Issues in VFD Applications - APQI

1 Issues in VFD Applications Assistant Professor in Electrical Engg., , Anna University, Chennai -25 E-Mail: Cell: 9841363144 Contents Introduction Applications Issues Technological solutions Case studies 2 The Motor market Nearly 5 Lakhs LT AC motors get added to the Indian industry use, each year The overall market size for rotating machines is `6300 crores in India- (IEEMA year 2011) India s import of motors is `690 crores and export is `115 crores worth- (IEEMA year-2011) The world motor market is set to grow by 30% in the next 10 years with the advent of new generation IE3 (premium efficiency)

2 Motors from Jan,2014 4 Motive energy Motors account for nearly 73% of industrial energy 2/3rd of the industrial motor Applications are centrifugal (utility) machines 1/3rd of the industrial motor Applications are for process and material handling Major motor Applications 5 VFD Market forecast Growing demand for electrical efficiency and increasing investment in modernization of infrastructure will drive the global VFD market to USD18,854 Million by 2017, with a CAGR of from 2012 to 2017 Variable Frequency/ speed Drives (VFD/VSD) Market - By Type (AC, DC, Servo), Voltage Range (Low, Medium), Power Range (Micro, Low, Medium, High)

3 And Application (Pump, Fan, Compressor, Conveyors & Others) - Global Market Trends & Forecast to 2017 6 Existing VFD Technology VFD uses power electronics to vary frequency of input power to the motor thereby controlling motor speed . Law of Affinity & Flow control table Major energy saving Applications Centrifugal machines (variable torque loads-2/3rd, ,75% of ) follow the law of affinity speed Volume Power 100% 100% 100% 90% 90% 77% 80% 80% 51% 70% 70% 34% 60% 60% 22% 50% 50% 13% 40% 40% 6% 30% 30% 3% Law of Affinity : Flow speed Torque Speed2 Power ( Energy consumption ) Speed3 Note: Energy saving through speed variation is phenomenal.

4 Centrifugal loads-Fans, pumps, compressors 9 Benefits of VFD Application examples 11 Scalar or Vector Scalar Drive Constant V/f pattern No true torque control Not suitable for <1:5 speed if your application needs accurate control below 10Hz, scalar may not work for you. Vector Drive Vector drives come in 2 types, Open Loop and Closed Loop precise control of speed or torque from exact vector of V & f Closed Loop Vector Drive uses a shaft encoder & can develop full torque at zero speed Open loop or sensor less vector drive takes the feedback within the VFD.

5 Not good for cranes and hoists 12 Control Technology Control Algorithms Voltage and current control PWM ---------------------------------------- Open loop Closed loop -Scalar V/F -Vector (FOC) Direct Indirect Sensorless DTC AI techniques Control Implementation DSP,c, p PLC, DCS, other n Adjustable speed drive VVVF Motor DSP Board Electronic gate drive circuit Control EstimationAlgoritm load Inverter Rectifier Torque & speed reference Voltage and current sensing Comparison of Control Algorithms Open loop Closed loop Scalar SensVector Sensorless Low speed perf.

6 Poor good Good (Ind) Poor (Dir) Medium Dynamic perf. V. poor poor excellent V. good Param. sensitive No No Yes (Ind) No (Dir) Yes Cost V. low Low high low Sp or pos sensor none simple complex none Appl to HPDr No No Yes (Ind-Dir) Yes Carrier frequency The output waveform of the VFD is not a pure AC sine wave. It is a simulated waveform that is actually made up of pulses of DC voltage from the VFD's DC bus. These pulses cause vibrations in the motor that are heard as a high-pitched squeal or whine. The pulses are generated by the switching of the VFD's output transistors.

7 The switching rate of the output transistors is controlled by the VFD's Carrier Frequency parameter. Increasing the Carrier Frequency will reduce the audible noise from the motor. However, doing so may require derating of the VFD, as increasing the switching rate of the transistors causes the VFD to be less efficient Standard range for carrier frequency for many commercial VFDs is KHz to 5 KHz 15 VFD Disadvantages Less efficient at 100% rated motor speed Possible winding insulation breakdown Inverter-rated motors recommended Harmonics Expensive preventive measures from damage Shaft current damaging the bearing Possible voltage reflected wave from long lead lengths Higher first cost 16 Insulation stress Dual coat wires and Vacuum

8 Pressure impregnation for insulation varnishing are recommended along with special care in selection of associated elements 17 Harmonics - Origins -2 -1 0 1 2 TOPOLOGY C e1 e2 e3 M i I Is VARIABLE speed DRIVES Draw high harmonic current of order 5, 7, 11, 13 The current is unstable CURRENT WAVEFORM HARMONIC SPECTRUM H5: - 81%; H7: - 74%; H11: - 42%; .. 0 50 100 H1 H5 H7 H11 H13 H17 H19 H21 H23 Non Linear Load Three Phase S = 23 KVA; Fc= ; THDI=124% All the energy efficient devices and drives are nonlinear in nature They will draw reactive- and harmonic-components of current from the source Power Quality Issues zero sequence All harmonics tend to distort the original fundamental 50 Hz sine wave creating operational problems with electrical equipment such as, PLC's, automation equipment, machine tools, and computers.

9 Odd Triplen harmonics, (3rd, 9th, 15th, etc.), do not cancel each other, but add together in neutral conductors of 3-phase, 4-wire systems to cause overheating in panels, neutral conductors, terminations and transformers. 20 Negative sequence 5th harmonics of sufficient magnitude result in motor inefficiencies and overheating. The negative sequence may produce sufficient counter-torque to cause excessive motor vibration. Generalized harmonic effects include: unexplained operation of protective devices, audible noise interference on telephone circuits, blown fuses on power factor correction capacitors and, erratic operation of generators with solid-state controls.

10 21 Positive Sequence 7th harmonics of sufficient magnitude result in losses. VFD are deployed in every industry. VFD s for the three phase load is predominant. 5th order and 7th order harmonics are predominant because most of the VFD s are six-pulse VFD. 12, 18, 24 and 30 pulses are available for VFD Applications . How can we reduce the harmonic current? 23 DC link choke within the drive Line reactor Passive filter Active filter Multi-pulse converters Active front-end Use of DC choke Power factor better than A large DC bus choke is effective at reducing 5th and 7th harmonics, which are the largest.