SYNCHRONOUS-MOTOR TROUBLE CHART

Symptom and Possible Cause	Possible Remedy
Motor Will Not Start
(a) Faulty connection	(a) Inspect for open or poorconnection.
(b) Open circuit, one phase	(b) Test, locate, and repair.
(c) Short circuit, one phase	(c) Open and repair.
(d) Voltage falls too low	(d) Reduce the impedance of the external circuit.
(e) Friction high	(e) Make sure bearings are properly lubricated. Check bearing tightness. Check belt tension. Check load friction. Check alignment.
(f) Field excited	(f) Be sure field-applying con tractor is open and field discharge contactor is closed through discharge resis- tance.
(g) Load too great	(g) Remove part of load.
(h) Automatic field relay not working	(h) Check power supply to so lenoid. Check contact or tips. Check connections.
( i) Wrong direction of rotation	(i) Reverse any two leads.
Motor Will Not Come Up to Speed
( a ) Excessive load	(a) Decrease the load. Check operation of unloading device (if any) on driven machine.
(b) Low voltage	(b) Increase voltage.
(c) Field excited	(c) Be sure field-applying con tactor is open, and field discharge contactor is closed through discharge resis tance.
Fails to Pull Into Step
(a) No field excitation	(a) Check circuit connections. Be sure field-applying con tactor is operating. Check for open circuit in field or ex citer. Check exciter output. Check rheostat. Set rheostat to give rated field current when field is applied. Check contacts of switches
(b) Excessive load	(b) Reduce load. Check opera tion of unloading device (if any) on driven machine.
(c) Load inertia excessive	(c) May be a misapplication consult manufacturer.
Motor Pulls Out of Step or Trips Breaker
(a) Low exciter voltage	(a) Increase excitation. Examine exciter. Check field ammeter and its shunt to be sure read ing is not higher than actual current.
(b) Open circuit in field and ex citer circuit	(b) Test with magneto and repair break.
(d) Reversed field spool	(d) Check with low-voltage and polarity indicator and reverse incorrect leads.
(e) Load fluctuates widely	(e) See “Motor Hunts.”
(f) Excessive torque peak	(f) Check driven machine for bad adjustment, or consult motor manufacturer
(g) Power fails	(g) Re-establish power circuit.
(h) Line voltage too low	(h) Increase if possible. Raise excitation.
Motor Hunts
(a) Fluctuating load	(a) Correct excessive torque peak at driven machine or consult motor manufacturer. If driven machine is a com pressor, check valve opera tions. Increase or decrease flywheel size. Try decreasing or increasing motor field current.
Stator Overheats in Spots
(a) Rotor not centered	(a) Realign and shim stator or bearings.
(b) Open phase	(b) Check connections and cor rect.
(c) Unbalanced currents	(c) Loose connections. Improper internal connections.
One or More Coils Overheat
(a) Short circuit	(a) Cut out coil as expedient (in motors up to 3.73 kW). Re place coil when the opportu nity arises.
Field Overheats
(a) Short circuit in a field coil	(a) Replace or repair.
(b) Excessive field current	(b) Reduce excitation until stator current is at nameplate value.
All Parts Overheat
(a) Overload	(a) Reduce load or increase motor size
(b) Over- or under-excitation	(b) Adjust excitation to name plate rating.
(e) No field excitation	(e) See that nameplate voltage is applied
(f) Improper ventilation	Remove any obstruction and clean out dirt
(g) Excessive room temperature	(g) Supply cooler air.

SUMMARY

A synchronous motor is one which is in unison or in step with the phase of the alternating current which operates it. In construction, synchronous motors are nearly identical with the corresponding alter nator, and consist essentially of the stator (armature) and the rotor (field).

When a balanced polyphase alternating current is supplied to the armature of a synchronous motor, it produces a rotating magnetic field which rotates in synchronism with that of the supply current. This rotating magnetic field acts on the damper or amortisseur squirrel-cage winding to produce a starting torque, causing the rotor to rotate.

The speed of a synchronous motor is determined by the frequency of the supply current and the number of poles of the motor. The equation for the determination of motor speed is:

in which;

r/min p

P == number of poles of the motor.

The torque required to operate the driven machine at all moments occurring between initial breakaway and final shutdown is important in determining the motor characteristics. The various torques associated with synchronous motors are starting torque, running torque, pull-in torque, and pull-out torque. The running torque is determined by the horsepower and speed of the driven machine, and at any given point, the torque in lb.-ft. is:

To obtain metric:take answer and multiply by 0.1383 to get kilogram- meters.

The power in watts (Ht} delivered through a single-phase ac circuit is the product of the current, voltage, and power factor, as shown in the equation:

In a three-phase ac circuit the power in watts can be determined by the equation:

Methods most commonly used in starting synchronous motors are:

(1) across-the-line (2) reduced voltage (3) reactance; (4) resistance; and (5) Korndorfer.

The various classes of service for which synchronous motors may be used are classified as: (1) power-factor correction; (2) constant-speed, constant-load drives and (3) voltage regulation.