Rated capacity (t)

Lifting gear (t)

Hoisting speed (m/min)

Efficiency η

Motors m

Cyclic duration factor JC

REQUIRED MOTOR POWER

32.01 kW governed by: Static power (P.1 / P.2)

Next standard rating 37 kW Preferred power ratings per GB/T 4772.1 — confirm against the actual motor catalogue.

Quick estimate only — this is the static power from Annex P. It is the starting point of motor selection, not the answer. Switch to Full verification to add the overload and thermal checks.

Calculation	kW	Governing
Static power (P.1 / P.2)	32.01

Calculation detail

Lifting load P_Q	(20 + 0.8) × 1000 × 9.81 = 204,048 N
Speed v_q	8 ÷ 60 = 0.1333 m/s
Static power	204,048 × 0.1333 / (1000 × 0.85) = 32.01 kW(P.1)

Why one formula is not enough

The familiar P = m·g·v / (1000·η) is formula (P.1) of GB/T 3811-2008 Annex P. It is step one of three. Skipping the other two is what lets an under-sized motor pass a paper check and then overheat in service.

Static power

What the load demands at steady speed. Annex P, formulae (P.1) and (P.2). Answers how much power the load needs — nothing about heat or torque margin.

Overload check

Can the motor still pull rated load when voltage dips and torque tolerance stacks up? Annex R, formula (R.1), with H = 2.5 for wound-rotor and cage motors, 2.2 for inverter-fed motors, 1.4 for DC. Answers will it stall.

Thermal check

At this duty factor and this many starts per hour, will it overheat? Annex S, formulae (S.2)–(S.5). Starting and braking are converted to equivalent full starts, then multiplied by the inertia increase rate. Answers will it burn out.

Formulae and clause references

1 · STATIC POWERAnnex P (normative) Hoisting P_N = P_Q · v_q / (1000 · η) (P.1) Travelling P_N = P_j · v_y / (1000 · η · m) (P.2) P_j = P_Σ (ω + m_α) + P_WI indoor P_WI = 0 2 · OVERLOAD CHECKAnnex R (normative) Hoisting P_N ≥ H / (m · λ_m) · P_Q · v_q / (1000 · η) (R.1) H = 2.5 wound-rotor & cage2.2 inverter-fed1.4 DC 3 · THERMAL CHECKAnnex S (normative) Z = d_c + 0.25 · d_i + 0.8 · f (S.3) C = (J_d + J_e) / J_d CZ = C · Z (S.4) Hoisting P_S = G · P_Q · v_q / (1000 · η) (S.2) Criterion P ≥ P_S — P being the catalogue power at that JC and CZ (S.5)

Symbol	Meaning	Unit	Clause
P_Q	Lifting load — rated capacity plus lifting gear, as a force	N	(77)
v_q	Rated hoisting speed	m/s	(83)
P_Σ	Total wheel load	N	(87)
ω	Travel resistance coefficient — 0.006 anti-friction / 0.015 sliding	—	(142)
η	Total mechanical efficiency of the mechanism	—	P.1
m	Number of motors in the mechanism	—	(89)
H	Overload coefficient — 2.5 wound-rotor & cage, 2.2 inverter-fed, 1.4 DC	—	R.1
λ_m	Maximum torque multiple — provided by the motor manufacturer	—	R.1
G	Steady-state mean load coefficient	—	Table P.1
JC	Cyclic duration factor	%	Annex Q
Z	Referred number of full starts per hour	1/h	(S.3)
C	Inertia increase rate, (J_d+J_e)/J_d	—	(S.4)
CZ	C × Z — governs motor heating from starting and braking	—	S.2.1.1.3.3

Applicable to

Overhead and gantry cranes — single and double girder
Hoisting mechanisms and rail-mounted travelling mechanisms
Crane-duty and metallurgical-duty motor series
Altitude ≤ 1000 m, ambient ≤ 40 °C, stable supply voltage
Pre-dimensioning and checking at enquiry and budget stage

Outside this tool — talk to an engineer

Foundry, explosion-proof, nuclear and other special-duty cranes — additional requirements apply
Electric hoists and cone-rotor brake motors — different thermal behaviour, results here are indicative only
Eddy-current brakes or thyristor stator-voltage control — thermal checking should use the mean-loss method (S.2.3)
Travelling mechanism starting inertia — the ΣJ·n²/(91 200·t_q) term needs mechanism design data and is often the governing term
Slewing and luffing mechanisms — not covered by this tool
Imported motors rated to S1 continuous duty — a different rating basis; do not mix conversion factors

Get the calculation verified by our engineers

A pre-dimensioning result is not a design calculation. λ_m comes from the motor catalogue, ΣJ and the starting time come from the mechanism design, and the catalogue power at your actual JC and CZ has to be read from the manufacturer data. Send us the figures above and we will return a verified motor selection with the supporting calculation.

Email our engineers

Typical response within one working day. No obligation.

Frequently asked questions

QIs the number I get the motor power I should order?›

Not on its own. In Quick estimate mode the figure is the static power from Annex P — what the load demands at steady speed. Motor selection also has to clear the overload check (Annex R) and the thermal check (Annex S), and the governing value is the largest of the three. Switch to Full verification to see all three side by side.

QWhy do you ask for the weight of the lifting gear separately?›

Because the motor lifts both. P_Q in formula (P.1) is the lifting load, which includes the grab, magnet, lifting beam or spreader. On a 16 t grab crane the grab alone can be 6 t — leaving it out understates the power by more than a third.

QWhat is the difference between JC and the number of starts? ›

JC is how long the motor is energised within a cycle. Starts per hour is how often it accelerates. They heat the motor by different mechanisms: JC drives steady-state heating, starting current drives peak heating. A short-travel, high-frequency crane can have a low JC and still overheat because of the starts — which is exactly why the thermal check converts jogs and electric brakings into equivalent full starts (Z = d_c + 0.25·d_i + 0.8·f) and multiplies by the inertia increase rate C.

QWhy will the tool not fill in λm for me? ›

Because the standard does not provide one. GB/T 3811-2008 R.1 states that λ_m is provided by the motor manufacturer. The only figure the standard gives is λ_m ≥ 2.2 for cage motors started direct-on-line, and that is a lower limit rather than a value. It matters: on a 32 kW static power, λ_m = 2.2 gives an overload requirement of 36.4 kW — which overtakes the static power and becomes the governing item — while λ_m = 3.0 gives 26.7 kW. A guessed λ_m produces a meaningless overload check, so the tool leaves it blank and skips the check instead.

QCan I use this for an electric hoist? ›

Treat the result as indicative only. The formulae in Annex P are written for crane-duty and metallurgical-duty motor series driving open mechanisms. Electric hoists commonly use cone-rotor brake motors whose thermal and starting behaviour differs, and hoist manufacturers rate their units as complete products rather than by motor power alone. Send us the duty and we will advise on the hoist model directly.

QOur specification names an imported motor rated to S1. Does this still apply? ›

The static, overload and thermal formulae still apply — they describe the mechanism, not the brand. What does not transfer is the power conversion basis: crane-duty motor catalogues are rated on an intermittent basis, while general-purpose imported motors are rated on S1 continuous duty. The conversion factors of the two systems are referenced to different baselines and must not be mixed. Please contact our engineers for that conversion.

Leave Message

Kinocranes is a professional crane manufacturer. We integrate crane research and development, manufacturing, sales, installation and service.

Email: [email protected]
Tel: +8619037303917

Please feel free to contact us!