Motor Sizing for a Conveyor with a Soft Start
A 12 m inclined conveyor lifts bulk material at 0.8 m/s up a 15 degree incline. Loaded mass on the belt is 400 kg, and the belt and rollers add 150 kg of moving mass. The drive is a 4-pole induction motor through a gearbox with ratio 20:1 and efficiency 0.92. Friction in the rollers is estimated at 250 N. The maintenance team reports the existing 7.5 kW motor trips on overload during start-up but runs fine at steady state. They propose fitting a larger 11 kW motor. The electrical engineer suggests a soft starter instead.
1. Calculate the steady-state power required at the belt and at the motor shaft, and judge whether the 7.5 kW motor is adequate for running.
Read the worked answer
Gravity component along incline: (400 + 150) x 9.81 x sin 15 = 550 x 9.81 x 0.2588 = 1396 N. Friction: 250 N. Total belt force = 1646 N. Belt power = F x v = 1646 x 0.8 = 1317 W = 1.32 kW. Motor shaft power = 1.32 / 0.92 = 1.43 kW.
So steady-state demand is only about 1.4 kW, far below the 7.5 kW rating. The 7.5 kW motor is more than adequate for running. The trip is therefore a starting problem, not a running-capacity problem.
2. Explain why the motor trips at start-up and evaluate the two proposed fixes.
Read the worked answer
Induction motors draw a high inrush current and produce high starting torque at low speed; the conveyor's breakaway friction and the need to accelerate the loaded belt demand more torque at zero speed than at running speed. The trip is a torque/current overload during acceleration, not a power deficiency.
Fitting an 11 kW motor raises available torque but also raises inrush current and may overload the supply and gearbox; it treats the symptom and wastes energy at steady state. A soft starter limits starting current and applies torque gradually, reducing the peak demand while keeping the correctly sized motor. The better judgement is a soft starter (or a variable-speed drive), not a bigger motor, because the steady-state calculation shows the existing motor is already oversized.