Metal Shredder Electric Motor: Impact Load and Drive Arrangement

When selecting the motor for a metal shredder, the real deciding factor is not the type of material being shredded but the path by which the impact reaches the motor. On a low-speed twin-shaft shredder the impact travels straight back to the shaft through the gearbox; on a hammer mill the flywheel absorbs the impact but lengthens the start; on a hydraulically driven machine the motor hardly feels the impact at all. So the drive arrangement must be understood first, and the motor then selected for that arrangement.

Three drive arrangements, three different motor loads

Machines that shred scrap metal, swarf, aluminium profiles, white goods or cable may look similar from the outside, but they contain different drive architectures. The load affecting the motor changes completely with this architecture.

Even with two machines of the same power, deciding on a motor replacement without knowing which arrangement each uses is risky. The machine nameplate and drive diagram are the first documents in motor selection.

Torque peaks on geared low-speed shredders

On a twin-shaft shredder, the bladed shafts turn at a few tens of revolutions per minute. The motor usually drives these shafts through a planetary or helical bevel gearbox; some machines have a separate motor-gearbox group for each shaft. When a hard piece gets between the blades, the shaft slows down and torque rises instantly.

Automatic reversing

The controls of these machines stop the shafts when motor current exceeds a certain value, reverse them briefly and then try forward again. Every change of direction means a new start in which the motor draws high current. When working with material that jams frequently, dozens of reversals per hour can occur; this determines motor heating far more than the rated power does.

• State the estimated number of reversals per hour in the order; frequent reversing is assessed separately in motor selection.
• Direction should be changed with the motor stopped; sudden reversing while running puts a heavy load on the winding and gearbox.
• On machines with a frequency inverter, reversing is done with a ramp and current is limited. Our three-phase series are suitable for speed control with a frequency inverter; for long periods of full-torque operation at low speed an external fan may be needed, and if the cable between drive and motor is long a dv/dt or sine filter is considered.

Mechanical protection elements

To keep torque peaks away from the motor and gearbox, the drive line may include a torque limiter, a shear-pin coupling or a hydrodynamic (fluid) coupling. A fluid coupling lets the motor accelerate unloaded first and absorbs sudden impacts by slipping. These elements belong to the machine builder's design; when replacing the motor, make sure they are in place and working.

Flywheel and starting time on hammer mills

On hammer-type metal crushers the rotor and its hammers form a large mass; on most machines an additional flywheel is also fitted. During operation this inertia smooths the impacts: the rotor slows slightly with each strike and the motor accelerates it again in between. The motor thus carries a load close to the average rather than the peaks.

The price is paid at start-up. Bringing a heavy rotor from standstill to full speed can take a long time, and the motor draws high current throughout. Every motor has a permitted starting time and number of starts per hour. If the rotor's moment of inertia (J) and the required starting time are known, it can be calculated whether the motor can handle this start; ask the machine builder for these data and pass them on to us.

• Start with the machine empty; open the feed only after the rotor has reached full speed.
• With star-delta starting, the low torque in star lengthens the acceleration of a heavy rotor; the changeover time is set accordingly.
• Repeated start attempts heat the motor; after a failed start, wait for the motor to cool down.

Motor selection on hydraulically driven machines

On some shredders the bladed shafts are turned by hydraulic motors; the electric motor only drives the hydraulic pump. In this arrangement the impact is limited by the hydraulic pressure relief, and the motor mostly runs at constant speed with a fluctuating but smoothed load. Motor power comes from the pump flow and operating pressure; power is not determined from the machine's name.

On this type of machine the motor works on the tank of the hydraulic unit with a pump bell housing and coupling. Mounting arrangements (B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15) are offered according to the frame and the flange size is confirmed with the order. For an overview of hydraulic applications, see the hydraulic pump motors page.

Whichever drive arrangement you have, if the power and speed of the existing motor are known, review suitable HEM Motor models and current prices.

Frame, environment and speed

Metal shredding sites are dusty, vibrating and often semi-open areas. A cast iron frame offers a more rigid structure against impact and vibration in this environment. IP55 protection is against dust and water splashes; the motor's fan inlet must not become clogged with metal dust and swarf, and the frame should be cleaned regularly.

On the speed side, on geared shredders the gearbox ratio has been calculated for the pole count of the existing motor; the pole count is kept in a replacement. On a hammer mill, the pulley ratio and rotor tip speed are decisive; if the motor speed changes, the pulleys must be recalculated.

Example models

Models from the power range commonly seen on metal shredders are listed below. All have a cast iron frame, IP55 protection, insulation class F and a 400/690 V (Δ/Y), 50 Hz supply.

• 250M-6: 37 kW, 6-pole, 985 rpm, IEC 250M, rated torque 358.7 Nm. For geared drives that need high torque at low speed.
• 280S-4: 75 kW, 4-pole, 1,486 rpm, IEC 280S, rated torque 482 Nm, service factor 1.15.
• 315S-4: 110 kW, 4-pole, 1,488 rpm, IEC 315S, rated torque 706 Nm, service factor 1.15.

A service factor of 1.15 is the motor's 15% power reserve; it is valuable as a safety margin under impact loads, but it is not used to select the motor one size smaller. Current prices are on the product pages; all high-power models are listed on the cast iron frame motors page.

Conclusion and what we need for a quotation

On a metal shredder, the motor is determined as much by the drive arrangement as by the type of scrap: torque peaks and reversing frequency on geared machines, starting time on hammer mills and pump power on hydraulic drives come to the fore. For a quotation, please send us:

• The machine's drive arrangement (geared, belt-and-flywheel or hydraulic)
• A nameplate photo and mounting arrangement of the existing motor
• Gearbox ratio or pulley diameters; on a hammer mill, the rotor inertia if known
• Starting method and number of reversals or starts per hour

You can find all models on sale and current prices on the buy electric motors page. Send the nameplate photo, speed and mounting details of your existing motor through the nameplate submission page and request a quotation for a suitable model. For additional documents such as the drive diagram, you can use our contact page.

Frequently Asked Questions

Why does the motor on a metal shredder reverse so often?

When current rises, the controls briefly reverse the shafts to clear the jam. Each reversal is like a new start; if it happens often, it becomes the factor that determines motor heating.

Why does the motor on a hammer mill take so long to start?

The inertia of the rotor and flywheel is large; it takes the motor a long time to bring this mass to full speed. Starting time and starts per hour must be assessed together with the rotor inertia.

Can a motor with a service factor of 1.15 be selected one size smaller?

No. The 15% reserve is a safety margin under impact loads. Motor power is selected for the machine's actual load; the reserve is a buffer on top of that power.

How is motor power determined on a hydraulically driven shredder?

Because the electric motor drives the hydraulic pump, power is calculated from the pump flow and operating pressure. The machine's name or capacity alone does not give enough information.