3-phase motor and contactor

[Skidood]

Hi folks, I know this is off-topic, but I'm sure someone will know this.
I have an older machine with a 3 phase 208 VAC 10 HP pump motor that is frequently blowing the fuses. The contactor is, of course, a 3-pole, with 3 fuses supplying power to the live side of the contactor. If one of the 3 poles in the contactor fails to make a connection when it pulls in, will this blow one or more of the fuses? I seem to recall that the answer is yes but I don't understand why, unless the missing phase provides impedance inside the motor to the other phases. I have this situation right now at a customer I am trying to help. I noticed that 2 out of the 3 fuses were blown.

 

[lfe]

When a three-phase motor loses a phase, it also loses the synchronism marked by the rotation of the 3 phases, the motor loses speed or even stops, the current of the remaining two phases can exceed the nominal current several times, so the trip of the protections, if they are well chosen, is insured.

If the contactor has repeated failures, it is probably not the right one for that motor and should be replaced with a larger one

 

[Skidood]

When a three-phase motor loses a phase, it also loses the synchronism marked by the rotation of the 3 phases, the motor loses speed or even stops, the current of the remaining two phases can exceed the nominal current several times, so the trip of the protections, if they are well chosen, is insured.

If the contactor has repeated failures, it is probably not the right one for that motor and should be replaced with a larger one

Yes, just looking at the size of the contactor, it seems small for this big motor. I am from the age of seeing big blocky black Square D contactors for motors like this. I will be able to go back and get the data off it on Monday and source a replacement. The fuses are 30 A.... Would not surprise me if the contactor was rated for 30 as well.

 

[BryanG]

I don't have standard tables that cover 208V, at 220V you would normally be looking at 27Amps, but it depends on the motor. So an AC3 30Amp rated contactor would be near the top end, but unless you have very frequent starts should be acceptable. I would hope that there is a motor thermal overload included in the design to protect the motor. The overload should also have single phasing detection. Again it depends on your circuit but I tend to prefer motor breakers that act as both an overload and short circuit protection device. More expensive but a whole lot easier to reset. Also check the control circuit voltage, a chattering contactor can cause high currents and burn out a contactor very quickly.

 

[Bolt]

According to my motor Square D Motor Data Calculator, a 208V 3 Phase 10 HP motor calls for either a 60 amp breaker or 50 amp time delay fuses (and a NEMA size 2 contactor). The rated FLA alone is 32.2, so one should readily expect nuisance tripping on 30 amp fuses.

 

[Skidood]

According to my motor Square D Motor Data Calculator, a 208V 3 Phase 10 HP motor calls for either a 60 amp breaker or 50 amp time delay fuses (and a NEMA size 2 contactor). The rated FLA alone is 32.2, so one should readily expect nuisance tripping on 30 amp fuses.

That's good to know. Thanks. The current rating shown on the motor data plate is 24 amps at 208/230V. Its labelled as a high-efficiency motor. Made by Hyundai of Korea.

Here in Canada (I could be wrong here but have always been annoyed at this) I believe electrical code (which in certain aspects is a little over the top) does not allow you to go that much higher (I mean as high as your calculator result) over/above the actual load when selecting fuses and circuit breakers. And yes, it does lead to nuisance tripping once in awhile.
On Monday I can see what their facility cct breaker is rated for, as it has also tripped at least once very recently.

 

[BachPhi]

Motor nameplate FLA should be using for OL size. But Fuse and wire size should be using FLC & NEC 430.250

 

[Rson]

That's good to know. Thanks. The current rating shown on the motor data plate is 24 amps at 208/230V. Its labelled as a high-efficiency motor. Made by Hyundai of Korea.

Here in Canada (I could be wrong here but have always been annoyed at this) I believe electrical code (which in certain aspects is a little over the top) does not allow you to go that much higher (I mean as high as your calculator result) over/above the actual load when selecting fuses and circuit breakers. And yes, it does lead to nuisance tripping once in awhile.
On Monday I can see what their facility cct breaker is rated for, as it has also tripped at least once very recently.

I'm not sure what fuses you are using - but if they are Bussman fuses, the manufacturer recommendation for a 3 phase 230V 10HP motor are the following:

LPJ fuses: 45A (Class J)
TCF fuses: 45A (Class CF)
LPN-RK-SP fuses: 30A (Class RK1)
FRN-R fuses: 30A (Class RK5)

#10 wire may be ok - but I would use #8.

The wire & fuse ampere calculation is 28A per NEC (not sure about Canadian applications)

I have build many motor control panels - and I've found NFPA motor sizing typically covers Canada & Mexico - it usually matches UL or CSA.

The fuse size will typically go well above the FLA of the motor nameplate due to the inductive nature of the motor. The FLA on the motor nameplate should only be used as an 'exact' setting when you are using a motor-protective circuit breaker such as Allen Bradley's 140M series.

 

[BachPhi]

208VAC 3P 10HP, not 230V.

 

[Rson]

208VAC 3P 10HP, not 230V.

My bad. At 208V, the fuse recommendations are:

LPJ fuses: 50A (Class J)
TCF fuses: 50A (Class CF)
LPN-RK-SP fuses: 45A (Class RK1)
FRN-R fuses: 40A (Class RK5)

30.8A FLA must be used for wire sizing, etc. which means #8 as mentioned above.

 

[Vic]

The Canadian Electrical Code states that the overcurrent protection for motor circuits must not exceed 175% of the motor full load current for time delay fuses, 300% for non time delay fuses and 250% for circuit breakers (table 29).

 

[doctord]

I would first ask has this always be happening or is this something new.
It sounds like it maybe setup wrong, but if it worked for years then what has changed. Could be a wiring issue, motor issue, even what the motor is driving could be adding additional load on the system.

 

[Gene Bond]

If the motor was recently (last 5 years or so) replaced, the problem likely lies with the fact that the newer, high efficiency motors draw much higher LRA than the older designs. The NEMA Code on the nameplate will show the range of LRA to FLA.

Either way, 30A fuses were always too small for a 24A motor. 24x1.75=42A.

 

[andrewwei]

Motor nameplate FLA should be using for OL size. But Fuse and wire size should be using FLC & NEC 430.250

I hope he reads your reply. This is step one.

 

[ValeoBill]

"doctord" nailed it: what changed? As others have mentioned, most hardware/components are undersized. Tho' it does seem odd to take out fusing without overloads being tripped. Have you verified wiring and motor windings with a megger?