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.
Thanks
Best Answer
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.