1. 7154/7156 Variable Speed Drives
Paul Weingartner

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 Duty cycle t-on t-off
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

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