Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Finding maximum current carrying capacity of a motor/cable

Status
Not open for further replies.
Z

zuq

Member level 3
Joined
Jul 29, 2011
Messages
67
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,288
Activity points
1,801
Hi guys,

I am a bit unclear on maximum allowable current in motors and cables so the question might sound a bit dumb. But any ways here goes;

I know cables can carry more current for short duration due to *thermal mass effect thing*, it takes certain time to bring the cable up to temperature i.e. thermal time constant.

Is it possible to work out motors and cables thermal time constant? and then determine how much current can my motor or cable carry for a certain time.

Articles explaining this would be of great help too.
 

Your limit will probably be a maximum insulation temperature, which varies by
material and its surroundings (open air > in-conduit, etc.). You have to work
back from mfrs' ratings or electrical codes, through the thermal calcs of
I2R heating and thermal transfer to ambient. Time averaging may help but
you can't do that for very long timescales where the temperature can rise
to steady-state within a "cycle"; here you need an idea of both the model
for current profile, and a thermal time constant.
 

The easy sign to watch for is whether it gets too hot to touch.

The factors affecting it are (listed from my memory):
* current flow
* duty cycle
* wire gauge
* winding density
* shape of waveform
* metal mass
* heat retention characteristics of insulation, varnish, wrappings, housing, etc.

Here is a table of wire gauges and ampacity. The specs appear to apply to wire in windings.

http://amasci.com/tesla/wire1.html
 

Maximum allowable current in your system will be based on the motor specifications which should be on the machines design plate or date sheet. Size the cables based on the motor FLA, typically you want to keep the current density below 2000A/in^2 however for short lengths of cable the current density can be significantly higher since the resistance will be very low.

BTW, you have to be carefull when operating the motor if you exceed the FLA rating, realize that your damaging the winding insulation which will decrease the machine life. Also, the once the machine begins saturating increasing the excitation current will yield nothing more than increased losses reducing the efficiency of your system.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top