T he market for variable frequency drives (VFD) for AC induction motor is found in most applications where DC drives were traditionally used and where belt and pulley or gears are used. Low cost and maintenance of AC induction motors were the factors that initially influenced the proliferation of AC drives. With increased market share and the advent of new generation of IGBTs, the cost of drives also has come down.
Simple applications have a potentiometer connected to the drive which is turned manually by an operator to vary the speed, while in automated applications logic controllers give a voltage or current command to the drive to control the speed. In more complex applications, feedback is taken from an encoder to achieve precision in speed control or synchronisation with another motion. This kind of feedback is used for position control in servo drives that use permanent magnet motors (BLDC or PM Synchronous) in place of induction motors. Presently energy saving in compressors, pumps and fans in HVAC application is an emerging market for AC drives. Most of these applications can be served by standard products. However, when an OEM uses AC drives in larger numbers, then custom designed drives could be boon to them, as demonstrated by the application stories below.
A manufacturer and supplier for AC drives received a curious request from a customer who had recently purchased a standard 1.1 kW AC drive: he asked if the appliance could be stripped of its cover, display and heat sink! A visit to the customer revealed that the standard product could not be mounted on the machine they were building. But they had already made the moulds and dies for the machine, and discarding or changing those would be costly for the small manufacturer.
The drives manufacturer presented a solution in a newly designed drive that was sized and shaped to fit into a recess in the proposed machine, and the machine’s body formed a part of the heat sink. In addition, radio controlled enable and controls for a small pump were also provided.
Meeting international norms
In another application, VFD was needed as part of a product that would be used in European homes. This product needed to conform to European Union standards for current harmonics. Hence to lower the harmonics in the provided VFD, a circuitry for power factor control (PFC) was added to the drive and input filters were designed to meet the European Marketing Confederation (EMC) standards.
Since the volumes were considerable, the drive was designed without keyboard, display and in open frame to bring down the cost. Additional advantage was achieved as the PFC circuit was designed to make the drive work from supply input range of 90V to 260V. This enabled the customer to use the same drive for his US market that used 110V as supply voltage or to any other market that used 220V. An additional advantage was reduced inventory and service costs. For an American customer who sold domestic booster pumps that are used to maintain the pressure of the water from municipal line, the drive was designed to have only the features that required by the pump and a ready interface to the pressure sensor. The use of this drive also saves power compared to on/off pump control. The salient feature of this model of drive was that this could be used with a three-phase or single-phase AC induction motor and this feature could be configured at the time of installation – once again reduced inventory. The variable frequency drive for single phase two-wire and three-wire configuration was a unique solution.
A novel solution
A winding machinery customer was given a unique product where one machine uses multiple drives. The customer’s electrical system was completely redesigned and economical and easy-to-use inverters that work common DC bus were supplied to them. Elimination of EMI noise filters for individual drives helped to reduce the cost and the real estate for the inverter was little compared the regular drive. The inverter had all different types of I/O required. In a variation in winding application, the drives were designed to implement electronic gearing using AC induction motors or brushless DC motors. These drives were equipped to interface with high resolution encoders.
Counting on benefits
OEM drives have to work in challenging environment, electrically and environmentally. A typical customer’s requirement will have to be fulfilled in terms of size, shape, features, I/O and conformity to electrical, safety and environmental standards. Custom drives would be found more suitable for applications in appliances, energy saving, transportation and machinery and in products like air conditioners, washing machines, grinders, fitness equipment, textile machinery, pumps, fans, compressors, mixers, conveyors, etc.
Today, customers want innovations in their products to cut costs, improve the features and do not want to compromise on their margins. An individual, customer-oriented approach helps achieve these targets as well as translates into handsome profits for the supplier.
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