Variable Frequency Drives 030302d Flashcards by Tyler Bohn

What are the two types of phase-converters?

Variable Frequency Drives ILM

Static Phase-Converters.
Rotary Phase-Converters.

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How many three-phase motors can a static phase-converter supply?

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One.

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How many three-phase motors can a rotary phase-converter supply?

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One or more.

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How does a static phase-converter start a three-phase motor?

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Use a capacitor to create a phase shift which creates a rotating magnetic field.

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When using a static-phase converter, how much power does the motor produce?

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50%-70% of its rated horsepower and will lack starting torque.

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What application are static-phase converters best suited for?

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Mechanical loads that do not vary significantly in their power demands.
Equipment that reaches its full operational load quickly. (No hard starts).

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What applications are static phase-converters not suitable for?

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Varying loads or where the motor requires full power.

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What are the types of capacitors that a static phase-converter uses?

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Electrolytic (Start Capacitor)
Film or Oil-Filled Paper. (Run Capacitor)

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What are the components of a static phase-converter?

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Capacitors (Electrolytic, Film, Oil-Filled Paper).
Potential Relay.
Contactor.
Autotransformer (Not always)

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What does an autotransformer in a static phase-converter allow you to do?

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Enables adjustment of generated line voltages through the several available taps.

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When is the start capacitor removed for a rotary phase-converter?

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When the idler motor is at full speed.

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What does the idler motor start to operate as when it reaches operational speed?

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Starts to operate similar to a generator.

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What are rotary phase-converters better suited for over static phase-converters?

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Harder starting and variable load conditions.

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What is the main part of a rotary phase-converter?

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Idler Motor

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What can be used on a rotary phase converter to ensure a load is not connected until it reaches full speed?

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Timing delay function or a voltage sensing relays.

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Why does the static phase-converter only allow a motor to operate at 50%-70% rated power?

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Static phase-converter starts the motor only. Once running the motor is running on a single phase instead of three-phase.

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How does a variable frequency drive change the rotational frequency of a motor?

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By adjusting the ac frequency supplied to the stator which changes the synchronous speed of the motor.

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What are some other names for variable frequency drives?

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Adjustable Frequency Drives (AFD).
Adjustable Speed Drives (ASD).
Inverter Drives.
AC Drives.

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How does the variable frequency drive adjust the frequency of the AC?

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Electronic Control Methodology.

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What are some of the primary functions a variable frequency drive can provide?

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Accurate Process Control.
Extended Equipment Life.
Energy Efficiency Improvements.

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What are some other functions that a variable frequency drive can provide?

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Speed Control.
Torque Control.
Reversing.
Motor Winding Short-Circuit and Overload Protection.
Ramped (soft) Start and Stop.
Regenerative or Dynamic Braking.

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What type of operation is open loop control for variable frequency drives meant for?

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Operations that do not require feedback like a fan or a motor that will run at constant speed.

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What is open loop control for a variable frequency drive?

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Motor operates on pre-set parameters programmed into the VFD.
VFD can estimate motor speed and temperature.
Does not receive feedback from the motor.

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What is closed loop control for a variable frequency drive?

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Motor operates based on pre-set values programmed into the VFD and VFD will respond to input from sensing devices.

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How can a variable frequency drive provide extended equipment life? ## Footnote Variable Frequency Drives ILM

Provide ramped starting which can reduce electrical stress by reducing inrush current. Provide ramped stopping so equipment does not stop abruptly. ## Footnote Variable Frequency Drives ILM Page 10

How can variable frequency drives provide energy savings? ## Footnote Variable Frequency Drives ILM

Allows motors to operate at reduced speeds which decreases power consumption. ## Footnote Variable Frequency Drives ILM Page 10

How much power and torque is used if a motor is operating at half speed using a variable frequency drive? ## Footnote Variable Frequency Drives ILM

1/8 power. 1/4 torque. ## Footnote Variable Frequency Drives ILM Page 10

What does a rectifier do in a variable frequency drive? ## Footnote Variable Frequency Drives ILM

Converts a.c. pulses to d.c. pulses using diodes. ## Footnote Variable Frequency Drives ILM Page 13

What does a dc Link do in a variable frequency drive? ## Footnote Variable Frequency Drives ILM

Smoothes and filters d.c. pulses using capacitors and inductors. ## Footnote Variable Frequency Drives ILM Page 13

What does the inverter do in a variable frequency drive? ## Footnote Variable Frequency Drives ILM

Converts d.c. to a.c. again at desired frequency.

What is pulse-width modulation (PWM)? ## Footnote Variable Frequency Drives ILM

Pulsing d.c. on and off both positive and negative for different lengths of time to simulate a three-phase ac voltage. ## Footnote Variable Frequency Drives ILM Page 14

What is a carrier frequency with variable frequency drives? ## Footnote Variable Frequency Drives ILM

Represents the number of d.c. pulses per second. Example: 5 kHz = 5000 switches per second. (Average switching time of 0.0002 seconds.) ## Footnote Variable Frequency Drives ILM Page 14

What do variable frequency drives use that are capable of rapid switching? ## Footnote Variable Frequency Drives ILM

Transistors. ## Footnote Variable Frequency Drives ILM Page 14

How is a higher average dc output voltage created with variable frequency drives? ## Footnote Variable Frequency Drives ILM

DC pulse is on longer. Longer "on" time = higher ac voltage. Longer "off" time = lower ac voltage. ## Footnote Variable Frequency Drives ILM Page 15

Why must the voltage reduce along with frequency for a variable frequency drive? ## Footnote Variable Frequency Drives ILM

If voltage does not reduce, the motor current is excessive at lower speeds. ## Footnote Variable Frequency Drives ILM Page 15

What is the Volts/Hz ratio for variable frequency drives? ## Footnote Variable Frequency Drives ILM

Ratio that is maintained to avoid excessive currents in the motor. ## Footnote Variable Frequency Drives ILM Page 15

What is a torque boost function used for with variable frequency drives? ## Footnote Variable Frequency Drives ILM

Torque boost function is used to adjust the volt/hz ratio to provide a higher voltage, current, and torque at low frequency operation to prevent a motor from stalling. ## Footnote Variable Frequency Drives ILM Page 16

What motor nameplate values are important for sizing a variable frequency drive? ## Footnote Variable Frequency Drives ILM

Motor Voltage. - VFD must provide output voltage to match the motor. Full Load Amperage and Service Factor. - VFD must have a current rating equal to or greater than motor FLA. Power. - VFD power rating must be equal to or greater than the motor power rating. Speed Range. - VFD must operate the motor within its proper minimum and maximum speed ranges. ## Footnote Variable Frequency Drives ILM Page 18

What type of motor is designed to be used with variable frequency drives? ## Footnote Variable Frequency Drives ILM

Inverter-duty motors. ## Footnote Variable Frequency Drives ILM Page 19

What do variable frequency drives have to help cool themselves? ## Footnote Variable Frequency Drives ILM

Cooling fans and heat sinks on the rear of the drive. ## Footnote Variable Frequency Drives ILM Page 20

What type of environment should variable frequency drives be installed in? ## Footnote Variable Frequency Drives ILM

Dry, non-corrosive, and dust-free environments. ## Footnote Variable Frequency Drives ILM Page 20

What does a higher carrier frequency with variable frequency drives cause? ## Footnote Variable Frequency Drives ILM

Quieter motor operation however higher carrier frequencies also can cause damage to cables and motor windings. ## Footnote Variable Frequency Drives ILM Page 20

What types of control can variable frequency drives offer? ## Footnote Variable Frequency Drives ILM

Speed control. Ramped starting & stopping. Fast start & stopping. Reversing. ## Footnote Variable Frequency Drives ILM Page 20

What is voltage ringing with variable frequency drives? ## Footnote Variable Frequency Drives ILM

Caused by changes in impedance within a circuit. Reflected sine waves adds on top of incoming wave and has a significant overvoltage (voltage spike). ## Footnote Variable Frequency Drives ILM Page 20

What causes Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI)? ## Footnote Variable Frequency Drives ILM

High frequencies which induce voltages into nearby circuits and motor cables. ## Footnote Variable Frequency Drives ILM Page 21

How should motor power cables and data cables be installed in relation to eachother? ## Footnote Variable Frequency Drives ILM

Should be separated by at least 300 mm. Should cross eachother at right angles. ## Footnote Variable Frequency Drives ILM Page 22

What can help reduce Electromagnetic Interference (EMI)? ## Footnote Variable Frequency Drives ILM

Copper Braided shield wrapped around the cable. ## Footnote Variable Frequency Drives ILM Page 22

What is the typical voltage insulation rating of inverter duty motors? ## Footnote Variable Frequency Drives ILM

1488 V ## Footnote Variable Frequency Drives ILM Page 22

What are some features of inverter duty motors to prolong their lifespan? ## Footnote Variable Frequency Drives ILM

Higher voltage rated insulation. Higher insulation temperature rating. Thinner stator laminations. Insulated or shunted (grounded) bearings. Additional cooling for lower speeds. (Totally Enclosed Blower Cooled TEBC Motor) ## Footnote Variable Frequency Drives ILM Page 23

What can using filters with variable frequency drives help with? ## Footnote Variable Frequency Drives ILM

Reduce transient overvoltages (Spikes). Reduce high pitched motor noises. Allows for longer cables. Create smoother sinewave voltages. ## Footnote Variable Frequency Drives ILM Page 23

What are the two types of filters used with variable frequency drives? ## Footnote Variable Frequency Drives ILM

Active (Electronic). Passive (Reactive). ## Footnote Variable Frequency Drives ILM Page 23

What are the three types of passive filters for variable frequency drives? ## Footnote Variable Frequency Drives ILM

Output Choke (Reactor). dV/dT. Sine Filter. ## Footnote Variable Frequency Drives ILM Page 23

What cable lengths are dV/dT filters generally used for? ## Footnote Variable Frequency Drives ILM

Lengths exceeding 300 ft and up to 1000 ft. ## Footnote Variable Frequency Drives ILM Page 24

What type of filter is used for cables over 1000 ft? ## Footnote Variable Frequency Drives ILM

Sine Filters (LC Filter) ## Footnote Variable Frequency Drives ILM Page 24

What do sine filters allow to pass through them? ## Footnote Variable Frequency Drives ILM

Only allows low frequencies to pass through. ## Footnote Variable Frequency Drives ILM Page 24

What lengths of cable are 3% reactor filters used for? | Output choke (reactor) ## Footnote Variable Frequency Drives ILM

Up to 300 ft. ## Footnote Variable Frequency Drives ILM Page 23

What is an electrical harmonic? ## Footnote Variable Frequency Drives ILM

Voltage and/or current that has a different frequency than the fundamental frequency of 60 Hz. ## Footnote Variable Frequency Drives ILM Page 25

What are the two main causes of variable frequency drive harmonics? ## Footnote Variable Frequency Drives ILM

Rapid switching of an inductive load. Output of a square waveform that inherently contains harmonics. ## Footnote Variable Frequency Drives ILM Page 25

What should be done for inverter-duty cables to reduce Electromagnetic Interference? ## Footnote Variable Frequency Drives ILM

Bonded on both ends. ## Footnote Variable Frequency Drives ILM Page 22