How to Choose the Right Three Phase Motor Size

How to Choose the Right Three Phase Motor Size

Choosing the right specifications for a three-phase motor is a common mistake for many beginners. If the motor is too small, it will overheat and trigger protection immediately upon startup, or even burn out completely. If it is too large, it will result in unnecessary additional costs and higher monthly electricity bills. This article explains in plain language how to select the correct specifications.

Step 1: Determine the exact power requirement of the equipment

When choosing an electric motor, it's not about the "model", but the "power". The unit is usually kilowatts (kW) or horsepower (HP). 1 horsepower is approximately equal to 0.75 kilowatts.

Check the equipment nameplate. Many devices are labeled with the recommended power when they are manufactured. For example, a 5.5-kilowatt water pump will have the instruction "Use a 5.5kW motor" directly on the nameplate. Just buy according to this, it's the most convenient way.

If there is no nameplate, just do the calculation. Let's take a water pump as an example. The power required by the water pump is approximately equal to (flow rate × head × liquid specific gravity) ÷ (367 × efficiency). The numbers in this formula are a bit complicated. I'll teach you a simpler method: Ask the manufacturer of the equipment. They know best how much motor power is needed for their machines.

When in doubt, leave some buffer space. For example, if a device is calculated to require 4.2 kilowatts, it is safer to purchase a 5.5-kilowatt motor rather than a 4-kilowatt one. It is common to leave a buffer of 20% to 30%. But don't leave too much. A 7.5-kilowatt motor powering a load that only requires 2.2 kilowatts is like "a big horse pulling a small cart", which is a waste of money.

Step 2: Check if the startup is difficult

Some equipment has a particularly challenging startup process. For instance, when the conveyor belt is filled with materials, once the motor is started, it has to pull a very heavy load and rotate. This situation is called "heavy-load startup".

Light-load starting equipment. Fans, pumps, and air compressors (in no-load condition) all fall into this category. The resistance during startup is not significant. Just select the power based on the normal calculated value.

Overloaded starting equipment. For example, crushers, mixers, and screw conveyors. When these devices start up, they may require two to three times more torque than during normal operation. At this point, ordinary motors may not be able to handle it. In this case, a motor with "high starting torque" should be selected, or the motor power should be increased by one gear. For instance, if it is calculated that 5.5 kilowatts are needed, directly choose 7.5 kilowatts.

For example. A concrete mixer that starts when there is still half a bucket of material left in the bucket. An ordinary 5.5-kilowatt motor might just hum but not turn. But when replaced with a 7.5-kilowatt motor, it starts up immediately.

Step 3: Determine the working environment of the motor

The environment will have a direct impact on the motor's output.

High-temperature environment. The power marked on the motor nameplate is usually measured under an ambient temperature of 40°C. If the temperature in your workshop is 50°C or even 60°C, the actual power output of the same motor will decrease. At this time, a larger-sized motor needs to be selected for compensation. How much it decreases? I don't know an exact formula down to each degree. But a commonly used rule of thumb is: for every 10°C increase above 40°C in the ambient temperature, the motor output is approximately reduced by 10% to 15%.

Highland environment. At an altitude of over 1000 meters, the air becomes thinner. The cooling performance of the motor deteriorates. The same motor can drive a smaller load at an altitude of 2000 meters compared to at sea level. Generally, when the altitude exceeds 1000 meters, capacity reduction for use is necessary.

Damp or dusty environment. This mainly affects the protection level (IP level) of the motor housing, but does not affect the power selection. However, a higher protection level may result in slightly poorer heat dissipation for the motor. In extreme cases, some margin needs to be left.

Step 4: Check the power supply and speed

What is the voltage in your factory? Common voltages include 380V, 400V, and 415V. Different voltages correspond to different wiring methods (star or delta). You don't need to worry about this; the electrician will handle it. However, when purchasing the motor, you should inform the seller of your voltage.

How to choose the speed? The common synchronous speeds for three-phase motors are 3000 rpm, 1500 rpm, 1000 rpm, and 750 rpm (corresponding to 2 poles, 4 poles, 6 poles, and 8 poles). The higher the speed, the smaller the motor size and the cheaper it is. However, the required speed of the equipment may not match. If the equipment requires 800 rpm, and you buy a 1500 rpm motor, you will also need to add a reducer. Buying a 1000 rpm motor directly might be more convenient.

For instance. For a fan that requires around 900 revolutions per minute, using a 4-pole motor (1500 revolutions) along with a reducer would result in a higher cost compared to directly using a 6-pole motor (1000 revolutions) without a reducer. Moreover, having an additional reducer means there is one more potential point of failure.

A simple selection process

Combine the above steps together.

Obtain the load requirements of the equipment, calculate or look up the required power, for example, 4 kilowatts.

See how difficult the startup is. If you are using a heavy load, increase the power setting by one level. Choose 5.5 kilowatts.

Check the environment. If it's a hot area or a high-altitude location, then increase the power setting by one level. Choose 7.5 kilowatts.

Check the voltage and speed. The voltage should be correct, and the speed should be as close as possible to what the equipment requires.

Look for a standard model on the sample from the motor manufacturer. The power indicated on the sample is always a standard value, not any random number.

Two common mistakes

Mistake 1: Taking the starting current as the basis. Some people think that the current is very high when the motor starts, so they believe that the power should be selected based on the starting current. No. The selection of the motor is based on the load current during continuous operation. Although the starting current is large, it lasts for a very short period of time. As long as the motor can start and rotate within a few seconds, it will be acceptable.

Error 2: Believing that "a larger one is safer". If the motor power is increased by 20%, the purchase price may only be 10% to 15% higher. However, after it is put into operation, the efficiency of the larger motor often tends to be lower under small loads. The extra electricity cost over the course of a year may even be more than the money saved by purchasing the motor. Therefore, it is better to choose the appropriate size, rather than the larger one being always better.

Summarize this.

When choosing the specifications of a three-phase motor, the key point is simply this: calculate the power required by the equipment accurately, and then leave an appropriate margin based on the ease of startup and environmental conditions. If you're unsure, it's better to call the equipment manufacturer rather than make a guess. A properly selected motor can be used for over a decade. A wrongly selected motor will either burn out frequently or incur extra electricity bills every year. Spending half a day calculating the specifications will make this calculation worthwhile in any case.