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    Choosing Varistor for AC motor Application.

    I would like to build a AC motor control board for bidirectional AC motors. I am trying to build a system similiar to this one: On semiconductor AND8017/D application, on page 2.
    link: http://www.onsemi.com/pub_link/Collateral/AND8017-D.PDF

    There are MOVs which are Metal Oxide Varistors and I am confused about choosing right varistor. There will be flybacks to system because of changing direction.

    Could you please give me suggestions?

    Thanks
    Best Regards.

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    Re: Choosing Varistor for AC motor Application.

    THe Triac is rated for 600Vp and the motor locked rotor current is (5.02 Amps rms)
    So the voltage of the MOV must be less than 600V at the rated locked rotor current when the triac is open circuit.

    The QUestion is, what is the inductive energy stored in a "locked rotor" or motor changing direction at zero speed, when switched off that must be absorbed by the MOV.

    THe motor 230Vrms, 1.9 Arms, 1/4 Hp impedance at full load if one assumes inductance, X(f)= 230/1.9A

    Can you figure that part out in Joules and then add a safety factor for temperature rise and a MUCHO bigger safety factor for the triac and MOV if the controller allows changing direction at full speed. ichi wawa
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975


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    Re: Choosing Varistor for AC motor Application.

    Dear Sunnyskyguy, I am using a 4 pole 230V, 0.18kW 1/4hp motor. In the technical document, running current is 1.6A and Locked rotor current is 4.7A. I will not turn the direction at the running time. I will stop motor for 3seconds and than turn it back. I will going to use bta16-800cw triacs and moc3041 triac driver.

    What you suggest me?

    How can i make the correct calculation for MOVs?

    Thanks
    Best regards



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    Re: Choosing Varistor for AC motor Application.

    800V triacs may be overkill. for a 230V line . One way to specify MOV's is the max tolerance or rated line voltage of 230V and twice the Locked Rotor Current or 10A @230Vac rated operating voltage. Other considerations are power interruptions on the line.

    This MOV comes in many packages from SMT to connectorized for Phoenix contacts as they have a limited number of operations for line surges but are often rated for 10kA line surges when an arc occurs across the motor or line.

    Panasonic make nice leaded or thruhole Square MOV's.

    - - - Updated - - -

    800V triacs might seem to be overkill. for a 230V line , but it is not.
    AC Line surge is rated at Surge Current, 8/20µs 10,000 A thus a continuous 230Vac line at +10 tolerance or 253Vac or >253Vac , one would choose 275V and find one where 10kA is << 800V

    I suggest this for your region.
    The iTMOV ® varistor differs from the TMOV ® varistor by the
    inclusion of a third lead for the purpose of indicating that
    the MOV has been disconnected from the circuit. This lead
    facilitates connection to monitoring circuitry.
    LITTELFUSE TMOV20RP275M is the iTMOV type with a 3rd sense lead and has 710V @100A max from a 8/20µs lightning surge. THese $2.26 but not normally stocked. So find what is available in similar sizes.

    They can be as cheap as $0.25 but remember they dont take surges forever and the above sense line is a feature.
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975


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