Post by thread:[EE] AC Motor control

All, I have been asked to look into controlling (turning on and off the power) to a couple of AC motors. It is not clear yet what size these will be but at this time they are estimated to be less that 3/4 Hp. This will be used as part of some animation at a fund raiser for a local charity so it needs to be safe but it does not need to be "production quality". As it stands right now the plans are that the motors will be used to drag a small cart (think of a 1/2 scale roller coaster car, without people) up a track and then release it at the top where it is free to roll back down the track. I know that the inrush current will be significantly greater than the nominal running current but that is about where my knowledge ends. Lets say for a 1/2 horse power 120v motor: - What would be the current carrying capabilities necessary for the relay? - I assume that I will need some form of over voltage protection (like I would use on a DC motor). Is that correct? - What are the issues that I need to be aware of? - Any web sites that you can point me to? Thanks for any pointers you can provide. Phil

Check this reference out http://www.nipsco.nisource.com/elecstd/er/er13560.pdf Looks like your are talking about a single phase motor. Characteristics for motors very depending on the type of motor ie a capacitor start, capacitor start and capacitor run etc. You can also get motor rated fuses that allow a brief surge of current during start up but give protection for a prolonged current draw as would happen with a locked rotor. Google has lots of hits for "Motor start current" _______________________________________ Roy Tauranga New Zealand _______________________________________ -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.308 / Virus Database: 266.10.4 - Release Date: 27/04/2005

Use a motor contactor/overload designed for this. Best prices I've seen is at whttp://www.automationdirect.com Probably need a limit switch to shut the motor off when it reaches the top of the ride, a start switch to enable it and of course an emergency stop switch to shut it all down in case of a problem. Probably a second limit switch at the bottom to know its in place and ready to go. Put it all in an enclosure, with a MCP on the front (motor circuit protector or breaker) and for under a few hundred be ready to roll. --- Hopkins <spam_OUTrdhopkinsTakeThisOuTihug.co.nz> wrote: > Check this reference out > www.nipsco.nisource.com/elecstd/er/er13560.pdf {Quote hidden}> > Looks like your are talking about a single phase > motor. > > Characteristics for motors very depending on the > type of motor ie a > capacitor start, capacitor start and capacitor run > etc. > > You can also get motor rated fuses that allow a > brief surge of current > during start up but give protection for a prolonged > current draw as > would happen with a locked rotor. > > Google has lots of hits for "Motor start current" > > > _______________________________________ > Roy > Tauranga > New Zealand > _______________________________________ > > > > -- > No virus found in this outgoing message. > Checked by AVG Anti-Virus. > Version: 7.0.308 / Virus Database: 266.10.4 - > Release Date: 27/04/2005 > > > --

Thanks for the reply but I think I may not have asked the right question or more than likely I do not understand the answers. What I was thinking of doing was to use a PIC to control the sequencing. As a person enters the room and a motion detector signals the PIC which turns on a spot light and releases the car that is parked at the top of the tracks. The car will roll down the tracks, driven by gravity, and "crashes" into a barricade with the appropriate lights flashing and sounds (Also controlled by the PIC). After a few seconds the spot light is turned off and the car is dragged back to the top of the slope by the motor to await the next trigger. At the top and the bottom I will have limit switches to ensure that the car is where I think it should be. I will most likely add redundant switches at the top to ensure that the car is not pulled over the top. So, -playing sounds on queue I can do. -turning on and off lights I can do. What I do not yet fully understand is how to safely start and stop the AC motor. I was thinking of using a relay but the ones that I have are rated at 15A @ 120v. Based on the reference that Roy provided (If I am reading it correctly) that would only be good for a 1/6 hp motor. It will be geared down but I doubt that it would be powerful enough to drag the car back up. I have Googled for hours but the sites all assume that you know what you are doing. Is there a web site "Motors for Dummies"? -How do I decide what size motor to use? (Assuming a 50 lb car and a 50% slope) -What do I need to do to start and stop the motor safely? -We are also thinking of using an electronic clutch as the release mechanism for the cable between the motor and the car. Then we could just leave the motor running and use the clutch only. Any ideas or thoughts are gratefully appreciated. Phil alan smith wrote: {Quote hidden}>Use a motor contactor/overload designed for this. >Best prices I've seen is at whttp://www.automationdirect.com > >Probably need a limit switch to shut the motor off >when it reaches the top of the ride, a start switch to >enable it and of course an emergency stop switch to >shut it all down in case of a problem. Probably a >second limit switch at the bottom to know its in place >and ready to go. Put it all in an enclosure, with a >MCP on the front (motor circuit protector or breaker) >and for under a few hundred be ready to roll. > > > >--- Hopkins <.....rdhopkinsKILLspam@spam@ihug.co.nz> wrote: > > >>Check this reference out >> >> >> >www.nipsco.nisource.com/elecstd/er/er13560.pdf > > >>Looks like your are talking about a single phase >>motor. >> >>Characteristics for motors very depending on the >>type of motor ie a >>capacitor start, capacitor start and capacitor run >>etc. >> >>You can also get motor rated fuses that allow a >>brief surge of current >>during start up but give protection for a prolonged >>current draw as >>would happen with a locked rotor. >> >>Google has lots of hits for "Motor start current" >> >> >>_______________________________________ >>Roy >>Tauranga >>New Zealand >>_______________________________________ >> >> >> >>-- >>No virus found in this outgoing message. >>Checked by AVG Anti-Virus. >>Version: 7.0.308 / Virus Database: 266.10.4 - >>Release Date: 27/04/2005 >> >> >>--

> I have Googled for hours but the sites all assume that you know what >you are doing. Is there a web site "Motors for Dummies"? >-How do I decide what size motor to use? (Assuming a 50 lb car and a 50% >slope) >-What do I need to do to start and stop the motor safely? >-We are also thinking of using an electronic clutch as the release >mechanism for the cable between the motor and the car. Then we could >just leave the motor running and use the clutch only. > >Any ideas or thoughts are gratefully appreciated. I guess you are using a motor that large to get the car back up the ramp in a reasonable time, but it does also bring up the thought, how do you return the pull hook to the bottom, or do you have an endless chain always going up? My thought was to use a winch as fitted to off-road 4WD vehicles to get themselves out of a bog or awkward spot, but that would probably be too slow, and the power supply you would need would also be hefty. At the end of it all, I think you just have to find a big enough contactor for the motor - any electrical supply warehouse (not DIY shop) should be able to supply one. You may need a smaller relay driven from the PIC to activate the contactor.

Phil Keller wrote: > -How do I decide what size motor to use? (Assuming a 50 lb car and a 50% > slope) Actually the slope has nothing to do with it. The relevant factors are the weight of the car, the total height it must be lifted, and how fast it must get there. Since you only supplied 1 of the three, I'll make up numbers for the other two as an example. The car weighs 50 pounds, which is about 23Kg. Let's say it needs to be raised up 5 meters in 10 seconds. The upward force on the car needs to be 23Kg x 9.8N/Kg = 225N. Times 5 meters lift is 1127 Joules. Divided by 10 seconds is 113 Watts = .15 horsepower. Assuming 80% motor efficiency and another 50% effeciency in the remaining mechanical system, .38 horsepower is needed. So a 1/2 horsepower motor can do this just fine. A 1/3 HP motor is close and maybe OK if the mechanical efficiency is not as bad as 50% or you're willing to wait a little longer. A 1/4 HP motor is enough if you're willing to wait 20 seconds instead of 10. Of course this all assumes proper gearing is in place to get the specified lift time at the optimal speed of the motor. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com