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  1. jpts

    Excitation current on induction motor as a generator via VFD

    Have you considered self-excitation with a 3-phase capacitor-bank instead of external power supply? The logic is that residual magnetism from the rotor causes small induced voltage, which then causes larger current because of the resonance in the capacitors, and the machine self starts. Here's...
  2. jpts

    Short Circuit Brake for wind turbine generator.

    Couple of things come to mind: 1) You should check is the generator designed to handle the short-circuit without demagnetization of the magnets. Some companies cut costs and use cheap magnets which are not designed to withstand the demagnetizing field due to short-circuit. If this happens...
  3. jpts

    Motor VFD open loop no-load current is much higher than closed loop no-load current. Why?

    Ok, but did you actually measure what was the terminal voltage in both open- and closed-loop operation? Even a rather small difference can mean relatively big difference on current especially in no-load. No-load current of a PM machine as a function of terminal voltage is a V-shape curve, where...
  4. jpts

    Motor VFD open loop no-load current is much higher than closed loop no-load current. Why?

    Did you check what was the used terminal voltage in each mode? I would guess that open loop had much lower or higher terminal voltage than closed-loop mode. Closed-loop control of PM machines often use so-called id=0 control which adjusts the terminal voltage in each load point so that the...
  5. jpts

    Dehumidifier inside azipod (pod propulsion system)

    Some corrections, Azipods (which is a trademark of ABB) don't have any kind of gear, the electric motor is always inside the pod and drives the propeller directly. In so-called L-drive there is one angle-gear and the electric motor is outside the pod, but this has nothing to do with Azipod...
  6. jpts

    Induction motors: magnetizing currents, output torque...etc

    No-load current of induction machine is mostly needed to produce the magnetic field, and small portion of it to cover the losses inside the motor (stator/rotor winding, stator core losses and windage and friction losses). You don't need to take these into account when matching the load. If the...
  7. jpts

    Phase-to-Phase Resistance Testing of MV Motor

    Residual magnetic field is a DC field, and shouldn't affect on results. I would check the contact from measurement device to motor terminals. Was the measurement done immediately after no-load testing -->Could small differences in winding temperatures between phases explain this? Although it...
  8. jpts

    VFD with Synchronous motors

    Synchronous machines are usually preferred in high-power, low-speed applications such as big conveyor motors, shaft generators (marine application) and so on. This is because induction machines don't work very well electromagnetically when they have to be designed for low-speeds. High pole-count...
  9. jpts

    Determination of iron alloy used in motor or generator laminations

    Yeah, then it's probably something very cheap and commonly used grade. According to my experience from Chinese motor manufacturing, 50WW470 -grade is one of the most common grades, and one of the cheapest (if not the cheapest) while still offering rather good performance. Attached you can find...
  10. jpts

    Determination of iron alloy used in motor or generator laminations

    Epstein frame test is the common way, but you can't apply it for the existing motor. You would need to make sample strips from the same batch where core laminates were manufactured and measure them in a special setup (so it's not actually that simple way) I would suggest to contact motor...
  11. jpts

    skewed slots in AC generator´s stator

    It shouldn't make big difference on torque ripple or noise, whether the skewing is applied on the rotor or on the stator. Smaller induction machines typically have Aluminium die-cast rotor (and no radial cooling ducts), which makes it very easy to apply rotor skewing during the manufacturing (no...
  12. jpts

    Skin effect in Generator power cables at 120Hz

    Hi Bill, You can think Litz wire working with same principle as Roebel bars. The main idea is that the single atrands are transposed/twisted along the conductor. Then the voltages induced by slot leakage will be equal in all strands, cancelling out circulating currents due to skin effect...
  13. jpts

    Skin effect in Generator power cables at 120Hz

    We have experience up to 240mm^2 cables. Yes, stranding is done for functional reasons (to give flexibility like you said), not to minimize skin effect (strands are not insulated either) We mostly use cables with very small strand diameter (such as Siwo-Kul from Nexans) since they are very...
  14. jpts

    Skin effect in Generator power cables at 120Hz

    Hi Single core cables are still made from bunch of smaller diameter strands (so it is not like one thick solid conductor). If the diameter of the strands is smaller than skin depth (5.95mm) skin effect should not be a problem. I assume this is the case. Often the strands are less than 1mm in...
  15. jpts

    Conversion of an induction motor into a synchronous generator

    Permeability means how well the material conducts magnetic flux. Iron has very high permeability, meaning that it has very small "magnetic resistance". Permanent magnets have very low permeability (about the same than air), meaning that they conduct magnetic flux very poorly. This means that...
  16. jpts

    Conversion of an induction motor into a synchronous generator

    Hi It seems that you have very thick magnets compared to mechanical air gap. Magnetic air gap (which defines the flux) is the thickness of magnet plus mechanical air gap length (because mpermeability of magnets is almost the same than with air). Between those two cases, I assume that the...
  17. jpts

    Tesla Motors

    I also found it strange that Tesla went into PM motors. Yes, they have somewhat smaller weight/size and bit better efficiency compared to induction motor, but well designed induction motor with squirrel-cage is not bad either. I think the biggest problem with PM motors is that they are highly...
  18. jpts

    1200 kW (12 poles) and 6200 kW (4 poles) (Both 11 kV Motors) windings coil comparison?

    I think it is quite well-known fact (for machine designer) that physical size of any electrical machine (and therefore active material consumption) is more or less depending on the torque rather than power. You can for example compare weight power ratio of a low-speed machine (e.g. hydro...
  19. jpts

    1200 kW (12 poles) and 6200 kW (4 poles) (Both 11 kV Motors) windings coil comparison?

    Probably the 12 pole -machine is a low-speed one (maybe few hundred RPM nominal), which means high torque and thus large active part volume. 4-pole machine is then probably running at higher nominal speed (maybe around 1500RPM), meaning that the torque is still in the same range even though hp...
  20. jpts

    hybrid induction/pm motor may be coming...

    Normal PM machines (whether SPM or IPM type) are typically not able to start directly on-line as mentioned also by cswilson, since there is not enough reluctance torque or special starter winding (squirrel-cage) in the rotor. On the other hand, there is often no need to DOL start, since they are...

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