Wye Delta Breaker Tripping

Location
USA
Occupation
Engineer
Hello all,

Need some insight on why motor is tripping “Main circuit breaker” and “Main cabinet breaker” immediately when “Start” is pressed. Brand new system under commissioning. There is no load on motor and spins freely. When motor leads are disconnected, star-delta transition works properly without trip.

Motor nameplate:

  • Voltage – 460VAC
  • Phase – 3 phase
  • Frequency – 60Hz
  • Size – 40HP / 30kW
  • Efficiency – 94.1%
  • RPM – 1780
  • Power Factor – 0.80
  • Full load amps (FLA) – 50A
  • Number of leads - 6
  • Motor code - K

Component sizing:

  • Main cabinet disconnect setting - 63A
  • Branch circuit breaker setting (supplies feeder contactor) - 50A
  • Feeder contactor size (supplies main, delta and star) - 65A
  • Main contactor size - 32A
  • Delta contactor size - 32A
  • Star contactor size - 25A
  • Inrush current measured - 169A

Edit: I feel the “Branch circuit breaker” and “Main cabinet disconnect” are undersized. But I need to make sure before I speak to design team. I calculated these values:

• Branch circuit breaker - 2.50*50 = 125A
• Main cabinet disconnect - 125A + FLA of other motors in system
 
Last edited:
Location
USA
Occupation
Engineer
If you disable the delta contactor does it still trip?
I haven’t tried that.

I only disconnected all 6 leads inside cabinet terminal. There was no trip then and voltage measured during Wye was only across U1,V1 and W1. After transition to delta after 12 seconds, voltage was across all 6 terminals.

Insulation test was done on motor at 500V and got 2Tohms.
 

paulengr

Senior Member
I haven’t tried that.

I only disconnected all 6 leads inside cabinet terminal. There was no trip then and voltage measured during Wye was only across U1,V1 and W1. After transition to delta after 12 seconds, voltage was across all 6 terminals.

Insulation test was done on motor at 500V and got 2Tohms.
That’s a bug hint. 2 T ohms I don’t even see that high on 13.5 kv motors. That tells me you didn’t Megger motor. Highest I’ve seen at 13.5 kv is 50-100 Gohns.
 

kwired

Electron manager
Location
NE Nebraska
40 HP @ 460 volts and you need reduced voltage starting methods?

Most the time that would be fine for across the line starting. Limited source would be reason to not start across the line.

Limited source with this being only load - the source is going to be what reduces voltage.

But yes make certain controller to motor connections are correct, next would be disable the delta contactor just to see if it will hold on initial starting current or even directly connect motor in wye configuration and try to start it with same breaker in circuit
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
40 HP @ 460 volts and you need reduced voltage starting methods?

Most the time that would be fine for across the line starting. Limited source would be reason to not start across the line.

Limited source with this being only load - the source is going to be what reduces voltage.

But yes make certain controller to motor connections are correct, next would be disable the delta contactor just to see if it will hold on initial starting current or even directly connect motor in wye configuration and try to start it with same breaker in circuit
I’ve rarely done reduced voltage starting. Is this reduced voltage or limitation of starting current?
 

kwired

Electron manager
Location
NE Nebraska
I’ve rarely done reduced voltage starting. Is this reduced voltage or limitation of starting current?
Both. It is reduced voltage starting because when connected in wye the motor coils only see 277 instead of full 480 volts. Won't be good to run for long in that condition, just a way to reduce initial starting current.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...
  • Main cabinet disconnect setting - 63A
  • Branch circuit breaker setting (supplies feeder contactor) - 50A
  • Feeder contactor size (supplies main, delta and star) - 65A
  • Main contactor size - 32A
  • Delta contactor size - 32A
  • Star contactor size - 25A
  • Inrush current measured - 169A

Edit: I feel the “Branch circuit breaker” and “Main cabinet disconnect” are undersized. But I need to make sure before I speak to design team. I calculated these values:

• Branch circuit breaker - 2.50*50 = 125A
• Main cabinet disconnect - 125A + FLA of other motors in system
The "breaker" sizes would indicate these are IEC devices, probably Motor Protection Circuit Breakers or what we used to call "IEC manual motor starters" that are thermal-magnetic with adjustable thermal trips, acting like the Over Load Relay (because you also didn't mention that, indicating there isn't a separate one). If the motor FLA is 50A and they used a 63A as the main breaker, that's cutting it close, but technically, legal.

The likely problem comes in when this is applied to a Wye Delta scheme. In the transition from Wye to Delta, there can be a HUGE current spike if the voltage is leading the current when it happens, and that is likely causing the non-adjustable magnetic trips on that device to take it off-line. If you want to get into the math of it all, read this link. Bottom line, this is a cheap way of making a Wye-Delta starter that is probably resulting in nuisance tripping. Your assessment of the breaker sizes would apply to a more traditional concept of having a Thermal-Mag breaker with adjustable mag trips, feeding contactors and an OL relay on the Delta contactor sized at 58% of the motor FLA. But that's not what you have there.

If it were me, I would rip it all out and put in a soft starter. I HATE Wye-Delta for this very reason. If that's not an option, there is a trick you can try called "rolling" the conductors. On the wires coming out of the Delta contactor, you swap them one lug over, i.e. A to B, B to C, C to A, so that the rotation is the same, it's just shifted over by one position. This basically tries to take advantage of changing the relationship to voltage and current in the transition to try to mitigate the phase shift that happens.
 

kwired

Electron manager
Location
NE Nebraska
Perhaps it's a high inertia load and they need to limit the current over a relatively long duration.
But the motor likely won't have sufficient torque to accelerate the load or not at a reasonable rate if it can accelerate when windings are only seeing 277 vs 480.
 

paulengr

Senior Member
The "breaker" sizes would indicate these are IEC devices, probably Motor Protection Circuit Breakers or what we used to call "IEC manual motor starters" that are thermal-magnetic with adjustable thermal trips, acting like the Over Load Relay (because you also didn't mention that, indicating there isn't a separate one). If the motor FLA is 50A and they used a 63A as the main breaker, that's cutting it close, but technically, legal.

The likely problem comes in when this is applied to a Wye Delta scheme. In the transition from Wye to Delta, there can be a HUGE current spike if the voltage is leading the current when it happens, and that is likely causing the non-adjustable magnetic trips on that device to take it off-line. If you want to get into the math of it all, read this link. Bottom line, this is a cheap way of making a Wye-Delta starter that is probably resulting in nuisance tripping. Your assessment of the breaker sizes would apply to a more traditional concept of having a Thermal-Mag breaker with adjustable mag trips, feeding contactors and an OL relay on the Delta contactor sized at 58% of the motor FLA. But that's not what you have there.

If it were me, I would rip it all out and put in a soft starter. I HATE Wye-Delta for this very reason. If that's not an option, there is a trick you can try called "rolling" the conductors. On the wires coming out of the Delta contactor, you swap them one lug over, i.e. A to B, B to C, C to A, so that the rotation is the same, it's just shifted over by one position. This basically tries to take advantage of changing the relationship to voltage and current in the transition to try to mitigate the phase shift that happens.
Motor protectors...let’s just call them UL type F’s, are UL locked by the silly breaker rule that doesn’t really apply to motor circuits to a max of 10x FLA. So there is substantial delay built into the “instantaneous” part. There has to be or they would trip on every IEC motor with their insanely high inrush.

Usually I can replace all 3 contactors with a good quality American made soft start for less than the cost of rebuilding wye-deltas and it runs very smoothly.

But if you insist on sticking with it the big trick is optimizing the transition time. Too short and you get a bigger spike. Too long and the overload trips.
 
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