55 kW motor..
- Thread starter theripley
- Start date
longshanks
Member
I suspect you will be overloading the output. The starter will more than likely draw more than that on pull-in. I would suggest an external relay with at least a 10 amp contact rating. It will depend on the voltage that you are using for the starter coil. An external relay will be a lot cheaper to replace than the micro if there is a failure.
longshanks
Member
Yes, even at 200vac, the inrush on the starter coil of the mag starter will more than likely exceed your mico PLC output capability. While it seems excessive to use an interposing relay, it's still much cheaper to use it rather than have your PLC output fried. A mag starter for that size of motor will have a very hefty input draw. The extra relay, with 10A contacts, protects your PLC.
most micro plcs have there individual relay output contacts rated in mA. example AB Micrologix 1000 individual relay contact is rated at 500mA, sevral others are rated lower or higher, highest i have ever seen is an add-on module for AB micrologix 1100 had relay contacts rated for 5 amps. If your skeptical pull in the starter with a power supply and use an amp meter set to hold peak, so you can see the max current draw of the starter. If its greater than 2 amps, you should use an external relay at a lower control voltage than 220VAC if avaible. a 24VAC or VDC relay, which is rated for 220VAC might cost 20-30$ max. As stated above definatly cheaper than replaceing a plc due to a burned out relay output.
-g
-g
longshanks
Member
While George makes a good point, keep in mind that the lower the control voltage, the higher the amperage required to pull in the relay. With a 55kw motor, at 460vac, you are looking at a size 4 starter or larger. I've seen a size 2 (yes, in poor condition) draw 22 amps on the starter coil on pull in. It blew a 5 amp fuse like a flash bulb !! A large 24vdc or 24vac relay with a rating of at least 10 amps on the contacts should be okay with a new starter. If the starter is questionable, I'd look for a relay with a 20 amp contact rating. I wouldn't, in any way, connect the starter coil directly to the PLC output. A larger relay might, in itself draw to much at 24v.... Whichever, put protection on your PLC output in the way of a fuse and/or spike protection.
Like George says, a $20-$30 relay is a lot cheaper than the PLC....
Like George says, a $20-$30 relay is a lot cheaper than the PLC....
TConnolly
Lifetime Supporting Member
With a motor as large as 55KW another option that you might want to seriously consider is using a solid state motor soft starter instead of starting the motor across the line with a contactor. The control inputs on most softstarters are high impedence (low current) so you could drive the soft starter directly from the PLC output.
I 2nd Alarics suggestion.
Since VFDs have gotten smaller and a lot cheaper nowadays, I'd look into a VFD to run the motor.
Then you get all kinds of protection and diagnostics build into the VFD itself.
55KW motors aren't cheap to repair or purchase new!
leitmotif
Member
While VFD and soft starts have lots of benefits I think it is important to know what the motor is driving, how often started stopped, reversing and if variable speed is needed wanted advantageous.
To steal someone elses line
"not wise to do it just cause you can".
Dan Bentler
To steal someone elses line
"not wise to do it just cause you can".
Dan Bentler
Or to quote Peter N's Good Engineering Rules #3:
Just because you can doesn't mean you should
TConnolly
Lifetime Supporting Member
I agree with the general premise that just because you can doesn't mean you should. However considering the modern soft starters have become so economical that they can compete head on with NEMA starters I don't see any reason not to use them, unless for some reason direct across the line starting is mandatory (I can't imagine why it would be), especially on larger motors and especially when you consider the large demands direct on line starting places on infrastructure. IOW, with the variety of start profiles offered for modern soft starters, for a larger motor you can, and probably should.
Last edited:
Hi all,
Is soft starter & vfd the same? I have only used vfds before but not soft starters. On this application, we need not to change speed because the motor we will control is a cooling tower fan motor & supply pump. It may sound extravagant to use plc on a simple start-stop application, but its a business strategy as well that our client be able to know that we have capability in plc programming and not only equipment installation & maintenance. With this, we may be ableto get other automation related projects with them
Is soft starter & vfd the same? I have only used vfds before but not soft starters. On this application, we need not to change speed because the motor we will control is a cooling tower fan motor & supply pump. It may sound extravagant to use plc on a simple start-stop application, but its a business strategy as well that our client be able to know that we have capability in plc programming and not only equipment installation & maintenance. With this, we may be ableto get other automation related projects with them
Is it usually use in big motors to prevent big current surges? Because on small motors we only use contactors across the line.
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