7154/7156 Variable Speed Drives Paul Weingartner 569-1776

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

Standards organizations NEMA IEEE IEC

NEMA Enclosures Motor characteristic curves

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

Problems Expensive Electrical (utility) issues Motor wear/tear

Solid State drives DC drives AC soft start AC Variable frequency drives AC vector drives

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

Basics Speed Torque Horsepower Efficiency

Power factor Real power Apparent power Leading power factor Inductive reactance Capacitive reactance

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

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

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

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

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

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

Motor Insulation class

Motor VFD issues Volts/Hertz ratio Constant volts range

VFD principle of operation 3 phase rectifier DC bus 3 phase inverter

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

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

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

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

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

VFD drives Scalar Vector

3 phase motor

NEMA Motor Curves

1336 picture

1336 – Description of L7E option

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

PWM inverter

Motor selection criteria

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

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

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,

Volts/Hertz

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.

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.

Constant torque loads Conveyor systems

Constant horsepower loads grinders, winders, and lathes

Variable torque loads fans and pumps

Motor ventilation TENV TEFC ODP High Altitude considerations

Motor soft start Limit inrush current Linear ramp S-curve

Skip freq

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