Burnt out motors

[swanaldo]

I know that motors burn out but what i want to learn is the reasons why. What are the main causes and the science behind it. I know for instance if a motor is stalled its not good for it and can burn out. But understanding the burnout bit is what i want to know and also long term burn out. Any cleverclogs that can give indepth explanation most welcome.

[woody]

Volts * Amps = Watts ( power )
In an electric motor most of the Watts is converted into rotational energy and some goes into warming the motor windings.
A stalled motor however has no rotational component and becomes the equivalent of an electric fire.

[woody]

cut and paste:-

Temperature Rise

Excessive heat shortens the life of a motor. Motor losses dissipate as heat and are removed from the motor by the motor cooling system. The motor cooling system includes internal or external fans, fins, and even coolant-based systems for special application motors. While losses increase with the load, the cooling ability of the motor does not; therefore, the temperature of the motor increases with load. When a motor application overloads the motor or restricts air flow over the motor, inhibiting the fans and fins most commonly used to cool the motor, the motor temperature can rise over the designed limit shortening the life of the motor.

Overloading the motor, improper supply voltage (high/low), frequent start/stops, poor input power quality, restricting the cooling system and using variable frequency drives increase temperature rise. This heat destroys motors in two main ways:

Heat reduces the expected life of lubricants and seals. At higher temperatures, the soap component of lubricants that time-releases oil becomes sticky increasing friction losses, reducing the life of the bearing and leaking the released oil onto the motor windings. Lubricating oils deteriorate winding insulation causing premature winding insulation failure.
Winding insulation in motors is classified by its life expectancy at various temperatures. When the rated temperature is exceeded, the insulation breaks down at a faster rate resulting in premature failure.

[swanaldo]

Excellent. Thanks Woody

[mightymouse]

A low gear ratio will also cause a motor to run hot.................

Trev

[kane]

I thought that was a high gear ratio... e.g. a low gear ratio being one where the input speed is reduced by the gearing to provide a low output speed but much greater torque (so the motor would be doing less work)?

[swanaldo]

yeah trev

[grant_ploughbot]

Trevs right Kane - High ratio is the bigger the ratio. 15:1 being a higher ratio than 5:1.

[psycho_fling]

so in a waty running a higher voltage is ok if you have a higher gear ratio?

[mightymouse]

Alex - no, if you exceed the wottys input voltage it will expire, rapidly.

A high gear ratio ie; 15:1, will load the motor less than a low gear ratio ie 5:1. A low gear ratio does not allow the motor to reach its peak revs as quickly as a high ratio therefore generating heat from the excess current it will draw..........yeah Swanny