Problem Definition
A 2300 V, 60 Hz, 1250 HP synchronous motor must operate in a plant supplied at 50 Hz. The question is whether a medium-voltage variable frequency drive can synthesize 60 Hz output from the 50 Hz source, and what drive topology and rating are required.
The short answer: yes. An MV drive rectifies the incoming supply to DC and re-inverts it, so the output frequency is decoupled from the line frequency. The engineering work is not in the frequency conversion itself — it is in the output voltage capability, the current rating, the excitation scheme for a synchronous machine, and the driven load's behaviour at the higher speed.
Frequency, Voltage and Flux
The nameplate defines the volts-per-hertz ratio the magnetic circuit was designed for:
- V/Hz = 2300 V / 60 Hz =
38.33 V/Hz - Constant-flux voltage at 50 Hz = 38.33 × 50 =
1917 V
Two distinct operating cases follow:
| Case | Output voltage | Available shaft power | Comment |
|---|---|---|---|
| Run at 50 Hz, constant flux | ~1917 V | 932.5 kW × (50/60) = 777 kW (~1042 HP) |
Rated torque preserved; ~17% power derate. Requires the drive to reduce voltage — a fixed-frequency 50 Hz feed at 2300 V would over-flux the machine by 20% and drive it into saturation. |
| Run at 60 Hz via VFD | 2300 V | 1250 HP (932.5 kW) full nameplate | Drive must deliver rated volts at rated frequency; input transformer must support it. |
Note the boundary condition on the drive: a voltage-source inverter cannot produce more fundamental output voltage than its DC link supports, and the DC link is set by the rectifier input voltage. If the 50 Hz plant bus is, for example, 3300 V or 6600 V, the drive's isolation/phase-shifting transformer is specified with the secondary tapped to produce a 2300 V-capable output. If the plant bus is itself 2300 V, expect the usable output to fall a few percent short of the input because of rectifier and inverter losses and modulation limits — verify the exact maximum output voltage figure against the vendor's rating sheet and specify a step-up or dedicated input transformer if needed. Do not assume 1:1 output capability.
Current and kVA Sizing
The nameplate does not state efficiency, power factor or full-load amps in the available data, so size the drive against labelled assumptions and then correct against the actual motor data sheet. Three-phase apparent power:
kVA = √3 × V_LL × I_line / 1000 → I_line = kVA × 1000 / (√3 × 2300), with √3 × 2300 = 3983.7
| Assumption set | Input kW | kVA | Line current at 2300 V |
|---|---|---|---|
| η = 0.95, pf = 1.0 | 981.6 kW | 982 kVA | ~246 A |
| η = 0.95, pf = 0.9 leading | 981.6 kW | 1091 kVA | ~274 A |
| η = 0.95, pf = 0.8 leading | 981.6 kW | 1227 kVA | ~308 A |
Shaft power basis: 1250 HP × 0.746 = 932.5 kW. Synchronous machines are commonly excited for leading power factor, which increases stator current for the same shaft power — the 0.8 leading case is the conservative selection basis. Size the drive to the motor's stamped full-load amps, not to a computed value, once the data sheet is in hand. Add margin for the required overload profile (a constant-torque load and a variable-torque load will not be quoted the same drive frame).
Drive Topology Selection for a Synchronous Motor
| Topology | Fit for this application | Watch items |
|---|---|---|
| LCI (load-commutated inverter, current-source, thyristor) | Classic choice for MV synchronous motors; commutation is provided by the motor's own back-EMF, so it only works with a synchronous machine | Produces pulsating air-gap torque at harmonic orders of the 6-pulse bridge; requires torsional interaction analysis of the full shaft train. Needs a starting method below the speed where back-EMF supports commutation (DC-link pulsing). Poor input and output waveform quality without multi-pulse arrangements. |
| Current-source PWM inverter | Sinusoidal output current with an output capacitor filter; low dv/dt at the motor | Output filter resonance and self-excitation must be checked against motor parameters. |
| Cascaded H-bridge multilevel VSI | Very low motor dv/dt and low harmonic torque; multi-pulse input via the integral phase-shifting transformer | Large integral transformer; cell count sets output voltage. Regeneration usually not available unless active front end is specified. |
| Three-level NPC VSI | Compact, common at 2.3–4.16 kV class | Higher dv/dt than CHB; long motor cable runs may need an output reactor or sine filter to control reflected-wave overvoltage. |
MV drive suppliers active in this class include Siemens (Robicon product line), Rockwell Automation / Allen-Bradley, ABB (formerly ASEA), Toshiba and other MV drive vendors. Issue the motor data sheet, driven-load torque-speed curve, plant bus voltage, available fault level and cable length with the enquiry; MV drives are engineered-to-order and the transformer secondary voltage is part of the order.
Synchronous-Machine-Specific Requirements
- Field excitation. Confirm the excitation type. A brushless rotating exciter derives its output from rotation and from the exciter field regulator — the regulator must be capable of holding field current across the full speed range including start-up, where a rotating exciter produces little or no output at zero speed. With slip rings and a static exciter, the exciter is fed from the plant supply and is speed-independent, which simplifies drive integration.
- Field/drive coordination. The drive and the excitation system must exchange field-on, field-forcing and pull-out signals. On an LCI the excitation control is integral to commutation. On a VSI, verify the vendor supports synchronous machine control mode — not every MV VSI firmware set does.
- Damper (amortisseur) winding heating. Inverter harmonic currents circulate in the damper cage. Confirm with the motor OEM that the machine is acceptable for inverter duty and identify the harmonic spectrum the chosen topology imposes.
- Starting and pull-in. With a VFD the motor is accelerated from near-zero frequency, so across-the-line pull-in torque and damper-cage induction starting duty no longer govern. Confirm the pull-out (maximum) torque margin at the intended load torque and at rated flux.
- Cooling. If the machine is self-ventilated, airflow follows speed. Continuous operation below rated speed reduces cooling; continuous operation above 50 Hz base speed on a machine originally applied at 50 Hz may exceed the fan and bearing design duty. Verify with the OEM before running above the frequency the mechanical design was validated for.
- Insulation stress. MV drive output imposes repetitive voltage steps and common-mode voltage the machine may not have been designed for if it is an older across-the-line unit. Confirm the winding insulation system, then apply output filters or reactors if the topology and cable length demand it.
- Bearing currents and grounding. Common-mode voltage can drive shaft currents. Verify shaft grounding, and insulated bearing arrangement where the OEM requires it.
Alternatives Worth Costing Against the Drive
| Option | Result | Trade-off |
|---|---|---|
| MV VFD, run at 60 Hz | Full 1250 HP available; speed control as a bonus | Highest capital cost; harmonics, transformer, footprint, torsional study |
| Run at 50 Hz at reduced voltage (~1917 V) from a dedicated transformer tap | Rated torque, ~1042 HP available | No speed control; needs correct voltage source; driven load must accept 5/6 speed. Confirm with OEM that the machine tolerates the reduced voltage/frequency point. |
| Rewind / re-rate the machine for 50 Hz | Purpose-matched machine | Outage, cost, and the driven load still runs at 5/6 of the 60 Hz speed |
| Replace with a 50 Hz motor | Simplest electrically | Foundation, coupling, shaft height and load speed match must be re-checked |
Verification Checklist
- Capture the full motor data sheet: FLA, efficiency, power factor, excitation type, field volts/amps, insulation class, damper winding data, pull-out torque, moment of inertia.
- Confirm plant bus voltage, frequency tolerance and available fault current at the drive input.
- Confirm the drive's guaranteed maximum output voltage at 60 Hz for the specified input voltage — in writing, on the rating sheet.
- Confirm the driven load torque-speed and power-speed curve at 60 Hz, including the 1.2³ power rise if the load is centrifugal.
- Require a torsional analysis of motor, coupling, gearbox and driven machine against the drive's harmonic torque spectrum; verify against the applicable rotordynamics/torsional standard for the machine class.
- Require an input harmonic study against the applicable power quality standard at the point of common coupling; specify multi-pulse (12/18/24-pulse) or active front end as the study dictates.
- Specify motor cable length to the vendor and obtain their filter/reactor recommendation for that length.
- Coordinate protection: differential, loss-of-field, and the interaction between existing motor protection relays and drive-fed non-sinusoidal current.
- Commission with the field excitation regulator in closed loop, log stator current, field current, and power factor at 25/50/75/100% load, and record motor and bearing temperatures at each step.
FAQ
Can a VFD run a 60 Hz motor from a 50 Hz supply?
Yes. The drive rectifies the 50 Hz input to DC and re-inverts it, so output frequency is independent of line frequency. The constraint is output voltage: the drive must be able to deliver 2300 V at 60 Hz, which usually means specifying the input/isolation transformer secondary accordingly.
What happens if I connect a 2300 V 60 Hz motor directly to a 2300 V 50 Hz supply?
Flux rises by the ratio 60/50 = 1.2, driving the core into saturation, with excess magnetizing current and overheating. Constant-flux operation at 50 Hz requires about 1917 V (38.33 V/Hz × 50).
How much current will a 1250 HP, 2300 V motor draw?
Using kVA = √3 × V_LL × I / 1000 with 932.5 kW shaft, assumed 95% efficiency and unity power factor, line current is about 246 A; at 0.8 leading power factor it rises to about 308 A. Size the drive to the motor's stamped full-load amps.
Which MV drive topology suits a synchronous motor?
The load-commutated inverter (LCI) is the traditional choice because the synchronous machine's back-EMF commutates the thyristors. Cascaded H-bridge and three-level NPC voltage-source drives are also applied, but confirm the firmware supports synchronous machine control and coordinate with the excitation system.
Do I need a torsional study for an MV drive on a synchronous motor?
Yes, particularly with an LCI, which produces pulsating air-gap torque at harmonic orders of its bridge. The study must cover the whole shaft train — motor, coupling, any gearbox and the driven machine — over the intended speed range.
