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  1. tmahan

    VSD DAMAGED - WHY?

    Back to the original problem of a failed VFD.... There are as noted above many reason for a drive (or any device to fail). It may very well turn out that neither the welding or the blocked fan was responsible. Having said that though , I am in the camp of the blocked fan. Although most...
  2. tmahan

    Wye/Delta current

    I'm just guessing, but it looks to me like the coils that make up the T3-T6 loop were put into parallel rather than series. That would explain why the currents on that winding loop are so much higher. The various phase currents are simply taking the path of least reistance and that happen to be...
  3. tmahan

    Motor Stall Times

    Electricpete With regards to small motor rotor construction. I once worked for a motor manufacturer in Houston... At that plant all small motors were made by pouring molten Al into the stacked laminations. Therefore all rotor bars were solid Al.and the rotor was a solid unit. In my opinion...
  4. tmahan

    Electronic Brake

    Your message indicates that the overload is tripping during the restart after braking. To me this suggest that the thermal overload is in the circuit (this is good) when the brake is actually firing. The DC current usually 2-3x the motor full load current (Jraef, is that right?) It further...
  5. tmahan

    Need Help Finding a Suitable VFD

    ABB and Toshiba both have very good products. Both have wide acceptance in the HVAC industry
  6. tmahan

    Power factor correction.

    as a short term solution is it possible to use a smaller "pony motor" to get the primary motor spinning? it sound like you could use the coupling to disengage the motor from the load. If this is possible, it will greatly reduce the current demand once the 3kV voltage is applied to...
  7. tmahan

    Exhaust fan motor with auto xfrmr starter sagging

    I think a decrease in current typically indicates a reduced load (or an increase in voltage if power output remained constant) Sounds to me like the fan is doing less work.....what is the pressure in the area being exhausted doing??
  8. tmahan

    Are winding RTD's affected by VFD operation ?

    my initial thought was that a RTD should not be affected by the use of VFD as a power source for the motor. Then I began to wonder if an induced voltage could cause a mis read by the actual device sensing the voltage drop across the RTD. Or could the RTD wires act as an antena for for the...
  9. tmahan

    SOFT STARTERS FOR OIL PUMPING MOTOR

    An additional problem that I had not considered has actually shown up on a similar application. Depending on how the softstarter manufacturer detects for shorted SCR's there could be phantom trips. In cases where shorted SCRS are detected by a voltage drop across the SCRs regeneration may...
  10. tmahan

    SOFT STARTERS FOR OIL PUMPING MOTOR

    Should not be a problem. Once a softstarter has finished starting the motor (figure a 5-10 second ramp time for a walking beam) the softstarter pretty much just acts like a contactor. There probably is a contactor in parallel with the softstarter in fact. Since there is no frequecy conversion...
  11. tmahan

    Pony Motor

    Actually for the case where the motors are coupled only a 150kW softstarter may be required. If the application permits it is possible to use a softstarter to start the first motor. Once the motor is "at speed", a contactor can be used to switch the second motor onto the line. This...
  12. tmahan

    POWERBOSS Motor Controller

    As Sed2developer mentions it is not simply enough to look at the "saved energy". the actual cost of the energy saved overtime must be discounted a some rate and compared over time to the cost of the NOLA based device. An additional point to mention is the issue of heat generation...
  13. tmahan

    Variable Frequency Drive

    JOmega, Typically Toshiba (the G2 and G3) has used a softcharge resistor on the DC bus. The resistor is switched out of the circuit once the DC bus caps were fully charged. The idea being that the Front end rectifier could short while the Caps were charging. Has this been removed from the G7...
  14. tmahan

    Variable Frequency Drive

    The comment "It happened twice" sound as though the drive was still functional. All of the problems above (shorted IGBT's, blown rectifiers ect would have been fatal to the drive. The G7 is smart enough to detect and announce that there was an IGBT failure. If the supposition...
  15. tmahan

    Best Motors and VSD's

    Every year more and more companies produce variable speed drives. In response to this pressure the major producers (guys like ABB, Toshiba, AB, Siemens) have had to struggle to keep their product from being "just another drive" . The way most have chosen to go is by being...
  16. tmahan

    Starting a large motor

    Several companies make solid state starters in that size range that could be used. With any reduced voltage starting method the load's starting torque requirements need to be considered carefully.
  17. tmahan

    MULTIPLE MOTORS ON AC DRIVE PACKAGE

    Prior to blaming the drive or the motor's lack of inverter duty rating I would check the simple and in-expensive stuff. In an installation such as this I think that each motor should have its own overload protection. Is that the case here?
  18. tmahan

    Frequent Starting of Low Inertia Loads

    This raise some additional questions to my mind as to how engineers (Users and designers) come up with "rules." The real issue is heat inside the motor. Rather than rely on a one size fits all standard why not use RTD or PTC to monitor the motor temperature directly, and then tie...
  19. tmahan

    Convert 400hz to 50 or 60 hz

    Lets check units 10kW as in the first post or 10,000kVA as in the last post.
  20. tmahan

    Convert 400hz to 50 or 60 hz

    The input frequecy wouldn't really make a difference would it? The drive in that size range would use a diode front end to convert to DC. I don't think heating would be much issue because the current wouldn't differ from the 60Hz input case.

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