ok. i'm designing a hoist. let's say i have a 400hp 3-phase motor. i couple it to the lifting cable with some gear differential. then i start lifting loads. for loads from zero up to the rated torque the speed will be close to the synchronous speed, (according to the given speed-torque curve). above this load, the speed will rapidly decline and the motor will stall.In other words, for the lifting range 0kg up to MAXkg the lift speed will be more or less the same.So, i shouldn't need a VFD right??
The speed will be more or less the same up to the rated torque. Above rated torque, the drop in speed will be greater, but the speed will not decline rapidly until the breakdown torque is exceeded. That will typically be more than 200% of rated torque. You need to consider what happens when you start the motor. If you start by simply connecting to a full voltage power source, the motor will draw up to 600% of rated current or so for a couple of seconds and accelerate quite rapidly. That puts a lot of electrical stress on the motor and power system and a lot of mechanical stress on the motor, gearing, cable and load. A hoist might need to be started and stopped quite frequently. A VFD would eliminate most of the stress and allow the load to be picked up and set down slowly and smoothly while still allowing it to be moved more quickly once it gets moving. There are other ways to reduce the stress of starting, but a VFD does the best job at the highest price. VFDs are often used for hoists. 400 Hp, is very high for a hoist, but the size doesn't matter much for discussion purposes. Re: Soft Start A soft start starts the motor with reduced voltage. Reducing the voltage reduces the torque that the motor can produce. The current is reduced, but the current is quite high in comparison to the torque produced. The motor must be allowed to come up to speed quite quickly. A VFD can produce rated torque at zero speed with close to the same current that is required to produce the same torque at full speed.
