Using a 440V 60hz motor on a 460V 60hz supply 2

[Atbalance]

I have a 440V 60 hz 184 amp motor (110 KW). The power supply is 460V (on an offshore drilling rig) with the cable and plug rated for 125 AMP, but has a 300 AMP breaker. The motor is connected to a pump and I expect we will not require more than 90 KW and will limit the pressure output of the pump to ensure we do not overload the supply.

Could someone comment if there is a problem or is acceptable using thjs this motor with the supply and what possible risks there may be with this type of setup. I have heard of the 5% rule which would suggest is acceptable to use a 440V motor on a 460V system.

Thanks.

 

[davidbeach]

The voltage is a whole lot less of a concern than that 125A cable and plug. Nothing in the circuit should be rated less than 230A.

 

[Atbalance]

Thanks for the information. I should have said limit to 90A / approximatley 55 KW, which would be about 1400 psi on the pump, so as not to overload the 125A cable and plug. I can use a pressure relief valve for this so indirectly limit the load on the motor. I realise this isn't the best situation and want to make sure I stay within the existing rating of the system though. If it is acceptable to use the 440V motor on the 460V system I can live with it.

 

[aolalde]

The 440 Volts motor can work at 460 Volts without problem. The standards allow for +/-10% voltage deviation from nominal or up to 484 Volts. The benefit is a reduction in current for the same kVA demand as far as the magnetic circuit is not saturated.
You should reduce the breaker current to properly protect your cable and plug.

 

[dpc]

I wouldn't worry too much about the voltage, but the wiring is definitely an NEC violation, if that is a concern.

 

[Atbalance]

How is the wiring a violation? 184A motor on a 125A system? I can request that the breaker be reduced. With a 184A motor expected to draw approx 100A max. could the breaker be reduced to 125A or would starting current cause a trip and need to be something above 184A?

 

[LionelHutz]

I don't see a problem running a 440V motor on a 460V supply. Motors are typically rated 460V or 575V and the supply is typically 480V or 600V so most motors are supplied with a voltage greater than their nameplate rating.

Code requires a wire size that is 125% of the motor current. To me, the reasoning seems to be that the overload is typically set to no more than l25% of the motor FLA so it will protect the wiring as well as the motor.

If you do not have to follow code you could use the breaker as long as you have the motor overload set to trip at 125A so that it protects the wire. The breaker would then protect against a short. This could be a problem during a start since this overload may trip.

I'd be leary of using that cable size without some type of overcurrent protection for it since someone could increase the motor load in the future and overload the plug or cable.

 

[dpc]

The NEC requires that the motor feeder be sized for 125% of the motor nameplate full load amps. Simple as that. The actual load on the motor is not a factor at all.

 

[jraef]

...and I expect we will not require more than 90(A)...

No offense, but there is the problem with your thinking. YOU expect that the motor will only draw 90A, but you cannot tell that to the motor. It will draw whatever is demanded by the load. You can attemp to control the loading of the motor, but you cannot control unforseen events, such as a component failure. WHEN that happens (not if), the motor will attempt to draw it's FLC and beyond, so in this case the wires will possibly catch on fire before the protective device has time to act. Contingencies are what the NEC is all about. That is why the NEC is actually part of the NFPA, the National FIRE Prevention Agency.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Atbalance]

If a 184A motor is started under at essentially no load (starting a pump at zero pressure) can this be done using a 125A breaker (or 100A) when started directly on line or with a soft starter? If the load then increases or there is a failure then the breaker would be sized properly for the cable and plug.

 

[dpc]

If the full load amps is 184 A, the starting current will likely by over 1000 A, not including the transient asymmetrical current. A typical 125 A breaker will probably trip on a full voltage start.

Not to keep beating a dead horse, but if you are required to meet the NEC, you just can't do what you are suggesting. Everything in the feeder is based on the motor full load amps. The fact that the breaker will protect the cable is nice, but if it is a motor feeder, it is still too small.

 

[Zoobie]

Even if you get away without meeting NEC (because your drilling rig is located at the far end of the earth) I bet your insurance company wouldn't be too impressed with you.

 

[Atbalance]

As you may have guessed - this is more like putting the cart before the (dead) horse As mentioned the system I was given which was 125A with a 300A breaker. Don't ask me what they had plugged in before - probably something with a short! Best I've managed to get them to do is reduce the breaker size and all they will go down to is 250A! I shall shock the horse and see if I can bring it back to life. Thanks to all for your input!

 

[pwrman]

Please READ THIS!!

The motor you wiring needs to be carefully thought out like any system. As I understand it you have a 440 Volt motor and the Full Load Amps are 185A. With just this basic information you can tell alot of what you need to do for a safe system. 1)440V Motor can run on a system with a 10% variant from nominal. 2)fla of 185Amps tells me I need a 250A MCP Breaker Thermal Magnetic (Not Just any Breaker) set at 13X the FLA. 3) The FLA of 185A tells me the overload size but I need the Service Factor (SF) of the motor to tell you how to set it. If the service factor is 1.15 then the overload will be set to 1.15 x FLA, if 1.00 then .95 x FLA. After the motor and wiring has been checked (Insulation Resistance Test) then you can bump the motor for rotation, if correct you may proceed with a 15min run to determine the run current of all phases. Please take this into consideration when determining a motor circuit and control of that motor circuit. This will help you in the Safety of personel and equipment. The Cable should be well thought out as well, you must take in account current, raceway and de-rating in the application.

Electrical Power Solutions Ltd.

http://www.e-p-solutions.com

 

[Atbalance]

The service factor is 1.0 I have convinced everyone that we need a larger cable and a 250A breaker. Hopefully they will put the right type in. The cable is quite short, approximately 50 meters. I still haven't had an explanation from the rig why a 300A breaker was on a 125A service but understood I wasn't happy about it!

 

[LionelHutz]

FYI;

Code allows a breaker rated up to 250% of the motor FLA. If that breaker won't permit the motor to start, the breaker is allowed to be increased to 300% of motor FLA for breakers rated greater than 100A.

So, you need wire rated for at least 125% of motor FLA and you can use a breaker rated up to 300% FLA (if a 250% rated one doesn't work). In theory, the installation could have been correct for a 100A motor. In reality, the likelyhood of needing a 300% rated breaker is slim.