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Old 12-05-2011, 11:04 AM   #21
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Originally Posted by dronai
Why wouldn't the OL's on the starter open if you lost a phase ? It seems like the amperage would go up on the remaining phases.
Losing a phase, there is also the chance that the other two phases can back-feed through that open phase if the motor is running at the time of phase loss. The motor slows down and the current will divide itself amongst the windings. Even if an increase, it may not be enough to trip the overloads.
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Old 12-05-2011, 11:15 AM   #22
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Originally Posted by dronai

If you have OCP protection, and OL protection at the starter, why do they use fusing as well ?
Sorry, missed this one, and possibly JRaef could back this one up. The fuses are basically for protection of your wiring as well as your motor. Some fuse types are sensitive to single-phasing conditions, but code still REQUIRES short-circuit protection, which overloads ARE NOT. If no fuse protection, at least a breaker sized for protection of the motor branch circuit.
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Old 12-05-2011, 11:33 AM   #23
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Quote:
Originally Posted by jmsmith

Sorry, missed this one, and possibly JRaef could back this one up. The fuses are basically for protection of your wiring as well as your motor. Some fuse types are sensitive to single-phasing conditions, but code still REQUIRES short-circuit protection, which overloads ARE NOT. If no fuse protection, at least a breaker sized for protection of the motor branch circuit.
Correct, fuses are for over current and short circuit protection.
O/L are for motor overload protection.

O/L can is some cases protect against single phasing. But not always. That is application specific. VFD's, Class 30 o/l's,, ect... The list goes on.
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Old 12-05-2011, 11:51 AM   #24
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Originally Posted by jmsmith View Post
Sorry, missed this one, and possibly JRaef could back this one up. The fuses are basically for protection of your wiring as well as your motor. Some fuse types are sensitive to single-phasing conditions, but code still REQUIRES short-circuit protection, which overloads ARE NOT. If no fuse protection, at least a breaker sized for protection of the motor branch circuit.
I know what you are saying here, but if the OCP, and wiring were oversized, and feeding several motors, that were all fusable disconnects, I can see the need for the fusing. If you were to remove the fusing in this case, you could potentially burn up wiring, and something else, before the OCP could do it's job.

One feed, one OCP matched for wire size, and motor start, should be suffic for code, and protection I would think. Then no fusies, and less chance of single phasing right ?
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Old 12-05-2011, 12:29 PM   #25
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Originally Posted by JRaef
Don't waste money on those voltage monitor relays, they can be fooled into not tripping if the phase loss occurs while the pump is running, because it will regen into the lost phase. You are better off using a current based phase loss detector, it cannot be fooled. Many many of the new solid state overload relays have this built in now, and they are far superior at motor protection anyway. Look at your starter and ask the mfr if they have an SSOL option for it. If they do, make sure it provides phase loss protection (a few don't) and swap out your cheap little bimetal OLR. If they don't, that 777 is a good aftermarket choice.

If you have a current based phase loss device, it no longer matters if you have fuses or a CB.
They current based phase loss system is what was installed on our newest bridge. It would def have a regen issue.
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Old 12-05-2011, 12:41 PM   #26
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Originally Posted by dronai

I know what you are saying here, but if the OCP, and wiring were oversized, and feeding several motors, that were all fusable disconnects, I can see the need for the fusing. If you were to remove the fusing in this case, you could potentially burn up wiring, and something else, before the OCP could do it's job.

One feed, one OCP matched for wire size, and motor start, should be suffic for code, and protection I would think. Then no fusies, and less chance of single phasing right ?
In theory, yes... But of you take a look at multi-motor equipment, the majority that I have seen is fused at each motor branch. Either fuses or individual breakers. That is the same thinking as using an MCC... We do not rely on the main breaker for protection of all motors together. You still have to have both branch circuit and overload protection for EACH motor, whether fuses or appropriately sized breaker. You can spec a combination starter for individual motor with either fuses or breaker, and I hope that this is what the suggestion I hope was to replace the fuses with an appropriately sized breaker, not do-away with the branch circuit protection altogether. Hope this makes some sense!
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Old 12-05-2011, 02:54 PM   #27
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When I suggested getting rid of the fuses "when I could" was, if it wasn't in the Manufacturers design specs & even then I would with proper permission. Then I would replace it with a non fused disconnect.
Usually Disconnects are a safety switch out in the field within sight of the equipment. They are usually fed from an MCC with a circuit breaker and a starter with O/L's so you have all your protection.. You can also have a circuit breaker disconnect with your starter & O/L Start/Stop on it

Granted fuses can detect & blow on a short circuit faster than a circuit breaker, but most times it is for a ground on ONE conductor or an overload that reached the limit of that fuse & that fuse blows and the motor keeps on churning on single phase until it is damaged or Burnt enough to have excessive current to trip the O/L's or breaker.
So what good was that fuse? It didn't take out the control circuit to shut off the motor. It was sitting there UNDETECTED until it was too late !
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Old 12-05-2011, 03:16 PM   #28
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When I suggested getting rid of the fuses "when I could" was, if it wasn't in the Manufacturers design specs & even then I would with proper permission. Then I would replace it with a non fused disconnect.
Usually Disconnects are a safety switch out in the field within sight of the equipment. They are usually fed from an MCC with a circuit breaker and a starter with O/L's so you have all your protection.. You can also have a circuit breaker disconnect with your starter & O/L Start/Stop on it

Granted fuses can detect & blow on a short circuit faster than a circuit breaker, but most times it is for a ground on ONE conductor or an overload that reached the limit of that fuse & that fuse blows and the motor keeps on churning on single phase until it is damaged or Burnt enough to have excessive current to trip the O/L's or breaker.
So what good was that fuse? It didn't take out the control circuit to shut off the motor. It was sitting there UNDETECTED until it was too late !
In the case of that remote disconnect, you are absolutely right, all it has to be is a disconnect. What I was referring to was the "main" disconnect for that branch. That is the case where I was referring to replacing the fuses with a breaker. Sorry about that....
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Old 12-05-2011, 09:37 PM   #29
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Why wouldn't the OL's on the starter open if you lost a phase ? It seems like the amperage would go up on the remaining phases.
It depends too much on the motor loading, and the motor can be damaged even if not overloaded.

When you lose one phase, the motor current on the remaining 2 legs will increase by 1.732X. But let's say you have a 10HP motor rated 14A FLC but the load is only pulling 5HP, so about 7A. Then you lose a phase and the motor current increases to 12A. If you had the motor starter OL relay set for 14A because that was the FLC rating, then it will never trip if the load is only making the motor draw 12A.

However, the motor is heating up disproportionately to the amount of current it is drawing because of the severe current imbalance. When you have a current imbalance, you create what are called "negative sequence currents" inside of the rotor cage. These in turn cause counter rotating torque, which essentially "fights" the normal torque required to do the work and the extra work this creates is totally wasted as heat inside the motor. The result is, the motor heats up disproportionately fast. The tripping curve of an OL relay is based on a motor's thermal damage curve, but in an imbalance, you no longer have a normal thermal damage curve. So even if the effect of the negative sequence current causes the motor current to increase above the OL relay trip threshold, the time lag to trip will be based on the normal damage curve and by the time the OL relay finally trips, the windings are fried.

Happens all the time, but most people are unaware of the reason. They think the OL relay malfunctioned.
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Last edited by JRaef; 12-05-2011 at 09:54 PM.
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