Motor Starting Torque Questions

I am used a 20hp VFD in Hoist application (415v three phase). In this application there is a problem that in the time of starting main motor Drum is Roll back first. So to avoid this Roll back problem it is any possibly to increase the output torque of the VFD by changing the Rotor resistance of motor in VFD motor parameter group. What happen if I increased or decreased rotor resistance in VFD motor parameter group?

One easy solution would be to introduce an encoder to allow maximum torque all the way down to zero speed. Another option is to allow the motor torque to speed set point to reach a certain point before releasing the brake (assuming the motor has a brake). If it is open loop then voltage boost is an option to allow the motor to hold the loads close to zero speed.

Insert Vector Control and perform a vector control circuit test, manage to install holding brake through a digital output and test it with different times. If hoist is to be lift it all speed ranges, insert encoder, Vector Control just works after 10% of setpoint with no encoder. Unless you are using series resistance with slip ring motor, do not mess with rotor resistance, just do it through the circuit test. Some VFDs also have an additional torque through moment of inertia calculation, but that´s another subject.

For open loop drive, a combination of boost, and ensuring the brake is not released until you have built up sufficient flux in the motor will help. Do this by switching the brake when the VFD is modulating at around 4 Hz. GoHz VFD has brake management parameters, to assist with this, maybe your VFD has something similar.

Use vector control and make sure that brake opens after there is sufficient flux. I have needed boost only in rather dynamic applications and normally 2 Hz has also been enough for brake opening if the dimensioning of the drive (VFD+GM) is done properly.

The best path forward would be a drive using "Torque Proving" control algorithm. All the math and functionality have already been designed in. All you will have to do is turn it on. Torque proving will ensure the motor is actively producing torque prior to releasing the break. This eliminates all the associated power transmission failures that could lead to a safety issue. This can be accomplished without an encoder, however, adding an encoder unlocks huge potential benefits. IE the ability to alarm and do a controlled staged decent if the mechanical brake system is failing, etc.

As stated already, you need to delay the release of the brake until the motor has enough torque (flux) to hold the load. When stopping the hoist, you will want to go to zero speed, hold torque, set brake then disable the drive. There are several built in features for this in many VFD manufacturers. GoHz drives have this built in as well. It also has a safe brake control. I've used this on over a hundred applications. It is highly recommended to use an encoder to produce the proper holding torque independent of the load and safe operation.

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