enter image description here I wants to understands possibility when torque overload occurs.

Suppose motor is pulling up load A through pulley.Now if extra load B is added to trolley.Now there is difference between load torque and electrical torque, motor speed momentarily decreases. Now motor control increase duty cycle and increasing the current and increasing the speed to SET Value.

But if current increases beyond max limit,We switch Off supply.Now motor will start rotating in reverse direction.It can gain very high speeds. and if motor is PMDC it can generate high voltage also.

My Question: what kind of damage it can do to a motor both mechanically and electrically? Any ways of reducing that.I read torque limiter can be used but what is best option when we have size limitation and we want the process to be automatic. Please correct me If I am thinking something Wrong.


  • \$\begingroup\$ You need a mechanical safety brake for any overhauling load. Nothing on the motor controller will help, it has already shut-down from an overload or fault. \$\endgroup\$
    – R Drast
    Dec 2, 2015 at 12:15

2 Answers 2


Hoisting applications such as this typically have a spring loaded brake that is released when the motor is energized. If the Control system responds to a motor overload condition the spring actuated mechanical brake brings the motor and pulley to a stop.

I've also installed Variable Frequency Drives (VFDs) on 3 phase hoisting applications. This requires ordering a constant torque drive and installing a braking resistor. When the Motor is called to come to a stop - the inertia of the hoisted load turns the motor until the brake is released. The coasting of the motor due to the inertia of the load results in generating voltage back into the drive. Installing a braking resistor gives the drive the ability to dump the generated power onto the resistor, which is dissipated as Heat. In my applications the combination of a mechanical brake and the braking action of the VFD has resulted in a quicker stop than the mechanical brake alone.

As far as mechanical damage - the motor brake and drive train should be designed to withstand a couple of times the dynamic forces that can occur within the system.

Mechanical Brakes will wear and need to be adjusted and replaced due to normal wear and tear.

As far as Electrical damage - again the Motor should properly fit the application. From an electrical damage standpoint - in dealing with a single or 3 phase motor - heat is what will damage the motor. Most VFDs and VFD rated motors have an integral heat sensor called a ptc. The VFD can be configured to sense the temperature of the motor and shut down the system if it gets to hot. In this condition you would be relying on the mechanical brake to bring the system to a safe stop.

  • \$\begingroup\$ The two applications I worked on had existing 15 HP 3 phase 480 Volt Motors with an integral brake. I converted the systems from a forward reversing starter to a VFD. \$\endgroup\$
    – Tinkerer
    Dec 4, 2015 at 23:11

The question really depends on the kind of motor used. The two real categories are brushed and brushless.

Brushless motors (induction and BLDC) are very difficult to damage with and over torque or stall condition. If a brushless motor has sufficient cooling it can remain stalled indefinitely.

Brushed motors are the ones that are easily damaged by an over torque or stall condition. The resistance of rotor in a brushed motor is a sum of the actual resistance and the back emf. The back emf increases with the speed of the rotor. As the motor slows down the back emf resistance becomes less and less dominant. This drop in resistance increases the current through the rotor. This extra current generates more heat. If the motor has sufficient cooling and the current is within the limits of the brushes the motor will tolerate it. As the motor slows down further there will be a tipping point where the back emf resistance is so low that the current through the rotor is too much. This current makes more heat than a cooling system can reject and overloads the brushes. When the motor stalls only the resistance of the rotor is left which is normally less then 10 ohms of effectively a short and bad things will happen.

With brushed motors, normally and over current condition signifies overloading and trips some type of shut down to protect the motor.

If lifting a load overhead then there should be a mechanism is place that can hold the load without intervention of the motor. All commercial cranes and lifts have electro-magnetic brakes that engage when the motor stops or power is lost.

To protect the motor the terminals of the motor can be shorted together. This effectively forms a brake. As the motor spins it will generate a potential, that potential is fed back into the motor but in a reverse direction to the spinning. This braking action can slow the motor down to a low speed but never to a complete stop. A good H-Bridge motor controller will have this feature. After a safety shut down engage the brake.

  • \$\begingroup\$ Agreed on all the parts but I wanted to know what damage could be done after we have shut down supply. Since In the condition shown above rotor of the motor will gain very high speed. \$\endgroup\$
    – jokeroor
    Dec 2, 2015 at 13:18
  • \$\begingroup\$ @jokeroor Sorry, didn't catch that part. I updated my answer. \$\endgroup\$
    – vini_i
    Dec 2, 2015 at 22:29

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