Delta vs Wye motor protection

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mbrooke

mbrooke

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Is it easier to protect a wye or delta motor against single phasing?


Also what about the following:


Blown primary fuse on a delta-wye transformer feeding a building?


Blown primary fuse on a wye-wye transformer feeding a building?


Phase A and B shorted together but only A phase fuse blown on a delta-wye vs wye-wye pad mount?

12.47kv Phase A fuse blown but shorted to ground on a delta-wye?


Thinking about this motor over loads protection be it fuse or heater must take into account over two dozen reasonably anticipated scenarios.


Where do I begin? :blink::?:?


http://www.cooperindustries.com/con...BUS_Ele_Protection_Against_Single-Phasing.pdf
 
Depends on exactly what you want to protect from and type of load driven.

Variable torque loads may not even trip certain types of overload devices in some instances, but at same time current in motor may not be at levels that will produce excessive heating.

High torque loads - most any type of overload protection will trip, as long as proper element selection or setting of adjustable trip was done as those kind of loads will usually have a definite high current on at least one leg in such incidents.
 
kwired said:
Depends on exactly what you want to protect from and type of load driven.

Variable torque loads may not even trip certain types of overload devices in some instances, but at same time current in motor may not be at levels that will produce excessive heating.

High torque loads - most any type of overload protection will trip, as long as proper element selection or setting of adjustable trip was done as those kind of loads will usually have a definite high current on at least one leg in such incidents.
Click to expand...

Fans, pumps and such I have in mind. Across the line or delta-wye starting setups.
 
mbrooke said:
Will it cover all the above conditions?
Click to expand...

I would think so. Various models are available to detect phase loss, phase reversal, phase shift error, over and under voltage:

https://www.macromatic.com/products-main/three-phase-monitor-relays

The relay output is typically put in series with the contactor coil to disable it when the parameter limits are exceeded. The macromatic relay should detect a problem with the 3-phase line voltages present at the contactor input. It won't sense an open circuit after the contactor, and it's not monitoring the current itself.
 
I just use Solid State OL relays that have phase current loss / imbalance protection built-in (most now do). They can't be fooled by regenerated voltage on a lost phase, and you need an OL relay anyway, so you kill two birds with one stone.
 
Jraef said:
I just use Solid State OL relays that have phase current loss / imbalance protection built-in (most now do). They can't be fooled by regenerated voltage on a lost phase, and you need an OL relay anyway, so you kill two birds with one stone.
Click to expand...

Agreed mostly with this. Whether motor internal wiring is delta or wye does not matter as it appears as a three phase delta connected load externally. Above a certain horsepower, around 500 HP or so, and on all medium voltage motors (2300+ V) IEEE and NEMA both have a much more extensive list of protective functions like what you are describing. Part of the issue is that thermally it switches from where the thermal limit is the stator to the rotor. So additional protections like stall or jam become necessary.

On a 460 V motor with say current limited fuses with type 2 protection such as say RK1 fuses and a euitectic overload if the voltage of one leg is low whether completely missing or partial voltage the current on the other two phases increases to compensate. In the case of full single phasing (0 V) it almost doubles. Torque is reduced by 1/3 so usually the current doubling is a minimum. Contrary to popular belief the motor won’t be damaged but instead the overload relay trips before the motor overheats if it is set properly. No damage to equipment though. This is for a simple old school heater system that has been around since WWII. Many distributors will paint all kinds of crazy scenarios about how this system does not give full protection. It is a lie. Look at the old Bussmann fuse protection handbook that you can download from cooper still for free. Look at the type 2 protection section. And by the way type 2 protection isn’t always necessary. It can be achieved with breakers but Cooper is in the fuse business and it requires more work to figure this out than just using a specific fuse type.

This used to be the way I recommend. But these days you can buy an electronic overload relay from just about everybody that is actually about the same price as the eutectic overload base relay without the overloads. Among other things they catch single phasing (voltage imbalance) earlier so even the stress of a stalled start under single phasing doesn’t happen. These are cheap, around $100-200.

With the medium voltage motors with undervoltage, stall, jam, single phasing, overcurrent, and ground fault protection, that is all covered and more. If you have a particular critical motor we usually use the one from Motortronics (cheap), the SEL-749M (better history data), the GE Multilin (if you have money to throw away), or the SEL-710-5 (the best motor relay money can buy). Prices range from $300-400 for the Motortronics to around $3500 for the 710 to $4500 for the GE.


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paulengr said:
Agreed mostly with this. Whether motor internal wiring is delta or wye does not matter as it appears as a three phase delta connected load externally. Above a certain horsepower, around 500 HP or so, and on all medium voltage motors (2300+ V) IEEE and NEMA both have a much more extensive list of protective functions like what you are describing. Part of the issue is that thermally it switches from where the thermal limit is the stator to the rotor. So additional protections like stall or jam become necessary.

On a 460 V motor with say current limited fuses with type 2 protection such as say RK1 fuses and a euitectic overload if the voltage of one leg is low whether completely missing or partial voltage the current on the other two phases increases to compensate. In the case of full single phasing (0 V) it almost doubles. Torque is reduced by 1/3 so usually the current doubling is a minimum. Contrary to popular belief the motor won’t be damaged but instead the overload relay trips before the motor overheats if it is set properly. No damage to equipment though. This is for a simple old school heater system that has been around since WWII. Many distributors will paint all kinds of crazy scenarios about how this system does not give full protection. It is a lie. Look at the old Bussmann fuse protection handbook that you can download from cooper still for free. Look at the type 2 protection section. And by the way type 2 protection isn’t always necessary. It can be achieved with breakers but Cooper is in the fuse business and it requires more work to figure this out than just using a specific fuse type.

This used to be the way I recommend. But these days you can buy an electronic overload relay from just about everybody that is actually about the same price as the eutectic overload base relay without the overloads. Among other things they catch single phasing (voltage imbalance) earlier so even the stress of a stalled start under single phasing doesn’t happen. These are cheap, around $100-200.

With the medium voltage motors with undervoltage, stall, jam, single phasing, overcurrent, and ground fault protection, that is all covered and more. If you have a particular critical motor we usually use the one from Motortronics (cheap), the SEL-749M (better history data), the GE Multilin (if you have money to throw away), or the SEL-710-5 (the best motor relay money can buy). Prices range from $300-400 for the Motortronics to around $3500 for the 710 to $4500 for the GE.


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Click to expand...

Ditto for the SEL :happyyes::happyyes:
 
Jraef said:
I just use Solid State OL relays that have phase current loss / imbalance protection built-in (most now do). They can't be fooled by regenerated voltage on a lost phase, and you need an OL relay anyway, so you kill two birds with one stone.
Click to expand...

Agree, but I still want to know the best way of protecting a motor with fuses and/or heaters.
 
mbrooke said:
Agree, but I still want to know the best way of protecting a motor with fuses and/or heaters.
Click to expand...

I thought I just stated it. Use the specific fuses given in the NEMA book:

https://www.nema.org/Standards/Pages/Type-2-Motor-Starter-Protection-Fuse-Guide.aspx

Read the Bussmann fuse protection handbook. Don’t worry about their special branding on the fuses. Bussmann does a lot of marketing to get higher margins, sort of like Klein for tools. Just stick to the NEMA guide on selection. I could give you a tutorial on motor protection but when I do training on this, I actually just copy and paste the Bussmann charts to a PowerPoint. It really is that good and why I recommend it if you want to understand how it works.

On eutectic (heater) overloads UNLESS the motor states something different on the name plate and only then or if you are using a general purpose motor under a Class 10 application, use Class 20 in NEMA. Higher classes are for specially built motors only. Also you said pump...submersibles are different. For them you can use an overload relay as backup protection only. Primary protection is from the thermal switch in the motor itself. Every manufacturer (Allen Bradley, Schneider, Cutler Hammer, etc.) has the same selection guides. It is brand specific though so a “B7” from one manufacturer will be a “W32” from another and they are not standardized so you can’t interchange the overloads or heaters themselves, only the whole relay.

That really is all there is to it. Going through all the charts, temperature correction, etc., is tedious but necessary.

Until about 10 years ago if you asked me what the best protection you can buy is that would be my answer. But microprocessor based electronic overloads are cheaper, more energy efficient, handle
conditions like stall and single phasing better, and immune to discrepancies between starter cabinet temperature and motor environment temperature. On the simplest ones you have a dial listed in FLA. Buy the right size overload relay. Set the dial to the FLA on the motor. That’s it. Same with the MX from Motortronics. This is vastly simpler to set up than heater selection on a Eutectic overload relay.

But an SEL as an example is more of the PLC of relays. The learning curve is STEEP. In the hands of a relay technician it can do just about anything. With it I can trigger all the contactors on the resistor controller of a wound rotor motor, command feeders to slow or stop on heavy loads, run multi pump staging, run the entire starter circuit, trigger multistage power factor correction banks, and waveform level diagnostics. As I said very cool but very steep learning curve.


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If you insist on using an OL relay with replaceable heater elements, those are only going to be NEMA starters and NEMA OL relays don’t have (as a generalization) the “differential bar” trip mechanism found in IEC OL relays that will skew the trip point lower if there is a phase loss. So I don’t like fuses on NEMA starters, I prefer circuit breakers; open all 3 lines.

If it’s an IEC starter and OL relay, then often those are NOT going to be UL listed with any kind of decent SCCR unless protected by fuses, so you may be stuck going that way anyway.
 
I liked your post, very informative. Didn't know the IEC starters had a balance beam.

I guess this boils down to breakers vs fuse vs overload- which I am clueless about to be honest. Motor protection is not my strong point. I come from a world where fuses simply remove failed equipment from service LOL.
 
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