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Power Conversion & Motor Basics
MOSFET for high voltage (AC drives), IGBT for low voltages (DC drives) – true or false?
False.
What is the in-rush current? How does Elmo solve the in-rush current problem?
When an amplifier is connected to the power supply and the power is activated, initial high level in-rush current can occur. This problem is solved by using negative thermal voltage transistors (NTCs).
What PWM switching method is used in Elmo drives?
Advanced unipolar with fast technology.
What kinds of current and voltage sensing are known to you?
Resistor sensing on low side power bridges, hall effect sensors on motor phases.
Why is voltage sensing required?
For drive protection (under- and overvoltage) and current GS control loop.
What type of electrical protections are known to you that are implemented in Elmo drives?
Short protection: Phase-phase, phase-ground, phase-VP+
Under/overvoltage, temperature protection
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Why do we need a shunt regulator? Describe the situation that will cause it to operate
To dissipate energy that is absorbed into the system, without harming the stage up to a certain rated level. This can occur during rapid deceleration of a high internal load.
Describe the principle of torque generation.
Passing current through the coil of a starter, creating a magnetic field perpendicular to the rotor’s field.
What is commutation? How is commutation performed in a DC brush motor?
The ability to drive and move a motor based on an electrical input current. Two phases of the brush motor have voltage applied to the brushes creating a magnetic field to repeatedly turn the rotor 90°.
What is the relationship between the motor rotation speed and the BEMF voltage?
BEMF (Back Electromotive Force) is the energy voltage developed on the system during rotation as a result of the inherent motor characteristic properties. This is always less than or equal to the rotation speed on a torque-speed curve.
What are the advantages and disadvantages of DC brush motors?
Advantage: They are simple motors where no commutation is required.
Disadvantage: They require maintenance and are not very efficient.
What is the difference between six-step and trapezoidal commutation? What method is used in Elmo’s digital drives?
Six-step and trapezoidal commutation are the same methods with different names. Elmo uses sine commutation.
What is the maximum torque ripple in trapezoidal current control in a sinusoidal wound motor?
About 13%–14%
What is the maximum torque ripple at a sinusoidal current control of a sinusoidal wound motor? and of a trapezoidal wound motor?
Very minimal, less than 1% of a sinusoidal motor.
13%–14% of a trapezoidal motor.
What are the main advantages of an AC brushless (sinusoidal) motor over a DC brush motor?
An AC brushless motor is more accurate (mainly regarding velocity) and requires almost no maintenance. However, the drive electronics are more expensive and more complex.
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