2 HP Motor Tripping OCPD and blowing fuses

[Transportation Guy]

Good afternoon. Havent performed motor calc in a few years. We have a customer with a 2 HP, 230V motor. FLA of 9.8 and SF of 1.6. We sized the OCPD @ 30AMPs. FLC = 12x250% = 30AMPs. The Fuses/overlaod at 125% of the FLA. 9.8x125% = 12.25.

SMH......Incorrect calcs or bad motor?

 

[ActionDave]

Good afternoon. Havent performed motor calc in a few years. We have a customer with a 2 HP, 230V motor. FLA of 9.8 and SF of 1.6. We sized the OCPD @ 30AMPs. FLC = 12x250% = 30AMPs. The Fuses/overlaod at 125% of the FLA. 9.8x125% = 12.25.

SMH......Incorrect calcs or bad motor?

Huh? What are you using for overload protection? Fuses?

If you are using 30A fuses for short circuit protection and you have a starter with heaters sized at 125% of FLA then I going with bad motor.

 

[Transportation Guy]

Branch circuit protection is a 30amp breaker. Overload protection are 20 amp fuses. Did not use a starter. Engineer didnt want to pay for it.

30 amp breaker feeding a fused disconnect which houses 20 amp fuses feeding the pump. Hope this is clear enough.


Thanks for your help.

 

[ActionDave]

Just a guess but I'm still going with bad motor or something bad in the circuit.

Is the fuse blowing right at start up?

 

[Ingenieur]

so the fused disconnect is being used to manually start the motor?
what is the load? pump, fan, etc.

 

[Jraef]

What type of fuses are you using? Time delay?

 

[Transportation Guy]

This is a pump. College is using to irrigate a field from a pond.


Yes, they want to use the disconnect to start when needed.

 

[Jraef]

This is a pump. College is using to irrigate a field from a pond.


Yes, they want to use the disconnect to start when needed.

So that doesn't exempt this from the laws of physics.

Using fuses as the running OL protection is tricky to pull off. 125% is the max you can use given the SF, but if the fuses are not time delay, the starting current may blow them. Even if they are time delay, it's often a craps shoot because you don't know the total starting current / torque/speed curve of the motor as applied to this pump used in this way. For example in your description, I've seen people not understand that if the output of the pump is just connected to a fire hose or huge pipe with no head pressure or flow restriction, the pump will go "open channel flow" and can easily overload. An OL relay may give you enough of a delayed reaction to allow the pipe or hose to fill, but that's really hard to pull off with fuses. They are not really made to be used this way.

A better plan, even if they don't want a contactor and controls, would be to repurpose the fuses and disconnect switch somewhere else and use an enclosed Manual Motor Starter, since you already have the Short Circuit protection with the up stream breaker.

But before spending any more $$ on hardware, take current measurements.

 

[just the cowboy]

service factor

service factor

I think I see your problem. The motor has a 1.6 SF that means,which is high, they can run it 1.6 times the full load current. Some manufactures use this when they can't get a motor to fit the machine, or when they don't make the size they need. I see this allot with 100hp they use the service factor of 1.15 that gets them to a 115 HP load. Use the service factor for sizing the fuses and overloads, this may mean a larger starter and feed. Check the current to see if this works out 14.4 amps

 

[ptonsparky]

Boils down to you probably should have a 3 or 5 HP instead of the 2.
Drives me nuts when they try to run them into the SFA continuously.

 

[Ingenieur]

What is the fuse spec?
either way there is an issue if both the cb and fuses go
trip instantly or after a time period? Sec, minutes???

put a peak reading current meter on it
see what the running fla is if you can

 

[Saturn_Europa]

So that doesn't exempt this from the laws of physics.

Using fuses as the running OL protection is tricky to pull off. 125% is the max you can use given the SF, but if the fuses are not time delay, the starting current may blow them. Even if they are time delay, it's often a craps shoot because you don't know the total starting current / torque/speed curve of the motor as applied to this pump used in this way. For example in your description, I've seen people not understand that if the output of the pump is just connected to a fire hose or huge pipe with no head pressure or flow restriction, the pump will go "open channel flow" and can easily overload. An OL relay may give you enough of a delayed reaction to allow the pipe or hose to fill, but that's really hard to pull off with fuses. They are not really made to be used this way.

A better plan, even if they don't want a contactor and controls, would be to repurpose the fuses and disconnect switch somewhere else and use an enclosed Manual Motor Starter, since you already have the Short Circuit protection with the up stream breaker.

But before spending any more $$ on hardware, take current measurements.

Agreed......Open heading a pump is a sure way to trip motor overloads.

Definitely need to pull amps on it.

I have had some luck with getting a missed sized pump , back on to it's flow curve by putting false head on it. Usually by partially closing a discharge valve.

Of course it's better to get the proper equipment, but it sounds like this job has budget constraints.

I'm assuming this is a centrifugal pump.

 

[GoldDigger]

If the problem is mainly associated with starting rather than running, starting the pump with valve closed and then opening it may make a difference.

mobile

 

[Jraef]

If the problem is mainly associated with starting rather than running, starting the pump with valve closed and then opening it may make a difference.

mobile

Assuming there is a valve...

 

[GoldDigger]

Assuming there is a valve...

If not, consider installing one.

 

[iwire]

If the problem is mainly associated with starting rather than running, starting the pump with valve closed and then opening it may make a difference.

mobile

If the pump won't start I don't see how closing the valve would change things.

A centrifugal pump will not start moving water right away and when it does it will be a ramp up not jump.

I would say using a valve to help could only change things during the steady running state.

 

[kwired]

If the pump won't start I don't see how closing the valve would change things.

A centrifugal pump will not start moving water right away and when it does it will be a ramp up not jump.

I would say using a valve to help could only change things during the steady running state.

I think what they are getting at is when filling the piping on the discharge of the pump there is high flow with little or no pressure.
A point on a timeline that is past the initial starting surge but before normal running conditions are stabilized.

I run into this occasionally on field irrigation systems, it takes some time to fill a 1/4 mile of 6-8 inch empty piping, at that time the motor is pulling more current then it pulls when the piping is full and under normal pressure. If it takes too long to fill the system the motor overload may trip, but a system that had a fair amount of water in it before starting fills fast enough the current drops before overload trips.