Motor Overloads Tripping

[steve66]

I been reading the other thread about motor overloads, and I'm having a similar issue. New exhaust fans for a commercial kitchen - I've had a 1/4 HP, a 3/4 HP, and a 1 HP motor each trip their overloads. All 1.0 SF motors.

I've never worried too much about overloads before. I usually just let the electrical contractor size and select them. Now I'm finding out overloads aren't as simple as I assumed they were. So any help would be appreciated.

1st Question:

The NEC says I can size the overloads at 115% of the motors full load current. However, if I follow the Square D guide for sizing thermal units (page 16-129 in the 174 Digest), they say to use 90%. Why the difference??

2nd Question: What are the differences between Melting alloy, and bimetallic? Are both resettable? Does one trip more easily?

3rd Question: I had "temperature compensated bimetallic" specified for the overloads. Now I realize that probably wasn't the right thing to spec, since the motors are outside on a rooftop, and the controllers are indoors at a constant temperature. Do you think its OK to keep the bimetallic temp. compensated, or do all the overloads need changed?

Thanks in advance:
Steve

 

[jim dungar]

1st Question:

The NEC says I can size the overloads at 115% of the motors full load current. However, if I follow the Square D guide for sizing thermal units (page 16-129 in the 174 Digest), they say to use 90%. Why the difference??

The NEC is not a design manual, it simply give the maximum allowable values. The Square D guide is specifically prepared for Square D oveloads and takes into account the ambient environment of the motor and the overloads. Some would argue that 110.3 requires you to follow the manufacturer's instructions.

2nd Question: What are the differences between Melting alloy, and bimetallic? Are both resettable? Does one trip more easily?

These are simply two different technologies each with disadvantges. There is usually no single reason to specify one over the other.

3rd Question: I had "temperature compensated bimetallic" specified for the overloads. Now I realize that probably wasn't the right thing to spec, since the motors are outside on a rooftop, and the controllers are indoors at a constant temperature. Do you think its OK to keep the bimetallic temp. compensated, or do all the overloads need changed

Ambient compensated overloads are usually thought of as a 'premium' design. They are a definite advantage when the motor is in a changing ambient (ie.e outside) and the controller is in a fixed environment.

My experience has been, if the overlodas are sized per the manufacturer's guides, OL tripping is usually a result of motor problems, like bad bearings or actual overloads.

 

[steve66]

Thanks for the response Jim. I've also been reading the Square D overload relay catalog. It gave me a little more detail also.

http://static.schneider-electric.us/docs/Motor%20Control/Overload%20Relays,%20NEMA%20Style/9065CT9701.pdf

Turns out for a Square D overload, the actual "trip rating" of the thermal unit is 1.25 times the minimum current listed. (At least, that's how I understand example #1 and "Protection Level" on page 23.) That does give a trip rating that is higher than the largest listed current rating. So it looks like if I base my sizing on the NEC calculation, I have to calculate all the trip ratings for the thermal units myself.

Example: Thermal unit AR 7.7: Listed current range: 4.66 - 5.29 amps
Actual trip point: 4.66 X 1.25 = 5.825 amps
Minimum rated motor current that this OL could protect (1.0 SF): = 5.825/1.15 = 5.06 amps.

I wish Square D would just list the trip amps.

Steve

 

[steve66]

The Square D catalog brings up another question. On page 14:

http://static.schneider-electric.us/docs/Motor Control/Overload Relays, NEMA Style/9065CT9701.pdf

they have a strap kit (for melting alloy only) that converts a three phase starter to work with a single phase motor. Why would one need that? Wouldn't just using two thermal units and leaving the third open work?

This kit also comes with selection tables for single phase motors. Do I need different tables to select thermals for a single phase motor??

STeve

 

[jim dungar]

Thanks for the response Jim. I've also been reading the Square D overload relay catalog. It gave me a little more detail also.

http://static.schneider-electric.us/docs/Motor Control/Overload Relays, NEMA Style/9065CT9701.pdf

Turns out for a Square D overload, the actual "trip rating" of the thermal unit is 1.25 times the minimum current listed. (At least, that's how I understand example #1 and "Protection Level" on page 23.) That does give a trip rating that is higher than the largest listed current rating. So it looks like if I base my sizing on the NEC calculation, I have to calculate all the trip ratings for the thermal units myself.

Example: Thermal unit AR 7.7: Listed current range: 4.66 - 5.29 amps
Actual trip point: 4.66 X 1.25 = 5.825 amps
Minimum rated motor current that this OL could protect (1.0 SF): = 5.825/1.15 = 5.06 amps.

I wish Square D would just list the trip amps.

Steve

There is no reason to calculate the 'trip current'. Follow the thermal unit selection procedure on page 19, up to step 4. You need to perform some adjustments to your particular FLA, because their tables are designed for 1.15 SF motors in the same ambient as the overload.

Step 5 is optional and would only be performed if you had the need to know the minimum trip value (i.e. for coordination purposes).

It is not possible to have a single 'trip amp' value, because any one thermal element may be used in a multitude of starters, enclosures, and environments.

 

[jim dungar]

The Square D catalog brings up another question. On page 14:

http://static.schneider-electric.us/docs/Motor Control/Overload Relays, NEMA Style/9065CT9701.pdf

they have a strap kit (for melting alloy only) that converts a three phase starter to work with a single phase motor. Why would one need that? Wouldn't just using two thermal units and leaving the third open work?

This kit also comes with selection tables for single phase motors. Do I need different tables to select thermals for a single phase motor??

STeve

The strap is required only when you hae decided to have a 3-phase motor starter control a single phase motor. The metling alloy overload relay requires the tripping mechanism be held in the 'active' position, either by a thermal unit of a jumper strap.

A thermal unit in a single phase overload relay experiences less mutual heating than when in a three phase unit. Notice how most of the slection tables have multiple columns.

 

[steve066]

The metling alloy overload relay requires the tripping mechanism be held in the 'active' position, either by a thermal unit of a jumper strap.

I see, and that also explains why they are only necessary for melting alloy OLs.

A thermal unit in a single phase overload relay experiences less mutual heating than when in a three phase unit. Notice how most of the slection tables have multiple columns.

I wondered what those columns were for. Now I know 1 T.U. = 1 trip unit, etc.

I also notice my temp. compensated bimetallic OL's don't need the separate columns since the temp. compensation would take care of the mutual heating.

Steve

 

[augie47]

In my experience restaurant exhaust fans, especially fractional horsepower ones, tend to have adjustable sheave pulleys. These allow for speed control by adjusting the pulley itself and very often they are not "set" for the particular application. If you O.L s are tripping the 1st thing I would do is check actual amps vs motor nameplate.

 

[steve066]

In my experience restaurant exhaust fans, especially fractional horsepower ones, tend to have adjustable sheave pulleys. These allow for speed control by adjusting the pulley itself and very often they are not "set" for the particular application. If you O.L s are tripping the 1st thing I would do is check actual amps vs motor nameplate.

Thanks. One is over the motor nameplate, and 2 are very close.