Everywhere I see slip explained it says that is the percentage difference between the speed, or frequency of the rotor and the magnetic field. However the only way I can imagine magnetic field and rotor having different speeds is if the angle between the two is constantly increasing. Is this what is actually happening?
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\$\begingroup\$ Yeah. Note that for a 60 Hz, two-pole induction motor, the magnetic field rotates at 60 Hz which is 3600 RPM. So 3600 RPM is referred to as synchronous speed for that type of motor (at 60 Hz). The difference between synchronous speed and actual speed can be used to calculate slip. To a first approximation, torque goes up linearly with slip. So when you apply torque to slow the motor down, the slip increases and the output torque increases until balance is achieved (or until the motor stalls and trips a breaker I guess). \$\endgroup\$– user57037May 8, 2018 at 5:59
2 Answers
However the only way I can imagine magnetic field and rotor having different speeds is if the angle between the two is constantly increasing. Is this what is actually happening?
Yes.
However, we normally express 'angle is constantly increasing' as 'angular speed is different'.
If the field is rotating at 50Hz, and the rotor is rotating at 47Hz, then the slip frequency is 3Hz, or 6%. This is the effective frequency seen by the 'transformer' whose secondary consists of the shorted rotor circuits. This is also a convenient explanation of why the torque of an induction motor would be zero at 0% slip, because the transformer will not induce any current at all at 0Hz.
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\$\begingroup\$ Thanks! I also assume that the larger the slip, the larger the current will be induced in the rotor? \$\endgroup\$ May 8, 2018 at 18:22
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\$\begingroup\$ @AnthropomorphousDodecahedron up to a point. The fixed rotor as used in small motors tends to have an L/R ratio suitable for normal running. When the slip frequency gets too big, the L term starts to dominate and the induced current drops, which is why small induction motors have pathetic starting torque. On big motors, wound rotors are more common, and extra resistance can be switched in for starting. \$\endgroup\$– Neil_UKMay 8, 2018 at 18:25
How about this: "Slip" pertains to "induction" motors and describes the time lag inherent in their design. If you think about the circuit, you are only applying electrical power to the Stator portion to create magnetic fields. For there to be repulsion / attraction of the magnetic fields to cause the Rotor to spin, the Rotor needs electrical energy as well. That electrical energy is "induced" to flow in the Rotor bars across the air gap via the constantly expanding and collapsing magnetic fields in the stator. Because this takes place as an inductive circuit, it is subject to the inductive time constant; current cannot change in an inductor faster than a specific rate. So that ends up as a delay in the formation of the magnetic fields in the rotor compared to the stator, and that delay is what we call "slip".