7154/7156 Variable Speed Drives

1. 7154/7156 Variable Speed Drives Paul Weingartner 569-1776

2. Overview • Variable Frequency drives (VFD) • Application of VFDs • Power quality issues • Human Machine Interface (HMI)

3. Standards organizations • NEMA • IEEE • IEC

4. NEMA • Enclosures • Motor characteristic curves

5. History of adjustable speed systems • Variable pitch pulley • Motor-Generator (MG) set • Eddy current clutch • Solid state drives

6. Problems • Expensive • Electrical (utility) issues • Motor wear/tear

7. Solid State drives • DC drives • AC soft start • AC Variable frequency drives • AC vector drives

8. DC drives • High torque • Large speed ratios • Regenerative braking • DC motors – high maintenance

9. Basics • Speed • Torque • Horsepower • Efficiency

10. Power factor • Real power • Apparent power • Leading power factor • Inductive reactance • Capacitive reactance

11. Electric utilities • Commerical customers are defined as users above 15KVA • Electric charge • Demand charge • Power factor penalties

12. Braking • None – let load coast to stop • Dynamic breaking – resistive load, uses generator effect • Plugging – reverse polarity across motor • DC injection – DC voltage is applied across two phases of an AC induction motor. Current must be limited and timing is critical for proper use • Regenerative • Mechanical brake

13. Goals • Ability to vary speed • Limit power factor issues • Sensitive to electric demand issues • Often need “soft start” • Cost savings

14. Ways to start a motor • Full voltage – Across the line starting • Reduced voltage starting • Soft start – limit current and rate of startup • VFD – great latitude over motor control

15. Relative cost difference for 1 HP motor • Full voltage - $120 • Reduced V - $200 • Soft state - $250 • VFD - $400

16. Motors • 3 phase squirrel cage induction motor • Principle of operation • Synchronous speed • Slip • Starting characteristics • NEMA classifications

17. Motor Insulation class

18. Motor VFD issues • Volts/Hertz ratio • Constant volts range

19. VFD principle of operation • 3 phase rectifier • DC bus • 3 phase inverter

20. VFDs – 1st Generation • VVI – Variable Voltage Inverters • 6 step drive • Uses SCRs on rectifier front end • Variable voltage DC bus

21. Problems with VVI drives • Motor signal – not very sinusoidal, causes problems • Sensitive to source voltage flucuations – 5-10% change will fault the drive • At low speed the drive will “cog” creating stresses on shafts, etc – freq should be above 15 Hz • Drive will reflect harmonics back to the line • Short power loss is bad

22. CSI – Current Source Inverter • Similar to VVI, but adds a line reactor on the DC bus • Supports regenerative braking without needing extra hardware • Creates harmonics

23. PWM • Operating frequency – carrier frequency • Increasing the carrier frequency decreases the efficiency of the drive electronics • Duty cycle • t-on • t-off • Transistor example • Linear operation vs. PWM • Power dissipation

24. PWM drives • Uses diodes for the rectifer, creating a Constant voltage DC bus • Constant power factor – due to diode front end • Full operating torque at near zero speed • No cogging • Can ride thru a power loss from 2 Hz to 20 seconds

25. VFD drives • Scalar • Vector

26. 3 phase motor

27. NEMA Motor Curves

28. 1336 picture

29. 1336 – Description of L7E option

30. 1336 Drive literature link • http://www.ab.com/drives/1336PlusII/literature/index.html

31. PWM inverter

32. Motor selection criteria

33. Synchronous speed • AC motors have a sync design speed that is a function of the number of poles and the line frequency • At sync speed ZERO torque is generated • Therefore, motors cannot run at sync speed

34. Motor slip • Since motors cannot run at sync speed, the will run at slightly less than this speed. • “Slip” is the term used to describe the difference between the sync speed and the maximum rated speed at full load

35. Motor slip calc • This formula includes a characteristic called slip. In a motor, slip is the difference between the rotating magnetic field in the stator and the actual rotor speed. When a magnetic field passes through the rotor's conductors, the rotor takes on magnetic fields of its own. These induced rotor magnetic fields will try to catch up to the rotating fields of the stator. However, there is always a slight speed lag, or slip. For a NEMA-B motor, slip is 3-5% of its base speed, which is 1,800 rpm at full load. For example,

36. Volts/Hertz

37. Drive frequency • The speed at which IGBTs are switched on and off is called the carrier frequency or switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3-4 kHz). As you can imagine, the higher the switch frequency, the smoother (higher resolution) the output waveform. However, there is a disadvantage: Higher switch frequencies cause decreased drive efficiency. The faster the switching rate, the faster the IGBTs turn on and off. This causes increased heat in the IGBTs.

38. High motor voltages • http://www.mtecorp.com/solving.html • High peak voltages • Fast rise times • Standard Motor Capabilities established by the National Electrical Manufacturers Association (NEMA)and expressed in the MG- I standard (part 30), indicate that standard NEMA type B motors can withstand 1000 volts peak at a minimum rise time of 2 u-sec (microseconds). Therefore to protect standard NEMA Design B motors, one should limit peak voltage to 1KV and reduce the voltage rise to less than 500 volts per micro-second.

39. Constant torque loads • Conveyor systems

40. Constant horsepower loads • grinders, winders, and lathes

41. Variable torque loads • fans and pumps

42. Motor ventilation • TENV • TEFC • ODP • High Altitude considerations

43. Motor soft start • Limit inrush current • Linear ramp • S-curve

44. Skip freq

45. Flux vector drives • http://www.mikrokontrol.co.yu/sysdrive/WhatInv.htm#FV