Motor Speed Control: VFDs, Dahlander, and Two-Speed Starts

Two-Speed Motor Starting Methods

Independent Windings Start

A two-speed motor with independent windings has two separate windings, each designed for a specific number of poles. The two windings behave as if they were different motors, each designed for a specific speed. The two windings must never be connected simultaneously; an interlocking mechanism between the two contactors is necessary.

Dahlander Start

The Dahlander start is a method that uses the same winding to achieve two or more speeds.

Variable Frequency Drives (VFDs)

A Variable Frequency Drive (VFD) is a power electronic device capable of modifying the motor's frequency in Hertz. When feeding a conventional machine, no special winding is needed. VFDs operate between minimum and maximum frequencies, allowing for easy regulation throughout the range. Drives often include a monitoring mode that permits the observation of parameters and magnitudes while the motor is running. Some models can exceed the synchronization frequency of the electrical grid.

VFD Programming Methods

VFDs have a set of parameters. Programming is done in various ways:

1. From a small operator terminal panel.
2. By external programming, which allows advanced programming.
3. From a personal computer via a cable connection and parameterization software.

Characteristic VFD Parameters to Adjust

The following are characteristic parameters that must be adjusted and programmed:

• Factory Setting: Resets all drive parameters to factory default values.
• Acceleration Ramp: The time (in seconds) required for the motor to reach the pre-programmed speed.
• Deceleration Ramp: The time (in seconds) required for the motor to slow down and stop.
• Maximum Speed: The fastest speed the motor can rotate.
• Minimum Speed: The slowest speed the motor can rotate.
• Speed Presets: Predefined joint velocities for the operator.
• JOG Program: Motor operation using impulses. This requires a preset speed and a braking input.
• Logic Braking Input: Sets the type of motor braking.

Variable Frequency Drive Connection Modules

VFD connections are typically divided into three main modules:

1. Power Module: Connects the grid, passes through the drive, and then connects to the motor. Drives can handle single-phase or three-phase input, but the motor output will always be three-phase.
2. Control Module: Contains logic inputs for connecting switches and buttons. Used primarily for starting and stopping the motor.
3. Speed Reference Module: This is typically an analog input with three terminals, connected to a rotary potentiometer. The manufacturer specifies the required resistance value (in kOhms).