Hydraulic Motor Sizing (Power Pack Motor kW)

The electric motor is the heart of a hydraulic power pack — and the most common thing engineers get wrong when specifying a unit. Size it too small and the pack stalls or trips on overload; size it too big and you pay for a frame, a starter and a running current you never use. This guide shows how to turn a working pressure and a flow into the right motor kW, from first principles.

Why motor power depends on both pressure and flow

Hydraulic power is not set by pressure alone or flow alone — it is the product of the two. Pressure (bar) decides how much force the system can develop; flow (litres per minute) decides how fast that force moves. A power pack can absorb the same motor power holding a high pressure at a trickle of flow as it does pushing a large flow at modest pressure.

That is why you can never size a motor from one number. Two systems that both run at 120 bar can need wildly different motors if one moves 6 L/min and the other 30 L/min. The motor has to supply the hydraulic power the pump draws — pressure times flow — plus the losses in the pump and drive. Get either variable wrong and the kW is wrong.

The motor sizing formula

For an electric-motor-driven hydraulic power pack, the input power in kilowatts is:

Where:

• P — working pressure in bar (the pressure the pump actually holds during the duty cycle).
• Q — pump delivery in litres per minute.
• η_o — overall efficiency, combining pump and drive losses. For a typical gear-pump power pack use 0.80 to 0.88 (85% is a sound default).

Where the 600 constant comes from

The 600 is not magic — it is a unit conversion that lets you work in the everyday units of bar and L/min. Hydraulic power in watts is pressure (in pascals) times flow (in cubic metres per second). Converting 1 bar to 100,000 Pa, and 1 L/min to 1/60,000 m³/s, the two conversions collapse to a single divisor: 100,000 ÷ 60,000 = 1.667, and dividing by 1000 to reach kW gives the tidy factor of 600. So P×Q/600 is the hydraulic (output) power in kW before losses.

Why efficiency divides, not multiplies

The pump does not convert electrical power to hydraulic power perfectly — friction, internal leakage and drive losses eat a slice. Because the motor has to supply more power than the fluid finally carries, efficiency sits in the denominator. Dividing by η_o = 0.85 makes the required motor power roughly 18% larger than the ideal hydraulic power. Never leave efficiency out — a "100% efficient" calculation will always undersize the motor.

Converting kW to HP

Motors in India and much of the world are catalogued in both kW and horsepower. To convert, divide kilowatts by 0.746:

So a 3.7 kW motor is about 5 HP, and a 5.5 kW motor is about 7.5 HP. The kW figure is what your calculation produces; the HP figure is often what the motor is called on the shop floor.

Worked example

Take a clamping power pack that must hold 120 bar while delivering 12 L/min, with an assumed overall efficiency of 0.85:

The calculation says 2.82 kW. But you cannot order a 2.82 kW motor — motors are built in standard frame sizes. So you round up to the next standard frame, which is 3.7 kW (5 HP). Rounding down to 2.2 kW would leave the motor running at or above its rating whenever pressure peaks, and it would trip.

Standard motor frame sizes

Three-phase induction motors come in a fixed ladder of ratings. The common frames for hydraulic power packs are:

• 0.75 · 1.1 · 1.5 · 2.2 · 3.7 · 5.5 · 7.5 kW (roughly 1, 1.5, 2, 3, 5, 7.5 and 10 HP)

Always select the next size up from your calculated figure. The headroom covers pressure spikes at valve shifts, cold-start viscosity, voltage dips and the fact that real efficiency drifts below the assumed value as the pump wears. A motor loaded to about 75–85% of rating runs cool, efficient and reliable — that is exactly what rounding up gives you.

Peak vs continuous pressure, and duty cycle

A hydraulic system rarely sits at one pressure. It might build to a high clamping pressure for a moment, then idle at low pressure, then repeat. Two pressures matter for motor sizing:

• Peak pressure — the highest pressure reached, often only for seconds. A motor can tolerate a brief overload.
• Continuous working pressure — the pressure held for sustained periods. This is the one that heats the motor and must be inside its rating.

You size the motor on the worst-case continuous working pressure — the highest pressure the pump must hold long enough to matter thermally. If the duty is genuinely continuous (the pump runs at pressure all day), be conservative and lean toward the upper frame. If the high-pressure phase is a brief fraction of a cycle, you can size on it as a short-term peak — but only with confidence in the duty cycle. When in doubt, size for continuous duty; a motor overheats far more easily than it stalls.

Starting current — DOL vs star-delta

An induction motor draws a large inrush at start — typically six to eight times its full-load current. For small motors (broadly up to about 5.5 kW) this is usually acceptable and the motor is started direct-on-line (DOL). Larger motors are started via a star-delta or soft starter, which limits the inrush and the resulting voltage dip on the supply. The exact changeover point depends on the supply capacity and local electrical rules, so confirm the starter choice with your panel builder — but keep it in mind, because a motor one frame larger can push you from a DOL starter to a star-delta panel.

Pressure × flow → suggested motor

The table below is illustrative — it applies the formula at η_o = 0.85 and rounds up to the next standard frame. Use it to sanity-check a selection, not as a substitute for calculating your own numbers.

Typical motor selection (η_o = 0.85, illustrative)

Working pressure	Flow	Calculated kW	Suggested frame
60 bar	6 L/min	0.71 kW	0.75 kW (1 HP)
100 bar	10 L/min	1.96 kW	2.2 kW (3 HP)
120 bar	12 L/min	2.82 kW	3.7 kW (5 HP)
150 bar	15 L/min	4.41 kW	5.5 kW (7.5 HP)
175 bar	16 L/min	5.49 kW	5.5 kW (7.5 HP)
210 bar	16 L/min	6.59 kW	7.5 kW (10 HP)

Note how the 175 bar row lands at 5.49 kW — just under the 5.5 kW frame. That is a comfortable fit; if your real efficiency or pressure runs higher, step up to 7.5 kW.

Common mistakes

• Sizing on relief-valve pressure, not working pressure. The relief valve is set above the working pressure and only lifts briefly. Sizing the motor to it inflates the kW, the starter and the running cost for a condition that almost never occurs.
• Ignoring efficiency. Leaving η_o out (or setting it to 100%) gives the ideal hydraulic power, not the motor power. The motor will be undersized by roughly 15–20% and will run hot or trip.
• Not rounding up. Picking the frame below the calculated figure — "2.82 is basically 2.2" — leaves no headroom for cold starts, spikes or wear. Always take the next standard size up.
• Forgetting continuous-duty heat. A motor that copes with a brief peak can still overheat if that pressure is held all shift. For continuous duty, size on the sustained working pressure and give the reservoir enough volume to shed heat.