Protection of soft starter by fuses

[SA07]

Hi
We have several soft starters driving pumps & fans ( 132 kW, 350 kW) which are protected by fuses. Is it possible to replace the fuses with a motor circuit breaker? We had some cases of burnt motor. There is a bypass contactor with each soft starter. Supply voltage is 3ph 690vac .Thks

 

[waross]

That is a code issue in North America. Where are you located?
But code issues aside, Motor Circuit Protectors do not provide overload protection, only short circuit protection.
Your soft starters should be providing overload protection. Check your settings.
In addition, dual element fuses provide fairly good motor protection.
Years ago, dual element fuses were marketed for motor protection.
Disclaimer: That was in the days of the "U" frame motors. Those old timers were much more rugged than the common "T" frame motors of today.
Nevertheless you may consider dual element fuses as a back-up to the protection provided by a properly configured soft starter.
Do you have a well balanced supply?
If the currents on all three phases to a motor are equal or within about 5% of each other your supply balance is good enough.
If there is a great difference between phase currents to a motor that is an indication of a supply balance issue.
An unbalanced supply causes extra circulating current and heat in the rotor. This heat radiates to the windings and may overheat the windings without being detected by the overload protection devices. Many O/L relays will trip out on phase current unbalance.
This may lead to the O/Ls being set progressively higher to avoid what may be perceived as nuisance tripping.
Check your phase currents and your PHASE TO PHASE VOLTAGES.
Checking the phase to neutral voltages may not be helpful.
But, once again:
A Motor Circuit Breaker will not help protect your motor. It is intended to protect the wiring and the source against short circuits, not overloads.
It will generally not trip until after the motor is burnt out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thebard3]

IMHO, properly sized fuses are better than circuit breakers (transmission voltages excepted).

Brad Waybright

It's all okay as long as it's okay.

 

[RRaghunath]

Randhir, From your post I presume the starter is in circuit only during running-up and then by-pass contactor cuts-in. In such a case, the soft starter is not available to protect motor while it is running with bypass contactor on.
Yes, you need motor protection. While type Gg fuses can only provide short circuit protection, motor fuses type aM can provide overload protection too, to some extent. Better still, if you can install motor circuit breaker that includes overload as well as short circuit protection.

Rompicherla Raghunath

 

[ScottyUK]

RRaghunath,

gG fuses are full-range general-purpose fuses, aM fuses are part-range types which provide short-circuit protection only and must be used with an overload relay in a combination starter. Mersen's notes might be helpful.

http://ep-us.mersen.com/fileadmin/c...Voltage_European_Fuses_aM_gG_IEC_60269_EN.pdf

 

[waross]

Add an overload relay to the bypass contactor. If it has one already, check the settings.
A decent overload relay will give you unbalance protection, something that fuses can't do.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SA07]

Yes the soft starter is used to start the motor. Then the bypass contactor supplies the motor. The motor is protected by the Siemens Simocode modules against overload and other protections.

 

[SA07]

We have 690,691,691 Vac on the voltage supply, the current taken by a pump is 346A, 346A, 343A .
The motor is 350 kW.

 

[RRaghunath]

If you already have motor protection modules, why do you think motor protection circuit breaker is required to prevent motor damage??
Fuses are better suited for short circuit protection, overload, earth fault and negative phase sequence protection is provided by Siemens protection modules.
If the motors are burning and if you think the protection is not doing its job, it is time to check the protection settings (not to add more protection). Isn't it!!

Rompicherla Raghunath

 

[waross]

Condensation? Dirty environment? Plugged or partially plugged cooling air passages? Corrosive atmosphere?
Your voltage and current balance looks very good.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

A breaker isn't necessary unless the motor failed due to a single phase blowing and single phased the motor thermally failing one phase. A proper soft-starter would protect against this ever happening.

Both the load should be reduced and the thermal overload should be set lower if the motor failed thermally on all 3 phases.

 

[jraef]

The Simocode is a very advanced solid state overload relay, more like a low cost Motor Protection Relay. No Motor Protection Circuit Breaker is going to be better than that. It would not allow a single phasing event unless someone deliberately turned it off.

If your motors are becoming damaged, it's either improper settings in the Simocode or a mechanical issue with the motors, as previously mentioned. The fuses ahead of the soft starter are only there as short circuit protection and the certification of the soft starter likely requires fuses. Often times when people want a circuit breaker to protect a soft starter they end up with a breaker, followed by the fuses anyway...


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