3 phase (delta) current

[JBender]

I am trying to size a circuit breaker for a 3 phase motor given it's power consumption. Note that I am not sizing a thermal overload.
Here goes:

Working with 480 VAC Delta and a motor rated at 3.5kW.
If the total power of the motor is 3500 Watts, than each winding is supplying 1/3 of that = 1167 Watts per winding.
At 480 VAC across each winding, there is a Line current of 1167/480 = 2.43 Amps.
Assuming an inrush current at a safe 10 times nominal, I would need a 25 Amp 3 phase breaker.

How close am I to the truth? Thanks for your help and corrections.

 

[weh3]

Your have correctly calculated the phase current, but the line current will be 1.732*2.43 = 4.2 A.

As for your overcurrent protection, use your country's standards. In the US, I would look the motor full load up in table 430-150, being nearly 5 horsepower, or 7.6 full load amps. You can in most cases use 1.25 times FLA, or 9.5 A, therefore a 15 A CB (next standard size up).

Breakers are not sized for the inrush but for full load. Select a breaker that will not trip on time-overcurrent on starting. That depends on your application. A high-inertia load with a long start time may need a softstart or an MCP (instantaneous trip at 800% FLA).

Ten times the FLA is high, six is most probable, but check your motor. You can look up starting amps in table 430-151(b), for most motors.

Will-I-Am

 

[JBender]

Thanks!
OK, so the current seen by the wire (line current) is multiplied by the sqrt of 3 to phase current to give me 4.2 Amps, that makes sense now. Why do you think NEC would list 7.6 Amps? I'll take a guess...because the motor rating is for output power and doesn't take into account other losses such as heat?

How can inrush be neglected when specifing a breaker? (NEC says 46 Amps) Wouldn't I end up with a breaker that trips whenever I start the motor? I am not familiar with time-overcurrent. I thought a breaker rated at a certain current would trip instantly at that current, by definition.

-Jason

 

[rbulsara]

Your guess for heat loss is right. But there is more to it and some correction method of calculation as follows:

When you are treating the 3 phase power as three single phase quantities, you need to use per phase voltage, regardless of the Y-delta configuration

Your per phase value is 480/1.732 = 277V or 460/1.732=265V

You also need to account for power factor and efficiency (losses).

The current is based on VA.

Your input power per phase in vA =w/(pf*eff.)

Assuming .8 pf and .9 eff.

VA = 1167/(.8*.9)=1620

Now the nameplate voltage rating for motors is 460V (not 480).
So your rated per phase voltage is 265V.

Current per phase (or line)=1620 VA /265V=6.1A.

NEC lists nameplate rating for HP not kW. For a 5HP motor it will be 3730 W not 3500. So now you adjust your calcs and you will have 6.5A.

May be 5HP motors are less efficient or have lower pf than I assumed..so 7.6A is not improbable.

As far as y-delta configuraiton of the motor is concnered, it has no bearing here, as it is just a black box consuming balanced three phase power.

By the way, NEC recommends rhe CB rated not more than 250% of the rated FLA of a 3 phase motor, to take care of the inrush.

Trust the Code, it has the experience of over 100 yrs.

 

[JBender]

Thanks rbulsara, for adding into the calculations the power factor and efficiency. That is helpful to see how NEC came up with the numbers they did.
I may not have made it clear that the power source for my motors is a 480 delta configuration, reguardless of the motor windings as you pointed out. Looking at the NEC codes they don't make mention of this - though it seems to me it would make a difference as there is only 480 Volts (Line or Phase) - no 277. Resulting in a lower FLA number?

 

[rbulsara]

The calculation method is correct for either 3 wire or 3 wire system.

You always use per phase voltages when calculating per phase quantities, it has nothing to do with having a phycial neutral.

May be you should go thru the following excercise to understand it better:

1. Calculate line amps for a 3 phase, 300kVA, 480V transformer using 3 phase formula of KVA=1.732*kV*A

2. Now draw this as a bank of (3)100kVA tranformers connceted in Wye. Calculate current per phase (single tranformer) which will be same as the line. Use the correct voltage across the transfomer. Compare to the previous calculated line currents.

3. Now draw this as a bank of (3)100kVA tranformers connceted in Delta. Calculate current per phase which will be same as the line. Convert phase to line currents, as this is delta configuration!! Compare to the previous calculated line currents. Use the correct voltage across the transfomer.

You should get the same line currents each time. Key is to see a 3 phase load as black box, not worry about is phase configuration, if all you want in the line currents.

 

[weh3]

You said it was 480V delta--I assumed you meant the motor was delta. The source configuration doesn't matter.

A delta motor has phase voltage equal to line voltage, and phase current equal to line current over root 3.

A wye motor has phase voltage equal to line voltage over root 3 and phase current equal to line current. It works out the same.

Rbulsara is right about the 250% for a CB. I said 125%, but I knew better.

A time-overcurrent or thermal CB is one where the higher the overcurrent, the quicker it will trip. Regular CB's are time-overcurrent, which is why they allow the starting current to pass.

A magnetic CB is one that trips instantaneously at a certain level. A magnetic CB protecting a motor can be set as high as 800% of the FLA.

The motor shaft puts OUT five mechanical hp. You are finding the INPUT current, so you have to account for efficiency and the power factor. Converting to kW, and allowing for 70% eff. and 70% pf:

(5 hp)*(0.746 kW/hp)/(.7)(.7) = 7.6 A.

Will-I-Am

 

[rbalex]

In 1996, Table 430-150 was revised slightly from previous editions for a very practical reason. The current values are based on the worst performing motor in a class that any NEMA motor division member makes. You can verify this by simply reading NEMA’s Proposal to change the NEC.

By creating the Article 430 Tables this way, it permits changing out NEMA frame motors without revising anything but overload protection. By the way using the Tables are a mandatory NEC requirement; see Section 430.6 in the 2002 Code, for example.

 

[rbulsara]

I meant "either 3 wire or 4 wire..." in my last post.

 

[JBender]

rbulsara,

yeah, I figured...Going through the exercise you mentioned I see that the Line Currents work out to be the same either way. It's basically a Voltage/Current mirror image of each other.

I also found the table 430.52(C) listing, like you said 250% max short circuit protection. It is listed under Inverse Time Breaker -- this must mean more overcurrent, less time analagous to Time Overcurrent I suspect.

Weh3 - you mentioned that most CB's are Time-Overcurrent. I would have thought most were Instaneous Trip as I've seen breakers trip many-a-times and it is imperceivably quick. My impression of motor inrush was that it would take a perceivable amount of time to reach a steady state (FLA) current.

Also, NEC lists 800% acceptable for such breakers - not the US industry preference?

This has all been incredibly helpful as I am the only EE in my company with a little too high ratio of responsibility vs. experience, though I like the challenge...Again, thanks

Jason

 

[weh3]

Jbender,

We all use the term "instantaneous", even though it doesn't really exist. You may be talking about the difference between tripping in 0.0083 second for a 50x overcurrent and tripping in 0.083 second for a 500x overcurrent, or between half a cycle and 5 cycles. Both would appear instantaneous to you and me, but they are different by an order of magnitude. This can make a whole lot of difference in terms of damage from a short circuit.

Trip curves are published for breakers and fuses that allow you to find out what you can expect from a given breaker.

William

 

[itsmoked]

JBender,

There are the regular "go to the store get the breaker" types that are generally used, then there are HVAC rated breakers. These are much tougher breakers that cost a little more but perform MUCH better. Refrigeration (HVAC)loads are probably the very worse motor loads commonly delt with. If you want less nusance trips, and less dead breakers search for and use HVAC rated breakers to run your motor loads.

 

[rbulsara]

Jbender:

You are again right..250% CB are thermal magnetic or electronic trip breakers with time-inverse characteristics.

The 800% CB are breakers with instaneous trip element (magnetic trip only)which does not have overload element.

A typcial time-inverse breaker will easily withstand currents 300% of its own rating for 8-10 seconds, which amounts to 600% to 700% of FLA of the motor and adquate for starting in most applications.

Thermal ones are more commonly used in the USA. The MCP (magnetic trip only) are also used, mostly by MCC mfrs.