Single phasing protection

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Chamuit

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
I am going to look at a job where the customer wants to protect their motors from single phasing. What kind of set ups or equipment do you use?
 
Assuming they all have magnetic motor starters it's as easy as breaking the control power through a phase monitor relay or relays. It depends on whether they want the whole facility protected or protection at each motor.
 
masterinbama said:
Assuming they all have magnetic motor starters it's as easy as breaking the control power through a phase monitor relay or relays. It depends on whether they want the whole facility protected or protection at each motor.
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There aren't any starters from the answers I got from the A/C tech. It will be protecting individual motors.

Can you point me to an example of a phase monitor relay.
 
Here is the Manufacture who makes some of the ones Granger sells that Bob posted about, I deal with them directly and save even more, they have a great tech support and back their products 100%

http://www.macromatic.com/products/phase-monitor-relays.html

they can even customize electronic relays to meet your need, I have designed a few for them.

Allot of relay for a great price in my book.

Also for HVAC equipment just wire the dry contacts into the LV control for the compressor contactor, if the unit also has a 3-phase condenser fan and or evap blower then put it in the main 24 volt transformer lead so it will kill all three, including the burners if it also has gas heat.
 
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Chamuit said:
I am going to look at a job where the customer wants to protect their motors from single phasing. What kind of set ups or equipment do you use?
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Most electronic motor overload relays also trip upon single phasing.

Careful selection of thermal overload relays will also protect from single phasing but will have slower response time. If you lose a phase, current in other two will increase and overload will trip.

If you want to shut off an entire machine immediately upon phase loss then use some of the phase monitors people have suggested. You don't interrupt motor current with these, you open a control circuit.
 
kwired said:
Most electronic motor overload relays also trip upon single phasing.

Careful selection of thermal overload relays will also protect from single phasing but will have slower response time. If you lose a phase, current in other two will increase and overload will trip.

If you want to shut off an entire machine immediately upon phase loss then use some of the phase monitors people have suggested. You don't interrupt motor current with these, you open a control circuit.
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I think they have had problems in the past with the heaters not working fast enough and allowing the motor to keep running (and fail.)

Thanks for all the suggestions.
 
Chamuit said:
I think they have had problems in the past with the heaters not working fast enough and allowing the motor to keep running (and fail.)

Thanks for all the suggestions.
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If overload heater is sized properly it will protect the motor. I still have better faith in thermal overloads than electronic, but they do have to be selected properly. If selected properly rise in temperature of the overload element will be similar to rise in temperature of motor windings.

Seen too many electronic overloads get turned to highest possible setting and not properly protect motor. At least with thermal elements you have to do more than just turn a dial. I have seen many electronic overloads just plain fail. Siemens seems to be the worst. Thermal elements will withstand more adverse conditions.
 
Just something else to remember, make sure when you install the phase monitor it doesn't shortcycle the motor if the power fluctuates. You need it to drop out the motor and keep it off.
 
Cow said:
Just something else to remember, make sure when you install the phase monitor it doesn't shortcycle the motor if the power fluctuates. You need it to drop out the motor and keep it off.
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Some of them are adjustable for how long or what level there needs to be a fail condition before opening the controlled circuit. Whether or not you want the load to automatically restart is an issue that really depends on nature of the load.
 
When using phase monitors with 480v motors, make sure the contacts of the monitor are rated for the control voltage used. Many of the lower priced units are not rated to switch 480v. At the same time, we put in Class CC fuses on the control for older units and those that do not already have them. (Talking Pump Panels) Seen to many coils fry and take anything in series with them.
 
kwired said:
If overload heater is sized properly it will protect the motor. I still have better faith in thermal overloads than electronic, but they do have to be selected properly. If selected properly rise in temperature of the overload element will be similar to rise in temperature of motor windings.

Seen too many electronic overloads get turned to highest possible setting and not properly protect motor. At least with thermal elements you have to do more than just turn a dial. I have seen many electronic overloads just plain fail. Siemens seems to be the worst. Thermal elements will withstand more adverse conditions.
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I would think there could be situations where the overload would not trip and motors could still overheat due to running for a long time on over 100% rated FLA. It really depends on how loaded the motor is before getting single phased, and what % of FLA the overload is set.
 
wirenut1980 said:
I would think there could be situations where the overload would not trip and motors could still overheat due to running for a long time on over 100% rated FLA. It really depends on how loaded the motor is before getting single phased, and what % of FLA the overload is set.
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On something like a centrifigal fan, yes it is possible and I have seen it. Most likely even can happen on a motor driving a high inertia load. Most other loads requiring constant torque, no. If a motor single phases on a constant torque load it will not provide enough torque to continue to drive the load, current on other two phases will increase enough to operate properly selected overload devices within reasonable time to protect motor. If motor stalls, which is likely with constant torque load the remaining two phases will be drawing locked rotor current, sometimes the branch circuit overcurrent device will open before the motor overload when this happens.
 
Chamuit said:
I am going to look at a job where the customer wants to protect their motors from single phasing. What kind of set ups or equipment do you use?
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The more proper term would be 'loss of phase' protection. I presume that you are using ordinary FVNR combination starters. Many overload relays, especially late designs do incorporate loss of pahse protection. That is considered perfectly adequate for run of the mill industrial or commmercial applications in the 150-1HP range. Since they only protect against winding thermal failure, they will not necessarily shut the motors down if they are lightly loaded, eg. the motor is capable to drive the load on two phases without overheating the windings. If the motor is >80% loaded the phase loss compensated function comes into play and will shut the motor down.

On large, MV motors the protection becomes more complex and comlicated and used in conjunction with the other protective functions in a multifunction, electronic relay.
 
kwired said:
If overload heater is sized properly it will protect the motor. I still have better faith in thermal overloads than electronic, but they do have to be selected properly. If selected properly rise in temperature of the overload element will be similar to rise in temperature of motor windings.
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I do not know of a major NEMA manufacturer (Rockwell, Schneider, GE, Eaton, Siemens) of overload relays that says their standard thermal offering will protect motors against single phasing. While they all do sell thermal overloads that have single phase sensitivity, for critical applications they usually recommend you use a dedicated type 'phase loss relay'.
 
jim dungar said:
I do not know of a major NEMA manufacturer (Rockwell, Schneider, GE, Eaton, Siemens) of overload relays that says their standard thermal offering will protect motors against single phasing. While they all do sell thermal overloads that have single phase sensitivity, for critical applications they usually recommend you use a dedicated type 'phase loss relay'.
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That doesn't mean they will not protect against single phasing either. I have seen many thermal overloads trip under single phasing conditions. I can't see any constant torque load not tripping a thermal overload if a phase is lost. Variable torque that is lightly loaded could single phase too long and cause some overheating in motor.
 
kwired said:
I have seen many thermal overloads trip under single phasing conditions.
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Good for you.

According to Rockwell, Publication 193-2.15 EN
"It is best to disconnect a motor quickly with the loss of a phase, since in the majority of cases the motor would stall due to the reduced torque. This would unnecessarily heat the motor if only a thermal function was available."

From Eaton's Freedom Bi-metal Overload Data Sheet 50-40011
"Per NEMA & UL:
Ultimate trip at 125% current (Typically 2-4 hrs from Cold Start)"

I stand by what I said before - If the motor is important/valuable, don't trust a standard thermal overload relay only, for single phase protection.
 
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