Starting Load When Choosing an Electric Motor for a Screw Conveyor

A screw (auger) conveyor pushes bulk material with a spiral flight turning inside a tube or trough. In continuous operation its load is fairly steady; the difficult moment is when the screw restarts after stopping full. Material that is packed inside the tube, has absorbed moisture and formed lumps, or has settled and compacted resists the flight as it begins to turn. So when choosing a screw conveyor motor, before the continuous running power you need to understand the torque demanded from the motor at the moment of starting and how that torque is transmitted through the reducer. This article covers the factors that determine starting load, the effect of starting methods on it and the information to gather before ordering.

What determines the starting load of a screw conveyor?

Turning an empty screw is easy; the difficulty comes from the behaviour of the material. The main factors that increase starting load are:

• Fill level: The fuller the screw tube when it stopped, the greater the mass of material in front of the flight. If the inlet was left open and the tube has filled completely, the starting load rises noticeably.
• Type of material: Fine powders such as cement, lime and flour pack down as they stand; damp sand or clay forms lumps; grain and pellets flow relatively freely. The same screw demands very different starting torque with different materials.
• Incline: Compared with a horizontal screw, on an inclined or vertical screw more of the material's weight bears on the flight; starting and running load increase together.
• Standing time: In a screw that has stood full overnight or over a weekend, the material settles. The first start in the morning is heavier than the starts during the day.
• Bearing and hanger condition: On long screws, if the intermediate hanger bearings are worn or packed with material, the friction load is passed on to the motor.

These factors cannot be read from the name of the machine. Two machines both called a "cement screw" can behave completely differently at start-up, with different lengths, diameters and inclines. The power decision should be based on the screw manufacturer's calculation or the existing motor's nameplate.

The difference between starting torque and rated torque

The rated torque on the motor nameplate is the torque the motor can deliver continuously at rated speed. At the moment of starting an asynchronous motor can produce more torque than this; the ratio depends on the motor design and is stated in the catalogue. What matters on a screw conveyor is that the torque demanded at start-up stays below the motor's starting torque and that the motor reaches rated speed quickly enough. If the motor stays at low speed under load for a long time, it draws high current and the winding heats up rapidly.

The reducer does not change this relationship; it only scales it: the torque reaching the screw shaft is multiplied by the reducer ratio. In other words, the reducer is a large torque multiplier, but it cannot deliver more than what the motor can give at start-up. You can compare motors' rated torques by kW and speed in the torque table.

The effect of the number of poles

At the same power, a 6-pole motor delivers about one and a half times the rated torque of a 4-pole motor, because it produces the same power at a lower speed. However, since the screw's rotational speed is determined together with the reducer ratio, if the number of poles changes the reducer ratio must change too. When replacing the motor on an existing screw, keeping the number of poles is the safest course.

How does the starting method affect starting?

The second major factor that determines whether a screw conveyor motor will start without trouble is the starting method.

We summarised whether an external fan and a filter are needed with a frequency inverter in the question on use with an inverter; these conditions must be stated with the order. If the existing panel is star-delta, the new motor's nameplate voltage must be suitable for it: at HEM Motor, models from 4 kW upwards are rated 400/690 V (Δ/Y) and run in delta on a 400 V supply.

Operating measures that ease difficult starts

Choosing a larger motor is neither the only nor the best way to solve a starting problem. In most cases, measures taken on the operating side relieve the motor:

• Before stopping, the feed is shut off and the screw is run for a short time so that it empties. A screw that stands empty or only partly full starts easily.
• If there is a slide gate at the silo or hopper outlet, it is closed while the screw is stopped; this keeps the tube from standing under pressure from above.
• With moisture-absorbing materials, the tube covers and seals are checked; water or steam entering from outside increases lumping.
• Hanger bearings and the flight-to-tube clearance are checked regularly; a worn flight lets material slip back and increases the load.
• The thermal protection is set to the motor's nameplate current. Raising the protection setting hides the problem and shortens the motor's life.

Example models

The models below were chosen from the power range common in screw conveyor drives and from the cast iron series suited to dusty environments. All have IP55 protection, class F insulation and 400/690 V (Δ/Y), 50 Hz. The decision on power and poles should be made with the screw's own data.

• 132S-4 cast iron: 5.5 kW, 4-pole, 1,460 rpm, IEC 132S, rated current 10.7 A, rated torque 36 Nm. The power class seen on short and medium-length horizontal screws.
• 160L-6 cast iron: 11 kW, 6-pole, 970 rpm, IEC 160L, rated current 22.0 A, rated torque 108.3 Nm. On geared arrangements designed for a low input speed.
• 180M-4 cast iron: 18.5 kW, 4-pole, 1,470 rpm, IEC 180M, rated current 33.5 A, rated torque 120.2 Nm. On long or inclined screws carrying dense material.

Further information on handling equipment is on the handling systems electric motors page. The mounting arrangement (B3 foot-mounted, B5 flange or both together as B35) is offered according to the frame; the reducer's input flange size is confirmed with the order.

Hypothetical example: a screw that trips at the morning start

The following situation is an example constructed for explanation. In a flour mill an inclined screw runs without trouble during the day; but after standing full over the weekend, on Monday morning it fails to turn on a star-delta start and trips the thermal relay. The motor nameplate reads 5.5 kW, 4-pole, 132S frame.

• The first step is to find the source of the starting load: check whether the screw was emptied before stopping and whether the silo slide gate was left open.
• If the screw is being stopped empty and the problem persists, the hanger bearings and flight clearance are inspected.
• If the operating routine cannot change, a starting method that provides higher starting torque than star-delta is considered; replacing the motor is discussed only after that.
• If the motor itself has failed, a model with the same power, poles and frame, for example the 132S-4 cast iron, is fitted after confirming the reducer flange.

Information to send with your order

• A photo of the existing motor's nameplate (power, speed, voltage, frame size)
• The screw's length, diameter, incline and the material carried
• Whether the screw starts loaded, and the number of starts per day
• Reducer type, ratio and how it connects to the motor (flange, coupling, belt)
• Starting method and supply voltage
• Environment: dust, humidity, outdoor installation

To see models suited to your screw conveyor and current prices, visit the buy electric motors page. If you are replacing a motor, send a photo of the motor nameplate; to assess the starting conditions together, get in touch with us.