im rather new in this field so please forgive my ignorance. im not so sure of the LRA but from the manufacturer catalogue, the DOL starting current / Nominal Current = 8.
The Canadian Electric Code calls for a maximum breaker rating of 200% of full load current for an Auto transformer start motor.
This is for a conventional inverse time breaker. The starting current factor of 8 is for direct online starting.
The starting current with an autotransformer starter will be much less. That's the point of autotransformer starting.
Although the Canadian code is somewhat harmonized with the NEC, the use and rating of instantaneous trip circuit breakers is one area with significant differences.
You had best state your location and the codes that you are subject to and let someone with local knowledge answer this.
respectfully
thanks a lot to itsmoked and waross for the quick reply.
So based on the motor / pump load, what kind of starter is hest for me? DOL , Star-Delta or AutoTrans ? Is it true that if i were to select DOL, i'll have to size my breaker to 8x the FLA of the pump ? and howabout for S/D starter?
Generally around here the S/D is heavily frowned upon. It has several bad gotchas.
DOL is the cheapest but can give voltage sag issues to surrounding equipment if your power source is spongy (dimming lights, resetting equipment, extinguishing HID lighting, raising your metering demand factor). Depending on the driven load it can also cause more wear and tear.
As I said autotrans is not in my experience.
The favored starter around here is a Soft Starter. These are electronic devices that let you set the maximum inrush current and the length of the starting time. They provide very gentle starts and disturb the power source the least. They can even allow starting some motors from sources that they couldn't possibly be started from normally.
They aren't cheap but they are starting to compete favorably with other systems like S/D as they are coming down in price. Your motor size is just at the top of reasonably priced units. You should at least research them. Scotty will tell you a make or three to look at.
Soft starters are usually less expensive than Autotransformers Keith, just so you know. At 45kW though it might be a toss up, however I don't know a lot of manufacturers who keep 45kW RVATs readily available.
Relaxjack, the type of starter you select depends on what your goal is.
Reduced Voltage Auto Transformer (RVAT) starters can provide the best bang for the buck in terms of line current to motor current because of the transformer action. Their limitation is in that they only have 3 steps of adjustment, 50, 65 or 80% voltage so if one of those doesn't work you have to take a big step to the next size up and that means more torque that you may not need. They also trade one big torque hit for two smaller ones, not as good for the motor as soft starters. Lastly they are very large and heavy and have a limited starting duty, usually much lower than the motor they are controlling. I only recommend RVATs when there is a severe problem with voltage drop or an undersized portable generator.
Reduced Voltage Solid State (RVSS) have infinite adjustability so they can be tuned to provide a very smooth transition-less acceleration, no excess torque (unless you want it). They are best for the load and motor, but motor current = line current, so they are a compromise when it comes to their effect on the incoming line voltage. Still, most people get along fine with them. They take up less space and floor weight than RVATs and typically have the same or better starting duty as the motor. They also cost less than RVATs (unless you are shopping poorly), and if you have a pump, they can be very handy for soft stopping to reduce water hammer. RVATs can't do that.
Forget Start-Delta, too many potential problems. I know that lots of people use them, but that's just because they are cheaper than RVSS. If they really knew what they are doing to the motor, power train and power supply they would run away.
The size of your breaker for direct online starting will be more like 200% or 250% than 800%, in the US or Canada, (I don't have my code book with me and can't check the exact percentage). However, this is a code issue which must be answered by someone in your location, using the same codes as you.
Given the low inertia and the speed/load characteristics of centrifugal pumps, direct online starting will work well on all but the weakest systems.
waross: according to the manufacturer catalogue, the DOL starting current is 8x the FLC. if i were to size my breaker to 200% - 250%, wouldn;t it trip the breaker during starting ? im a bit confused now because i just read a book saying that circuit breakers must be sized to take the starting current of the motor during the instant of starting. please enlighten me. anyone?
Code will have limits on the breaker size as Waross said, but the reason it will work at 200% FLA is because of the Inverse time function of the breaker. It trips instantly on short circuit (huge amperage), slower on starting surges (maybe 4X nameplate amps), and maybe never on a slight overload (1.2X nameplate?). So if the starting surge is short (it should be) the breaker is designed to ride through it.
Breaker and conductor sizing is done based on the FLC, not the LRC. As mentioned by ccjersey, the breaker's inverse time tripping characteristic will prevent it from tripping on LRC during startup. Conductor sizing rules for motor leads take starting current into account as well.
The only thing you may have trouble with is the breaker's instantaneous trip settings. Motors who's LRC is 800% of FLC rather than the usual 600% are typically the new high efficiency motors. One aspect of their increased efficiency design is often a tendency to have higher peak magnetization current, the brief instantaneous inrush that happens as the stator core becomes magnetized. In standard motor designs this has been from 800 - 1300% or FLC for less than 1 cycle, and the instantaneous magnetic trips of circuit breakers were adjustable to those levels to accommodate it. In the newer energy efficient designs it can be as high as 2000% FLC and some breakers are not capable of being adjusted high enough, or the default settings from the factory will be too low and nuisance trip. It's not a big deal, just something to look out for.
North American standards for motor conductors is 125% of the currents given in the NEC or failing that 125% of full load current. When in doubt, use the higher rating.
jraef; There is a note in the 2006 Canadian Code that says that instantaneous trip breakers may be provided with a "dampening means" to allow ride through of the first cycle transient.
The CEC also allows a setting of 215% of locked rotor current for instantaneous trip breakers.
A conventional inverse time with instantaneous trip breaker will often have an instantaneous setting that is 500% to 1000% of rated full load current.
For a 100 amp motor load, the breaker would be rated at 250 amps.
The instantaneous trip will be adjustable from 1250 amps to 2500 amps.
I hope that this explains how we get a motor started when the starting current appears to exceed the rating of the circuit breaker.
respectfully
Do you realize that there are breakers just for motors? You buy them for the motor's FLA, (plus a little), Then they have a screw adjustment that you set to a point that just allows the motor to start without tripping.
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