Motor Short Circuit Ground Fault Protection

paulengr

Senior Member
But won't the amount of current drawn (which you asked about) still depend on where in the coil the fault occurs? Higher the fault current the more likely it will take out the SC/GF protection. As far as further damaging the winding - most the time motor needs rewinding anyway even if damaged was minimized by fast overcurrent device response.
Do not forget that further damage is potentially worse.

Under 25 HP unless it’s something special, throw away the motor and get a new one. The rewind cost at a reputable shop exceeds the new price unless either they aren’t a reputable shop or you need to shop around for motors.

That leaves rewinds. On newer motors they are pushing energy efficiency so much that the laminations in the core are ridiculously thin and this is causing not only rewinding issues but also a slot fault can easily damage the core. Restacking a core often again means scrapping a motor. Even on large ones.

On large motors (over 1000 HP) and under a variety of conditions that increase common mode currents another common ground fault issue is bearing currents. Currents are induced in the shaft by common mode currents and circulate either through the motor internally or returning via the frame/ground path. These always exist. Part of the conductive path is the bearings. IEEE has scientific papers going back to the 1920s referencing papers back 20 years before that so these have existed since the beginning. If the current exceeds what the bearings can tolerate it arcs through the bearing resulting in bearing fluting. This issue has always existed but normal practices keep it under control. As VFDs naturally create common mode currents the issue has returned with a vengeance.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
What is causing these common mode currents? I always though that the current at the neutral point of a motor's windings would be zero hence hence why its insulated from ground and we don't pull a 4th white or grey wire to the motor.
 

paulengr

Senior Member
What is causing these common mode currents? I always though that the current at the neutral point of a motor's windings would be zero hence hence why its insulated from ground and we don't pull a 4th white or grey wire to the motor.
Many things cause common mode currents. In.VFDs the output on any phase at any time is either +VDC or -VDC. Each one is 120 degrees apart. We have three phases. With sinusoidal voltages normally V1+V2+V3=0. But with a VFD no matter what combination you use it will never be zero. Instead it oscillates where the average is zero.but it’s roughly a sinusoid equal to about V/6.

Other causes are anything that makes the three input voltages assymetrical. Things like if all three cables are not the same length, high resistance connections, more load and more voltage drop on one line than another especially with utilities (external), or ground faults. Another one I’ve seen recently is a contactor with a worn out contact. Or leaving leads long and cooling them up in the box turning them into air core inductors. Or when one of the capacitors only a surge or power factor capacitor fail, or a short develops in a load reactor or VFD output filter.

Now you can argue that the neutral is still forced to zero voltage and this is especially true in a wye connected motor. But this is only true if we have assymetrical currents too. The motor acts like constant power so it tends towards assymetrical currents. So we get back to your original question.
 

kwired

Electron manager
Location
NE Nebraska
What is causing these common mode currents? I always though that the current at the neutral point of a motor's windings would be zero hence hence why its insulated from ground and we don't pull a 4th white or grey wire to the motor.
Wye connection, the vector sum of the lines coming in would be zero, there still is current, there has to be or no current flows in the winding(s).
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Wye connection, the vector sum of the lines coming in would be zero, there still is current, there has to be or no current flows in the winding(s).

Good catch! I was thinking of taking a noodle to the neutral point... in that no current would flow on said noodle. But you are correct, there would still be current on the point itself as each phase adds or takes current. My error.
 

Jerry Solar

New User
Location
Orlando, Florida
Has anyone ever seen or had problems with Short Circuit and Ground Fault protection tripping when set at 115-125% of the motor's Full Load Amps?
New guy here. Maybe you already found the solution but you typically set the overload protection in the overload relay between 115% to 125%.
The circuit breaker you are allowed to set it as per NEC table 430-52 which for squirrel cage motors the maximum is 250% and for wound-rotor motors is 150%. For fuses the maximum is 175% for squirrel cage motors and 150% for wound-rotor motors. These settings will allow the motor to start under load and they will still protect for short circuit and ground fault. I hope this helps.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
New guy here. Maybe you already found the solution but you typically set the overload protection in the overload relay between 115% to 125%.
The circuit breaker you are allowed to set it as per NEC table 430-52 which for squirrel cage motors the maximum is 250% and for wound-rotor motors is 150%. For fuses the maximum is 175% for squirrel cage motors and 150% for wound-rotor motors. These settings will allow the motor to start under load and they will still protect for short circuit and ground fault. I hope this helps.

Yup.

Just make sure you calculate the adiabatic limits on that EGC.
 

JoeStillman

Senior Member
Location
West Chester, PA
New guy here. Maybe you already found the solution but you typically set the overload protection in the overload relay between 115% to 125%.
The circuit breaker you are allowed to set it as per NEC table 430-52 which for squirrel cage motors the maximum is 250% and for wound-rotor motors is 150%. For fuses the maximum is 175% for squirrel cage motors and 150% for wound-rotor motors. These settings will allow the motor to start under load and they will still protect for short circuit and ground fault. I hope this helps.
Welcome to the forum! And, for the record, the word "adiabatic" does not appear in the NEC.
 

paulengr

Senior Member
There are lots of optional things with motors, too.

Case in point on Saturday a customer called about a 4160, roughly 800 HP motor on a vacuum contactor starter. It has a basic overload relay, overcurrent relay (stall detection), and a ground fault relay. The ground fault had tripped.

As you can probably guess the motor had burned up and was dead shorted to ground. The R rated fuses, overload relay, and overcurrent relay all never tripped.
 
Top