Vertical Shaft Impactor (VSI) Motor

A vertical shaft impactor (VSI) drops stone into the centre of a rotor sitting on top of a vertical shaft and throws it against a surrounding bed of stone or against steel anvils, producing sand and fine cubical aggregate. Breakage happens as stone strikes stone or anvil, so all of the machine's crushing power comes from the rotor's rotation. For its place in the crushing line and its relation to the other machines, see the electric motors for crushers and stone crushing plants page; this page covers the conditions of the three-phase motor that turns the VSI rotor.

Rock-on-rock and rock-on-anvil VSI: what is the difference for the motor?

In the rock-on-rock layout, a bed made of the material itself builds up inside the rotor and around the crushing chamber; thrown stone strikes this bed. Wear is low and grain shape is good, but part of the stone makes more than one pass and crushing efficiency is relatively low. In the rock-on-anvil layout, the stone hits steel anvils directly; more breakage is obtained per unit of energy, but the anvils wear quickly.

Seen from the motor, the difference lies in how stable the load is. In the rock-on-rock layout, current fluctuates in the first minutes until the bed forms, then settles. In the anvil layout, the load depends more directly on feed rate. In both, the exit speed of the material from the rotor is proportional to speed; plants mostly adjust sand fineness with rotor speed.

Tip speed and speed adjustment

On a VSI the main variable that determines the product is the rotor's tip speed. Since rotor diameter is fixed, that speed changes only with rpm. With pulley adjustment the motor is usually 4-pole, and rotor speed is obtained from the ratio of pulley diameters; when a different sand fraction is wanted, the pulley set is changed.

In plants where the product changes often, a frequency converter does this without removing pulleys. Our three-phase series are suitable for drive operation. On a VSI, the torque the rotor needs falls as speed is reduced, so cooling at low speed is usually a limited concern; still, if the motor is to run well below rated speed at full torque for long periods, an external fan may be needed. With long motor cables, a dv/dt or sine filter is considered. State these conditions in your order; details are on the inverter compatible electric motors page.

One motor or two?

On small VSIs a single motor turns the rotor shaft through belts. On larger models two motors are placed on either side of the shaft, each driving the same rotor pulley with its own belt set. This arrangement lets two mid-size motors share the load instead of one very large motor.

Load sharing matters in a twin-motor layout. Both motors must be the same model and speed, pulley diameters equal and belt tensions close to each other. The motor with slacker belts takes less load, the other takes more and runs hotter. Each motor gets its own overcurrent protection; watching the two currents side by side reveals imbalance early. The motors turn in the same direction and start together. Keeping the spare motor of the same model is a practical way to make sure both motors remain identical when one has to be replaced.

Mounting and bearings on a vertical shaft machine

In many VSI designs the motors sit vertically on a slide base, shaft pointing up, parallel to the rotor's vertical shaft. This is a different mounting from a horizontal foot motor and must be stated clearly in the order. Depending on frame size, foot-mounted B3, flange B5 and B14, combined B34 and B35 and vertical V1, V3, V5, V6, V15 are offered; flange and shaft dimensions are confirmed with the order.

In a shaft-up vertical layout, pulley weight and belt pull put both axial and radial load on the bearings. Water or fine sand collecting at the shaft exit on top of the frame is a separate risk; a canopy above the motor and keeping the shaft exit clean are recommended. Where the belt line is long, slide base bolt tightness and pulley alignment should be checked regularly.

Starting, imbalance and rotor wear

Because the rotor turns at high speed, it stores a lot of energy and starting takes long. The motor should be started with no material in the rotor, and stone flow should begin only once the motor has settled at rated speed. Whether star-delta, a soft starter or a drive is suitable depends on rotor inertia, transformer capacity and, in a twin-motor layout, whether the two motors start together or one after the other.

A known risk of the VSI from the motor's point of view is rotor imbalance. If one of the wear tips inside the rotor breaks, or the stone bed builds up unevenly on one side, vibration rises suddenly. This vibration passes through the belts to the motor and strains bearings and base connections. A vibration sensor on the crusher and a PTC thermistor or PT100 in the motor give early warning; these are order options. An anti-condensation heater can also be requested for motors that stand idle in winter.

Wet sand and dust environment

A VSI spreads heavy dust around it because it makes very fine product; some plants spray water to suppress the dust. Our motors come with IP55 enclosures and class F winding insulation; this protection is for dust and rain. If the motor is to be placed where water is sprayed directly onto it or where it is washed with pressurised water, a standard IP55 motor is not designed for that condition; a canopy or a special motor is needed, so state it in the order. Muddy dust collecting between the fins also reduces heat dissipation, and cleaning should be part of the maintenance plan.

Sample motors for VSI

The listed motors are for guidance; which motor fits is decided by looking at rotor load, required speed, mounting arrangement and site conditions together. For reference: the 315M-4 model delivers 847.2 Nm rated torque at 1,488 rpm and the 315L2-4 model 1,282 Nm at 1,490 rpm; both have a service factor of 1.15.

Information needed for a quote

Send the VSI's make and type, whether it has one or two motors, the required power (kW) and speed (rpm) per motor, supply voltage, mounting type (especially vertical or horizontal), starting method and quantity. A nameplate photo of the existing motor speeds up the comparison.

For models and current prices see our shop, and for larger ratings the cast iron frame motors list. Vertical mounting options are summarised in the mounting arrangements question. For WhatsApp and phone, use the contact page.

Frequently Asked Questions

On a twin-motor VSI, why might one motor run hotter than the other?

Usually because of a difference in belt tension or pulley diameter; the motor on the tighter side takes more of the load. Tension the belts equally, measure the pulleys and compare the two motor currents. Also check that both motors are the same model.

Can sand fineness on a VSI be adjusted without changing the motor?

Yes; rotor speed can be changed with the pulley set or with a frequency converter. Any speed increase must not exceed the rotor manufacturer's limit, and motor power should be checked for the new operating point.

What else should be watched for a vertically mounted VSI motor?

The mounting type must be written clearly in the order, because bearings carry a different load in a vertical layout. If the shaft exit points upward, a canopy above it and preventing water and sand build-up are recommended.

Is the motor damaged when a wear tip in the rotor breaks?

Sudden imbalance raises vibration and strains the bearings; if the crusher is not stopped immediately, the motor bearing and base connections can be damaged. Vibration monitoring with automatic shutdown reduces this risk.