

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 5:14 AM

A circuit breaker or fuse is designed to protect the wiring,not the motor.

It is physically far removed form the motor,so it cannot really tell the most important

thing about the motor:The temperature.

A "Klixon" ,or other brand of thermal overload,located in,or in close proximity to the motor winding is the best motor protection that can be provided at a low cost.

Look at a central air conditioner units,and you will likely see one mounted inside the junction box of the compressor.

It will usually have a red reset button on it.

If the motor is overloaded,the temperature will rise,and the thermal overload will trip.

If the motor or wiring short circuits,the circuit breaker will trip.

You should spec the temperature of the thermal protective device based on the insulation type of the motor,and allow for the fact that,if it is not embedded into the winding,(usually by the manufacturer), there will always be an error and time delay in the action,therefor a trip point slightly below the max temperature of the motor should be selected.

Remember,heat is the enemy of motors,and like Arsenic,it is accumulative.

A motor cannot heal itself.

If you intend to use the circuit breaker as a switch,buy a Switch-Rated circuit breaker.

They are designed for switching,and are mechanically constructed to endure many off-on cycles.

If these motors have high starting current,use a HACR rated circuit breaker,which are designed for heavy starting current.

The bitter results of a cheap shortcut will endure long after the sweet taste of low cost.

Good luck.

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North of 60
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#9
In reply to #7

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 9:31 AM

Well said. Have a GA point on me.

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redfred
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#11
In reply to #9

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/02/2016 11:36 AM

Yes!

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Phys
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#13
In reply to #7

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/04/2016 8:38 AM

Very good - someone who understands what a circuit breaker is meant to do - protect the wiring. Yes - GA from me. (I have to repeat that at least once a week here)

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PWSlack
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#12

Re: Thermal Overload or Circuit Breaker for Single Phase Motor

05/03/2016 5:46 AM

<...instead of...>

Overload devices protect motors.

Circuit breakers protect wiring.

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Users who posted comments:

Ed Watts (1); HiTekRedNek (1); JIMRAT (1); kvsridhar (2); North of 60 (1); Phys (1); PWSlack (1); redfred (1); Signode (1); TonyS (2); Tornado (1)

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