DC motor burns out at low voltage

[triden]

I am trying to understand why a DC motor will burn out at low voltages. This idea comes from my dad who is a mechanic. He says that people that always start their car with a half-dead battery tend to burn out their starters and weld parts inside it together from the heat.

Personally I think it has something to do with back EMF, but I am not sure. In a resistive load, less voltage across a load means less current flows right? Shouldn't this be the same with a car starter? Or is there something about an inductive load that allows more current to flow.

regards,
Chris

 

[LURCH]

I believe that you are right about the counter EMF; the lower voltage of a half-charged battery causes the motor to turn more slowly than it normally does, and this prevents Counter EMF from building up, and excessive current travels through the motor. This causes the motor to burn out before its time.

 

[Ivan Seeking]

Although it may not apply to all motors, there is also the issue of cooling. In industry, there is something known as the half-double rule of thumb: A motor may be run at as little as half, and as much as double the rated speed. Of course any particular application still requires proper engineering, but the half-double rule of thumb is commonly referenced.

The biggest concern about running most motors too slowly is that the speed of the motor determines the air flow that it gets for cooling. If a motor is run too slowly, it doesn't get enough air flow, it overheats, and the motor is damaged. However some motors - perhaps some starter motors, which have a very low duty cycle, and some specialty motors - may depend only on heat sinking to provide the cooling.

 

[Averagesupernova]

I've noticed that trying to start some motors with a low battery voltage will cause the solenoid to stick and the starter will continue to run after the ignition switch is returned to the off position. I believe it to be due to the fact that the solenoid does not slam into its contacts quickly enough and the contacts spend to much time 'halfway'. They heat and partially weld together. Also, when the contacts DO make, the voltage is lowered that much farther which does not hold the contacs tightly enough together to form a low resistance connection. The result is heating. It's a theory anyway...

 

[Naty1]

In a resistive load, less voltage across a load means less current flows right? Shouldn't this be the same with a car starter? Or is there something about an inductive load that allows more current to flow.

You are correct regarding a resistive load.

A starter is designed to work at a certain power which is voltage times current. When battery voltage is low, the motor still wants its power and so pulls more current than it otherwise would since voltage is low. BUT! Because a motor which cranks slowly is using less power, I'm not at all sure what typical relationships really are...testing is the only way to know for sure.

Whatever the precise reason, it likely varies with dc motor designs...some will be capacitive loaded and some inductance loaded. My ac swimming pool motor, for example, has a BIG capacitor within. Starters on big diesel engines do not have capacitors, but they are D.

As you may know, maximum power is delivered when voltage and current are in phase...how this relationship is disrupted in a dc starting motor is not at all clear to me...

I searched online 'DC STARTER DESIGN' and got a lot of hits...:

At: http://www.electricmotors.machinedesign.com/Reg_SubForm.aspx they say...

"...The content deals with the following product types of motors: brushless, brush, fractional hp, frameless, linear, stepping, subfractional hp, synchronous, universal, ac, dc, servo among others..." Dc motors may be armature or field controlled ...

Your question is an intersting one, but requires practical application/design or failure testing experience to answer...

In the past I have sometimes contacted the manufacturer of a product and said "I bought your model ABC widgit and am interested in knowing..." As a supposed customer you'd be surprised how helpful some tech people can be..they don't all that often get to explain what they know...give it a try..."