August 2000 Torque in an AC motor is calculated using a constant, the volts over the frequency squared, and the line current. If you are running at a fixed speed and K is a constant, the Torque is directly proportional to the motor current. As it increases and decreases so does the torque.
August 2000 Sure! if we maintain voltage and increase resistance, the current will begin to drop. We are now in the constant voltage mode of operation, and Torque begins to fall off.
August 2000 The Sine weighted PWM voltage output to the motor looks like this. The frequency of the switch from positive to negative is determined by the drive based on the speed reference input, and the RMS or Average voltage value for that frequency is determined by the number and width of the pulses. If I vary or "Modulate" the pulse width, I vary the RMS Voltage to the motor.
August 2000 That voltage creates a current waveform in the motor that is very nearly a sine wave; certainly much closer to a true sine wave than the other technologies used in AC Drives. Here are the PWM waveforms. So by modulating or changing the Width of the voltage pulses and the frequency that those pulses create we create a very close approximation of a sinusoidal current waveform. The near sinusoidal nature of the current accomplishes two of our four goals; minimizing the low order harmonics ‑ you can see that the spikes are much smaller than in other technologies‑ and maximizing the transfer of power in the fundamental frequency.
August 2000 33 Scope traces from a 10 HP, 460 VAC VFD with 500 feet of cable between the VFD and the motor. The top wave shows the frequency at the drive output terminals. The bottom wave is the same wave at the motor terminals. An effect, called reflected wave, has raised the peak voltage at the motor terminals.
August 2000 At a minimum, variable-speed AC moors should meet NEMA MG1 Part 188.8.131.52 standards. That standard is depicted here. They should also have a minimum CIV rating of 1,600 V at rated operating temperature for 460 VAC applications and should have a higher voltage rating for 575 VAC applications. Always follow the lead length recommendations of the VFD manufacturer. Most have done extended testing to understand the reflected wave voltage amplitudes and dv/dt created by their products. Use reactors and filters when the distance between the drive and the motor exceeds the manufacturers recommendations. Use power-matched motor/drive packages that have been tested for compatibility in a wide range of operating conditions.
August 2000 Optional Motor review slide
August 2000 Optional Motor review slide
August 2000 Speed Regulation, as a Percentage, is how much the speed will change between no load (Minimal slip) and Full Load (Maximum Slip).
August 2000 Here's a curve for a standard Induction motor. 3% drop in speed. But a standard DC Drive typically has a 1‑2% speed Regulation, and, out of the box, a motion control drive provides .1% speed regulation. Why the difference?
August 2000 The difference is that most DC drives and all Motion Control Drives are what's known as closed loop. That means that some sort of feedback device attached to the motor feed speed information back to the drive for use in correcting any speed discrepancies. Open loop, like most AC Drives, means no such feedback exists, and the drive assumes that what it told the motor to do is actually being done.
August 2000 That's where DC Boost comes in. In order to drop enough voltage across the inductance, we raise or boost the output voltage above what it would be normally, until there is enough voltage across the inductance to provide the necessary torque to turn the motor or "Break" the motor away. Once that voltage boost reaches the level that it would have been on the standard curve, the boost is turned off and operation proceeds as normal. We accomplish DC Boost by widening the pulses in the PWM waveform, creating a higher average voltage, and therefore more current.
August 2000 Now that we understand the technology of AC drives, we need to apply what we know to the characteristics we already know about the AC Motor. Only then can we know how the two will react together. Here is our standard speed torque curve for our NEMA B design motor. An AC Drive has a fixed Maximum Continuous Current limit which we have shown here as a dotted line representing 100% of drive current. In addition, most drives have an intermittent ability to supply current up to some additional level. We have chose the 150% level found in drives like the BUl 1336. Since the drive will be limiting the current available to the motor, we will no longer see the entire speed torque curve. We will not be able to get full breakdown torque from the motor and will not see 200% starting torque as we did across the line. Remember that 200% required 600% current. We are now limited to 150%. What we create then, is an operating range on the torque curve for a motor use with a drive. the area you see here is for full voltage at rated frequency. A motor controlled by an AC Drive will always operate somewhere in this range.
Basics of an ac drive - with motor basics
Review of How Motor WorksMotor converts Electrical Energy to Rotating Mechanical EnergyCoils placement in motor creates rotating, magnetic field in statorRotating magnetic field cuts rotor bar and induces current in rotorRotor current creates magnetic field on rotorAttraction of rotor to stator creates torque and, hence, horsepower
AC Motor ReviewIn an AC Motor, speed varies by: Motor Speed (rpm) = 120 x Frequency - Slip # of Poles Since you can not change the number of poles in an AC motor, the frequency is changed to vary the speed.
Varying the Speed of an AC Motor 1800 1800 = 60 x 120(rpm) (rpm) 4 900 900 = 30 x 120(rpm) (rpm) 4 30 Hz 60 Hz
AC Motor ReviewIn an AC motor, Torque Varies by: E 2T = K x ( ) x I Line F Where: K is a constant E is applied voltage F is input frequency I Line is motor current
AC Motor Review Torque/Current RelationshipWhat you really need to know…... • Current is roughly proportional to load torque • The higher the load torque the higher the current
AC Motor ReviewHorsepower of an AC motor can be determined by: HP = Torque x Speed 5252 Where: Torque is in lb-ft Speed is in RPM 5252 is a constant
Motor nameplate Horsepower is achieved at Base RPM: HP = Torque * Speed / 5252 Constant Torque Constant Horsepower Range Range PM R Note that motor na horsepow meplate er is only achieved at and ab Horsepower base spe ov ed , NOT B e EFORE. d ee Sp10 u e 0% e rq asTo B
AC Motor Review IMPEDANCEIMPEDANCE: Resistance of AC Current flowingthrough the windings of an AC Motor NOTE: Impedance decreases as frequency decreases
Volts/Hertz Relationship I = Current V = Voltage I=V Z = Impedance ZTo reduce motor speed effectively:• Maintain constant relationship between current & torque• A constant relationship between voltage and frequency must be maintained
Volt/Hertz Relationship460 V The AC variable speed drive controls voltage & frequency230 V simultaneously to maintain constant volts-per-hertz relationship keeping current flow constant. 30 Hz 60 Hz
AC Drive Rectifier DC Bus InverterAC Power Supply M V V V V T T T•Rectifier • Inverter - Converts AC line voltage to Pulsating DC voltage - Changes fixed DC to adjustable AC - Alters the Frequency of PWM waveform • Intermediate Circuit (DC BUS) - Filters the pulsating DC to fixed DC voltage
PWM WAVEFORM VLL @ Drive 500 Volts / Div.+ DC Bus 1- DC Bus 3 Phase Current 10 Amps / Div. M2.00µs Ch1 1.18V PWM waveform is a series of repetitive voltage pulses
Drive and Motor Compatibility Voltage Wave VLL @ Drive @Drive Output 500 Volts / Div.PotentiallyDamagingVoltagePeaks VLL @ Motor 500 Volts / Div. Voltage Wave @ Motor Conduit Box
How to Specify -- NEMA Standards MG1-1993, Part 184.108.40.206 Maximum of 1600 Volt Peaks VpeakVoltage Steady-state voltage 100% 90% ∆ V dV ∆ V = dt ∆ t 10% ∆ t Time Rise time Minimum Rise Time of .1 Microseconds
GV3000/SEV/Hz OperationOutput 460Voltage Ratio @ 460VAC = 7.67 V/Hz 230 115 Hz 0 15 30 60 90 Output Base Frequency Frequency At Base RPM or 60Hz, the Motor sees line input voltage
GV3000/SEV/Hz OperationOutput 460Voltage Ratio @ 460VAC = 7.67 V/Hz 230 115 Hz 0 15 30 60 90 Output Base Frequency FrequencyAt 25% of Base RPM or 15 Hz, Voltage & Frequency is 25%
VECTOR DRIVE Magnetizing Current 25.0 (8.5 Amps) Amps Full Load Torque - Producing Current (23.5 Amps)Vector calculates Torque-Producing Current byknowing actual amps and magnetizing current.
GV3000/SEVector Control - Torque can be produced, as well as regulated even at “0” RPM Motor Current is the VECTOR SUM of Magnetizing Motor Current is the VECTOR SUM of Magnetizing & Torque Current, & Torque Current, 100% this is where the term VECTOR DRIVE is derived this is where the term VECTOR DRIVE is derived Torque Current Motor Torque Current Current Motor 10% Current 90° 90° Magnetizing Current Magnetizing CurrentMotor Current is the Vector Sum of Torque & Magnetizing
GV3000/SEFlux Vector Drive - simple diagram reviewA Vector Drive always regulates current “LEM” Current Sensors L1 L2 Motor L3 E Micro PEncoder feedback provides rotor speed & position information for calculations
GV3000/SESensorless Vector Control - simple diagram reviewSVC estimates rotor speed & position to the stator field “LEM” Current Sensors L1 L2 Motor L3 Micro P ( FVC + Speed Estimator )A “Speed Estimator” calculates rotor speed & position to maintain 90° to the field
INVERTER DUTY MOTORSNEMA Design ‘B” Motor w/ 3% Slip - Across the Line Start BDT 200% Operating LRT Region on AC PUT Drives 100% FLT Slip Base RPM AC Drives regulate Motor Speed based on designed slip
INVERTER DUTY MOTORS Blowers may be added to Blowers may be added to motors to allow operation at low motors to allow operation at low speed including “0” RPM with speed including “0” RPM with 100% Torque continuous 100% Torque continuous Some motor frames are sized so that Some motor frames are sized so that just the surface area is suitable to just the surface area is suitable todissipate motor heat w/o the need of adissipate motor heat w/o the need of a fan or blower fan or blower
GV3000/SE with “Inverter & Vector Duty” AC MotorsVXS Motors Based on Reliance XEX Motor Designs TENV, TEFC-XT and TEBC Enclosures Ideal for; Positive Displacement Pumps and Blowers Extruders and Mixers Steel and Converting Process lines Standard Features; Encoder Mounting Provisions Motor Shaft Tapped for Stub @ ODE Accessory Face @ ODE Motor Winding Thermostats, 1/Phase 10:1 to 1000:1 CT speed ranges w/o derating
GV3000/SE with “Inverter & Vector Duty” AC MotorsRPM-AC Motors Laminated Steel, DC-style construction DPFV, TENV, & TEBC enclosures Ideal for; Extruder applications Web processing & mill applications Retrofitting existing DC Drive & Motor systems Standard Features; High torque to inertia ratios Encoder Mounting Provisions Motor Winding Thermostats, 1/Phase Infinite CT speed range, 0 RPM continuous CHp Range of 2:1 on TENV & TEBC Frames Base Speeds from 650 RPM to 3600 RPM
Speed RangeSpeed Range - Designed operating range of an inverter duty motorExample1800 rpm motor10:1 Speed Range = 180 -1800 (rpm)
CONSTANT TORQUE REGIONSpeed / Torque Curve of an AC Drive & Inverter Duty Motor 100 Torque 90 % 80 Torque T 70 O 60 R 50 Q 40 Acceptable Region U 30 for Continuous Operation E 20 10 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 HZ Inverter Duty Motors operate at 1/4th Base RPM
CONSTANT HP REGIONSpeed / Torque Curve of an AC Drive & Inverter Duty Motor 100 Torque 90% 80 TorqueT 70O 60 Torque aboveR 50 base RPM =Q 40 100%U 30 % Base RPME 20 10 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 HZ CHp Operation above Base RPM is typically limited to 150%
CONSTANT TORQUE REGIONSpeed / Torque Curve of a Vector Drive & Vector Duty Motor 100 Torque 90 % 80 Torque T 70 O 60 R 50 Q 40 Acceptable Region U 30 for Continuous Operation E 20 10 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 HZ Vector Duty Motors operate at “0” RPM w/ 100% Torque Cont.
CONSTANT HP REGIONSpeed / Torque Curve of a Vector Drive & Vector Duty Motor 100 Special motor & drive Special motor & drive 90 designs can allow operation designs can allow operation% 80 up to 8 * Base RPM up to 8 * Base RPMT 70O 60 TorqueR 50 TorqueQ 40 Vector Duty Motors may haveU 30 CHP Ranges ofE 20 2 * Base Speed or more 10 depending on their design 0 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 HZ Some Vector Duty Motors can provide CHp ( 2 * Base RPM )
Drive Terminology V/Hz Restart DC Boost Preset Accel / Decel Jog Frequency Current Limit Voltage Analog / Digital HP Power Factor Speed Harmonics Skip & Bandwith Ride - Thru Braking Speed Range DB Speed Regulation Regen Frequency Regulation Injection Cogging Coast Efficiency Ramp
Accel/Decel Acceleration Rate - Deceleration Rate Rate of change of motor speed. 100 % Example:Frequency 0 Speed - 1750 rpm 30 seconds 30 sec TIME
Full Voltage Bypass Drive Bypass Branch Disconnect Fusing Switch GV3000/SE M InputDisconnect Switch Bypass Option
Speed RegulationHow Much Will the Speed ChangeBetween No Load and Full Load? Expressed as a Percentage
Voltage BoostVoltage Boost over prolonged operating periods may result inoverheating of the motor’s insulation system and result inpremature failure. CAUTION: Motor Insulation Life is decreased by 50% for every 10°C above the insulation’s temperature capacity Unable to perform like DC, the industry looks to Vector Control
Critical Frequency An Output Frequency of a Controller that Produces a Load Speed at Which Severe Vibration Occurs.A Frequency at which Continuous Operation is Undesirable
AC Drive InputsAnalog Inputs: Digital Inputs:• 0-10 VDC • Start• ± 10 VDC • Stop• 4-20 mA • Reset • Forward/Reverse • Run/Jog • Preset Speeds
GV3000/SEHigh Bus Avoidance ( SVC & FVC ) For Trip Free Deceleration if low to medium inertia loads SPEED TIME Trip Free Deceleration when enabled
Snubber/Dynamic Braking Rectifier DC Bus InverterAC Power Supply M• Snubber/Dynamic Braking - Addition of Snubber Resitor Kit 7th IGBT - Dissipates excess energy to regulate braking Braking Resistor - Regulator monitors DC bus voltage - Signal sent to 7th IGBT - Handles short term regenerative loads - Less expensive than AC line regeneratiion braking
AC Regenerative BrakingAC Power Supply AC Line Drive 1 Drive 2 Drive 2 Regeneration Module• Severe Regenerative Braking - Drives powered through DC bus instead - Addition of AC Line Regeneration Module - Monitors DC bus voltage of through the Rectifier bridge - Sends Excess voltage back to AC line - Share regenerative energy between - Handles long term regenerative loads motoring and regenerating drives - Run Multiple Drives off 1 Module - Send energy back to AC Line instead of dissipating as heat
Auto - RestartHow will the drive react after being shut down by a fault condition? Will the drive resumeRunning after the Fault condition is Cleared? (Sometime restricted to certain Faults)
Preset SpeedsA Pre-Programmed Command Frequency That can be activated via Mode Select or Input Device
Current Limit The ability of a drive to react to the increased current caused by momentarilyincreasing the load on the motor (Shock Loading) without tripping the drive on Overcurrent.
Power Loss Ride-Through The Ability of a Controller to sustain itself through a loss ofInput Line Voltage for a specific period of time.
Operating Range ForVariable Frequency AC Drives