FAQ - Synchronous Motors

  • Motor is getting excessively hot? I cannot touch it!
    Yes. It is possible. Motor is rated accordingly. (a) Motor generates heat due to resistive watt loss (b) Inductive / Eddy current watt loss (c) Continuous operation/ Higher duty cycle.
    Lastly motors are made with class "F" insulation. That means it will withstand higher temperatures like motor body temp up to 90 Deg. Solution is to use heat sink or use cooling fan or use both.
  • What is heat sink? How to fit heat sink?
    Heat sink is an aluminum body with fins. Supplied by Srijan in two parts can be fitted on motor. Please remember to fit the heat sink every time with the layer of 'heat sink compound' for even/equal heat distribution.
    Please note: 'Heat sink compound/chemical' is available in any electronic solder shop.
  • What is R-C network?
    Resistance Capacitor network. It is a phase shift circuit. It converts a 1-phase supply in to 2-phase supply. The RC network is provided along with motor as it is designed for that particular type of the motor.
    Please note: Cost is included in the price of motor. It is not inter-changeable between different motors.
  • What is a Scott transformer?
    This transformer has input 3-phase 415 VAC and output is 2-phase 230 VAC (90 Deg phase shift). Since Srijan motors requires this (2-phase) input, few customers use it.
  • What are the advantages / disadvantages of Scott transformer?
    Advantages: Scott transformer runs AC Synchronous motor with best possible performance! Highest torque, motor runs cooler and with least noise and least vibration. A small size of motor can produce more torque. Higher wattage Scott can drive multiple motors in synchronization.
    Disadvantages: It is expensive. Requires 3-phase supply.
  • Why motor is not lifting weight as per specifications on label?
    It should and it will provide inertia if the load is taken care of!! Means use flexible coupling or flexible rope / string to lift the load. Or help the motor (a bit manually) to lift the load. Alternatively, it can lift the load if it is evenly distributed. Please read FAQ>Stepper motor> Acceleration. Stepper motors can be accelerated using digital controls. It is not possible with synchronous motors.
  • What is the RPM (Revolution per Minute) of motor?
    1. The RPM of AC Synchronous motor is directly proportional to the frequency of AC supply.
    2. In India, frequency of AC supply is 50Hz +/- 2% and hence RPM is 60 RPM +/- 2%
    3. When AC supply freq is 60 Hz, RPM is 72. RC Network is changed for 60Hz supply.
    Please Note: Torque reduces proportionately when RPM increases.

    1) Stall Torque = Weight (Kg) * Distance (cm) / 9.8
    2) Torque Constant = Peak Torque / Current
    3) Torque(N.m)= Force(N) * Radius(meter)
    4) Torque = J * α
                        = J * (ωfinal-ωinitial )/time(sec)
    α = Angular acceleration in radians/sec2
    J = Moment of Inertia in kg.meter2
    ω= radians/sec
    5) Wattage = Torque (Nm) * ω (rad/sec)
    Important Conversions:
    1 radian = 57.2957 °= 360/2*pi
    1 rad/sec = 9.554 RPM
    1 N = 0.10197 Kgf = 0.10197 Kg
    1 kgf = 9.81 N (Note:This is force)
    1 kg = 2.2046 pounds(lb) = 35.273 961 95 ounce(oz) (Note: This is weight/mass)
    1 N.m = 10.2 kgf.cm = 141.612 Ozf.in = 0.7375 lb.ft
    1 HP = 746 Watts (approx)
    6) Motor Power:
                        P (Watts) = Torque (N.m) * ω (rad/sec)
                                          = 2*pi* N (RPM)*Torque (N.m)/60
    7) Back EMF constant is calculated as follows.
    Eb= Generated Voltage / (Rads/Sec) = Vb*9.554/RPM
    8) Step angel calculation

    φ = Step angle
    NS =Number of teeth on stator
    Nr = Number of teeth on rotor
    Steps per Second = (rpm * steps /revaluation)*60
Please Note: Torque reduces proportionately when RPM increases.