Episode #39: Types of Electric Motors: DC Brushed, Asynchronous, and Synchronous

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The Motion Control Show

Having already talked about what an electric motor is, now I'm going to talk about a few different types of electric motors.  Specifically, I'm going to talk about DC brushed motors, asynchronous and synchronous.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.

There are plenty of people who know a way more about some types of electric motors than I do, so I reached out to a good friend of mine and colleague, John Brokaw, to give some of his input on some of these types.

COREY:  John, what can you tell me about the DC brushed motor?

JOHN:  This is the oldest school down-and-dirty motor in creation.  This thing has been around.  See it right here on the slides, invented by Faraday in 1821.  So, it's a 200-year technology.  It actually is still used in a lot of applications because it's relatively inexpensive.  It does have a few known issues that everybody's aware of.  Most commonly being the wear on the brushes.  You have these ceramic carbon brushes that are passing the current to the rotor that's rotating and the wear on those brushes they just, like anything, they eventually wear out and they have to be replaced.  This takes your motor down whatever vehicle it is working on down, and it's just a nuisance. 

COREY:  So, where the DC brush motor here is commutated by a break in the wires and these brushes here, the AC motor actually is commutated by the sinusoidal frequency of the AC current coming in and goes to the contactors here.  Now this is showing the coil being on the inside, but really, usually, the coil is the stator on the outside with the rotor on the inside that is the one that's turning.  But this is good for comparison.  The difference between the AC and DC and how they are commutated. 

Before I talk anymore about AC Motors, let's talk about synchronous versus asynchronous.  An AC asynchronous motor doesn't have any magnets in it, so it actually turns slower than the synchronous speed of the frequency coming into it.  I already talked about how an AC motor commutates off the AC frequency coming in, 60 Hertz here in the US, but an asynchronous motor, since it doesn't have the magnets, will actually fall behind that, and it will always be running to catch up.  So you can see here that it's the frequency coming in times 120 divided by the number of poles minus some slip.  So it's always just going to be running to catch up, whereas a synchronous motor has some permanent magnets in it, so it's locked into the frequency being controlled that's coming into it, and it's always going to turn at that synchronous speed.

I have to jump back to John Brokaw for this one.  John, are AC induction and asynchronous motors the same thing?

JOHN: All AC induction Motors are asynchronous.  But you can get synchronous, pseudo-synchronous, applications out of them by pairing them with feedback and doing vector control on them.  That's where you're actually controlling the angle between those two and controlling that slip frequency to be exactly where you want to be to generate the torque/speed performance of the application. 

And here we have the guts of an AC induction motor.  You can see that this is a classic induction motor where you don't see any brushes or anything coming to it.  You have that rotor assembly there in the middle that's tied to the shaft going through it.  The only wearing component on a typical AC induction motor are the bearings which you can see at the ends of the motor.  There are a number of accessories that can be added to an induction motor depending on what the application is.  One of the primary one wants to think about is cooling.  This one has a fan.  This looks like a totally enclosed fan-cooled motor.  You can also have non-ventilated motors which are sealed.  You can have an open motor where you actually have air flowing through the motor.  You can have forced air on these things.  You can put hydraulic cooling jackets on motor.  There's lots of different ways to cool a motor off.  The thing to remember at the end of the day is that an electric motor is a coil of copper and you're putting electricity through it.  Whenever that happens, that's an electric heater.  So you are going to generate heat in the system and in some way you've gotta get that heat out.  Heat management is one of the key issues in selecting and sizing and operating motors.

Other wearing points you can see are the bearings.  Bearings, like any bearing, like bearings on your car, eventually you're gonna have to replace them 'cause they do wear out.  There are some other accessories, gaskets, seals, different things depending on the environment you're actually putting your induction motor into and what the application is.

COREY:  Let's talk about AC Motors and the VFDs, the variable frequency drives, that run them.  What do you think of them?

JOHN:  VFDs are great.  It really depends on the application because you’re usually talking about a couple different things.  One is what do you want?  How do you want the motor to start?  And there's a number of different ways of doing this.  You can start across the line.  What that means is basically you just have a switch and you're basically going YAK and all of a sudden current starts blowing off of the electric grid.  Issues with this one.  It's a little hard on the motor because you're putting a surge into the motor.  This can also affect your local power grid and computers there on that system really don't like it when you do this.  It's a really rough way to start a motor.  It's done.  It's done a lot of different places where it doesn't matter.  Say if you're running a pump for irrigation facility, you're usually on a dedicated line.  There's not a lot of computers around that are going to be sensitive to it.  You're just throwing a thing and starting a pump.

The other method is using a soft start.  These are electronic components that basically slowly ramp up the voltage over 5, 10, 15 seconds to allow that closure to be a little bit smoother.  It’s a lot easier on the motor and a lot less noise gets back-generated onto your electric grid.  That's the down and dirty old school, been done for a couple 100 years, way of starting an electric motor. 

Since the 60s, we've had variable frequency drives.  With the advent of semiconductors, we’re able to do different pulse-width-modulations to starting the control of the frequency of an AC induction motor.  Remember, motors follow the frequency coming into them.  So, by adjusting the frequency, you can adjust the speed of a motor.  This has a lot of advantages.  Pump application: you can actually control how much water you're pumping in where you're working on the pump motor curve.  Gets you a little bit more efficiency.  You can optimize the application.  You can also then slowly ramp up the velocity so you're not just closing the line, so it makes it a lot smoother and a lot cleaner for the power grid.  Note some VFDs you may need to put some filter in 'cause they do create some harmonics that can back feed into your power grid, but in general a VFD is a lot cleaner, electrically, way of installing and starting a motor.

COREY:  To drive home the point of what a synchronous motor is, it is characterized by the constant speed of the rotation, which is independent of the load, but linked to the supply frequency or the current depending upon the drive type.  That's where the term synchronous comes from, and this is mostly done via the permanent magnets that are on there.  If you look here at the construction, it looks quite a bit different than the AC induction motor.  I'm going to have John Brokaw point out a few things for us.

JOHN:  Note on a synchronous motor there are a couple things that are always going to be there.  You will always have feedback on a synchronous motor.  You do this because you need to know where the actual magnets are, 'cause they're alternating North, South, North, South around the rotor.  As you can see in this in the diagram in the lower right, you can see all the little surface mounted magnets on there, and they're actually, if you actually put a magnet on there you would actually see their alternating North, South, North, South, North, South as you go around the rotor.  This is what then the coils react against and are able to actually spin that by alternating.  Without the feedback on the device, you won’t know where you are to turn the right coil on or off and you can end up with the system fighting itself.

COREY:  So, John this really begs the question, are synchronous and servo motors the same thing?

JOHN:  All AC servo motors are synchronous motors.  All synchronous motors are not servo motors.  There are some oddball motors out there that are synchronous that are not servo motors; switch reluctance motors, stepper motors are synchronous because they're following the frequency, but they are not servo motors. 

COREY:  To put the two types of motor side by side, you can see how the construction is similar, but also how it is different.  The asynchronous motors can be huge.  They can be absolutely huge, the size of a small room.  The synchronous motors, the magnets get too expensive, so they're really are not going to be any bigger than a large cat, usually at the most.  But there's a few similarities, a few differences.

Now, John really wanted to make sure that I explained the importance of the horsepower calculation.  Power equals torque times speed.  Power can be in horsepower or can be in Watts.  The calculation I like to use, just off of memory, is that horsepower is equal to torque in ounce-inches times speed in revolutions per second divided by 16,800.  Now this is important because the asynchronous motors and the AC Motors are rated in horsepower, but if you have a servo motor we have speed/torque curves that often times look like this where you have torque over here and have speed here.  This is pretty much all the same power from start to finish, but it is a production of the torque and speed, so we don't talk about sizing a servo motor, or a synchronous motor often times, in terms of power.  We talk about it in terms of speed and torque.  (One motor may have high torque whereas another has high speed, but the same power.)  So, if somebody wanted to go from an AC motor to a servo motor, they can't just say, hey, give me a 1-kilowatt motor.  They do and we try to accommodate, but really the better information is how much speed and torque do you need?  So that's really important here.  One horsepower is equal to 756 Watts. 

One last comparison here.  The important part about this graphic here is the different types of applications.  The asynchronous motors are really better at continuous velocity sort of applications where synchronous motors are needed for more accurate velocity, but also the positioning type applications.  So, I hope that helps.

I'm Corey Foster at Valin Corporation.  Reach out to us here.  Thank you, John Brokaw, for assisting.  I learned a lot today.  I hope this helps.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.