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Why should a DC series motor never be started on a no-load?

The DC series motor attains dangerously high speed when we run it on no load. The main reason of over speeding is that at no load the flux produced by the field winding is very less and the reduced flux cause over speeding of the motor. The speed of the motor is inversely proportional to the flux.

Many people may often hear the advice not to start at no load when they first buy or use a DC motor. Do you know why this is? If you happen to have questions about this issue, I hope the next introduction will be helpful to you.

The first reason is that when running at no load, the DC motor will reach a very dangerous high-speed state. When a DC series motor starts at no load, the magnetic flux produced by the excitation winding is very small, and since the magnetic flux is inversely proportional to the speed of the motor, at this time, the motor will over speed. If the motor is in an over-speed condition, it will create a serious vibration, and this violent vibration may accelerate the friction of the motor, leading to damage to the motor flying apart, and even threatening the safety of other equipment and people around the risk.

DC Motor at the no-load start, the initial current consumed by the motor is much higher than the current consumed by the motor when it is running under normal conditions. This initial high current can damage the motor's armature windings and commutator. The armature winding, which is responsible for generating the magnetic field that rotates the motor, can become overheated and damaged due to the high initial current. The commutator, which is responsible for switching the current in the armature winding, can also be damaged by the high initial current.


In summary, the no-load starting of DC series motors can easily damage the motor and is also a safety hazard. To prevent this from causing you problems, you should make sure that the DC series motor is running under load when you start it.

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