motor overload protection...

Status
Not open for further replies.

eric dolphy

Member
Location
California
in the 2011 NEC, 430.32 A 1 lists a 'separate overload device,' A 2 lists a 'thermal protector integral with the motor,' and A 3, 'a protective device integral with the motor.'

please clarify the three motor protection devices, two being 'integral.'

thank you
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
in the 2011 NEC, 430.32 A 1 lists a 'separate overload device,' A 2 lists a 'thermal protector integral with the motor,' and A 3, 'a protective device integral with the motor.'

please clarify the three motor protection devices, two being 'integral.'

thank you

a separate overload device is typical part of the motor starter assembly.

some (typically small) motors have overload protection built in to the motor. they may have a temperature switch embedded within the motor that is wired in series with the motor leads so the motor is de-energized on high temperature.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
(1) Would be the heaters or electronic sensor on a motor starter.

(2) Would be a thermal bi-metal device like a Klixon that heats up and snaps open when too much current is drawn by the motor. Think of the reset button on your garbage disposer.

(3) Is similar to (2) but might use electronics or a current sensing relay.
 

iwire

Moderator
Staff member
Location
Massachusetts
in the 2011 NEC, 430.32 A 1 lists a 'separate overload device,' A 2 lists a 'thermal protector integral with the motor,' and A 3, 'a protective device integral with the motor.'

please clarify the three motor protection devices, two being 'integral.'

thank you

When a motor had a reset button on it, like most if not all garbage disposals have, that would be an example of 'A3'.
 

kwired

Electron manager
Location
NE Nebraska
(1) Would be the heaters or electronic sensor on a motor starter.

(2) Would be a thermal bi-metal device like a Klixon that heats up and snaps open when too much current is drawn by the motor. Think of the reset button on your garbage disposer.

(3) Is similar to (2) but might use electronics or a current sensing relay.

When a motor had a reset button on it, like most if not all garbage disposals have, that would be an example of 'A3'.

Dave's #2 is Bob's #3.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
OK, I'm awake now...

#1 is fine.

#2 would be the cheap little "snap switch" thermal device (Google the word Kilxon for more info) in the motor frame, it clicks open if the motor over heats, then either resets on its own after it cools down or you push the red button on the back of the motor.

#3 would be a thermistor or RTD sensor embedded in the motor, but requires an external device to interpret the information.

So a thermistor is a PTC (Positive Temperature Coefficient) resistor that rapidly changes its resistance based on temperature; it has a "knee point" that can be detected. You would have a Thermistor Tripping Relay that is wired into the controller circuit to act as an overload protection device. It sends a low voltage into the thermistor and measures the voltage coming back to determine if the temperature is above or below that knee point. PTC means that as the temperature INCREASES, the resistance INCREASES. So if the motor gets hot, the voltage coming back from the thermistor drops to nearly zero and the relay changes state. If the wire breaks it interprets it the same way so it is "fail safe". This kind of sensor is used a lot in VFD motors because there can be added heating to the motor that does not track with the current going to it. Also, a lot of foreign soft starters only monitor 2 legs of current, not 3 (because it's cheaper and IEC allows it), but the NEC requires that overload relays are to be in all 3 legs. So rather than redesign their product to monitor all 3 legs, they use the clause in A3 to get around the issue by providing a thermistor input to the PC board, because over there, Explosion Proof motors are REQUIRED to have thermistors as supplemental protection, so the circuit has to be there for that anyway. It's not their problem if YOU don't actually use the thermistor here in the US.

You can also use RTDs (Resistive Thermal Devices) embedded in the motor. RTDs are similar to thermistors, but there is no knee point threshold, the resistance changes constantly with temperature. So another special type of external transducer is used to constantly monitor the actual winding temperature inside of the motor and either trip, or send that information to something else. It's much much more accurate than thermal overload relays, but also much much MUCH more expensive to implement. RTDs are generally only used on very high value assets where the cost of an unscheduled shutdown is so catastrophic to a facility that they spare no expense. There are a few low cost relays that only monitor the RTDs themselves and just open a contact, but generally, RTDs are associated with very expensive Motor Protection Relays, like a Multilin 469, which will cost around $10k to add to the starter cost. You see that in motors over 500HP and a lot in MV motors.
 
Status
Not open for further replies.
Top