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motor starts per hour limit

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  • motor starts per hour limit

    If using a VFD with conservative ramps, couldn't the limit for starts per hour on the motor be exceeded since the large starting current is avoided?

    In this case ramp up is 3 seconds and ramp down is 10 seconds (coast with no braking).
    Last edited by JPR; 11-30-2006, 02:53 PM.
    John

  • #2
    You've hit a critical issue exactly. The across the line starts per hour for an induction motor are limited by the connected inertia. The more interia (flywheels etc) the fewer the starts. Motor manufacturers have in their engineering guides the charts and formulae for calculating the starts under varying inertia, ambient temperature, and power quality.

    With a VFD all these things fly right out the window because the VFD controls slip and max current. When the VFD parameters are correctly set the motor "motors" the load to speed following the accel ramp or the max current setting which ever is greater. Starting surges are practically eliminated. It also regenerates when it ramps to stop storing the energy in the DC buss and then dissipating it via the DB resistor.

    I looked seriously into repowering a 50 HP table motor Cincinnatti HyPro planer replacing the original 75 HP Ward-Leonard drive with an induction motor and a VFD. All the numbers worked and the savings in power were significant. There would have been some loss in table thrust at lower table speeds (those Ward-Leonard drives were like a geared transmission when it came to constant HP curves) but nothing major if the drive and motor are selected with that in mind. I selected a 75 HP 875 RPM motor connected for 230 Volts and a 460 Volt 150 HP drive with the V/Hz set to 3.83. This provides 50 HP at 582 RPM at 100% duty cycle. Given the speed torque characteristics of mill motors they are capable of 150% rated torque at reduced duty cycle and so on. The result is that only in the lowest table speeds is the table thrust limited. It's almost a direct change out. Peak line currents are less than with the original GE Ward-Leonard drive. The motor never sees full load current above 300 RPM. The motor natrually has to be separate blow cooled.

    Table speeds and accellerations are increased. In fact the reversals could be set to be so violent that a washer or clamp left lying loose will slide up to six feet on the table depending on cut and return speed settings. I selected settings to keep the table from lurching under "Jog" speed to aid set-up.

    I presented the study to the customer but he didn't like the price tag for Plan "A" which used all new components. He went with Plan "B" which I presented using scrounded used stuff and having reduced performance. Poppleton Electric executed the proposal which was designed by a moonlight EE out of Boeing. Figgers. Plan "B" was about 1/4 the cost of Plan "A".

    Anyway if your VFD parameters are set correctly, you can plug reverse the motor to the limits of accel/decel time without let or hindrance.

    Parenthetically an unloaded three phase motor can be plug reversed across the line about 20 times a minute.
    Last edited by Forrest Addy; 11-30-2006, 04:33 PM.

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