Electric Motor Buying Guide (Video)

By Yin on 21st Sep 2014

Video Transcript

How many of you would walk into a car dealership and say, “I’m here to buy a car and I want your very cheapest car”? Well, nobody would do that, right? Because what kind of a car would you get? And yet at the same time, they would go online - they will go online - they will go to a distributor, and they’ll say, “I want your very cheapest motor”. What kind of motor are you going to get? You are going to get the very lowest quality motor, and you may be sorely disappointed when you find out that that motor is not able to do the job that you expected of it.

Let’s find out now, how to tell the difference, between a very low quality motor or a good quality that will deliver the kind of performance you need. Let’s get started. We have here a very high-quality motor made specifically for variable frequency drives. It is a Motor Drives International motor, and it is 1 horsepower. This motor has a stainless-steel nameplate. Arguably, the nameplate is the most important part of the motor - how else do you know what the motor is? It gives you the voltage, the speed, the amps. This happens to be a 1 horsepower motor. It also gives you the service factor. A service factor tells you how much you can overload that motor. This is a 1.25 service factor motor, meaning you could actually overload this 25%. We recommend that you get at least a 1.15 service factor motor for any motor used with a variable frequency drive.

Now let’s turn the motor around here. This motor has been cut away so that we can see the internals of the motor. You will notice that we have the stator on the outside – that’s the part that does not move, or is static - and we have the rotor on the inside. That’s the part that rotates. Looking at the frame here, this is a cast iron frame. There are two types of motors generally, there is a rolled steel frame – that’s the cheapest way to build a motor. You simply roll some plate steel around, weld it, and then stuff the innards in to create a motor. This is a full cast-iron motor. A cast-iron motor is cast in a mold, and you can tell it very quickly by the heat dissipation ribs on the outside of the motor. A rolled-steel motor, generally, is called Open Drip Proof, and generally the air goes right through the motor. Whereas this is a Totally Enclosed Fan Cooled Motor, you can see that it’s completely sealed, and although it can breathe, the air is actually pushed by this fan over the outside of the motor keeping the internal clean, and its cooled through these ribs on the outside of the motor. The main advantage of the cast-iron motor is that it’s very stiff, and it keeps those bearings in alignment. Keeping the bearings in alignment is a very important part of longevity for a motor.

There are two ways that a motor fails. One is by failed insulation and the other is by failed bearings. Failed bearings is the number one reason for motor failure. The first thing you want to do is use oversized bearings. Oversized bearings will mean that those bearings are running at a lower amount than their full capacity. Then, you want to protect the contamination from things like shaft slingers and oil lip seals that you can see right here. Next, you want to get an oversized rotor. Now how do you know what oversized means? Well the best way to get an oversized motor is to get a premium efficiency motor. When you get a premium efficiency motor they have to have more iron and copper in here, and thus the motor runs cooler and is better for use with a variable frequency drive. The next thing you are going to want to do is get a rotor with skewed rotor bars. Now, this is the rotor, and you can see the red is the iron laminations, the silver is the edge of the rotor bars. (In this case aluminum, in some motors it is copper). You notice that these are at a slant, that is called skewed rotor bars, and skewed motor bars cause this motor to have more starting torque, quieter operation, and run smoother. The rotor bars are the part that conducts the electricity, the iron laminations are the part that conducts the magnetic flux.

Next, we want to talk about the temperature rise of the motor. In this case, this is a Class B Rise class of insulation. Pretty much all motors these days have a class of insulation, but getting a Class B rise as opposed to Class F rise will allow that motor to run cooler. You may end up having to get a motor that’s only 15 horsepower in this frame rather than a 20 horsepower if you do this, but it will run much cooler and cleaner.

Finally, vacuum pressure impregnation uses vacuum to actually evacuate all the air before introducing insulation into the windings. Vacuum impregnation is the best way to get a motor that’s very solid and high-end insulation.

I want to talk about the insulation itself. Most motors are rated for 1000 Volts, VFD motors should be rated for 1600 Volts. VFDs put out sharp pulses, and many times they go over 1000 Volts. You want to make sure that these have what is called high spike wire, and the high spike wire is rated for 1600 volts and will assure that your windings have a very long life.

Well there you go, now you are armed with all the information you need to make sure that the motor you select for your variable frequency drive is high quality and will give you years of reliable service. Make sure you check us out on VFDs.com for our extensive inventory of variable frequency drives and accessories, or call one of our knowledgeable sales associated for more information.