How Is the Electric Motor Power of a Feed Grinding Machine Determined?

A feed grinding machine, in other words a hammer mill, is one of the machines in a feed plant that consumes the most energy and puts the most strain on its motor. Barley, maize, wheat or bran is broken up by hammers on a rotating rotor and passes through a screen to reach the desired fineness. Since the range is wide, from small farm machines to plant mills processing tonnes of product per hour, the question "what kW motor does a feed grinder need?" has no single answer. In this article we explain the factors that determine motor power, from whom and how the power should be obtained, the choice of speed and transmission, and the information to be provided with the order.

Factors that determine the power requirement

The power drawn by a hammer mill depends less on the value written on the machine's nameplate than on what is fed into it that day. The main variables that change the power requirement on the same machine are:

• Type of raw material: Hard grains such as maize and barley require much more energy than bran or pre-broken product. Fibrous materials such as dry hay or straw load the hammers in a different way.
• Screen hole diameter: When a screen with small holes is used for fine grinding, the product stays longer in the mill and the power for the same capacity rises markedly. Changing the screen actually means changing the load on the motor.
• Moisture: Moist grain does not break easily, clogs the screen and builds up around the rotor; as a result the current rises.
• Capacity (feed rate): The more the feed screw or gate is opened, the more the motor is loaded. When the mill is fed more than the motor can carry, the speed drops first and then the protection trips.
• Number of hammers and their wear: Hammers with blunt tips rub the grain instead of breaking it; more power is spent for the same work and heat increases.
• Operating time: A farm machine running one or two hours a day and a plant mill running non-stop through a shift wear the motor differently, even at the same power.

That is why giving a fixed table such as "this capacity needs this kW" would be misleading. The right way to determine motor power is to follow the sequence described below.

Who determines the power, and how?

On a new machine, the power is the machine manufacturer's value. The manufacturer has designed the rotor diameter, the hammer arrangement, the screen area and the target capacity around this power; fitting a smaller motor limits the machine, while fitting a much larger one may stress mechanical parts such as the rotor, bearings and belts beyond their design limits.

If the motor of an existing machine is being replaced, the information to rely on is the old motor's nameplate: power, speed, voltage and IEC frame code. If the old motor ran for years without problems, a motor with the same values is the safest choice. If the motor trips the thermal relay frequently or overheats, the following questions should be asked before increasing the power: Was the screen replaced with a finer one? Did the raw material change? Are the hammers worn? Was the feed setting changed? Are the belts slipping? Most of these problems are solved not by enlarging the motor but by correcting the machine itself.

If moving to a larger motor is genuinely necessary, for example when a capacity increase is planned, the decision should be taken together with the machine manufacturer. Whether the rotor, shaft and belt arrangement can withstand the new power can only be assessed on the machine side.

What does a current measurement tell you?

The practical way to understand how heavily the motor is really loaded on a running mill is to measure the phase currents with a clamp meter under normal feed. If the measured current is very close to or above the rated current on the nameplate, the motor is working at its limit. If it is clearly below, the motor is comfortable and the source of the overheating problem should be looked for elsewhere. This measurement bases the replacement decision on data rather than guesswork.

Speed and transmission: direct coupling or belt and pulley?

In hammer mills the rotor usually turns at a high peripheral speed; for this reason, on many machines a 2-pole (3,000 rpm synchronous) motor is connected directly to the rotor with a coupling. In this arrangement the rotor speed equals the motor speed and the motor speed cannot be changed. On some machines a 4-pole motor turns the rotor through belts and pulleys; the pulley ratio determines the rotor speed.

• Direct coupled arrangement: The motor speed must be the same as the speed required by the machine manufacturer. The foot-mounted (B3) motor is aligned with the rotor on a common baseplate; the shaft height and foot spacing are compared with the old motor.
• Belt and pulley arrangement: If the motor speed changes, the rotor speed changes too. Replacing a 1,500 rpm motor with a 3,000 rpm motor doubles the rotor speed if the pulleys are not changed; this is dangerous for both the hammers and the bearings. Speed is not changed without recalculating the pulley ratio.
• Belt tension: An over-tightened belt puts a side load on the motor's front bearing; a loose belt slips, heats up and wastes part of the power.

Because the rotor speed is high, imbalance also matters. If one of the hammers is broken or missing, the resulting vibration is transmitted to the motor bearings. If the real cause of the old bearing failure was such an imbalance, the new motor will soon meet the same end.

Starting and electrical conditions

The rotor of a hammer mill is heavy and behaves like a flywheel. For this reason the starting time can be long even when empty. If the mill was stopped full, the motor has to accelerate both the rotor and the product left inside it at start-up.

• Not stopping the mill before emptying it and starting it empty significantly reduces the load on the motor and the starter.
• At medium and high powers, star-delta or soft starters are common. Because motor torque drops in the star position, the changeover time must be set correctly so that the heavy rotor reaches sufficient speed in star.
• Operation with a frequency inverter allows soft starting and capacity adjustment. The general rule is explained in the inverter use question; long operation at full load at low speed and long cables are stated with the order.
• A long start requires the motor protection setting to be chosen accordingly. For protective devices, see the article on choosing thermal relays and fuses.

Feed plants are dusty environments. IP55 protection provides protection against dust and water splashes; however, if the motor's cooling fins and fan cover are covered with dust, cooling becomes weaker. Regular cleaning is the simplest and most effective measure for motor life. If the plant has a special zone classification because of dust density, this information should be stated separately with the order.

Example models

The cast iron frame models below are examples of the power ratings and speeds often encountered on feed grinding machines; the right power for your machine is determined by the machine manufacturer's value and the existing motor's nameplate. All models have IP55, class F insulation and a 400/690 V (Δ/Y), 50 Hz supply; on a 400 V network they are connected in delta and are suitable for star-delta starting.

• 160L-4: 15 kW (20 HP), 4 poles, 1,465 rpm, IEC 160L, 27.3 A at 400 V, rated torque 97.8 Nm. For machines that turn the rotor through belts and pulleys.
• 180M-2: 22 kW (30 HP), 2 poles, 2,955 rpm, IEC 180M, 38.5 A at 400 V, rated torque 71.1 Nm. For mills that drive the rotor directly through a coupling.
• 200L1-2: 30 kW (40 HP), 2 poles, 2,960 rpm, IEC 200L, 52.1 A at 400 V, rated torque 96.8 Nm.

The cast iron frame stands out in vibrating and dusty feed plants thanks to its rigid structure. General information on other machines in feed plants is on the feed machinery motors page, and information on other grinding and milling plants is on the flour mill motors page. For dimensions, see the 2D technical drawings by frame size.

Hypothetical example: a mill that keeps tripping

This section is not a real customer record but an example built to show the order of decisions. Suppose that on a direct coupled hammer mill at a livestock farm, the 22 kW 2-pole motor has been tripping frequently in recent months. The first thought is to switch to a 30 kW motor. However, the inspection shows that the screen was replaced with a finer one and that the hammer tips are worn. When the hammers are turned and renewed and the feed is reduced slightly, the current drops below the nameplate value. Even if the old motor has failed, the right replacement is a 180M-2 with the same values. Had the machine manufacturer approved a higher power for a capacity increase, the 200L1-2 could have been considered together with a check of the rotor and shaft.

What to send with the order

So that we can identify the right motor for your feed grinding machine, send the following information together:

• The power and speed recommended by the machine manufacturer, or a photo of the old motor's nameplate
• Transmission type: direct coupling or belt and pulley; if pulleys, the pulley diameters
• Mounting arrangement (foot, flange) and the IEC frame code of the old motor
• Network voltage and starting method
• The raw material processed, daily operating time and the dust conditions of the environment

You can review suitable models and current prices on the buy electric motors page. If you are replacing a motor, send us a photo of the motor nameplate and we will identify the equivalent model together; for other questions, the contact page is available.