Guide · Sizing
Sizing a 48 V drive: reading continuous power, phase current and derating correctly
“10 kW” on a datasheet says little unless it also says for how long, at which temperature and with which cooling. This guide explains which figures really count for inverter and motor, why phase current and DC current are different things, and how to factor thermal derating into sizing.
At a glance
| Continuous power | the power the drive sustains thermally without limit – not the peak value |
| Phase current | determines torque and the losses in motor and power stage |
| DC current | determines the load on battery and DC link – at 48 V a different figure from the phase current |
| Derating | the controlled reduction of power with rising temperature – better than shutting down |
| INV-60V-120A | 6 kW continuous, 17–60 V, 120 A rms continuous phase current, from 108 g, air-cooled with thermal derating logic |
Published September 25, 2026 · Persystems GmbH, Regensburg, Germany
Peak power is not a sizing figure
Electric drives tolerate considerably more for a short time than continuously – the thermal time constants of winding, heat sink and housing make it possible. That is exactly why peak values are so popular on datasheets and so uninformative. For sizing, continuous power counts: the power the drive sustains without limit at the intended ambient temperature and cooling, without a protection function stepping in.
That always includes the boundary conditions. An air-cooled inverter in a propeller slipstream sees different conditions from the same inverter in a closed enclosure. Comparing continuous power means comparing the cooling condition as well – or better, measuring the drive under your own conditions.
Phase current, DC current and torque
In 48 V systems two currents are easily confused. The phase current flows between inverter and motor; it determines the torque (with field-oriented control directly via Iq) and the losses in winding and power stage. The DC current flows from the battery into the DC link; it follows from the delivered power, the DC-link voltage and the efficiency.
The relation is given by the power balance, not by equality of the currents: at low speed and high torque the phase current can be a multiple of the DC current because the inverter works as a step-down converter. For sizing that means: the phase current decides whether the inverter can deliver the requested torque; the DC current decides what battery, cables and fuse must carry. The INV-60V-120A provides both values in its telemetry – phase current (rms) and DC current every 100 ms over PersyCAN.
Voltage level: what “48 V” really means
A 48 V system is rarely at 48 volts. A 12s lithium-ion pack swings between about 50 V full and just under 40 V empty; during recuperation the voltage rises briefly. The inverter must cover the whole range – the INV-60V-120A works from 17 to 60 V – and the sizing must consider the worst corner: with an empty battery less voltage is available for the same power, the current rises, and the motor’s maximum speed drops because the back-EMF reaches the DC-link voltage earlier.
For drones that means concretely: the thrust reserve at the end of the flight is the relevant one, not the one with a full battery. For vehicles: top speed must be reachable with an empty pack as well.
Derating: protection that keeps the drive running
When a drive gets hotter than allowed there are two strategies: shut down or back off. Shutting down is not an option for an aircraft and unpleasant for a vehicle. Thermal derating instead reduces the permissible power in a controlled way as temperature rises – the drive keeps running, only with less reserve. The INV-60V-120A works with such a derating logic and reports MOSFET and motor temperature cyclically over PersyCAN; over DroneCAN an impending overtemperature appears as a health level before a shutdown becomes necessary.
In the sizing, derating should be considered as a curve: what continuous power remains at 40 °C ambient, what at 50 °C? Knowing only the cold-start value means sizing too tight.
Sizing motor and inverter together
Inverter and motor are not a kit in which any parts fit together. The motor’s inductance determines the current ripple and the demands on the current control; pole-pair count and maximum speed determine the electrical frequency the inverter must handle; the back-EMF constant sets the speed at which the voltage runs out. A high-pole-count outrunner on a drone poses different demands from a geared traction motor.
The safe way is to compute motor, inverter and control together before hardware is ordered: with OverDrive, current ripple, saturation and the behavior at the voltage limit can be simulated physically exactly – and the results verified on the test bench. That is exactly how we work at Persystems: what we simulate, we measure.
Checklist for your inquiry
To size a drive for you, these details help: voltage level and battery (cell count, nominal voltage, range), required continuous and peak power with duration, torque and speed range, motor data (pole pairs, inductance, resistance, back-EMF constant if known), cooling condition and ambient temperature, interface (CAN, DroneCAN, PWM) and position sensing (sensorless, sin/cos, incremental encoder). We determine missing values together – if necessary on the test bench.
Frequently asked questions
Why is the phase current higher than the DC current?
Because the inverter converts the DC-link voltage into a lower motor voltage. At the same power, lower voltage means higher current – at low speed and high torque the phase current can therefore be a multiple of the DC current.
Is a 10 kW inverter enough for a 10 kW motor?
Only if the boundary conditions match: cooling, ambient temperature, voltage range and the actual phase current at the required speed. We recommend computing or measuring the drive as a whole.
What happens in the INV-60V-120A on overtemperature?
The derating logic reduces power in a controlled way; temperatures are reported cyclically. Overcurrent, overtemperature and undervoltage switch the drive into a safe state – Safe Torque Off or Active Short Circuit, configurable.
Contact
Size your 48 V drive together with us?
Send us voltage, power, motor and interface – you will get an assessment straight from the engineering team, on request with a measurement on the test bench.