Motor Control 101

welcome to Eton's motor control 101 the first topic we'll be discussing is what controls a motor now way back in the day when people were just learning how to harness the power of electricity the three-phase AC motor was invented by this guy and with this new motor they needed a way to turn it on and off so they created this thing called a knife switch when an operator grabbed the handle he would move it from one side to the other and it would connect the motor to power but this wasn't particularly safe for the operator since when the switch was almost closed there were arcs formed at the contacts which could injure the operator ouch so they said scratch that let's just put the switch inside of an enclosure to protect the operator they put a handle on the outside and now they could turn on and off the motor and the arcs were contained inside the box so what's the problem with this well you have to have someone stand there and physically operate the switch if you've got 40 motors spread around your plant you better have an operator who has great endurance if they're gonna run all over your plant to operate those motors clearly that isn't going to work for a lot of applications so they invented this thing called a contactor inside the contactor there is a coil of wire wrapped around a piece of metal above that piece of metal there's another piece of metal that is attached to an electrical bridge which connects contacts on one side to the other side this top portion the metal attached to the electrical bridge is called an armature when you apply an electric current through this coil of wire you end up turning the bottom piece of metal into a magnet the fancy word for this is an electromagnet when you've turned on the electromagnet it attracts the top piece of metal which is part of the armature and pulls it down which in turn closes the electrical contacts on both sides when those contacts close it allows current to pass through the contactor and down to the motor well okay that's pretty cool but how do I turn off the motor well the first thing you do is you kill power to the coil which will turn off the electromagnet but that's not enough to separate the electrical contacts there is a spring inside the contactor when the electromagnet is turned on the top piece of metal overpowers and compresses the spring but when the electromagnet is turned off the spring is able to push apart the electrical contacts so what's the point of all this well it allows for remote operation of the contactor which thus allows remote operation of the motor if you put something like a push button on the other side of the circuit that is connected to the contactors coil then you can turn on the coil from wherever you want so imagine a control station with 40 push buttons installed on it and now the operator can stand there and control all the motors around the facility without even taking a step what I've done here is I've connected a green start button to control power and connected the other side of the push button to the coil inside of one of Eaton's freedom series contactors when I push the start button you can see the small black rectangle on top of the contactor move up and down that is the top of the armature showing through which is moving inside the contactor you can also hear the contacts inside opening and closing as well as the hum of electricity flowing through the contactor coil next I'll remove the top off of the contactor so you can see the electrical contacts inside watch the armature and contacts move as I energize and de energize the coil here's a close-up view of the electrical contacts if the contactor were hooked up to a motor right now this is where the power would flow through the contactor and out to the motor next let's talk about auxiliary contacts if we're looking at a contactor from the top down we've got three-phase power coming in the top three contacts on the inside one for each phase and three-phase power going out the bottom well there is a very common accessory that gets added to contactors called an auxiliary contact often it's mounted to the side of the contactor and inside the auxiliary contact module we have an electrical contact that can be normally open or normally closed I've drawn a normally open auxilary if you grab a normally open auxilary off of the shelf it will not pass electricity through it unless it gets changed by an outside force when it gets attached to the side of a contactor there is a mechanical connection between the auxiliary contact and the moving parts inside the contactor in this close-up view of the contactor you can see the black plastic clip on the side which moves up and down with the rest of the armature this is where the auxiliary contact connects to the inside of the contactor so when the contact are pulls down it causes the auxilary contact to also pull down which closes the contact inside of the auxilary so it's nifty about this is you can connect a circuit to the auxiliary contact and it will open and close the circuit whenever the contactor turns on or off the motor here I've got the same contactor hooked up to a push-button but now let's attach an auxiliary contact to the side with the aux contact attached let's wire a red light to it to indicate that the motor is running now that there is a red indicator attached to the auxiliary contact I'll dim the lights and watch what happens when I energize the coil basically the auxiliary contact gives you a status as to whether the contactor is open or closed you now that we understand how to turn on and off a motor how do we protect it let's briefly talk about short circuits well what can cause a short circuit when we're talking about motors one of the ways we can see a short circuit is when we get a phase 2 phase short so let's say a forklift operator accidentally Nick's the wires going to a motor he doesn't cut all the way through the wire but enough to cause two of the phases to touch each other well when the wires no longer have the motor in the way they have an unrestricted path for current to flow so you end up getting a very large amount of current very quickly through the wire well what's wrapped around wire insulation and you know what insulation doesn't like heat well when you put a huge amount of current through the wire you build up heat very quickly thus burning up the insulation and now you've got yourself a fire another way you can get a short is if you have a face to ground short let's say that a wire ended up touching the rail on a garage door and now you've got a path to ground it's the same problem one of the popular ways to protect against a short-circuit is to use a molded case circuit breaker an inside of that circuit breaker there are two forms of circuit protection the first looks like a coil of wire wrapped around a metal core sounds familiar right well next to that coil of wire there is this thing called a trip bar when the current passes through the breaker and through this coil of wire it starts generating magnetism when you get a large blast of current passing through this coil it creates a magnet so strong that it pulls the trip bar towards it and causes the circuit breaker to trip this is called magnetic circuit protection next let's talk about a motor overload or too much current being drawn by the motor well what could cause this to happen let's look at an example here we have a conveyor belt driven by a motor and on this conveyor belt we have several small packages that are being moved this motor is designed to move these small packages at a certain speed and is rated to move this load well what happens if we get rid of the small boxes and instead put a jumbo package on the conveyor well the motor is still going to try to move the load but it's too heavy for the motor the motor will try and try to move that load but it will be pulling more and more current as it tries to turn eventually so much current will be pulled by the motor that it will overheat the inside and smoke that motor circuit breakers have a technology inside that prevents against overcurrent that technology is based around two thin strips of metal bonded together this is called bi-metal next to the bi-metal strip there is a trip bar well what happens the metal when it gets warm it expands and if we have two different types of metal bonded together it's likely that one side will expand faster than the other which will cause that strip to Bend so as we pass current through the bi-metal strip it starts to heat up from the current if the current becomes too great it bends towards the trip bar until it finally trips the circuit breaker this is called thermal protection okay now let's have a little sidebar conversation about how an electric motor functions here I've drawn a graph of current versus time when I start an electric motor from a dead stop you get a big rush of current right at the beginning to get the motor up and running after that big rush of current it levels out and runs at about a hundred percent speed at a hundred percent speed it will pull a certain amount of continuous current until you turn the motor off this current at full load or full speed is called full load amps or Fla that big spike of current you get when you start the motor is called in rush in rush is usually about six to eight times the full load amps of the motor you going back to the circuit breaker we've got magnetic protection built in and thermal protection built in magnetic protection reacts to a very high level of current very quickly so it is suited to protect against short circuits thermal protection is a slower reacting protection and correlates to the amp rating you see on the front of the circuit breaker this is to prevent against over current but what are molded case circuit breakers designed to protect they're hooked up to all sorts of things you have circuit breakers feeding light circuits outlets motors heaters coffee makers televisions the list goes on well since the circuit breaker doesn't really know what it's hooked up to it instead focuses on protecting the wire that feeds all those devices if it sees too much current going through those wires it trips and allows the wire to cool down well if you remember from before when we start an electric motor we get in rush at the beginning causing a much larger draw of current than usual that inrush current looks an awful lot like an overcurrent condition to a circuit breaker since it isn't specifically designed for motor protection fortunately there is a type of molded case breaker where the thermal protection has been removed but we're still left with the magnetic protection which still protects us against a short circuit and that type of circuit breaker is called a motor circuit protector or an MCP frequently you'll see Eaton's version called an HM CP with the H standing for high interrupting so if we get rid of the overcurrent protection provided by the Moldy case breaker we still need to find a way to protect the motor from an overload condition here we've got a contactor with our power coming in the top and out the bottom but now let's add a new component to the contactor this is called an overload relay the overload relay has three bi-metal strips inside that are designed to heat up at the same rate as the windings of the motor this is different from the bimetals inside the breaker because those bimetals didn't know to ignore the inrush when you start the motor when you pair an overload relay with a contactor we call this a starter now that we've covered motor protection and motor control let's put it all together at the top of the circuit we have our HM CP breaker or alternately we could use a fuse disconnect switch next we have the contactor followed by the overload relay lastly we have the cables connecting the overload relay to the motor in this setup the HM CP provides a way to disconnect the entire circuit from power as well as providing short-circuit protection since it still has the magnetic protection inside the contactor provides a means of controlling the motor and then finally the overload relay protects our motor from overload conditions that concludes our lesson on motor control but if you'd like to look for more training opportunities and resources please visit the electrical training group's website at WWE 10.com ford slash training you

Channel: EatonVideos

Views: 1,654,891

Rating: 4.8752642 out of 5

Keywords: 3807882799001, USA, electrical, youtube, MotorControl, public, English

Id: aml0VGzNXEo

Length: 15min 59sec (959 seconds)

Published: Tue Aug 04 2015