The choice between a synchronous motor and an induction motor comes down to how each machine produces torque and how tightly it locks to line frequency. A synchronous motor runs at exactly the speed set by supply frequency and pole count, with no slip, while an induction motor always runs slightly slower than synchronous speed because it depends on slip to generate rotor current. That single distinction drives almost every practical difference between them: excitation, power factor behaviour, starting method, efficiency, and where each belongs on a plant floor.
An induction motor (also called an asynchronous motor) works purely on transformer action. The stator winding, fed from the three-phase supply, creates a rotating magnetic field at synchronous speed N_s = 120f/P, where f is line frequency in Hz and P is the number of poles. This rotating field cuts the rotor bars and induces a current in them by mutual induction, exactly like a transformer secondary. That induced current then interacts with the same rotating field to produce torque. Because current can only be induced in the rotor when there is relative motion between the rotor and the field, the rotor must always turn slower than N_s. This speed difference is slip, s = (N_s - N_r)/N_s, typically 1 to 5% at rated load for standard cage motors.
A synchronous motor produces torque differently. Its rotor carries either a DC-excited field winding or permanent magnets, so it generates its own independent magnetic field rather than relying on induction. Once the rotor locks into step with the stator's rotating field, the two fields interlock magnetically and rotate at exactly the same speed, hence "synchronous." There is no slip at steady state. If the load torque exceeds the maximum pull-out torque, the rotor falls out of step and stalls rather than simply slowing down, which is a key operational difference from an induction machine.
Excitation is the clearest structural divide between the two designs.
The DC field current in a wound-field synchronous motor is the control handle for reactive power. Increasing field current over-excites the machine and makes it look capacitive to the supply; reducing it under-excites the machine and makes it look inductive. This is the basis of the V-curve behaviour that synchronous motors are known for.
This is the single biggest advantage synchronous motors hold over induction motors. An induction motor always draws lagging (inductive) reactive power to build its magnetic field, typically running at 0.80 to 0.90 lagging power factor at full load and considerably worse at partial load. A synchronous motor, by contrast, can be over-excited to run at leading power factor or exactly unity, effectively acting as a rotating capacitor bank while it does mechanical work. Large synchronous motors are routinely specified at 0.8 leading purely to correct the power factor of an entire substation feeding a bank of induction loads, avoiding utility penalty charges without a separate capacitor installation.
Induction motors are inherently self-starting: as soon as voltage is applied, slip is 100% and torque is produced immediately, though inrush current can reach 5 to 8 times full-load current (locked-rotor code letters per NEMA MG 1). Starting methods such as star-delta, autotransformer, soft starters, or variable frequency drives are used mainly to limit that inrush and the mechanical shock of full-voltage starting.
Synchronous motors cannot start on their own from a DC-excited or PM rotor at synchronous speed, because a stationary rotor field and a rotating stator field produce zero average torque. Practical starting methods include:
Synchronous motors hold exactly constant speed regardless of load, up to their pull-out torque limit, which typically falls between 150% and 250% of rated torque depending on design. This makes them the natural choice anywhere multiple drives must stay in precise ratio or phase, such as synchronised paper machine sections or reciprocating compressor trains. Induction motors show a speed droop with load: slip rises from perhaps 0.5% at light load to 3 to 5% at rated load on a standard NEMA Design B motor, which is acceptable for the vast majority of pumps, fans, and conveyors but unsuitable where multiple shafts must never drift relative to one another without a common VFD reference. Because slip generates real losses in the rotor (rotor copper loss is proportional to slip times air-gap power), it also has a direct maintenance signature: rising slip under constant load is one of the earliest indicators of rotor bar damage, and comparing measured slip against nameplate slip is a standard diagnostic covered alongside broken rotor bar detection methods.
Synchronous motors, particularly PMSM designs, generally achieve higher efficiency than induction motors of equivalent rating because they eliminate rotor I²R losses from induced current altogether (PM types) or allow the field to be tuned to the load (wound-field types). Efficiency gaps are most visible at partial load, where induction motor efficiency falls off faster due to fixed magnetising current. IEC 60034-30-1 and the IE1 through IE4 efficiency classes apply to both technologies, but IE4 and the emerging IE5 ultra-premium class are dominated by synchronous reluctance and PMSM designs precisely because they avoid rotor slip losses. For a full breakdown of the class boundaries and where each design tends to land, see motor efficiency IE classes.
| Characteristic | Induction motor | Synchronous motor |
|---|---|---|
| Rotor speed | Below N_s (slip 1 to 5%) | Exactly N_s, zero slip |
| Rotor field source | Induced current (transformer action) | DC field winding or permanent magnets |
| Starting | Self-starting | Needs damper winding, pony motor, or VFD |
| Power factor | 0.80 to 0.90 lagging, load-dependent | Adjustable, unity to leading via excitation |
| Typical efficiency class ceiling | IE3 to IE4 for cage designs | IE4 to IE5 (PMSM, sync reluctance) |
| Overload behaviour | Slows down, higher slip, eventually stalls | Holds speed to pull-out torque, then stalls abruptly |
| Relative cost and complexity | Lower cost, simpler, less maintenance | Higher cost, exciter/magnets, more complex control |
| Typical applications | Pumps, fans, conveyors, compressors, general drives | Large compressor trains, power factor correction, precision speed processes |
Choose an induction motor for the great majority of general-purpose plant drives: centrifugal pumps, fans, compressors below a few megawatts, and conveyors, where robustness, low cost, and simple maintenance outweigh the modest efficiency and power factor penalty. Pairing the motor with a properly sized pump (see centrifugal vs positive displacement pump selection) usually matters more to overall system efficiency than the motor technology itself.
Choose a synchronous motor when constant speed independent of load is essential, when the motor is large enough that leading power factor correction offsets its higher first cost, or when multiple drive trains must stay in fixed speed ratio. High-speed PMSM and synchronous reluctance motors are increasingly specified on VFD-driven applications purely for the IE4/IE5 efficiency gain, even on modest-sized fan and pump loads.
Whichever technology is installed, the failure signatures differ: induction motors reveal degradation through slip creep and rotor bar signatures, while synchronous motors reveal problems through pole-slipping events, field winding insulation breakdown, or damper winding damage during repeated starts. Continuous condition monitoring feeding into a CMMS lets maintenance teams set technology-specific alarm thresholds and auto-generate work orders the moment a synchronous motor's excitation current drifts or an induction motor's slip trends upward, rather than waiting for a trip. Tracking that data alongside OEE also shows whether motor-related slow losses are quietly eating into availability before a hard failure occurs. Teams standardising this across a fleet of mixed motor types can Book a Fabrico demo to see how condition data and work order triggers are configured per asset class.
Yes. At a fixed mechanical load, changing only the DC field current shifts the stator current phase, moving the operating point along a V-curve from lagging through unity to leading power factor without changing output torque.
The damper winding exists specifically to provide starting torque and to damp oscillations. Without it, the DC-excited or PM rotor produces no net starting torque from standstill and would also be prone to hunting (speed oscillation) after minor load disturbances.
No, slip is fundamental to how the motor produces torque at all. What matters for condition monitoring is whether slip at a given load matches the nameplate value; a rising trend, not the presence of slip itself, indicates rotor deterioration such as broken bars or high-resistance joints.
A VFD lets an induction motor's synchronous speed itself be varied by changing supply frequency, narrowing the speed-control gap with synchronous machines, but it does not add slip-free operation or inherent leading power factor capability, both of which remain structural properties of the synchronous design.
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