Motor slip

[domnic]

When talking about a motor what is slip ?

 

[mivey]

When talking about a motor what is slip ?

The difference in how fast the field in the stator rotates vs how fast the rotor rotates. Slip is needed to produce torque. The more load you add, the more it slips (the rotor slows relative to the stator field).

 

[just the cowboy]

Difference in RMP

Difference in RMP

It is the calculated RPM verse the actual RPM.
The formula is
n= 2 x f x 60/p
Where
N=speed
2 = number of time poles change in 1 cycle
f= frequency
60 = number of seconds in a minute
p= number of poles in the motor
Example : A motor rated at 1725 RMP motor is calculated at 1800 RPM. There is a 75 RPM slip for that motor, at unloaded RMP. The slip can change with the load and in part contributes to the change in current, no slip is known as locked rotor current.

 

[kwired]

It is the calculated RPM verse the actual RPM.
The formula is
n= 2 x f x 60/p
Where
N=speed
2 = number of time poles change in 1 cycle
f= frequency
60 = number of seconds in a minute
p= number of poles in the motor
Example : A motor rated at 1725 RMP motor is calculated at 1800 RPM. There is a 75 RPM slip for that motor, at unloaded RMP. The slip can change with the load and in part contributes to the change in current, no slip is known as locked rotor current.

Wouldn't locked rotor current occur when there is total slippage? No slip means rotor is synchronous with the stator field rotation doesn't it?

Slip does change with the load. AFAIK if a motor is rated 1725 RPM that means it will turn at 1725 RPM if rated voltage and frequency is applied and it is delivering rated horsepower. Change any of those factors and speed will change.

 

[iwire]

There is a 75 RPM slip for that motor, at unloaded RMP.

I have assumed the 1725 was the RPM at rated HP output.

 

[iwire]

AFAIK if a motor is rated 1725 RPM that means it will turn at 1725 RPM if rated voltage and frequency is applied and it is delivering rated horsepower. Change any of those factors and speed will change.

That was my assumption / impression.

 

[dkidd]

Slip is needed to produce torque.

So synchronous motors can't produce any torque? :?

 

[dkidd]

no slip is known as locked rotor current.

slip = current? :?

 

[mivey]

So synchronous motors can't produce any torque? :?

When the rotor slips, it cuts the magnetic field lines and induces a current in the rotor and is why we call it an induction motor.

This current creates a field that will oppose changes in the rotor current and thus we wind up with a balance between the induced current and applied torque.

A synchronous motor does not use slip to create the magnetic field in the rotor but it is separately created by magnets rather than cutting field lines.

 

[mivey]

slip = current? :?

No. He meant the condition of slip.

 

[mivey]

Wouldn't locked rotor current occur when there is total slippage?

Yes.

 

[dkidd]

created by magnets rather than cutting field lines.

I thought that an external source of DC current was usually applied to the rotor.

 

[mivey]

I thought that an external source of DC current was usually applied to the rotor.

That is for the electromagnets (electrical magnets).

 

[Strathead]

So synchronous motors can't produce any torque? :?

To be clear, a "synchronous motor" is a different style of motor than the ones that are referred to when talking about slip. The two that immediately come to mind are wire wound motors, and permanent magnet motors. So a synchronous motor is different than a motor running at synchronous speed.

 

[Ingenieur]

Electrical speed - Mechanical speed delta
as others noted required to produce torque
the driven load causes it
Think of it as 'stretching' the magnetic fields and them resisting

 

[GoldDigger]

Electrical speed - Mechanical speed delta
as others noted required to produce torque
the driven load causes it
Think of it as 'stretching' the magnetic fields and them resisting

Except that you could imagine the field stretching through a angle offset without a speed slip.
That makes the drag metaphor more suitable than stretch.

 

[Ingenieur]

Except that you could imagine the field stretching through a angle offset without a speed slip.
That makes the drag metaphor more suitable than stretch.

Sure
what ever you say
lol

you could also 'drag' at a constant angle and slip at no load

 

[gar]

160309-1956 EST

Strathead:

To be clear, a "synchronous motor" is a different style of motor than the ones that are referred to when talking about slip. The two that immediately come to mind are wire wound motors, and permanent magnet motors. So a synchronous motor is different than a motor running at synchronous speed.

I don't understand what you were trying to say.

A synchronous motor is an AC motor where the rotor runs at exactly the same average speed as the rotating magnetic field from the AC power source. The synchronous motor is built with the same kind of stator coils as in an induction motor. These stator coils produce a magnetic field that rotates in space. This is equivalent to putting a bar magnet on the end of a shaft and rotating the bar magnet.

The rotor of a synchronous motor has a fixed magnet on it. This can be from a permanent magnet, or a wound coil magnet supplied via slip rings from a constant current source (DC source).

Now we have two magnets phased to attract each. One is the permanent magnet or DC excited coil magnetic on the rotor and the other is the rotating magnetic field in space created by the stator coils with AC excitation. There is an attractive pull between these two and they run in exact synchronization.

There is an angular shift between between the rotating field and the motor shaft that is dependent upon the torque load on the shaft. The larger the load the larger is the shift until a break-a-way torque. Beyond this point the shaft stops rotating but gets pulses of torque.

The the instantaneous speed of the rotor can fluctuate very slightly above or below the synchronous speed as the torque load changes. In other words frequency modulation of shaft speed. But average speed must equal synchronous speed or the motor will slip out of synchronization.

Stepping motors are synchronous motors. So called brushless DC motors are really AC synchronous motors.

Synchronous motors can be built with the fixed magnet as the stator and the rotating field on the rotor. But generally this is not done because it is less efficient at removing heat from the motor.

.

 

[Ingenieur]

http://www.usmotors.com/TechDocs/ProFacts/Motor-Machinery-Terminology

Coupling Angle: The mechanical degree relationship between the rotor and the rotating electrical field in a motor. While present in both synchronous and non-synchronous A.C. motors, it is usually of concern in synchronous applications. At no load, the rotor poles line up exactly with the field poles and the coupling angle is considered to be zero. When a load is applied, the lines of force coupling the rotor with the stator field are stretched, causing the rotor to fall behind the field. The mechanical angle by which the rotor lags behind the field is called the coupling angle. The coupling angle will continue to increase with load until it reaches the "pull-out" point. The maximum angle which is possible prior to pull-out is dependent on motor type and rotor design.

 

[Tony S]

slip = current? :?

In effect it does but not in the stator, the induced current flows in the rotor cage.

One way it was shown to me as an apprentice was the rotor current of a slip ring motor. It’s weird watching as it pulses in time with the slip frequency.