Chuck Yung
EASA Technical Support Specialist
Member Question: We recently received a rotating frequency converter for repair. It appears to be a wound-rotor motor coupled to an induction motor. The drive motor is not the same speed as the wound-rotor motor. How does this work?
The rotating frequency converter is exactly as you describe. Usually the drive motor has fewer poles than the wound-rotor motor, so the wound-rotor motor is driven faster than its synchronous speed to increase the frequency of the output.
The poles for the stator and rotor of the wound-rotor motor must be equal. When rewinding any wound-rotor motor, watch for consequent-pole connections. Grouping alone is
not a reliable indicator of poles.
The best way to understand the process is to visualize the rotating electrical field in the stator of a wound-rotor motor. The field rotates one direction, so that when the rotor is driven in the opposite direction the passing frequency is increased. If the speeds are the same (i.e., the same number of poles as the drive motor) the output frequency will double.
So a 4-pole wound-rotor motor operating from 60(50) Hz could be driven ‘backwards’ (against the stator field) at 1800 (1500) rpm to produce 120(100) Hz power from the output leads. The input leads of the wound-rotor motor are normally energized from the same power supply as the drive motor.
To obtain other frequencies, we can use a variety of combinations of drive motor / wound-rotor motor poles. Since the stator field is rotating at a constant 60 (50) Hz, it is fairly straight forward to calculate the output frequencies that can be obtained. While most frequency converters are direct-coupled (or integral units on a common shaft), the use of pulleys permits almost any reasonable frequency to be produced.
Calculations should be done using the formula below:
[(N1 + N2) x F1] / N1 = F2, where:
N1 = Original speed of wound-rotor motor
N2 = Driven speed
F1 = Frequency input into the stator
F2 = Frequency output of the frequency converter
For example, if a 2-pole motor is used to drive a 4-pole wound-rotor motor:
60 Hz example
[(1800 + 3600) x 60] / 1800 = 180 Hz
50 Hz example
[(1500 + 3000) x 50] / 1500 = 150 Hz
The tables below offer a quick reference of some of the frequencies that can be obtained by driving a direct-coupled wound-rotor motor at synchronous rpm.
As the tables indicate, there are several options to get the same frequency output. If called upon to build a rotating frequency converter, it is more practical to keep the drive motor and wound-rotor motor as close to the same poles as possible. [For example, we could obtain 180 (150) Hz by driving a 16-pole WR motor at 900 (750) rpm, but driving a 4-pole at 3600 (3000) rpm is more practical.] Since the stator field remains a steady 60 (50)Hz, driving the wound-rotor motor against the direction of the stator field rotation adds the difference in Hz, while driving it with the rotation subtracts the difference (see following tables).
When testing a rotary frequency converter: If the output frequency is too low, simply reverse any two input leads of the wound rotor motor or of the drive motor (not both).
If building a frequency converter, the mechanical construction of the wound rotor may limit the speed at which it can be driven. For example, a 20-pole wound rotor driven by a 2-pole would probably fail quickly because of centrifugal force. The peripheral speed of the rotor precludes safe operation. A review of EASA’s database of wound-rotor motors suggests that wound-rotor peripheral speed rarely exceeds 7,500 feet (2,286meters) per minute. That should be a safe guide- line when setting up a frequency converter.
Lamination thickness also limits the frequency that a motor can operate / produce. A typical 60 Hz motor is likely to experience high eddy-current losses above 200 Hz. A 60 Hz machine typically has lamination thickness about .024” while lamination thickness for a 400 Hz unit is approximately .012”.
Tip — For a variable-frequency output, a wound-rotor motor can be driven by a variable speed DC motor, or by an induction motor operating from a stock VFD (to obtain frequencies above VFD capabilities).
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