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MATING VFD’S TO EXISTING MOTORS Jack Gibbs – Instructor Cincinnati State College 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 2 High Carrier Frequency Side Effects are: HEAT, Audible Noise, and Vibration. Extremely High and Rapid Voltage Rise. Carrier Frequency is a ByProduct of Obtaining Current at a VARIABLE 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 • Pitting Due to Bearing Currents. (Disintegration) • Air Flow Problems ~ 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 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 NonLinear Drives or Variable Frequency Anything. • Measurements It All. ARE the Key to 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 NONPEAK 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 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 < 40V 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 19 The MOTOR • Measure Voltage at the Drive AND at the Motor!!! DROP AND Volts Buildup!!! • Take Temperature Readings!! • Look For Voltage 20 Introduction • • • State the purpose of the discussion Identify yourself Topics of Discussion • • State the main ideas you’ll be talking about Topic One • • • • Details about this topic Supporting information and examples How it relates to your audience Topic Two • • • • Details about this topic Supporting information and examples How it relates to your audience Topic Three • • • • Details about this topic Supporting information and examples How it relates to your audience Real Life • • • Give an example or real life anecdote Sympathize with the audience’s situation if appropriate 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 Next Steps • • • Summarize any actions required of your audience Summarize any follow up action items required of you