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Russell Bull
08/26/05 03:37
Read: 1072 times
Melbourne
Australia


 
#100126 - Mechanical things
Responding to: U Kaz's previous message
Being a mechanical engineer - I would have thought your first choice would've been a relay! First thing to note about an electric motor - startup current is very high. As to how long this exists depends on how fast it spins up. If the motor seizes then we get high current until the fuse blows or smoke happens.

With a triac there's two main things that will kill it - voltage spikes and excessive i2t (change of current vs time). With a relay the failures are caused by arcing and mechanical wearout. The manufacturer specifies the expected life of the relay under certain conditions so wearout is reasonably easy to estimate.

If you really want to use a triac, lets assume the startup is 10 times the running current (I'm not sure of the startup characteristics of a single phase motor). It may be more than 10 times. So we need a triac rated for 30 amps so a bit of safety. Triacs also have loss - about 1-2V as a guess. So heat loss at 2.9A would be about 6W worst case. So for a triac such as a BTA41-600B you will need a heatsink as the temp rise (at a rough guess) of 50degrees C per watt will give you a case temperature of 300degrees! Even at a loss of 3W thats still 150degrees - ouch! To cope with voltage spikes you'll need a snubber circuit- something like a 100nF X2 capacitor and a 100R 2W resistor would be a start. To cope with i2t is the hard part. One would normally use a high speed semiconductor fuse which are a bit more expensive that your average 3AG slow blow. i2t is an issue when dealing with an overload. Relays normally just take the kick. There's heaps of app notes on the web regarding triac design - maybe a read of these will convince you that the humble relay is probably much easier.

List of 23 messages in thread
TopicAuthorDate
What is a safer?      U Kaz      08/25/05 01:52      
   Think about it      Russell Bull      08/25/05 02:34      
   hmmmm      U Kaz      08/25/05 03:24      
      Why not just use a relay?      Russell Bull      08/25/05 03:30      
         Agree but....      U Kaz      08/25/05 03:37      
            Hmm...      Andy Neil      08/25/05 04:08      
         Relay      Andy Neil      08/25/05 04:13      
      philips      Abhishek Singh      08/25/05 09:47      
   No buts      Russell Bull      08/25/05 05:54      
      Is it rocket science then?      U Kaz      08/25/05 06:09      
         What's non-ideal?      Andy Neil      08/25/05 06:49      
            even with a triac you still need a relay      Erik Malund      08/25/05 07:08      
            Relays      U Kaz      08/25/05 07:57      
               the life of a relay      Erik Malund      08/25/05 08:17      
                  Try this      Steve M. Taylor      08/25/05 09:53      
                  Contactors      Russell Bull      08/26/05 01:54      
                     motor specs      U Kaz      08/26/05 02:20      
                        Mechanical things      Russell Bull      08/26/05 03:37      
                     Yebbut      Steve M. Taylor      08/26/05 05:30      
               Use the math Luke      Neil Kurzman      08/25/05 10:00      
   Dumbass customers      Russell Bull      08/26/05 02:15      
      War Stories - New Thread?      Andy Neil      08/26/05 05:49      
         Since Craig is adept at moving posts, I'      Erik Malund      08/26/05 07:41      

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