Motor & gearbox sizing
Size the drive properly: torque rather than power, derated for the real ambient and altitude, geared down to pump speed, and specified for the area classification. Set the minimum and maximum speed you intend to run at and the motor's own curves show whether it can hold that torque continuously — or where it needs forced ventilation.
Enter the drive duty
Take the absorbed power and the speed from a pump selector, add your site conditions, and this will size the motor and gearbox, apply the deratings, and list the accessories the installation needs.
How drives are sized
Four things that decide the motor
Torque, not power
A motor is a torque machine: T [Nm] = 9550 × P [kW] ÷ n [rpm]. Halve the speed and you double the torque for the same power. That is why a positive-displacement pump turning at 200 rpm needs a gearbox and a torque-rated drive, not simply a bigger motor.
Derating for the real site
IEC ratings assume 40 °C and 1000 m. Hotter or higher and the motor cannot shed its heat — roughly 1% of rating per °C above 40, and about 3% per 500 m above 1000 m. Ignore it and the winding runs hot: every 10 °C above the insulation limit halves insulation life.
Constant torque versus variable torque
A centrifugal pump's torque falls with the square of speed, so it turns down easily. A PD pump demands full torque at any speed, so a self-cooled motor runs out of cooling long before it runs out of torque — which is why constant-torque duty below about half speed needs forced ventilation.
Reading the curves
A motor's torque curve has three points that matter: locked-rotor torque at standstill, the pull-up saddle as it accelerates, and breakdown torque near full speed. If the load line crosses the saddle, the motor hangs at part speed drawing near locked-rotor current until something trips. The curves drawn with each selection show the load line against all three.
Speed range and field weakening
Below base frequency the drive holds V/f constant so full torque is available — but a shaft-mounted fan is turning slowly, so a self-ventilated motor must be derated. Above base frequency the voltage is already maxed out: flux falls, rated torque drops as 1/n and breakdown torque as 1/n². Set your minimum and maximum speed and the tool checks both ends against the motor it selected.
Starting torque
PC and lobe pumps have a break-away torque well above their running torque, and it rises as the stator ages. Star-delta only delivers about a third of DOL torque, so it is a poor match; a VFD or soft starter with torque boost is usually the right answer.
Hazardous area, at a glance
| Zone | Protection | Meaning |
|---|---|---|
| Zone 1 (gas) | Ex db / Ex eb | Explosive atmosphere likely in normal operation |
| Zone 2 (gas) | Ex ec / nA | Unlikely, or present only briefly |
| Zone 21 (dust) | Ex tb IIIC, IP6X | Combustible dust cloud likely in normal operation |
| Zone 22 (dust) | Ex tc | Dust cloud unlikely or short-lived |
| Underground mining | Ex db I Mb (Group I) | Group I certification — not the Group II used on the surface |
Certification covers the whole assembly — motor, terminal box, glands, gearbox, fan and any encoder. Always confirm the gas group and temperature class for the actual substance present.
Need the whole package?
RDJ supplies the pump, drive, gearbox, instrumentation and control panel as one engineered skid.