Technology / Technical guide
Synchronous & Induction Generators: Fields, Speed and Slip
Wind-generator terminology describes how mechanical rotation interacts with a magnetic field. Synchronous and induction machines use different relationships between that field and the rotor. Neither name alone establishes the terminal voltage, converter functions or complete electrical arrangement.
Compare rotor motion with the field, not with a product label.
Synchronous speed belongs to a defined electrical field frequency and pole count; induction slip expresses relative motion against that field.
01 / Rotating field
A magnetic pattern and a physical rotor are distinct.
An electrical winding arrangement can produce a rotating magnetic pattern.
An electrical winding arrangement can produce a rotating magnetic pattern. Its mechanical rotation rate depends on electrical frequency and pole pairs. A physical rotor can follow that pattern synchronously or move relative to it, depending on the machine and operating condition.
A synchronous machine’s rotor field can be provided by permanent magnets or electrical excitation. Permanent-magnet construction is one synchronous-machine form, rather than the definition of every synchronous generator. Induction machines create rotor electrical behaviour through interaction with changing linked fields.
02 / Synchronous speed
Use pole pairs consistently in the speed relationship.
For electrical field frequency fe in hertz and p pole pairs, mechanical synchronous speed is ns = 60fe/p revolutions per minute.
For electrical field frequency fe in hertz and p pole pairs, mechanical synchronous speed is ns = 60fe/p revolutions per minute. If total poles P are used instead, the same relation is ns = 120fe/P. The two counts must not be interchanged.
DOE’s generator reference explains the frequency, rotational speed and pole-count relationship. A hypothetical 40 Hz field with two pole pairs has ns = 1,200 rpm. These assumed values identify no MARSTON pole count, electrical frequency or operating speed.
03 / Slip definition
Relative speed is different from synchronous rotor angle.
Using the conventional induction-machine definition, slip is s = (ns − nr)/ns, with rotor speed nr and nonzero ns expressed in matching units.
Using the conventional induction-machine definition, slip is s = (ns − nr)/ns, with rotor speed nr and nonzero ns expressed in matching units. Slip speed is a speed difference; slip itself is dimensionless. MathWorks distinguishes rotor electrical speed from synchronous-field speed in its model.
In the hypothetical 1,200 rpm field example, a 1,260 rpm rotor gives s = −0.05. This is a mathematical sign example. A synchronous machine’s load angle instead describes an angular relationship with the field; it is not the dimensionless induction-slip variable.
04 / Induction action
The generating interpretation needs a declared machine model.
In the basic steady squirrel-cage induction model with an established stator field, rotation above synchronous speed corresponds to generating action.
In the basic steady squirrel-cage induction model with an established stator field, rotation above synchronous speed corresponds to generating action. Induced rotor currents interact with the field and the electromagnetic torque resists the supplied mechanical rotation.
That interpretation requires the magnetic excitation and electrical boundary assumed by the model. Rotor-fed induction arrangements introduce another electrical path and should not be collapsed into the same simple speed rule. A rotating shaft alone does not establish the complete excitation or power-transfer arrangement.
05 / Conversion boundary
Machine frequency is not necessarily delivered output frequency.
If a synchronous generator rotates at variable mechanical speed, its generated electrical frequency changes with that speed and pole count.
If a synchronous generator rotates at variable mechanical speed, its generated electrical frequency changes with that speed and pole count. A separate conversion stage can introduce another output-frequency boundary; the architecture and implemented functions determine that relationship.
The output-frequency guide concerns the delivered interface, while the terminal-voltage guide separates internal induction from loaded port conditions. Neither electrical variable can be established from synchronous speed alone.
06 / Product scope
Keep the architecture statement within its confirmed model.
FD14-50KW is confirmed as a brushless permanent-magnet direct-drive generator.
FD14-50KW is confirmed as a brushless permanent-magnet direct-drive generator. The generator types of the other four current models are not inferred from that confirmation. Electric yaw describes orientation actuation and does not identify the generating machine’s electromagnetic architecture.
This explanation supplies no generator conversion, excitation connection, synchronisation procedure or accepted speed range. Its speed and slip examples clarify physical relationships; they are not operating permissions, measured performance or evidence of any implemented converter capability.
Related technical guides: Generator Internal EMF, Terminal Voltage & Load Response · Three-Phase Imbalance & Symmetrical Components · Converter Instantaneous Power, Storage & Dissipation.
General background: DOE: generator frequency, mechanical speed and pole-count relationship · MathWorks: rotor electrical speed and induction slip-speed model. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Pole count | p pole pairs; P = 2p total poles | The chosen notation accompanies the formula. |
| Synchronous speed | ns = 60fe/p in rpm | Defined rotating field and frequency. |
| Rotor speed | nr in rpm | Physical motion differs from field motion. |
| Slip | (ns − nr)/ns | Matching speed units and nonzero ns. |
| Electrical boundary | Machine terminals versus converted output | Architecture alone does not establish the delivered interface. |
Engineering questions.
Is every synchronous generator a PMSG?
No. Permanent magnets and electrical excitation are different ways of providing a rotor field.
Are slip and rotor angle the same quantity?
No. Slip is a normalised speed difference; rotor angle describes position relative to a reference.
Does negative slip always establish every induction generator arrangement?
The simple interpretation belongs to the stated established-field squirrel-cage model; other rotor paths need their own model.
Is the 1,200 rpm example product data?
No. It uses an assumed field frequency and pole-pair count.
Does generator electrical frequency equal delivered AC frequency?
Not necessarily when a separate conversion stage defines another boundary.
Are all five MARSTON machines identified as PMSGs?
No. That architecture is confirmed here only for FD14-50KW.
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Wind-generator terminology describes how mechanical rotation interacts with a magnetic field. Share the model and applicable operating or design information for an engineering discussion.