MATING VFDS TO EXISTING MOTORS Jack Gibbs Instructor
MATING VFD’S TO EXISTING MOTORS Jack Gibbs – Instructor Cincinnati State College 10/27/2020 1
The “Existing” Motor • Motors Connected to VFD’s Receive Power That Includes A Changeable Fundamental Frequency, a Carrier Frequency, and a Rapid Voltage Buildup. • These Factors Have a Negative Impact When Existing Motors are Used. 2
High Carrier Frequency § Side Effects are: HEAT, Audible Noise, and Vibration. § Extremely High and Rapid Voltage Rise. § Carrier Frequency is a By-Product of Obtaining Current at a VARIABLE FUNDAMENTAL Frequency! 3
• With Frequencies Other Than the Fundamental the Motor Runs With Excessively HIGH Slippage. • The Fact that Many More Lines of Flux are Being Cut Contributes to Additional HEAT, and Additional WATTS in the Motor. 4
• • • The SYNCHRONOUS SPEED of This CARRIER Frequency is 120, 000 rpms in a Four Pole Motor! Carrier Frequencies are Typically 2 to 15 KHz. This ADDITIONAL Current is Transmitted to the Rotor Bars and Dissipated as “Rotor Losses” in the Form of Heat. 5
• • This ADDITIONAL HEAT Represents another 5% to 10% Thermal Buildup in the Motor. Because of These and Other Conditions One May Need To DERATE the Motor. 6
Other Side Effects~ NOISE~Component Resonating • VIBRATION~Skip Frequency • • Bearing Problems (Disintegration) • Air Flow Problems ~ Pitting Due to Bearing Currents. COOLING!!! 7
Conductor Insulation Breakdown • In PWM Circuitry, with HIGH Voltage Buildup at HIGH Frequencies Insulation WILL Break Down!!! • Turn to Turn Shorts. Skin Insulation Breakdown. Stator Shorting and Meltdown. • • 8
Motor Application • • A Motor is a CONSTANT TORQUE Machine ~ at RATED Torque and RATED Speed it, it will Produce a CERTAIN HP!! When Speed is REDUCED by Voltage and Frequency Reduction, the Motor, by Consuming MORE Current, will TRY to Maintain Constant Horse Power. 9
• THE MOTOR MUST THEREFORE BE SIZED TO MOVE THE LARGEST LOAD AT THE SLOWEST DESIRED SPEED! 10
• • At Times, This May Make the Motor Appear Too Large. How Much HP is Required at the LOW END of the Speed Range? Voltage is Also Reduced in a VFD meaning That at 50% Speed We’re at 230 VAC or ½ Rated HP at Full Speed. !! The Motor Will Have to have Sufficient Thermal Capacity to Handle the Load. 11
Troubleshooting Your Drive • • The Problem is Fourfold~ Simply by Design. Nobody PLANNED on Non-Linear Drives or Variable Frequency Anything. • Measurements ARE the Key to It All. 12
The Facility Power Supply • “Over and Under” Voltages Greater Than 10%+/- WILL TRIP MOST DRIVES!!! • A Voltage Unbalance of 3% to 5% Can Cause Tripping of a Drive’s Overload Device. 13
What To Do~ • Measure the INCOMING Line Voltage at the INPUT Side of the Drive Itself. • • Measure ALL Three Phase to Phase. Look for Over and Under Conditions. 14
• • • Measure at PEAK and NON-PEAK Times. Look For Unbalances, Too! Is the Facility, Itself, Causing These Problems? Take a Current Reading of ALL Three of these Phases on the Line Side of the Drive. 15
Take VFD Readings • • Measure DC BUSS Voltage. Actual DC BUSS Voltage (Line plus x 1. 414). DC BUSS Under Load ~ At The Drive Capacitor Connections ` Take = and – Readings (Line + 1, 414). Now Take the SAME Reading on “AC”. NOTHING Above 5 VAC!!! 16
Volts Per Hertz Meas. • Maintaining a 7. 67: 1 Ratio? ? ? • Leakage Current ~ A Transistor Does • not Actually “Open Up” like a Mechanical Switch ~ it Just Reduces the Amount of Current it Lets Through. It Should be < 40 V While Energized In RUN w/ Speed @ Zero (Meas Between Phases). 17
The LOAD!!! Rotational Direction Correct? ? ? • Why FAST Acceleration Speed? ? ? • What Type of Load? ? ? • Harmonics and Overvoltage Reflections. • 18
METERS TO USE • DON’T Throw Out That Old Simpson 260 Rev. D ANALOG!!! • Use it on the Inverter!! (Smoothing Effect of the IGBT Switch. ) 19
The MOTOR • • Measure Voltage at the Drive AND at the Motor!!! Look For Voltage DROP AND Volts Buildup!!! • Take Temperature Readings!! 20
Introduction • • • State the purpose of the discussion Identify yourself 21
Topics of Discussion • • State the main ideas you’ll be talking about 22
Topic One • • Details about this topic Supporting information and examples How it relates to your audience 23
Topic Two • • Details about this topic Supporting information and examples How it relates to your audience 24
Topic Three • • Details about this topic Supporting information and examples How it relates to your audience 25
Real Life • • • Give an example or real life anecdote Sympathize with the audience’s situation if appropriate 26
What This Means • • • Add a strong statement that summarizes how you feel or think about this topic Summarize key points you want your audience to remember 27
Next Steps • • • Summarize any actions required of your audience Summarize any follow up action items required of you 28
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