MCB to turn AC motors on and off.

G

grotor

Member
Location
Wisconsin
Occupation
Appliance technician
The 30A 2 pole QO series Square D breaker supplies 240V to 2 separate pieces of equipment. The third phase is Grounded B. Both pieces of equipment aren't supposed to run independently, but this rule is routinely disregarded in real life. One is run by the 3-phase, 3Hp motor. It draws 5A during operation. The second is run by 3-phase 10Hp motor. It draws 9A during operation, so the combined running current is 14A.
Can the customer use the breaker to start both motors in the morning and turn them off by the end of the day? It will be one on-off cycle per day. I don't want to bother the esteemed members of this forum with all details of this strange setup, but I will provide them upon request.
I exchanged e-mails with Square D engineer. He told that based on the trip curves and UL489 endurance test, the breaker should handle inrush currents and 6,000 on-off operations. My last question for him was, "Since AC motor is an inductive load, can arcing when the circuit is interrupted damage the breaker's contacts in the long run?" I still haven't received an answer.
What do you think?
 
Mcb on off switch manufacture recommendation seek, Use mpcb type breaker. Q0 for general purpose. See your single phase breaker suitable corner grounded.
 
The breaker in question does have overload protection. Besides, there is an overload relay attached to the motor contactor.
 
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Breakers are rated to be used as on/off switches but how long they will last with motor loads such as this is another question. I think you can go on what the SquareD engineer told you 6000 operations. I think it would be best to install a disconnect or safety switch and use that to operate the motors.

-Hal
 
Most 3 phase motors have no overload protection built in. If that is the case you need a VFD or a Motor starter with overload protection. Then wire a switch or a pushbutton into the start stop circuit of the VFD or motor starter.
 
If this is some kind of machine tool or something that could hurt or kill someone, what are the provisions for an E-Stop? Not to mention if the contactors close when you energize the power circuit if there is a power interruption the motors will start when the power comes back on. Operators can get hurt or killed when that happens. And if there is no disconnect, how are you doing lockout-tagout? The whole idea of controlling a machine from a breaker panel is kind of dangerous.
 
@ retirede
As I mentioned in my original post, the breaker supplies 240V to 2 motors: 5HP and 10HP. They aren't supposed to run independently - either both are one or both are off. The key word is supposed. There are no electrical interlocks between them.
@hal
You are the only one who has the patience to read everything that I post. There is a three-position switch, but the manufacturer jumpered the "off" circuit. For almost 25 years the restaurant was working 24/7. They needed both units running constantly. They work from 7AM to 9PM now. There is no need for both pieces of equipment to run 24/7.
To everyone else,
I do appreciate your time, expertise and desire to help me, but please, limit your responses to the QO series Square D breaker's suitability for turning motors on and off once a day.
 
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Joethemechanic said:
If this is some kind of machine tool or something that could hurt or kill someone, what are the provisions for an E-Stop? Not to mention if the contactors close when you energize the power circuit if there is a power interruption the motors will start when the power comes back on. Operators can get hurt or killed when that happens. And if there is no disconnect, how are you doing lockout-tagout? The whole idea of controlling a machine from a breaker panel is kind of dangerous.
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Both units are on the roof. They are completely enclosed and don't require operators. There are disconnects that are also on the roof by the machines.
 
grotor said:
I do appreciate your time, expertise and desire to help me, but please, limit your responses to the QO series Square D breaker's suitability for turning motors on and off once a day.
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All breakers listed to UL489 are rated for multi motor switching.

The only problem with switching motors is when the Inrush/Starting current is interrupted. Once a motor is up to speed stopping it has very little impact on a breaker, except when the motor full current is near the breaker rating. You are switching only 50% of breaker rating. Your application is fine.
 
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jim dungar said:
All breakers listed to UL489 are rated for multi motor switching.

The only problem with switching motors is when the Inrush/Starting current is interrupted. Once a motor is up to speed stopping it has very little impact on a breaker, except when the motor full .had current is near the breaker rating. You are switching only 50% of breaker rating. Your application is fine.
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Thanks, Jim.
 
Corner ground 240 Delta in a restaurant seems unusual. How are they getting their 120? There must be another transformer
 
Little Bill said:
I still don't understand, he has a 2-pole breaker. How is the B-phase being used?
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According to his post, B phase is grounded (corner grounded system) phase A & C is switched by the two pole breaker.

On a corner grounded system the grounded conductor (B) is not to be fused or disconnected unless all poles are disconnected simultaneously. It should be using a straight rated 2 pole breaker (240V) not slash rated 120/240, for a corner grounded system.

For a downstream disconnect or motor starter at the unit's location on the roof, it is permissible and desirable to disconnect all three phase conductors simultaneously, supplying the motors and rooftop equipment.
 
MTW said:
According to his post, B phase is grounded (corner grounded system) phase A & C is switched by the two pole breaker.

On a corner grounded system the grounded conductor (B) is not to be fused or disconnected unless all poles are disconnected simultaneously. It should be using a straight rated 2 pole breaker (240V) not slash rated 120/240, for a corner grounded system.

For a downstream disconnect or motor starter at the unit's location on the roof, it is permissible and desirable to disconnect all three phase conductors simultaneously, supplying the motors and rooftop equipment.
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ActionDave said:
As the third phase
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Thanks, I've never dealt with this before.
So where do you pick up the B phase? Does it land in the panel, without landing on the breaker, or in a bucket without landing on OCPD?
Hard for me to picture how the phase conductor is ran to the disconnect, or control.
 
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