Ward Leonard Speed Control Circuit Schematic and Working Principle

Implement a feedback loop with a tachogenerator to maintain closed-loop stability–critical when operating near zero RPM. Any deviation above 0.5% in reference voltage introduces oscillations, particularly in high-inertia loads like rolling mills or textile machinery. Use a compensating winding on the rotating exciter to counteract armature reaction effects, which can distort the generated voltage by up to 8% under full load.

A three-field generator configuration–main, differential, and control–allows fine-tuned adjustment of the motor’s armature voltage without relying solely on field rheostats. This setup reduces copper losses by 12-15% compared to single-field excitation. Ensure the differential field is wound opposite to the main field; incorrect polarity reverses torque direction, risking unintended acceleration or deceleration.

For dynamic braking, connect a braking resistor across the motor terminals with a dedicated contactor. Resistance values should be sized for 2-3 times the motor’s armature resistance to dissipate regenerative energy without exceeding 1.2x rated current. Overshooting this threshold risks commutator damage due to excessive sparking. In multi-motor applications, synchronize load sharing by matching the slope of the generator’s external characteristics within ±2%.

Isolate all field circuits with interposing relays rated for at least 2,000 operations to prevent back-EMF transients from damaging control components. Replace carbon brushes when wear exceeds 50% of original length–delayed maintenance increases voltage drop across the commutator by 3-4%, reducing speed regulation accuracy. For continuous duty cycles, use a motor-generator set with class F insulation; thermal margins narrow significantly below 80% of rated airflow.

Constructing the Variable Voltage Drive Circuitry

Start by wiring the separately excited generator’s field winding to a dedicated DC supply with adjustable rheostat–this regulates output voltage precision to ±0.5% at full load. Ensure the motor’s armature connects directly to the generator’s armature terminals, bypassing intermediate relays to minimize voltage drops; typical losses here should not exceed 1.2V per 100A. Ground the generator’s frame to the motor’s frame using a 16mm² copper busbar to prevent circulating currents in bearings.

Component Selection Criteria

Use a generator with a shunt field resistance between 50Ω and 150Ω–values outside this range risk either insufficient excitation or thermal runaway. The prime mover must deliver 110% of the generator’s rated output at the lowest required shaft velocity; for a 50kW system, a 55kW induction motor with a 1.15 service factor suffices. Replace standard carbon brushes with electrographitic grades to reduce sparking under rapid load transients, extending brush life to ~3,000 hours.

Install a series compensating winding on the generator if the motor’s no-load current exceeds 8% of full-load current; this counters armature reaction and maintains linearity across the entire velocity range, particularly below 30% of base speed. Connect a 0.1μF capacitor across the motor’s armature terminals to suppress high-frequency noise generated during commutation, reducing electromagnetic interference by 40dB in the 10kHz–100kHz band.

For velocity feedback, mount a tachogenerator with an output of 10V/1,000rpm on the motor shaft–this provides ±0.1% accuracy when coupled with a 12-bit ADC. Use shielded twisted-pair wiring for all feedback signals, grounding the shield at the controller end only to eliminate ground loops. Calibrate the system by adjusting the generator’s field rheostat to match the motor’s rated voltage at 1,500rpm, then verify linearity by plotting output voltage against field current at 20%, 50%, and 80% of maximum velocity.

Key Elements of the Motor-Generator Variable Drive Arrangement

The primary driving unit–a separately excited DC generator–must be sized to handle 120 % of the continuous torque demand of the driven machinery. Field windings should be wound with Class H insulation (180 °C rating) and carry a full-load excitation current not exceeding 1.5 A per pole to prevent excessive copper losses.

A synchronous alternator rated at 50 Hz, 400 V line-to-line feeds both the driving generator and auxiliary circuits. Its prime mover must sustain 1.2 pu steady-state overload without voltage droop beyond 2 %. Flywheel inertia (WR²) is calculated at 7.5 kg·m² per kW of generator capacity to smooth transient spikes.

Connect the motor armature directly to the generator armature via busbars sized for 150 % of peak current. Use bolted joints with silver-plated surfaces; torque fasteners to 45 N·m to eliminate micro-arcing under dynamic braking.

Component	Rated Voltage	Peak Current	Time Constant (ms)
Driven DC motor	440 V	320 A	28
Generator armature	460 V	340 A	22
Exciter set	110 V	8 A	12

Exciter output windings regulate generator field voltage between 0–110 V dc using a carbon-pile rheostat. Position the rheostat physically adjacent to the exciter set to keep lead inductance under 0.3 µH.

Fit each shaft with two proximity sensors spaced 90° apart; set thresholds at 3 mm air gap to trigger overspeed shutdown within 18 ms of detecting 115 % rated speed.

Enclose the entire set in an IP54 cubicle with forced-air cooling rated at 0.5 m³/s per kW of losses. Mount intake filters upstream of the fans to maintain ≤ 50 mg/m³ dust concentration.

Interpose a DC contactor (500 A, 600 V dc breaking capacity) between generator and motor armatures. Include auxiliary contacts wired into the excitation circuit to ensure the main contactor only closes when generator output is ≤ 5 % above motor rated voltage.

Step-by-Step Assembly of the Motor-Generator Pair Wiring

Begin by securing the generator’s armature terminals to the motor’s field winding using 10 AWG copper wires. Ensure the connections are crimped with insulated spade terminals and tightened to 12 Nm torque to prevent voltage drops under load. Label each wire at both ends with heat-shrink tubing marked “GEN-A” (armature) and “MOT-F” (field) to eliminate cross-wiring errors during troubleshooting.

Mount the motor and generator on a common baseplate with a maximum flatness deviation of 0.5 mm/meter. Align the shafts using a dial indicator, targeting a runout under 0.05 mm. Misalignment increases bearing wear and introduces harmonic distortions in the output voltage, reducing efficiency by up to 8%. Fix both machines with grade 8.8 bolts, applying anti-seize compound to threads exposed to temperature cycles.

Connect the generator’s field winding to a separate DC supply rated for 125% of the nominal excitation voltage. Use a 5 A fuse inline to protect against overcurrent during startup transients. The supply must feature galvanic isolation; opt for a linear power source if switching regulators introduce ripple exceeding 2% of the output voltage. Verify polarity with a multimeter before energizing–reversed fields demagnetize the iron cores irreversibly.

• Install a rheostat in series with the motor’s armature circuit to limit inrush current. Size it for 150% of the armature’s full-load current (e.g., 20 Ω for a 10 A armature). Bypass the rheostat with a normally-open contactor once the motor reaches 80% of rated speed to avoid unnecessary power dissipation.
• Wire the generator’s armature output to the motor’s armature via a polarized plug connector (e.g., Anderson SB50). Avoid screw terminals here–vibration loosens them over time. Apply dielectric grease to connections in humid environments.
• Splice a current shunt (75 mV, 10 A) into the generator’s armature circuit to monitor load conditions. Route the shunt’s voltage taps to an analog meter or PLC input, scaling the signal to 1 mA per 1 A of armature current.

Test the setup in stages:

1. Energize the generator’s field only. Measure output voltage at no load–it should match the nameplate rating (±5%). If low, check for residual magnetism by momentarily applying full excitation voltage.
2. Apply 25% of the motor’s rated voltage to its field winding. The motor should start smoothly without jerking; if not, inspect brush alignment or commutator surface condition.
3. Gradually increase the generator’s field current while monitoring the motor’s speed. At full excitation, the motor should reach its base speed within 3 seconds. Prolonged acceleration indicates undersized wires or excessive friction.

Install suppression components to mitigate EMI:

• Fit RC snubbers (0.1 µF + 100 Ω) across each commutator segment-to-segment to damp voltage spikes. Failure to suppress spikes causes arcing, reducing brush life by 40%.
• Add a varistor (MOV) rated at 1.5× the peak supply voltage across the generator’s output terminals. This clamps transient voltages from sudden load changes.
• Use ferrite beads on all signal wires to attenuate high-frequency noise above 1 MHz.

Document every connection in a wiring schedule table, including:

• Wire gauge, color code, and insulation rating (e.g., THHN 90°C).
• Terminal numbers for both source and destination (e.g., “GEN-T1 → MOT-T3”).
• Crimp/cable lug specifications (e.g., “Panduit LSO10-14 crimped with CT-300”).
• Torque values for all mechanical fasteners (e.g., “Coupling bolts: 25 Nm”).

Store spare brushes, fuses, and connectors in a climate-controlled cabinet–relative humidity above 60% accelerates corrosion on commutators. Replace brushes when their length reaches 5 mm; worn brushes increase voltage ripple by 15% and generate carbon dust that contaminates windings.