Why DC series motor should not be started without load?

[Manoj Sahu]

Today a professor of mine who teaches Electrical machines told us that a DC series motor should not be started without load. I wonder why is that so. Please provide a detailed explanation of this PF members. Thank you very much in advance.

 

[anorlunda]

A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed. This is called a runaway condition.

 

[jim hardy]

Never connect one to a load by a belt , either.
If your car starter fails to engage and you hear it speeding up, , release the key .

In a Runaway motor centrifugal force throws the armature wires out of their slots and they jam against the poles.

electric car propulsion motor - the guy had gearbox in neutral and "put the pedal to the metal". .

 

[Averagesupernova]

Once had a snowmobile that used a small v-belt that connected the starter to the flywheel. Naturally it finally broke. It wouldn't Rev high enough to destroy itself before even the slowest of mechanically inept would realize to release the ignition from the start position. What a tank that thing was. Not a piece of aluminum on it. Would sink like a rock in a pond.
https://www.google.com/search?q=joh...Xh6YMKHQUUC1IQ_AUIswQoAg#imgrc=O-CgEG7gkB1kDM:

 

[jim hardy]

Once i had to fix the starter on an Opel Kadet that'd oversped and slung a few conductors.

Every generator shop in town told me "Sure, we'll rewind it for you just bring it in, won't cost much" but when they saw it said "Cant rewind that you got to buy a new one for two hundred fifty bucks"
Now in 1970 my salary was $801 a month. $250 was more than a week's wages !
So i replaced the chewed up conductors by soldering in new wires , carefully sanding the cut ends to matching oblique angles so as to increase surface area at the splice.
Then i wrapped the whole repaired area with fifty pound test dacron fishing line(tournament grade) .
Then i soaked it with marine spar varnish to surround the new conductors both insulating and locking them into the slots, and put it back together.
It worked perfectly for years. Cost was about a dollar for some #12 solid house wire.

Not long afterward i had occasion to take apart my 1962 Chrysler starter.
Found inside it a little shunt field that some practical minded engineer had designed into prevent runaway.
Sigh. Another unsung hero, that guy .

old jim