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"I have an 11 kW motor, give me an 11 kW drive" is one of the requests we hear most. It often gives the right answer, but in some cases the drive trips on first load. The reason: a drive's kW label is a reference; what actually decides is the continuous current it can deliver.
Four values to read from the nameplate
- Rated current (A) — the primary criterion. Take the figure on the 380–400 V line of the nameplate.
- Voltage and connection (Δ / Y) — how the motor will run determines the drive's output voltage.
- Rated power (kW) — useful for comparison, not sufficient on its own.
- Speed (rpm) and frequency (Hz) — needed for the speed range and the field-weakening region.
Why current rather than kW?
Two motors with the same kW rating can draw different currents depending on efficiency and power factor. An older, lower-efficiency motor pulls more current for the same work. A drive delivers current, not kW. So the comparison is always between the motor's rated current and the drive's continuous output current.
Load type changes the selection
| Load type | Example | Overload need | Drive selection |
|---|---|---|---|
| Variable torque | Fans, centrifugal pumps | 110% for 60 s | Can be sized close to motor current |
| Constant torque | Conveyors, mixers, compressors | 150% for 60 s | One frame size up is the safe choice |
| High breakaway torque | Crushers, hoists, extruders | 180–200% | Heavy-duty drive or two frame sizes up |
On variable-torque loads such as fans and centrifugal pumps, torque demand falls with speed and the drive is rarely stressed. On constant-torque loads such as conveyors and mixers, full torque is wanted even at low speed, which means sustained high current. On crushers and hoists, breakaway torque can reach twice the rated figure.
Ambient temperature and altitude: the silent derating
The current figure in the catalogue applies at a stated ambient temperature, usually 40 °C. If the temperature inside the panel goes above that, the current the drive can deliver falls. In plants where the panel reaches 55 °C in summer, a drive that looks correctly sized is in fact running below its capacity. The same applies above 1000 metres of altitude.
What to buy alongside the drive
- Input fuse or circuit breaker — of the type the manufacturer specifies; an arbitrary choice affects the warranty.
- Shielded motor cable — for noise control, bonded 360° at both ends.
- Braking resistor — on high-inertia loads and where fast stopping is required.
- Output reactor — where the drive-to-motor distance is long.
- EMC filter — where the specification calls for it or the plant is noise-sensitive.
Send a photo of the motor nameplate and tell us what the load is; we will draw up the drive, protection and accessory list together. Commissioning and parameter setting can be done by the same team.
Frequently asked questions
Is there any harm in choosing a drive larger than the motor?
One frame size up is usually helpful and leaves margin. Going much larger brings two problems: cost rises, and the drive's current protection loses sensitivity for your motor. A greatly oversized drive may not notice an overload on a small motor, in which case separate thermal protection is needed.
Can one drive run more than one motor?
It is technically possible but conditional. The motors must start together and run at the same speed, and each must have its own thermal protection, because the drive sees only total current and cannot isolate an overload on one motor. The drive is also sized for the sum of all motor currents. Where different speeds are needed, separate drives are essential.
What if the motor is old and the nameplate is unreadable?
Two routes. First, measure the motor under load with a clamp meter to establish the current it draws. Second, use the drive's auto-tuning function; the drive measures winding resistance and derives the parameters itself. Even so, the pole count and connection type should be confirmed.
Can we convert a star-delta panel to a drive?
Yes, and it is a common conversion. The existing contactors and timer come out and the drive goes in. The point to watch is the motor connection: a motor in a star-delta panel was selected to run in delta, and it is connected in delta on the drive too. Since the drive adds space and heat load inside the panel, the existing enclosure's adequacy should be checked.
Does buying a drive guarantee energy savings?
No. Savings depend on how the load responds to speed change. On variable-torque loads such as fans and centrifugal pumps, reducing speed cuts power consumption cubically, and the gain is large. On constant-torque loads such as conveyors, reducing speed saves proportionally, not cubically. On a load that must run at fixed speed, a drive brings control rather than savings.