Choosing Between 1,500 and 3,000 rpm for a 22 kW Electric Motor

For anyone looking for a 22 kW (30 HP) motor, the first question is usually "1,500 or 3,000?". Both motors have the same power, but what they deliver to the machine is very different: a 3,000 rpm motor turns its shaft twice as fast and gives half the torque, while a 1,500 rpm motor gives twice the torque at half the speed. A motor at the wrong speed either drives the machine faster than necessary and overloads it, or never brings it to its operating point. This article explains how to choose the speed at 22 kW according to the way the machine is driven, and what to watch out for when considering a switch between the two speeds.

Values of the 1,500 and 3,000 rpm models at 22 kW

At HEM Motor, 22 kW is offered with a cast iron frame. The nameplate values of the 1,500 and 3,000 rpm models side by side are as follows:

Both models have IP55 protection and class F insulation. The currents are close to each other; the real difference is in torque and frame length. To carry twice the torque, the 1,500 rpm model sits in a longer 180L frame. Even though the shaft height is the same, the two frames differ in foot hole spacing and shaft end dimensions.

The machine decides the speed, not the motor

The starting point for choosing the speed is how fast the machine's shaft needs to turn. The connection between the motor and the machine determines how that speed is reached.

Direct coupled drive

If the motor shaft and the machine shaft are joined by a coupling, the machine turns at the motor's speed. In this case the speed is fixed by the machine's design speed and cannot be changed. If a 1,500 rpm motor is fitted to a centrifugal pump designed to run at around 2,950 rpm, the pump reaches only about a quarter of its head; conversely, fitting a 3,000 rpm motor to a pump designed for 1,450 rpm overloads both the pump and the motor. On a coupled machine, changing the speed means changing the machine.

Belt and pulley drive

In belt drives the machine speed is set by the pulley ratio, so on paper either motor can be used. However, to reach the same machine speed with a 3,000 rpm motor, the motor pulley must be smaller or the machine pulley larger. A small pulley leads to more frequent belt bending, higher belt tension and a greater side load on the shaft. For this reason, in belt drives that need a high reduction ratio (that is, when the machine turns slowly), a 1,500 rpm motor usually gives a better-balanced arrangement.

Gearbox drive

Gearboxes are selected for a particular input speed, and most standard gearboxes are rated for a 1,500 rpm input. Connecting a 3,000 rpm motor to the same gearbox doubles the output speed, halves the output torque and can push the bearings and seals on the gearbox input side beyond their permitted limit. On a geared drive, check the gearbox manufacturer's input speed limit before changing the motor speed. Other points to watch when connecting a motor to a gearbox are summarised on the motors for gearboxes page.

1,500 or 3,000: decision list

• 3,000 rpm (180M-2) may make sense: for direct-coupled centrifugal pumps and fans designed for high speed; for high-speed blowers and some compressors; in applications where the machine builder specifies a 2-pole motor.
• 1,500 rpm (180L-4) may make sense: for general-purpose machine drives; for geared conveyors, mixers and screw conveyors; for heavy-starting or high-torque loads; for belt drives that need a high reduction ratio.
• Check both options: belt-driven fans can use either speed; here the pulley size, belt speed and the permitted speed of the fan impeller are assessed together.

This list shows general tendencies; an exact speed or power cannot be recommended from the machine's name. Load, duty time, starting conditions and connection type must be evaluated together.

Additional issues that come with high speed

A 3,000 rpm motor may look attractive because it is smaller and has lower torque than a 1,500 rpm motor of the same power. Before choosing it, take the following into account:

• Noise: 2-pole motors are usually louder because of the higher fan and rotor speed. In enclosed areas where people are constantly present, this difference is noticeable.
• Bearings and lubrication: high speed may require more frequent bearing maintenance. Pay attention to the lubrication interval and bearing temperature.
• Balance and alignment: at high speed, coupling misalignment and imbalance increase vibration more quickly. Coupling alignment must be done more precisely.
• Shaft end: in some frames the shaft end of the 2-pole model may differ from the 4-pole one. Before ordering a coupling or pulley hub, read the dimension for your own model in the 2D technical drawings.

Example: changing speed on a belt-driven fan

The following sequence is a hypothetical example. Suppose the 1,500 rpm, 22 kW motor of a belt-driven radial fan at a plant has failed, and a speed change is being considered because a 3,000 rpm motor is on hand.

• Keep the fan speed: the operating speed of the fan impeller must not change. To reach the same fan speed with a 3,000 rpm motor, the motor pulley diameter must be roughly halved.
• Check the pulley: such a small pulley may fall below the minimum diameter permitted for the belt profile; belt life shortens and the side load on the shaft increases.
• Check the base: moving from a 180L to a 180M frame changes the foot hole spacing; the slide rail or base may need adapting.
• Conclusion: in this example, the least problematic path is to return to the 180L-4, at the same speed as the old motor. A speed change can only be considered if the pulley, belt and base are recalculated.

If the same plant has a low-speed mixer that needs high torque, neither option may be suitable; in that case the 200L2-6 (22 kW, 6 poles, 980 rpm, IEC 200L, 400/690 V, rated torque 214.4 Nm) can be considered.

Information needed for your order

• The machine's operating speed and connection type (coupling, belt and pulley, gearbox)
• For belt drives, the motor and machine pulley diameters; for geared drives, the gearbox input speed
• Mounting arrangement (B3, B5, B35, etc.) and, for a replacement, the old motor's frame size
• Supply voltage and starting method
• For a replacement, a photo of the old motor's nameplate

You can review the 22 kW models and current prices on the buy electric motors page. If you are unsure about the speed, send us the nameplate of your current motor and we will choose the right model together; for other questions you can use our contact page.