8.2 Wound-Rotor & Synchronous Motors: Construction, Starting & Power Factor

8.2 Wound-Rotor & Synchronous Motors: Construction, Starting & Power Factor

Quick Answer: Wound-rotor induction motors (WRIM) use three-phase insulated rotor windings connected through bronze slip rings and carbon brushes to external resistance banks. Adding external resistance shifts maximum breakdown torque (250% to 300% FLT) directly to zero speed ($s = 1$) while slashing starting inrush current to 150%–200% FLA, with resistors cut out in steps as the machine accelerates. Synchronous motors run at exact synchronous speed ($s = 0$) by locking their DC-excited rotor electromagnets with the stator rotating field. They start as induction motors using amortisseur (damper) windings embedded in the pole faces; DC field excitation is applied at ~95% speed via a polarized field application relay. By varying DC field current, a synchronous motor can operate at unity, lagging, or leading power factor; in an overexcited state, it acts as a synchronous condenser supplying leading kVAR to correct plant-wide power factor.

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1. Wound-Rotor Induction Motors: Architecture & Slip Ring Assembly

While squirrel-cage motors dominate general industrial applications, the Wound-Rotor Induction Motor (WRIM)—also called a slip-ring motor—is specified for heavy industrial machinery requiring controlled, low-inrush starting under extreme inertial loads, such as mining ball mills, overland conveyors, overhead crane hoists, and large water treatment pumps.

   [ 3-Phase Stator ] <─── 3-Phase AC Line (600 V / 4160 V)
          │ (Air Gap)
          ▼
   [ 3-Phase Wound Rotor ] (Wye-connected, same pole count as stator)
          │
          ├──> Rotor Shaft Lead 1 ───> Slip Ring 1 ───(Brush)───> Terminal M1 ──┐
          ├──> Rotor Shaft Lead 2 ───> Slip Ring 2 ───(Brush)───> Terminal M2 ──┼──> [ External Resistor Bank ]
          └──> Rotor Shaft Lead 3 ───> Slip Ring 3 ───(Brush)───> Terminal M3 ──┘    (Stepped Contactor Control)

Stator & Rotor Construction

• Stator: Identical in construction to a standard three-phase squirrel-cage motor. It contains distributed three-phase windings housed in a laminated silicon steel core, generating a rotating magnetic field when energized.
• Rotor: Instead of cast aluminum or copper bars shorted by end rings, the WRIM rotor contains a fully insulated, three-phase distributed winding constructed from formed copper coils. The rotor windings are wound for the exact same number of magnetic poles as the stator. The internal phase coils are connected in a balanced wye ($Y$) configuration.
• Slip Rings (Collector Rings): The three open ends of the rotor wye winding are brought out through the hollow center of the motor shaft and terminated onto three insulated bronze, brass, or copper alloy slip rings mounted on the shaft.
• Brush Rigging: Stationary carbon-graphite brushes ride continuously on the rotating slip rings. Flexible copper pigtail braids connect the brush holders to heavy external junction terminals designated M1, M2, and M3 on the motor terminal box.

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2. External Rotor Resistance: Torque Maximization & Acceleration Dynamics

The Physics of Breakdown Torque vs. Resistance

In induction motor theory, the breakdown (maximum) torque ($T_{\text{max}}$) developed by the machine depends on stator voltage and rotor inductive reactance, but its magnitude is completely independent of rotor circuit resistance:

$$T_{\text{max}} \propto \frac{V_t^2}{2 X_{r0}}$$

However, the slip at which breakdown torque occurs ($s_{\text{max}}$) is directly proportional to total rotor resistance:

$$s_{\text{max}} = \frac{R_2}{X_{r0}}$$

Where:

• $R_2$ = Total rotor circuit resistance per phase ($R_{\text{rotor}} + R_{\text{ext}}$)
• $X_{r0}$ = Standstill ($60\text{ Hz}$) rotor inductive reactance per phase

   Torque (% FLT)
 300 |             R_ext = Max (s_max = 1.0) ──┐
     |             [ Maximum Torque at Start ] │    R_ext = Medium (s_max = 0.5)
 250 |               * *                       │        * *
     |              *   *                      └───>   *   *           R_ext = 0 (Short-circuited)
 200 |             *     *                            *     *             * *
     |            *       *                          *       *           *   *
 150 |           *         *                        *         *         *     *  <── BDT (250%)
     |          *           *                      *           *       *       *
 100 |         *             *                    *             *     *         * <── FLT (100%)
     |        *               *                  *               *   *           *
   0 +───────*─────────────────*────────────────*─────────────────*─*─────────────*────> Speed
             0 RPM (Standstill, s=1.0)                                      1800 RPM (s=0)

Sizing External Resistance for Maximum Starting Torque

In a standard squirrel-cage motor, internal rotor resistance is fixed and low, meaning breakdown torque occurs at high speed ($s \approx 0.15 - 0.25$). Starting torque at standstill ($s = 1.0$) is significantly lower (150% to 200%), while inrush current is massive (600% to 800% FLA).

In a wound-rotor motor, external resistance ($R_{\text{ext}}$) is inserted into the secondary circuit through terminals M1, M2, and M3. By selecting $R_{\text{ext}}$ such that total resistance equals standstill reactance ($R_{\text{rotor}} + R_{\text{ext}} = X_{r0}$):

$$s_{\text{max}} = \frac{R_{\text{rotor}} + R_{\text{ext}}}{X_{r0}} = 1.0$$

The Operational Result:

1. The motor develops 100% of its breakdown torque (250% to 300% of FLT) right at zero speed (standstill).
2. Because the added resistance limits rotor current and shifts the rotor power factor toward unity, starting current is slashed from 600%–800% down to only 150%–200% of full-load amperes.

Step-Wise Resistor Cut-Out & Acceleration

A wound-rotor motor is never started with its slip rings shorted. It starts with maximum resistance inserted in the secondary circuit. As the rotor accelerates, secondary accelerating contactors (controlled by adjustable timing relays, frequency relays, or a programmable logic controller) short out resistor steps in sequential stages:

1. Step 1 (Breakaway): Maximum resistance in circuit. Motor develops 250% starting torque with ~150% line current, breaking away heavy static friction.
2. Step 2 (Intermediate Acceleration): Timing relay 1TR energizes contactor 1A, shorting out the first bank of resistors. The torque curve shifts to the right, maintaining high accelerating torque.
3. Step 3 (Near Full Speed): Timing relay 2TR energizes contactor 2A, shorting out the second resistor bank.
4. Final Step (Run Condition): Contactor 3A closes, completely short-circuiting terminals M1, M2, and M3. The motor now operates identically to a high-efficiency squirrel-cage motor, with low slip (1% to 2%) and minimal rotor $I^2 R$ heat losses.

Speed Control Limitations

Historically, WRIMs were used for variable speed drives (e.g., crane hoists or mine ventilation) by permanently operating with intermediate resistance inserted. However, this method is thermally inefficient: all slip power ($P_{\text{slip}} = s \times P_{\text{airgap}}$) is converted to heat and dissipated across the resistor banks. At 50% speed ($s = 0.5$), 50% of the mechanical power delivered across the air gap is wasted as heat. Modern retrofits replace secondary resistor banks with Variable Frequency Drives (VFDs) or slip-power recovery systems (Scherbius drives).

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3. WRIM Maintenance: Slip Rings, Carbon Brushes & Rigging

Because the secondary circuit carries high rotor currents, brush and slip ring maintenance is critical to prevent flashovers, ring grooving, and unexpected plant shutdowns.

Carbon Brush Maintenance & Rigging Standards

• Brush Pressure: Brush springs must apply uniform, calibrated pressure across all brushes, typically 14 to 25 kPa (2.0 to 3.5 psi), measured using a handheld spring scale. Insufficient pressure causes brush chatter, electrical arcing, and ring pitting. Excessive pressure causes rapid mechanical brush wear and ring grooving.
• Brush Box Clearance: Brushes must slide freely in their brush holders without binding. Recommended radial clearance is 0.05 to 0.13 mm (0.002 to 0.005 in). Accumulated carbon dust, grease, and atmospheric dirt must be blown out periodically using dry, oil-free compressed air (maximum 200 kPa / 30 psi).
• Brush Wear Limits: Replace brushes before they reach their stamped wear-limit line or before the embedded copper pigtail braid touches the slip ring surface. If a pigtail strikes the ring, severe circumferential scoring occurs.
• Patina Formation: A healthy slip ring develops a uniform, glossy, chocolate-brown film consisting of copper oxide, graphite, and microscopic moisture layers. This film lubricates the interface and minimizes electrical wear. Streaking, threading, or raw bright copper indicates incorrect brush grade, low current density, or excessive humidity.
• Resurfacing Rules: If rings become eccentric (runout > 0.08 mm) or grooved, they must be trued using a commutator stone or lathe tool. Never use emery cloth to dress slip rings or seat brushes. Emery contains conductive aluminum oxide or iron particles that embed into brush carbon and ring insulation, causing catastrophic phase-to-phase flashovers across the rings. Use non-conductive flint paper, aluminum oxide sandpaper, or dedicated dressing stones.

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4. Three-Phase Synchronous Motors: Construction & Exact Synchronous Operation

Unlike induction motors, which must slip behind the stator field, the Three-Phase Synchronous Motor operates at exact synchronous speed ($N_r = N_s$) with zero slip ($s = 0$) under all steady-state operating conditions from no-load up to full design load.

                     +--------------------------------------------------+
                     |              SYNCHRONOUS MOTOR STATOR            |
                     |    3-Phase AC Winding creates RMF at N_s RPM     |
                     +--------------------------------------------------+
                                              │
                                   Magnetic Interlocking
                                  Across Air Gap (s = 0)
                                              │
                     +--------------------------------------------------+
                     |              SYNCHRONOUS MOTOR ROTOR             |
                     |    DC Electromagnets lock with Stator Poles      |
                     +--------------------------------------------------+

Machine Architecture

1. Stator: Constructed with a three-phase distributed AC armature winding housed in a laminated core, identical to an induction motor stator. When energized from a 60 Hz line, it produces an RMF rotating at $N_s = 120f/P$.
2. Rotor: The rotor is an electromagnet consisting of magnetic poles energized with Direct Current (DC). The DC current creates alternating North and South magnetic poles that physically lock with the opposing magnetic poles of the rotating stator field.
   • Salient-Pole Rotors: Projecting pole pieces bolted to a rotor spider. Used in low-to-medium speed machines (typically 150 to 1200 RPM, 6 to 48 poles) such as reciprocating compressors, pulp refiners, ball mills, and hydro generators.
   • Cylindrical (Non-Salient) Rotors: Solid, smooth steel forgings with distributed field slots. Used in high-speed 2-pole and 4-pole machines (1800 and 3600 RPM) such as industrial gas turbine drives and boiler feed pumps.
3. Excitation Source: DC excitation is supplied to the revolving rotor windings by:
   • Brush-type Excitation: DC current fed from an external DC rectifier or generator through two bronze slip rings and carbon brushes.
   • Brushless Excitation: A small, shaft-mounted AC exciter alternator whose AC output is converted to DC by a shaft-mounted three-phase rotating silicon diode bridge rectifier. Because the rectifier spins with the rotor, no brushes, slip rings, or exposed contacts exist, making brushless designs ideal for hazardous (Class I) locations.

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5. Starting Synchronous Motors: Amortisseur (Damper) Windings & Field Application

The Zero-Starting-Torque Dilemma

A synchronous motor cannot start from standstill on DC excitation alone. At the instant of energization, the rotor has high mechanical inertia and is stationary ($N_r = 0$). The stator rotating magnetic field sweeps past the stationary rotor poles at 1800 RPM (for a 4-pole machine). A given rotor pole is pulled forward for half an electrical cycle ($8.3\text{ ms}$ at 60 Hz) and then pushed backward for the next half cycle. The net average starting torque is zero. The rotor merely vibrates violently and hums loudly without rotating.

Amortisseur (Damper) Windings

To achieve starting capability, synchronous motors incorporate amortisseur windings (also called damper windings or squirrel-cage starting windings). Heavy copper, brass, or bronze alloy bars are embedded in axial slots across the pole faces of each rotor pole piece. At both ends of the rotor, heavy copper shorting segments braze all bars together, forming an integral squirrel-cage induction winding on the rotor surface.

   Pole Shoe ───>  [ o   o   o   o   o   o ] <── Amortisseur (Damper) Bars (Copper/Brass)
   Pole Core ───>  [                       ] 
                   [   DC Field Coils      ] <── Excited with DC after 95% speed
                   [   (Insulated Copper)  ] 
   Rotor Spider ─> [=======================]

Step-by-Step Starting Sequence

Starting a high-horsepower synchronous motor requires a synchronized sequence coordinated by a specialized solid-state motor controller:

1. Induction Acceleration: Three-phase AC line voltage is applied to the stator. The rotating magnetic field induces high currents in the amortisseur cage, producing induction torque that accelerates the rotor from standstill, exactly like a NEMA Design B or C induction motor.
2. Field Discharge Resistor: During acceleration, the DC field winding is disconnected from the DC supply and closed across a heavy-duty Field Discharge Resistor (FDR).
   • Why this is critical: Because the stator RMF sweeps past the multi-turn rotor field coils at high relative speed, intense transformer action occurs. If the field circuit were left open-circuited, tens of thousands of volts would be induced across the field terminals, puncturing coil insulation and destroying the exciter. The discharge resistor safely discharges this induced AC current and produces additional induction starting torque.
3. Synchronizing Speed (95% – 97%): As the rotor accelerates, slip drops. When the motor reaches approximately 95% to 97% of synchronous speed ($s = 0.03 - 0.05$), the induction torque curve flattens out, and the rotor cannot accelerate further on amortisseur torque alone.
4. Polarized Field Application Relay (PFR): At ~95% speed, the control system opens the field discharge contactor and immediately connects the DC excitation source across the rotor field coils.
   • Optimal Timing: The solid-state PFR monitors the frequency and phase angle of the induced AC voltage across the discharge resistor. It triggers DC excitation at the exact microsecond when a rotor North pole is slipping past a stator South pole.
   • Pull-In: The magnetic attraction pulls the rotor across the remaining 3% to 5% speed gap, locking the rotor into synchronism with the stator field. The motor accelerates to exact synchronous speed ($N_r = N_s$, $s = 0$).
5. Steady-State Damping Function: Once locked in synchronism, relative motion between the rotor and the stator field is zero. Consequently, no current flows through the amortisseur bars during steady-state operation. However, if sudden load variations cause the rotor to oscillate or hunting occurs around the magnetic axis, relative velocity develops momentarily. Currents are instantly induced in the amortisseur bars, generating damping torque that quenches oscillations and stabilizes the drive.

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6. Power Factor Control, V-Curves & The Synchronous Condenser

The most remarkable operational attribute of the synchronous motor is its ability to operate at a variable power factor—unity, lagging, or leading—simply by adjusting its DC field excitation current ($I_f$).

   Armature Current (I_a)
     ^
     |   /                  \
     |  /                    \         ── Full Load
     | /                      \       ----
     |/       Unity (1.0)      \     - - - Half Load
     |\         PF Line        /    ...... No Load
     | \           │          /
     |  \          │         /
     |   \.........*......../
     |             │
   0 +─────────────*───────────────────> DC Field Current (I_f)
       LAGGING PF  │   LEADING PF
      (Underexcited)  (Overexcited)

The Physics of Excitation & Power Factor

• Normal Excitation (Unity Power Factor, $\text{PF} = 1.0$): The DC rotor field current is adjusted to provide exactly 100% of the magnetic flux required by the air gap. The AC stator line needs to supply only active (real) in-phase current ($I \cos\theta$) to produce shaft mechanical horsepower. Stator current ($I_a$) is at its absolute minimum.
• Underexcitation (Lagging Power Factor): The DC field current is reduced below normal. The rotor electromagnet is too weak to support the required air-gap flux. To compensate, the motor stator must draw lagging reactive current ($I_{\text{mag}}$) from the AC distribution system, functioning like an induction motor. Operating power factor is Lagging.
• Overexcitation (Leading Power Factor): The DC field current is increased above normal. The rotor produces more magnetic flux than the air gap requires. This excess magnetic field induces a counter-EMF in the stator windings that drives leading reactive current out of the motor and into the AC power line. The motor behaves like a giant three-phase capacitor bank, operating at a Leading Power Factor (typically 0.80 leading at rated full load).

Synchronous Condensers in Heavy Industry

In large Canadian industrial installations—such as pulp and paper mills, steel rolling mills, and mining concentrators—hundreds of three-phase induction motors operate simultaneously, drawing massive lagging inductive reactive power (kVAR) that degrades the overall plant power factor down to 0.70 or 0.80 lagging. Canadian electric utilities impose severe financial billing penalties for facility power factors below 0.90.

To solve this, industrial facilities run large synchronous motors in an overexcited state. The motor delivers mechanical shaft power (driving a primary plant compressor or chipper) while simultaneously feeding leading kVAR into the plant distribution bus. This cancels out the lagging kVAR drawn by surrounding induction motors, elevating the plant power factor above 0.95.

• Synchronous Condenser Definition: When a synchronous machine is installed with no mechanical shaft load (shaft spinning freely in air or uncoupled) and operated in an overexcited state solely to supply leading reactive power (kVAR) for power factor correction and grid voltage stabilization, it is termed a Synchronous Condenser.

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7. Concrete Industrial Troubleshooting Scenario: Synchronous Ball Mill Pull-In Failure

Facility Background & Problem Statement

At a copper-gold mine in British Columbia, a 2500 HP, 4160 V, 3-phase, 60 Hz, 24-pole (300 RPM) synchronous motor drives the primary ore grinding ball mill. During commissioning after a scheduled mill reline, the motor starts across-the-line on its amortisseur windings and accelerates smoothly to 285 RPM (approx. 95% speed). However, the instant DC field excitation is commanded, the motor fails to pull into synchronism. It hums with a loud low-frequency beat, draws severe surging stator currents (300% to 400% FLA), and trips out on Damper Winding Thermal Protection (Device 46/51) within 6 seconds.

Diagnostic Procedure

1. Visual & Mechanical Verification:
   • Electricians confirm the ball mill mechanical drive train rotates freely; charge load is normal.
   • Amortisseur bars show no visible thermal discoloration or fractured joint brazing at the pole shoes.
2. Field Discharge Circuit Investigation:
   • The synchronous motor starter uses a magnetic field contactor (Contactor $FC$) that alternates between the Field Discharge Resistor (FDR) during starting and the DC exciter during running.
   • Lockout/Tagout is executed per CSA Z462 protocols.
   • Visual inspection of Contactor $FC$ reveals that the normally closed auxiliary contacts controlling the discharge resistor are severely pitted, eroded, and mechanically warped from repetitive cycling.
   • Resistance Measurement: A digital low-resistance ohmmeter (DLRO) is placed across the discharge circuit. The measured resistance is $14.8\ \Omega$, compared to the factory blueprint specification of $1.85\ \Omega$. High-resistance contact oxidation was choking the induced rotor starting current.
3. Exciter DC Voltage Verification:
   • The static DC exciter power supply is tested on a dummy load. DC output voltage under load reads 125 VDC, matching nameplate specifications.
4. Polarized Field Application Relay (PFR) Timing:
   • A multi-channel digital recording oscilloscope monitors induced field voltage and DC contactor closure.
   • The recording reveals that because the discharge resistor contact was oxidized, the induced AC feedback signal to the PFR was severely distorted. The relay triggered the DC field contactor out of phase—applying DC voltage when opposing poles were adjacent, generating massive repulsion braking torque rather than attraction pull-in torque.
5. Corrective Action Plan:
   • Contactor $FC$ contacts and coil mechanism were replaced.
   • The field discharge resistor connections were cleaned, torqued, and verified at $1.85\ \Omega$.
   • The PFR pull-in frequency setpoint was recalibrated to trigger at 96.5% speed ($f_{\text{slip}} = 2.1\text{ Hz}$).
   • On restart, the motor accelerated to 289 RPM, the PFR applied DC excitation at the optimum angle, the rotor locked into synchronism at exactly 300 RPM ($s = 0$) in under 350 milliseconds, and stator current settled smoothly to 240 A at 0.85 leading power factor.