6 Mar 2014
Motor Overload Protection
Many people plan their motor protection around a fuse or breaker. Fuses and breakers look at short-circuit current that happens after the motor has failed. Their job is to protect the power system from damage due to the level of short-circuit current. When they go, it's already too late for your motor. For motor protection, you should look to the overload relay in your starter.
Overload relays are defined by their protection Class. Protection is based on the amount of time it takes the overload to trip at locked rotor current. A Class 10 overload is faster that a Class 20 or Class 30 overload.
3 Mar 2014
Greasing Electric Motor Bearings
Over the past years, we have been researching the problem of grease and
electric motor bearings. The major problems are the type of grease, the
proper application of the grease, and the frequency of application. This
is the result of our own research involving grease manufacturers,
bearing manufacturers, motor manufacturers, our industry's technical
association, and our own experience in our facility.
Probably the least understood part of the problem is the grease itself. Without going into great detail, grease is approximately ninety percent oil and ten percent thickener. The oil does the lubricating; the thickener keeps the oil in place.
Probably the least understood part of the problem is the grease itself. Without going into great detail, grease is approximately ninety percent oil and ten percent thickener. The oil does the lubricating; the thickener keeps the oil in place.
26 Feb 2014
Wye vs Delta Motor Connections
In general, three phase motor windings are connected either in a wye or a
delta. I say "in general" because some motors are not pre-connected
either way. We'll discuss that in a minute. The primary reason for
connecting in wye or delta is basically for manufacturing convenience.
It DOES NOT have anything to do with the way the upstream transformer is
connected. So, all we can do is connect the motor for the correct
voltage.
Most motor nameplates have connection data for low or high voltage. If not, there are many charts and publications out there that will show you those connections. One great source is a small handbook put out by our trade association, EASA, that is literally pocket sized and contains a wealth of information. It is called the "Electrical Engineering Pocket Handbook." But don't worry, there's a lot more electrical than engineering. These should be available from your local motor rewind shop. Another common source is George Hart's "Ugly's" book. This has a lot more general electrical information than the EASA book, so it is larger. It sits nicely in a mechanic's toolbox, though. Also, most manufacturer's catalogs have connections in them too. Because of that, this article will not cover which wires to connect for every possible winding you may see. So, let's look at some of the things you will see when you take the cover off the connection box.
In the US, it is not uncommon to see motors up to around 25HP wye connected, and delta connected above that. It is common to find nine leads which locks your connection into either a wye or a delta right out of the box. The thing to remember is that, for either voltage, you always connect the incoming lines to motor leads one, two, and three. The problem is what to do with the other leads.
A low voltage connection is a parallel connection, while a high voltage connection is series. Those of us who like to fish have been doing this with trolling motor batteries forever. Paralleling two twelve volt batteries still gives us twelve volts but with lots of current. Connecting those batteries in series gives us twenty-four volts with less current. But that's OK, because a twenty-four volt motor draws half the current of a twelve volt motor at any given speed.
Many IEC motors will use a delta connection for all ratings. It is also common to see IEC motors with twelve leads, which does not pre-configure the windings into a wye or delta connection. Because motors are made all over the world for shipment all over the world, it is becoming more common to see this on some US motors also. Motors connected like this can be started across-the-line, part-winding, or wye-delta, depending on the final run connection. This allows a "soft-start" of the motor, which becomes more common in larger horsepower motors. We'll cover "soft-start" connections in another article.
Once you are familiar with the numbering system for nine or twelve lead motors, it's much easier to figure out the correct lead numbers if a couple leads have lost their tags. Also remember the jumpers which connect the motor for the correct voltage must be connected before doing any motor testing.
Most motor nameplates have connection data for low or high voltage. If not, there are many charts and publications out there that will show you those connections. One great source is a small handbook put out by our trade association, EASA, that is literally pocket sized and contains a wealth of information. It is called the "Electrical Engineering Pocket Handbook." But don't worry, there's a lot more electrical than engineering. These should be available from your local motor rewind shop. Another common source is George Hart's "Ugly's" book. This has a lot more general electrical information than the EASA book, so it is larger. It sits nicely in a mechanic's toolbox, though. Also, most manufacturer's catalogs have connections in them too. Because of that, this article will not cover which wires to connect for every possible winding you may see. So, let's look at some of the things you will see when you take the cover off the connection box.
In the US, it is not uncommon to see motors up to around 25HP wye connected, and delta connected above that. It is common to find nine leads which locks your connection into either a wye or a delta right out of the box. The thing to remember is that, for either voltage, you always connect the incoming lines to motor leads one, two, and three. The problem is what to do with the other leads.
A low voltage connection is a parallel connection, while a high voltage connection is series. Those of us who like to fish have been doing this with trolling motor batteries forever. Paralleling two twelve volt batteries still gives us twelve volts but with lots of current. Connecting those batteries in series gives us twenty-four volts with less current. But that's OK, because a twenty-four volt motor draws half the current of a twelve volt motor at any given speed.
Many IEC motors will use a delta connection for all ratings. It is also common to see IEC motors with twelve leads, which does not pre-configure the windings into a wye or delta connection. Because motors are made all over the world for shipment all over the world, it is becoming more common to see this on some US motors also. Motors connected like this can be started across-the-line, part-winding, or wye-delta, depending on the final run connection. This allows a "soft-start" of the motor, which becomes more common in larger horsepower motors. We'll cover "soft-start" connections in another article.
Once you are familiar with the numbering system for nine or twelve lead motors, it's much easier to figure out the correct lead numbers if a couple leads have lost their tags. Also remember the jumpers which connect the motor for the correct voltage must be connected before doing any motor testing.
25 Feb 2014
Single Phasing Three Phase Motors
When a three phase motor is "single phased", it is a power system
problem, not a motor problem. A three phase motor needs three EQUAL
phases in order to operate properly. When the symmetry of the motor is
interrupted by the loss of a phase, the motor will die quickly unless
the controls have single phase protection. Many heater type overload
relays do not have this.
Single phasing occurs as a result of several possibilities. A loose wire, a bad connection, bad starter contacts, overload relay problems, a bad breaker, a blown fuse, and other things can cause this destructive condition. Obvious signs are a louder than normal humming from the motor and/or a shaft that vibrates rather than rotating.
Testing for this possibility needs to be done quickly since motors are not happy with this condition at all. The obvious test is to look at the current in each phase. This is where multiple meters will help so you can see all three phases at once. You can also look at the voltage, again with multiple meters if possible. I look at the phase to ground readings first. The phase to ground voltage will equal the phase to phase voltage divided by 1.7; thus 480 volts phase to phase will be 277 volts phase to ground. The advantage of taking phase to ground measurements is that each reading is independent of whatever is happening in the other phases. However, you can read phase to phase if you want. You would see an unbalance there too. The phase to ground reading will show you the bad phase, though; this will make troubleshooting easier.
These tests need to be made as close to the motor as possible, preferably in the motor's connection box while the motor is driving the load. If the motor is not connected, or you take your readings at the starter or breaker with the motor off, you can get fooled. A bad set of contacts in a contactor or breaker can just barely touch and still tell you that you have good voltage. Ask those same contacts to deliver enough current to run a loaded motor, and the voltage will take a dive.
You could continue to test at various stages of the power system upstream of the motor, but that keeps subjecting the motor to the stress of running in the single phase condition. Otherwise, make sure the circuit is off and locked out, and then start taking things apart.
The first place to look is at those suspect contactor contacts. But, Bo Diddley said "you can't judge a book by looking at the cover." Contacts can be like me - real ugly but still functional. Contacts that are gone don't work very well, though. Also look at the connections in and out of the contactor. Loose or burned wires or terminals are probably the second most frequent offenders.
If the contactor looks good, take continuity readings from the line to the load side of each phase of the overload relay. It should look like a short circuit. A word about overload relays here. With today's motors being smaller than their U-Frame predcessors, you need a fast overload relay with single phase protection. An IEC Class 10 thermal overload relay works very well. You can get more expensive, solid state models, but the Class 10 thermal relays work well for all but the most sensitive applications. Be careful with replaceable element versions, though. They are usually Class 30 (slower) and don't have single phase protection.
If everything is good at the starter, check wires, connections, and devices ahead of the starter until the problem is found and corrected. Once you have three good phases again, you should see a voltage balance of within two or three percent. Your motor will be happier, healthier, and have a shiny coat.
Single phasing occurs as a result of several possibilities. A loose wire, a bad connection, bad starter contacts, overload relay problems, a bad breaker, a blown fuse, and other things can cause this destructive condition. Obvious signs are a louder than normal humming from the motor and/or a shaft that vibrates rather than rotating.
Testing for this possibility needs to be done quickly since motors are not happy with this condition at all. The obvious test is to look at the current in each phase. This is where multiple meters will help so you can see all three phases at once. You can also look at the voltage, again with multiple meters if possible. I look at the phase to ground readings first. The phase to ground voltage will equal the phase to phase voltage divided by 1.7; thus 480 volts phase to phase will be 277 volts phase to ground. The advantage of taking phase to ground measurements is that each reading is independent of whatever is happening in the other phases. However, you can read phase to phase if you want. You would see an unbalance there too. The phase to ground reading will show you the bad phase, though; this will make troubleshooting easier.
These tests need to be made as close to the motor as possible, preferably in the motor's connection box while the motor is driving the load. If the motor is not connected, or you take your readings at the starter or breaker with the motor off, you can get fooled. A bad set of contacts in a contactor or breaker can just barely touch and still tell you that you have good voltage. Ask those same contacts to deliver enough current to run a loaded motor, and the voltage will take a dive.
You could continue to test at various stages of the power system upstream of the motor, but that keeps subjecting the motor to the stress of running in the single phase condition. Otherwise, make sure the circuit is off and locked out, and then start taking things apart.
The first place to look is at those suspect contactor contacts. But, Bo Diddley said "you can't judge a book by looking at the cover." Contacts can be like me - real ugly but still functional. Contacts that are gone don't work very well, though. Also look at the connections in and out of the contactor. Loose or burned wires or terminals are probably the second most frequent offenders.
If the contactor looks good, take continuity readings from the line to the load side of each phase of the overload relay. It should look like a short circuit. A word about overload relays here. With today's motors being smaller than their U-Frame predcessors, you need a fast overload relay with single phase protection. An IEC Class 10 thermal overload relay works very well. You can get more expensive, solid state models, but the Class 10 thermal relays work well for all but the most sensitive applications. Be careful with replaceable element versions, though. They are usually Class 30 (slower) and don't have single phase protection.
If everything is good at the starter, check wires, connections, and devices ahead of the starter until the problem is found and corrected. Once you have three good phases again, you should see a voltage balance of within two or three percent. Your motor will be happier, healthier, and have a shiny coat.
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