Motors

[john37]

Hi. I'm just starting out in the electrical field. Can someone please explain the difference between Induction & Synchronous motor types in layman terms? Also, what type of motors does air handlers usually use, induction or synchronous? Thanks.

 

[jim dungar]

The vast majority (maybe 99%) of all AC motors are the induction type. Synchrounous motors are normally only used in special applications ( large volume pumping), although they are common in some industries (i.e. steel mills).

Induction motors require relatively simple starters. Synchronous motors require more complex starters which include exciters.

 

[charlie b]

When you connect a source of three phase power to a three phase motor (of either type), it creates a magnetic field in the stator (the stationary part). This field is felt within the rotor (the moving part). The field will rotate around the stator at a speed determined by the frequency of the source (generally, this is 60 hertz in the US). When I say that the field ?rotates,? I mean that there will be a Magnetic North Pole at the top of the machine at one moment, that a moment later that Magnetic North Pole can be observed at the side of the machine, and a moment later it is at the bottom of the machine, and around the circle it goes.

The term ?synchronous? refers to the fact that the rotational speed of the motor is in exact synchronization with the rotating speed of the magnetic field. If the field rotates at a rate of 20 rotations per second (1200 revolutions per minute), then by golly the rotor will rotate at that exact same 1200 revolutions per minute. It can rotate at no other speed. That is due to the way it is constructed.

The rotor of an ?induction? motor will also rotate in response to the magnetic field that is rotating around it. But it cannot rotate at the same speed. Here again, due to the way it is constructed, it will rotate at a slightly slower speed than the speed of the field. The more load you put on the motor, the slower it will run.

This of course brings up the questions, (1) What is the difference in the way the two are constructed, that causes them to behave in the manner described above, and (2) What are the advantages of one over the other? Those will have to wait for another day.

 

[Gregg Harris]

You might also be looking for synchronous speed of induction motors,which deals with the frequency such as 60 hertz and the number of poles that the motor contains gives you the motor RPM.?:

 

[john37]

Thanks for the explaination guys. That was really helpful.
After reading through Article 440 (specifically Article 440.22(A) if I'm understanding it correctly to size the overcurrent protection device you would do the following for an a/c unit.

For example:

ACCU-1
240volts - 1 phase
Compressor: 12RLA
Fan: 0.75FLA

15RLA x 175% + 0.75FLA = 27.0A
Therefore you would use a 1P30A breaker.

Am I on the right track here?

 

[bob]

Thanks for the explaination guys. That was really helpful.
After reading through Article 440 (specifically Article 440.22(A) if I'm understanding it correctly to size the overcurrent protection device you would do the following for an a/c unit.
For example:
ACCU-1
240volts - 1 phase
Compressor: 12RLA
Fan: 0.75FLA

15RLA x 175% + 0.75FLA = 27.0A
Therefore you would use a 1P30A breaker.

Am I on the right track here?

The A/C unit should have a name plate that gives you the minimum ampacity
for the conductor and the max ampacity for the breaker.

 

[72.5kv]

Rotor contruction is different for a synchrounous motor and asynchrounous induction motor.

Synchrounous motors have wound rotors that are wired for DC. This is the exciter. Once the synchrounous motor start, at about 95% of full speed, DC is supplied to the rotor that that brings the rotor up to sychrounous speed. Now the rotor is rotating at the same speed at the rotating as the magnetic field in the stator.