Motor Overload protection not required?

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philly

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I am looking at an application with MV motors that do not have overload relays and only have fues located in the contactor for protection of the motor. It looks like 430.32 requires seperate overload protection for the motor for all continuous duty motors. However 430.33 states that for any non-continuous duty motors the short circuit protective device per 430.52 is allowed to be used for the motor overload protection.

This particular application is for MV compressor motors. Would compressor motors be considered non-continuous such that they would not require seperate overload protection per 430.33 thus allowing the fuses to be used as the motor overload protection?
 

augie47

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Note the FPN at 430.31 that sends you to 430 Section XI for over 600 volts,
 

templdl

Senior Member
Location
Wisconsin
Have you consulted with the motor manufacture? Besides simple overload protection there is the issue of the mumber of starts per hour that would be allowed before the motor overheats. As I recall a common thermal OLR as used in a LV motor starter could be used but using CTs. By coordinating the OLR and heaters with properly sized CTs it is one way to do it.
Then there is the programable solid state relay device.
Unless the motor manufacture recommends fuses for overload protection, fuses probably are a very poor choice in my opinion.
 

philly

Senior Member
I agree that fuses are a poor choice for motor overload protection. My question is weather or not they can legally protect an MV motor.

430.225(B) for overload protection of an MV motor states that "Each motor shall be protected against dangerous heating due to motor overloads and failure to start by a thermal protectior integral with the motor or external current-sensing devices, or both."

Does this statement mean that the fues are allowed to provide the overload protection?

For the motor MV short circuit protection do the same maximum values for fuses in 430.52 apply?

This is a motor at a customers facility so I have them checking weather or not there is a thermal overload block or smalll thermal electronic relay somewhere in the enclosure. It has been confirmed that there is no motor protection relay.
 

mbeatty

Senior Member
Location
Illinois
I agree that fuses are a poor choice for motor overload protection. My question is weather or not they can legally protect an MV motor.

430.225(B) for overload protection of an MV motor states that "Each motor shall be protected against dangerous heating due to motor overloads and failure to start by a thermal protectior integral with the motor or external current-sensing devices, or both."

Does this statement mean that the fuses are allowed to provide the overload protection?

For the motor MV short circuit protection do the same maximum values for fuses in 430.52 apply?

This is a motor at a customers facility so I have them checking weather or not there is a thermal overload block or smalll thermal electronic relay somewhere in the enclosure. It has been confirmed that there is no motor protection relay.

Philly,
I believe the above statements are correct.
Mark
 

david luchini

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I agree that fuses are a poor choice for motor overload protection. My question is weather or not they can legally protect an MV motor.

430.225(B) for overload protection of an MV motor states that "Each motor shall be protected against dangerous heating due to motor overloads and failure to start by a thermal protectior integral with the motor or external current-sensing devices, or both."

Does this statement mean that the fues are allowed to provide the overload protection?

For the motor MV short circuit protection do the same maximum values for fuses in 430.52 apply?


This is a motor at a customers facility so I have them checking weather or not there is a thermal overload block or smalll thermal electronic relay somewhere in the enclosure. It has been confirmed that there is no motor protection relay.

I would say that neither of these two statements is correct.

1) 430.225(B)(3) says that operation of the overload interrupting device shall simultaneously disconnect all ungrounded conductors. Fuses protecting the circuit will not do that.

2) 430.51 says that "the provisions of Part IV shall not apply to motor circuits rated over 600 volts, nominal." I think you'd use 240.100 for the short circuit protection of the MV motor circuit.
 

Jraef

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Philly,
I believe the above statements are correct.
Mark
Permissible: under the right circumstances. As long as the fuse sizing falls within the protection requirements for the motor as outlined elsewhere, i.e. the same rules as used for OL relays apply.

Does it happen: yes, when someone wants a controller to be ass cheap ass possible. It's usually used with Blown Fuse Indicator pins that hit snap switches which cut power to the contactor coil if one fuse blows (which makes it satisfy the "all ungrounded conductors" rule.

Adviseable: Never.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I'd be curious how you could properly protect a motor with just fuses if they are sized for short circuit as well as O/L protection. I'm just thinking out loud here, but if they have enough of a time delay to allow a motor to start you'd think that'd allow a motor to run overloaded for quite some time before blowing. If only slightly overloaded, maybe forever? I might be way off base here...
 

jdsmith

Senior Member
Location
Ohio
I'd be curious how you could properly protect a motor with just fuses if they are sized for short circuit as well as O/L protection. I'm just thinking out loud here, but if they have enough of a time delay to allow a motor to start you'd think that'd allow a motor to run overloaded for quite some time before blowing. If only slightly overloaded, maybe forever? I might be way off base here...

You're correct that the fuses will typically not provide overload protection. Take a look at the attached TCC for a 900 HP motor fed from a fused (E2 class) contactor. Right above the motor inrush characteristic there is a short blue line with a dot inside a circle at one end. The dot/circle is at about 35 sec and 650% on the graphs and the line sweeps up to about 100 sec and 475A. That blue line is the hot safe overload curve provided by the motor manufacturer. For any relay, fuse, etc. to protect against motor overload it must lie below that blue overload curve. The fuse shown on the attached TCC is a 12R fuse, which is the smallest we can use that will allow the motor to start. You can clearly see that the fuse does not lie below the overload curve, hence the fuse cannot protect against overload conditions.

Note that there is a green curve that lies slightly below the blue overload curve - that green curve represents the characteristic of the thermal overload element in the motor protection relay.
 

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