Magnetic/Short Circuit Trip Setting for 200 kW Induction Motor

Refer to the applicable codes for your area and the manufacturers' data.

There are probably web sites available as well, if you search for them.

Remember, overloads protect the conductors, not the motor.

Remember, circuit breakers and fuses protect wires and not loads or people. All of your provided information is useless in determining the peak current, cable capacity or even if this is a single phase (current too small for power) or three phase (current too big for power{but not by much]).

I will then follow useless information with a useless answer. The MCCB should be chosen to not cause frequent nuisance trips while still protecting the cabling. Don't forget to consider the maximum current (arc flash) this breaker will have to actually interrupt when a short circuit happens. The last thing you want is your protection device to become a bomb instead of protection.

Thermal (Overload) Setting: It is done at 350 A.

I am concerned about Magnetizing/Short Circuit Trip setting.

Normally it is done at 13 x FLA (In most cases).

Breaker trips if Magnetizing/Short Circuit Trip setting are done at 10 x FLC.

200 kW motor is 3 phase. Read the name plate of motor.

350?

It is common (outside the US at least) to protect motor circuit/motor against short-circuit faults using a mag-only-trip breaker (typically called a motor circuit protection circuit breaker) in combination with a thermal overload/line contactor to provide overload protection.

You are right, 13 x FLA has traditiionally been used. You need it to be higher than the inrush current of the motor.

Be aware, however, that some of the newer high-efficiency motors have an inrush that is closer to 18 x FLA, and so your breaker may need to be up-sized.

A couple of years ago here, I had a bank of 11 x 30KW motors of the new, you-beaut, high-efficiency type, and ended up having to buy 11 new MCPCB's because the first choice wouldn't hold in on startup.

Best is probably to talk to your motor supplier and ask if they can tell you what is the inrush current of your motor.

Thanks for considering / understanding the question in right context and sharing your valuable knowledge. Our motor is of Design Type B.

"Be aware, however, that some of the newer high-efficiency motors have an inrush that is closer to 18 x FLA, and so your breaker may need to be up-sized."

According to my knowledge, these are Design type E motor. For these SIEMENS recommends Magnetic/Short Circuit Trip setting to be no more than 17 x FLC.

Thermal-magnetic Circuit Breaker

Breakers has a trip unit that senses heat to detect an overload and senses a magnetic field generated by current to detect a short circuit. This type of circuit breaker trips immediately when a short circuit occurs, but delays an appropriate amount of time before tripping in the event of an overload.

In: Current Rating of MCCB = 500 A

Ir: Thermal ("Overload") Tripping Setting = 350 A (0.7 x In)

Im: Magnetic/Short Circuit Trip setting = ?

Icu: Breaking capacity

Performance curve of a circuit-breaker thermal-magnetic protective scheme

If the motor FLA is 385A as per the nemeplate details given by you, setting it at 350A would cause nuisance tripping, wouldn't it? You really need to set thermal trip at 385 A. However, the thermal trip of an MCCB would not be as good as a thermal overload relay. Maybe an electronic one since the motor seems expensive. The thermal device protects the motor against overload (and maybe single-phasing) while what you are seeking, the magnetic trip, is for protcting the cables and the source. So, you really need to decide what SC level would be appropriate to protect the cables

You really need to refer to your local electrical codes, Mildred, for something like this, as it isn't a job for an amateur armed with a reel of bell wire.

The magnetic settings for your application should be as follows: The MCCB should not trip during starting of the motor. Therefore setting should be 1.3 times the starting current.Normally the starting current will be 6 times the full load current of the motor. Therefore it will be 7.8 times the full load current. However you have to calculate the short circuit current at the motor terminal.this depends on source fault level at the bus feeding the motor and cable impedance between the MCCB and motor.The setting on the MCCB should be less than the calculated short circuit current. Another aspect is the accelaration time should be less than the time set on the MCCB for the magnetic setting. Further you have to calculate the currents in absolute values and settings to be done based on MCCB rating. For example, if the full load current is 385 amps and starting current is 6 times then the magnetic current setting would be> 7.8 x 385 =3003 amps. and setting would be > 3003/500 = 6. In this case the calculated short circuit current should be > 3003 amps

It is important to note the distinction between inrush current and starting current.

Motor starting current is the current drawn while it is accelerating to full speed, typically given as 6-8 x FLA, and usually given in the motor datasheet.

Inrush current is the current drawn between switch on and when the magnetic fields are established in the motor, and as mentioned above had been around 13 x FLA for older motors, but can now go to 17 x FLA or more.

It is the inrush that trips circuit breakers, be they thermal-magnetic or magnetic-only.

+1 for the gnome, dead on.

The phenomenon of energy efficient designs in motors creating higher (than previous) magnetizing inrush current is now well documented, with may papers explaining that levels of up to 2200% are theoretical, and 2000% have been recorded. Those extremes are dependent upon rare coincidental occurrences of mitigating factors, such as residual magnetism, phase angle at closing of contactors, contactor speed etc., but the point is, the old rules are now often causing nuisance tripping of breakers or clearing of fuses.

In years past, "Instantaneous Trip" (IT) aka Magnetic-Only MCCBs or MCPs were used in combination motor starters because they allowed for higher levels of magnetic trip adjustments than were built-in to fixed trip Thermal-Mag MCCBs at the time. The somewhat went away when MCCB manufacturers started shipping T-M breakers with adjustable mag trips anyway, but the practice continued simply because the IT breakers were cheaper for the mfrs. That has now begun to change again, at least here in North America.

Our rules, as in other parts of the world, are still based on the old concepts, so the IT breaker trip setting rules are unrealistic ally low, but there are exceptions by which they can be raised; first to 1100%, then to 1300%, finally upward to 1700% if it is proven and documented that a lower setting results in consistent nuisance trips, an arduous process. The problem is however, that you often run into frame size restrictions and the fact that the mag trip settings stop at 10x the maximum current rating of the breaker, then increasing the breaker frame size becomes restricted for other reasons. But a little known (until recently) trick is to take advantage of the combination of the relaxed rules on T-M breaker sizing allowances, done to possibly compensate for the previous fixed trip setting situation, and the fact that many are now coming with adjustable mag trips. So because of these rules, you can usually apply a larger frame size T-M breaker, but because they now have adjustable mag trips, you can have an adjustment range that exceeds the rating of an IT breaker in previous designs. This usually results in an adjustment range that extends beyond the 1700% limit.

Now, I'm not condoning this, so don't shoot the messenger, but here is what I observe is taking place <wink, wink, nudge, nudge>. People are setting the trips at 1700% just as they are supposed to, then passing inspection. But if they get nuisance tripping after the fact, they nudge it up by an RCH a few times until the problem goes away.

A slight delay of 70 msec will solve the situation, I think. The inrush current does not exist more than 20 msec. IMO all regular circuit breaker presents such a delay.

In this case the trip setting has not to be more than 10 times the rating.

However, a short-time delay -6 cycles [0.1 sec]- could be a dangerous arc-flash exposure.See:

http://www1.cooperbussmann.com/pdf/d46eafdc-7fa7-4eec-9ba1-3d7cfb6b8323.pdf

If local codes allow it, I'd simply use a moulded case circuit breaker with an electronic trip unit for motor protection (not to be confused with a power distribution trip unit). It will offer short circuit, overload (with motor protection trip curve) and phase unbalance protection. Just make sure the specs allow a high enough short-circuit current setting and an overload setting which corresponds to the rated current of the motor (rating plate current at corresponding line voltage).

Using a breaker which has a higher maximum nominal current than the used motor allows to exceed the typically found 1300% short-circuit threshold setting while still setting the overload trheshold at nominal motor current (at least if the short-circuit setting is x times the maximum settable overload current and not x times the actually set overload current).

For small motors breakers with electronic trip release or dedicated digital motor protections relays are too expensive but for a 200 kW it can be discussed.

If you motor has thermistors (PTC) installed, they should be used. Most new large motors from good manufacturers have factory-installed PTC sensors (check the terminal box for small wires and terminals, very large as well as medium voltage (above 1 kV) motors have one or more separate auxiliary terminal boxes for the PTC, (or Pt100), heater, brake, encoder and other options). Check insulation coordination for the winding temperatur sensors.

Consult a local qualified Electrician.