Whitepapers

Variable speed drives, due to their operating principle, produce a series of unwanted secondary effects, such as: motor winding isolation stress, bearing stress, acoustic switching noise in the motor and electromagnetic interference. In most applications these effects are at an acceptable level – but in some cases these effects need to be mitigated.

Electromagnetic interference (EMI) is the degradation of the performance of equipment caused by electromagnetic disturbance. An example of EMI, is the noise on a microphone, if there is a cell phone next to it doing a handshake with a communication tower to process a call. In this example the cell phone is the source of the interference and the microphone is the victim equipment.

Potential ways to save energy can be found in almost all sectors, like building services, conveyor belt systems or chemical processes. The life cycle costs of an application can be roughly divided into four parts: the initial investment cost, the cost for the operation and maintenance of the application, the cost of the energy required by the application and the disposal cost at the end of the life cycle of the application.

Machines can create potential or kinetic energy which we want to remove from the machine. The dynamic characteristics of some loads require 4-quadrant operation. A reduction in the stator frequency (and voltage) by the AC drive allows the motor to act as a generator and convert mechanical energy into electrical energy. Motors connected directly to the mains deliver the braking energy straight back to the mains.

The major technical characteristics of a motor are found on its nameplate. The information shown is very relevant for the electrical installer because values for voltage, frequency and full load current are given, but important information for the mechanical design is missing and can be found in the datasheet, catalogue, or by direct contact to the motor manufacturer.

A general overview of the standard current control principles for Danfoss AC drives is illustrated in Fig. 3.26 Basic principles of current standard AC drive from Danfoss. The PWM switching patterns are calculated for the inverter using the selected control algorithm.

Since the late 1960s, the AC drive has developed at a tremendous rate. Major advances have been made thanks to developments within the fields of microprocessor and semiconductor technology and the associated price reduction. However, the basic principles of the AC drive remain the same.

The synchronous motor is defined by the fact that the rotor rotates at the same speed as the magnetic field created by the stator windings. The design of the stator is in many cases similar to that of induction motors, with distributed windings. Some manufacturers use concentric windings (in slot) which enable a more compact motor design and require less copper. The energy savings achieved by the reduced use of copper are however often eaten up by additional losses, which result from harmonics in the air gap flux caused by the construction.

An electric motor is an electromechanical device that converts electrical energy into mechanical energy. The reverse process of producing electrical energy from mechanical energy is performed by a generator.

In 1968, Danfoss was the first company in the world to commence mass production of AC drives, for variable speed control of three- phase induction motors. Today AC drives are an increasingly important component for optimizing motor operation, and the system attached to the motor.