What is the Single Phase Motor Humming Problem?

Single Phase Motor Humming Problem is the alternating magnetic field trying to move the rotor in both directions at the same time. 

Reason No. 1

Testing of start circuit: A quick test for start winding failure I frequently use is wrap about 15 turns of string around the shaft, switch the motor on, and as its buzzing, very positively pull the loose end of the string till it comes off the shaft: This will give it the starting momentum and direction to run. 

If the motor runs, there is a single phase motor humming problem with the start winding. Look at the capacitor located inside or near the connection box. The buzzing indicates that the RUN winding is good.

Reason No. 2

There may be the reason If the motor shaft cannot be easily turned, the following may apply:

The rotor is dragging on the Field laminations inside the Squirrel Cage. The bearings or Bushes could be worn, or, if the motor end shields were removed, they were not correctly put back on, and need to be lined up.

The load could also be either seized, jammed or be just too big.

What happens if we increase or decrease running winding turns of a single phase induction motor?

If we increase turns

Idle current and HP will decrease in proportions to the square of the change in turns. Locked rotor current and starting torque will decrease in direct proportion to the change in turns.

If we decrease turns

Idle current and HP will increase in proportion to the square of the change in turns, Locked rotor current and starting torque will also increase in direct proportion to the change in turns.

Decreasing in turns leads to core saturation. For example, a 5hp motor, and that core (rotor and stator), are not going to do well if you try to make it a 10hp motor.

We can’t decrease turns much unless you want to make a hot motor with say a 10-minute duty cycle (10 minutes per hour).

If we have capacity margin in motor

If you had a motor made before 1950 you could probably increase the HP by 25% because they had large frames, low flux density, and low design temperature.

Your starting and accelerating torque will drop a little unless you change the start winding too.

Why are circular coils preferred in transformers?

The circular coils in the transformer have the following advantages over rectangular form coils.

• The circular coil has a minimum mean length of the turn. If the diameter of the circular coil is d, the length of one turn πd. The length of the one turn of the rectangular coil is 2d. Thus, the circular coils have about 1/3 rd length in comparison with rectangular coils. The less length of the wire means the less ohmic value because the resistance of the wire is directly proportional to its length. The heat loss in the case of the circular coil is minimum.

• Under fault conditions, the coils of the transformer experienced heavy electromagnetic forces. The force act on the central axis of the coil. In a circular coil, the force is even throughout the coil, whereas the force on the rectangular coil is uneven and the force is maximum at the center and weaker at the edges of the coil. Thus, the rectangular coils are prone to damage under fault conditions.

• The winding of the circular coil is easy to form. There is a chance of insulation damage at the corners of the rectangular coils.

Related Post:

Why is the core of power transformer grounded?

What is PS Class CT?

Protection special class CT  'PS" or 'PX' is used for protection of the electrical equipment like motor, transformer, generator, and bus bar. The protection or P class current transformer is used for the protection of feeders. 

The central theme of any electrical protection scheme is that the current should not saturate at the time when fault current flows through the current transformer. Under CT saturation conditions, it behaves as an open circuit, and protection of the electrical equipment is not guaranteed. 

The protection class or 'P' class CT like 5P10, 5P20 is used for the protection of feeders. The knee point voltage of protection class CT  is more than the knee point voltage of the metering class CT. It means the protection class CT saturates at much higher CT secondary voltage.

For the protection of the alternator, generator, high tension motor and bus bar, protection zone, differential protection, or unit protection scheme is used. The unit protection scheme provides tripping to the breaker in case there is a fault within the equipment. For instance, the differential protection trip in the transformer feeder shows that there is a fault in the transformer. If the differential protection relay operates, it is essential to check the transformer before switching on the equipment again.

The differential protection scheme is designed by selecting the PS class CT. The PS class CT has a higher knee point voltage. The following parameters of the current transformer of CT used for the PS class must be defined.

• Knee Point Voltage Vk(KPV) Minimum - Volts

• Excitation Current(Maximum) at Vk/2 - Amperes

• Rct (secondary winding resistance) at 75 Degree Centigrade (In Ohm)

Specifications of the PS class CT are as given below.

The PS class CT used for the differential transformer must have identical magnetization characteristics. Any Aberration in the CT characteristics used on the primary and secondary side may lead to spurious tripping during the through fault condition.