Allowable starting time for small DC motor

[PAP]

I've tried to find the safety starting time limit for a small DC motor and got some idea about the motor thermal time constant. Not sure is it appicable during starting period or not as the cooling is normally low. If it is yes, then the final heating = ( I start/I rate)^2 * (1-e^(t start/ time const))
anyway the small motor time constant is also difficult to find. Do you have any idea for typical value for that time const according to motor size (esp for the small one let say 1.5 hp)?

 

[waross]

Series, shunt, compound, PM, universal? Normally the manufacturer provides safe starting information.

Bill
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"Why not the best?"
Jimmy Carter

 

[mikekilroy]

small motor time constant is also difficult to find. Do you have any idea for typical value for that time const according to motor size (esp for the small one let say 1.5 hp)?

thermal time constant is a function of mass of motor, so if you just compare similar size motors to yours you will have a good idea what YOUR thermal time constant is.

Keep in mind if your I start/I rate is much more than 1.5 then the thermal time constant of the COIL itself will override the thermal time constant of the mass of the motor, as the coil will not be able to get the heat out into the motor mass to dissipate before it overheats. Very complex relation ship.

any small dc motor rated 1.5hp is probably around 3-4" dia x 5" or so long: typical TC for this size motor is probably around 20 minutes. TC of coil is probably 10 seconds.

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