Replacing the Electric Motor of a Milking Vacuum Pump

In a milking system, when the vacuum pump stops, milking stops too. Because cows have to be milked at set times, a pump motor failure is one of the jobs on a farm that cannot wait. In this article we look at what information you need to collect when replacing the motor of a milking vacuum pump, how to check the connection according to pump type, how the barn environment affects the motor, and what farms with a single-phase supply can do.

Vacuum pump types and motor connection

There are three pump arrangements most often found in milking installations. Motor selection starts with how the pump is connected to the motor:

• Rotary vane vacuum pump: The most common type on farms. The motor usually stands foot-mounted on a common baseframe and is connected to the pump by belt and pulleys. On some models the pump is directly coupled to the motor.
• Lobe vacuum pump: Found in larger milking parlours. It generally runs with a standard foot-mounted motor, by belt or coupling.
• Liquid ring pump: Less common; pump and motor are often a single unit joined by a flange.

In a belt-and-pulley arrangement the pump speed is set by the pulley ratio; so the new motor must have the same number of poles as the old one. In a coupled arrangement the motor's rated speed is directly the pump speed. In units where the pump and motor are joined by a common flange, the flange and shaft fit decide whether the replacement is possible. If the shaft end on a flanged unit is non-standard, a standard motor cannot be used; in that case, ask about the special motor requirement with the nameplate and technical information.

Information to collect for the replacement

The soundest way to replace a motor is to start from the old motor's nameplate. Note the following fields from the nameplate:

• Power (kW or HP) and rated speed (e.g. 1,420 or 1,445 rpm)
• Voltage and connection (230/400 V Δ/Y; if only one voltage is written, it may be single-phase)
• IEC frame code and mounting arrangement (B3 foot-mounted, B5 or B35 flanged)
• Degree of protection (IP) and insulation class

If the nameplate cannot be read, look at the pump's nameplate; the pump manufacturer usually states the required motor power and speed. If you have increased the number of milking units, the pump capacity and therefore the motor power requirement may also have changed; in that case the power is determined according to the calculation of the company that installed the milking system. An exact power cannot be recommended by looking at the machine's name.

If the frame code cannot be read, measure the foot hole spacing, shaft height and shaft diameter and compare them with the technical drawings.

The barn environment: humidity, wash water and ammonia

The vacuum pump usually stands in the machine room next to the milking parlour. Although this room is more sheltered than the rest of the farm, it still has conditions that are demanding for a motor:

• Humidity and wash water: Water splashes and steam occur during washing after milking. IP55 protection guards against dust and water jets from any direction; however, washing the motor directly with pressurised water is not appropriate.
• Ammonia and manure gases: Gases coming from the barn speed up corrosion on metal surfaces. Ventilating the machine room and separating it from the barn is important for motor life.
• Pump exhaust and oil mist: On lubricated rotary vane pumps, oil mist comes out of the exhaust. If the motor fan draws in this air, oily dust builds up on the frame and cooling weakens. The exhaust needs to be discharged outside.
• Winter conditions: In unheated machine rooms, night cold and daytime warming can cause condensation inside the frame.

In an enclosed, clean machine room an aluminium frame is sufficient and its light weight makes replacement easier. Where the risk of dust, knocks and corrosion is high, a cast iron frame can be considered. All our models on sale have IP55 protection and class F insulation.

Operating time and starting

The vacuum pump runs without interruption for one to several hours at each milking, and stays on during washing after milking. As the motor is under continuous load throughout this time, a motor designed for continuous duty, with S1 on its nameplate, is suitable. Rotary vane and lobe pumps do not generally create a heavy starting load; however, oil that thickens in cold weather can increase the load on the shaft at start-up.

In some new milking systems the pump speed is adjusted with a frequency inverter according to the vacuum level. Our three-phase series are suitable for speed control with a frequency inverter; for prolonged operation at low speed with full torque a separately driven fan may be needed, and if the cable between the drive and the motor is long a dv/dt or sine filter is considered; these conditions are stated with the order.

Farms with a single-phase supply

In rural areas, some farms have only a single-phase (230 V) supply. In this case there are two options: use a single-phase motor, or feed a three-phase motor from the single-phase supply through a suitable drive. Which is appropriate depends on the pump power, the capacity of the supply line and the recommendation of the milking system manufacturer.

HEM Motor also has a single-phase motor series in the 0.18–5.5 kW range; however, details of these models are not included in the product list in this article. If you are looking for a single-phase motor, write to us via our contact page with the old motor's nameplate; we will let you know the suitable model and price separately. When replacing a three-phase motor with a single-phase motor, also take into account that the starting torque and the current drawn from the line will be different.

Example scenario: motor replacement on a belt-driven rotary vane pump

The following situation is hypothetical. Suppose the motor of the rotary vane vacuum pump on a medium-sized dairy farm has burnt out. The nameplate reads 2.2 kW, 1,420 rpm, 230/400 V Δ/Y, 100L, IM B3. The motor stands foot-mounted on a baseframe and drives the pump by belt; the farm has a three-phase 400 V supply.

• If the power, number of poles and frame are kept the same, the matching model is the aluminium frame 100L1-4 (2.2 kW, 4-pole, 1,450 rpm, IEC 100L, 230/400 V, 4.52 A). On a 400 V supply it is connected in star.
• The new motor's rated speed is slightly higher than the old motor's (1,450 vs 1,420 rpm); since both are in the 1,500 rpm synchronous class, no pulley change is needed and there is a small difference in pump speed.
• If the farm has increased the number of milking units, the pump manufacturer's new power value is requested; if the requirement rises to 3 kW, the 100L2-4 in the same 100L frame is considered.

Example models for vacuum pumps

• 90L-4 aluminium: 1.5 kW (2 HP), 4-pole, 1,445 rpm, IEC 90L, 230/400 V (Δ/Y), 3.30 A. For small milking systems.
• 100L1-4 aluminium: 2.2 kW (3 HP), 4-pole, 1,450 rpm, IEC 100L, 230/400 V (Δ/Y), 4.52 A.
• 100L2-4 aluminium: 3 kW (4 HP), 4-pole, 1,450 rpm, IEC 100L, 230/400 V (Δ/Y), 6.02 A.

For other power ratings and agricultural applications, see the agricultural machinery motors page. The current price is on each model's product page.

What to send with your order

• A photo of the old motor's nameplate and the pump's nameplate details
• Power (kW/HP) and speed
• Connection (belt, coupling, flanged unit) and mounting arrangement
• Supply: three-phase 400 V or single-phase 230 V; starting method
• Machine room conditions: humidity, wash water, ventilation

Review suitable models and current prices on the buy electric motors page. To ask for the suitable model with your motor's nameplate details, send the nameplate photo; for single-phase needs, use our contact page.