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.

Understanding Motor Frames

The purpose of the motor frame system is to allow you to substitute one manufacturer's motor for another brand, if it's not a custom OEM frame. In order for a motor to list a specific motor frame on it's nameplate, it must have mounting dimensions common to that frame. Notice I said mounting dimensions. That doesn't mean every 184T frame motor is exactly alike, just that they will mount in the same location as other 184T frames. So, what are the mounting dimensions?

The most common mounting dimensions are as follows (the dimension designations are in parentheses - NEMA first, then IEC):

Shaft height (D), (U)
Shaft length (N-W), (E)
Shaft diameter (U), (D)
Distance from front mounting to back of shaft (BA), (C)
Front-to-back mounting holes (2F), (B)
Side-to-side mounting holes (2E), (A)
C or D flange mounting hole diameter (AJ), (M)
C or D flange mounting lip (AK), (N)

Let's discuss NEMA frames first. NEMA frames are grouped into families in which all the mounting dimensions are the same except the 2F dimension. Dimensions other than the mounting dimensions can be different. This is most easily seen in the "C" dimension, which is the distance from the tip of the shaft to the back of the motor. Since it's not a mounting dimension, there are no standards from one brand to another. Today's frames are "T" frames. From 1952-1964, the "U" frames were standard. The "original" frames were built prior to 1952.

Now, notice I put shaft height first. There's a good reason for that. If you take the first two digits of a NEMA "T" frame and divide it by four, you will get the shaft height. For example, on a 184T frame, that would be 18 divided by 4, or 4.5 inches. Conversely, if you measure the shaft height of a NEMA T frame motor and multiply by 4, you'll have the first two digits of the frame size.

Let's say you have a motor where you can't read parts of the nameplate. If you measure the shaft height as 4.5" and multiply by 4, you know you have a 182T or 184T frame motor. You have just eliminated all the other possible frames with one quick measurement. Now, all you have to do is check the 2F dimension to see which of the two frames you have. If you know any two out of the horsepower, frame size, and RPM, you can usually figure out the third. So, in our motor with the 4.5" shaft height, if we know its speed is 1750 RPM, it's typically a 5 horsepower motor. This is no guarantee because sometimes OEMs don't follow the rules when having a manufacturer build their motors. Also, in the larger frames, there can be frame size differences between an open (ODP) motor and an enclosed (TEFC) motor. If you find a frame size where the last letter is a Z, that indicates a special frame, generally only available from the OEM that had the motor built for them.

The IEC, or metric, motors usually make it a little easier. First of all, the shaft height in millimeters is the frame size. So a 90mm shaft height is a 90 frame. In many cases, the is a letter afterward, like a 90S or 90L, which distinguishes between frames in the 90 family. Again, this will show up in the B (like the NEMA 2F) dimension. The other nice thing is that all dimensions are in whole millimeters. There are no standard motors with a 90.5mm shaft height. The hard part about the IEC motors is they have no shame about not following these standards when they don't want to; and they don't tell you when this happens, like with a NEMA Z designation. This occurs quite regularly with their flanges. A NEMA C flange is designated a B14 flange in the IEC world. A NEMA D flange is called a B5 flange. You pretty much need a metric ruler with you to make sure you know what IEC motor you have.

This is a start in understanding motor frames. Most all manufacturers put dimensional sheets in their catalogs and/or on the internet. Take a look and this will make more sense.

Testing Electric Motors

Testing electric motors in the field really only requires two meters - an ohmeter and a megohmeter.

First, the best place to test the motor is in the connection box with the leads disconnected. If that's not possible, test as close to the motor as possible. The more wire and electrical devices you have between you and the motor, the more you have to assume there are no problems with them. We all know what happens when we ASS-U-ME. I've seen many motors removed from service because someone had a cable problem outside of the motor.

To test the windings, use an ohmeter. Make sure any jumpers for voltage selection remain connected. For a nine lead, dual voltage motor connected for 480 volts, these would typically be the 4-7, 5-8, and 6-9 connections. Measure the resistance from 1-2, 2-3, and 3-1. The resistance readings should be between 1-2% of each other - in other words, BALANCED! There is no way to tell how much resistance you should get, but it is typically low. Realize that you are testing with a DC battery. Therefore, you are reading the resistance of the copper wire. In many cases, this will look like a short circuit. GOOD! It's when it doesn't look like a short circuit that you have problems. As long as the readings are low and balanced, you're ready to go to the next step. I can't tell you how many motors are brought to us because "the windings are shorted." Don't make that mistake.

To test the insulation system, use a megohmeter. This is commonly called a "megger", but that is a registered trade name belonging to Biddle, the long-time megohmeter manufacturer. Without permission from Biddle, I use "megger" as a generic name for my tester. Everybody recognizes it, and it takes less typing. When using a megger, make sure there is an open set of contacts between you and any upstream electronic device, such as a VFD, so you don't send 500VDC into some sensitive electronic components. Making sure that the voltage selection jumpers are connected, set your megger for 500VDC to test motors of a 230/460 nature. 1000 volts is not better - it's wrong! Connect one megger lead to the ground lug in the connection box, connect the other megger lead to any motor lead (we'll come back to that). Turn the megger's crank, or push the button if you have a digital tester. You should read hundreds of megohms. If you get a zero reading. Stop testing - the motor is shot. Many times I do this test first. If I get a bad reading, I don't have to do anything else.

The type of readings you can get vary widely. If you get under 5 megs, it's bad. From 5 through around 100 megs means your insulation is about to fail completely, or you have nasties living in you motor. The nasties can be water, oil, excess bearing grease, or anything else that will provide a high resistance path over the surface of the windings to ground. Many times, a motor with these readings can be steam cleaned, baked dry, and retested with marked improvement in the insulation readings. What we are trying to determine is if the insulation system is good. This consists of the enamel extruded on the magnet wire when it is made, the insulating paper we put in the motor slots and between phases, and the varnish that glues it all together. Don't let a dirty motor get in the way of proper readings.

OK, back to the reason we only need to connect the megger to one lead. The motor windings are connected to each other in either a wye or a delta configuration. This is an economical decision made by the motor manufacturer. No, it doesn't matter how the secondary of the upstream transformer is connected. Most American motors will be wye connected (wye not?) up to about 25HP. Above that, they are typically delta. Most IEC motors will be delta because of their way of voltage selection. That's for another post. The point is, the motor windings are connected phase-to-phase. Therefore, you can get from any point in the windings to any other point in the windings without leaving the motor. So, the test current generated by applying the megger's DC voltage can "find" a problem anywhere in the windings.

By the way, some motors are connected externally, not internally. This includes motors made for wye-delta or part-winding starting, among others. This requires a modification of our testing methods. Stay tuned, and we'll have another post covering motor connections in the future.

So here's your Final Exam:

To test the windings for continuity, use:
An ohmeter
A megger

To test the insulation, use:
An ohmeter
A megger

Did you pass?


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. The problem arises when you mix greases which have different thickeners. The most common thickener, or base, used in today's electric motor bearings has a polyurea base. The most common grease used by maintenance departments has a lithium base. Polyurea and lithium don't like each other. If you mix the highest quality polyurea based grease with the highest quality lithium based grease, the result can be a severe reduction in the effectiveness of the base. The result is that your grease can become pure oil and flow into the motor, leaving you with no bearing lubrication. This explains why we sometimes see motors which are full of oil, the bearings have failed, and the customer says there is no oil anywhere near that motor.

Most bearing manufacturers build their bearings in plants all around the world. Some have codes that define the grease that is in their bearings. But sometimes, these codes are hard to decipher. We specify polyurea based grease in all the bearings we use in motor rebuilding. This brings us in line with the motor manufacturers. The most readily available brands of polyurea grease are Exxon/Mobil Polyrex, Chevron SRI, Shell Dolium BRB, and Rykon Premium #2. Of these, Exxon/Mobil Polyrex is by far the most recommended brand.

Many customers ask when and how to grease their bearings. Our recommendation is to clean the grease fitting and remove the grease relief plug. With the motor running, pump in new grease until clean grease comes out of the relief port. Leave the relief plug out until grease stops coming out; this may take several minutes or more. Reinstall the relief plug. Your bearings will have the proper amount of grease in them. When you see grease coming out of the end bell around the shaft, you have put in far too much grease. The proper fill is about half of the volume of the bearing cavity, allowing for expansion. More motors fail from too much grease than too little. "A couple shots a week" is not a good policy, especially if the relief plug is never removed.

So, how often should you grease your motor bearings? This is where you will find a wide range of answers. We have seen manufacturers that say never grease a shielded bearing, the most common type supplied in new motors. Other manufacturers give varying intervals. Some take duty cycle into consideration, and some don't. Even though this may not be the answer you want to hear, we believe each application has its own timetable. Our suggestion is to develop a schedule based on the condition of the grease that comes out of the relief port. If the first grease that comes out looks exactly like the grease you are putting in, you can extend your greasing interval. If a solid glob of old grease that has to be, like the constipated mathematician, worked out with a pencil, comes out, you need to shorten your interval. This may result in a chart with greasing intervals that vary from motor to motor. As long as you do it properly, you could set your interval based on the "worst case" motor in your plant. This would require more grease, but it would minimize bearing related motor failures.

In today's manufacturing plants, we see a lot more repairing/replacing and a lot less maintaining of electric motors. Hopefully, this approach will help you save down time and cut repair costs.


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.

Another important feature is the overload's ability to recognize a single-phase condition and trip faster. Many overloads have two trip curves covering symetrical or single-phase tripping; most replaceable heater overload relays do not.

The biggest mistake I see is the application of overload protection at the motor nameplate current. That assumes the motor is completely loaded. Many motors run at 75% to 90% load. Overcurrent protection should be applied slightly above RUNNING load current, not full load current. This allows the overload relay to tell you something is wrong when you exceed normal running load current but before you exceed full load current. Take care not to get too close to the running load current in order to reduce nuisance tripping.

Motor Horsepower

Perhaps the biggest misunderstanding when looking at a motor nameplate concerns the motor's horsepower rating. You should consider this the same as looking into a bucket. Each has a capacity. The motor's horsepower is an expression of the amount of work a motor can do WHEN THE LOAD ASKS FOR IT, not how much work it will shove into the load. Therefore the load current, and consequently the horsepower the motor develops, is determined by the load, not by the motor.

For example, if a pump's impeller is turned at 1750 rpm, it will require a specific amount of horsepower as shown on the pump curves. Let's assume those curves call for a 10HP motor. The load on the motor will be the same whether a 10HP or a 20HP motor is supplied. This, of course, ignores things like efficiency and no-load current which will have minor effects on the motor current. But, you won't get any more output from the pump or draw twice the current by using the larger motor.

So, consider this when you size your overload protection.