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Compressed air is, in most production facilities, the second lifeline after electricity. When the compressor's motor burns out, the pneumatic lines stop; CNC machines, packaging machines, sandblasting cabins and paint lines all go on hold at the same moment. For this reason, the decision to replace a compressor motor is less a question of "which motor fits" than "which motor can I find and have fitted by tomorrow morning". As an electric motor manufacturer producing in Turkiye since 1979, HEM Motor delivers, from its Turkiye stock, the standard-frame motors used in the great majority of reciprocating and screw compressors; this guide was prepared so that purchasing and maintenance managers can describe the old motor correctly and order the right replacement in a single pass.
Replacement or Rewinding? A View from the Purchasing Side
The operation whose motor has burned out has two paths ahead: have the existing motor rewound or buy a new one. Rewinding looks cheap at first glance; however, three more items must be added to the commercial picture. The first is time: between the trip to the rewinding workshop, disassembly, rewinding, bearings and testing, a wait of usually three to seven days arises, and the compressor does not run during that time. The second is performance: with every rewinding the efficiency of the motor drops a little, and in an application such as a compressor that runs thousands of hours a year, this loss quietly shows up on the bill. The third is the warranty: a rewound motor does not carry the factory warranty of a new motor. On the new-motor side, choosing an IE4-class replacement instead of IE3 means lower operating cost in the same frame; those who want to examine the investment logic between the two classes in detail can read our investment comparison between IE3 and IE4. The practical rule is this: if the motor is 7.5 kW and below, or if the frame age is over ten years, a new motor is in almost every scenario the healthier decision rather than rewinding. Even at high powers, if there is ready stock at the manufacturer, the delivery speed of a new motor often leaves rewinding behind.
In facilities with more than one compressor, it is also possible not to leave this decision to the moment of breakdown: the age, rewinding history and criticality of the motor fleet are mapped out; a replacement plan is made today for the two or three most at-risk motors. Planned replacement has two concrete advantages over replacement during a breakdown: you replace the motor not with whatever happened to be on the market that day but with the correct configuration, and you take the replacement into a window that does not stop production, such as a weekend or shift break. From the purchasing-budget point of view too, planned purchasing is always cheaper because it is stripped of the freight and downtime costs of an emergency purchase.
The Old Motor's Nameplate: Eight Pieces of Information to Read Before Ordering
The path to the right replacement runs through reading the burned-out motor's nameplate completely. When requesting a quotation, conveying the following eight pieces of information allows the other side to set aside the right motor for you in a single pass:
1. Rated power (kW/HP): In compressor motors, power is never chosen "approximately"; one size down trips the thermal, while an unnecessary size up can cause a change of panel and contactor. 2. Speed (rpm): Although 2-pole (~2900 rpm) and 4-pole (~1450 rpm) motors look physically similar, they give a completely different result on the compressor side; if the speed is wrong, air flow changes in a belt-driven system and screw speed in a direct-coupled system. 3. Frame size (e.g. 112M, 132S, 160L): Determines the foot hole spacing and the shaft height. 4. Mounting form: B3 foot-mounted, B5 flange-mounted, B35 foot-and-flange, or B14 small-flange. 5. Voltage and connection: Is it 230/400 V or 400/690 V; this line is of vital importance in compressors with a star-delta starter. 6. Efficiency class: IE2, IE3 or IE4. 7. Duty type (S1, S3, etc.): If it is written on the nameplate, be sure to report it. 8. Protection class (IP): Compressor rooms are dusty and full of oil vapour; IP55 should be accepted as standard. If the nameplate is too burned to read, clear photos of the motor and the shaft end together with the make and model of the compressor are usually enough for a replacement.
Next to the nameplate, information about the operating environment should also be added; because the motor nameplate is written for standard conditions (altitude near sea level and 40 °C ambient temperature). If the compressor room reaches 45–50 °C in summer, or if the facility is at high altitude, the thermal reserve of the motor decreases and a motor of the same power runs hotter. In this case the solution is not to increase power but to report the situation at the ordering stage; the recommendation is shaped accordingly and the thermal relay setting is made according to these conditions at commissioning. Improving the ventilation of the compressor room is also often the cheapest investment that can be made in motor life.
Shaft and Flange Matching: Measure Without Dismantling, Fit in One Pass
The nameplate information gives the electrical identity of the motor; the mechanical identity is on the shaft and mounting surface. The biggest cause of returns and delays in replacement orders is failing to check the shaft dimensions. Take three measurements with a caliper: shaft diameter, shaft extension length and keyway width. Standard frame-shaft pairings are known (for example, a 38 mm shaft is typical on a 132 frame); however, some compressor manufacturers may have machined a special shaft. On flanged motors, the flange outer diameter, the hole spacing (pitch-circle diameter) and the spigot register diameter should be measured; in B5 the holes are plain, in B14 they are threaded. On belt-driven reciprocating compressors, the pulley may need to pass onto the new shaft with the same tightness as on the old one, and on taper-bush pulleys the bush may need replacing; talking about this at the order stage means not having a surprise on installation day. The HEM Motor technical team carries out the matching on your behalf with the nameplate photo and shaft dimensions you send; all the standard frames on the compressor electric motors page are stocked according to this matching logic.
Installation and First Start: Commissioning-Day Checklist
The right motor wears out quickly with the wrong installation; that is why we also consider replacement day part of the supply. In belt-driven systems the first job is alignment: the motor and compressor pulleys must be brought into the same plane with a straightedge or laser, and the belt tension must be set according to the manufacturer's value — a loose belt means loss and heating, an over-tight belt means front-bearing damage. In direct-coupled systems the coupling gap and shaft misalignment are checked. On the electrical side the sequence is as follows: bridging the terminal connection according to the voltage on the nameplate (star or delta), running the motor off-load and verifying the direction of rotation, then measuring the currents of the three phases under load with a clamp meter and comparing them with the nameplate value. A marked current imbalance between phases is a sign of a problem on the supply line and should be caught on the first day. The last step is first-week monitoring: noise from the bearing area, frame temperature and the setting value of the thermal relay are checked. HEM Motor sends the connection diagram with the shipment and provides engineering support by phone during commissioning; the early-failure rate is markedly lower on the motors of customers who share with us the values measured at first start.
Load Character in Reciprocating Compressors: High Starting Torque and Flywheel Effect
A reciprocating compressor is a demanding load for a motor. The crank mechanism produces a compression pulse on every revolution; the motor has to carry not only the average load but also this pulsating torque. Moreover, most reciprocating compressors run on pulley-belt transmission, and the inertia of the large flywheel pulley causes the motor to draw high current for a long time at start. For this reason, a motor selected for a reciprocating compressor must have a high starting torque and a rotor resistant to pulsating load. In classic reciprocating compressors running on a tank pressure switch, the motor stops and starts dozens of times a day; this start-stop regime stresses the winding and bearing life more than a continuous regime. The purchasing equivalent of this is: when selecting a reciprocating compressor motor, you must preserve the nameplate power exactly, prefer a motor with Class F insulation and IP55 protection, and not deviate from a standard industrial motor with a front-bearing structure capable of carrying the pulley load. In compressors with an unloading (off-load) start valve, starting is easier; fitting a new motor to a compressor with a faulty valve, on the other hand, drives the new motor to the same fate — this check must definitely be carried out at commissioning.
Matching in Screw Compressors: Continuous Load, Precise Coupling, Correct Starting
In a screw compressor the picture reverses: the load is pulse-free but continuous. The motor turns the screw block, usually without interruption, through a coupling or gear drive; this requires a high-efficiency motor suited to the S1 continuous duty regime. The great majority of screw compressors start with star-delta starting; therefore the motor must be suitable for delta connection on a 400 V grid (with a 400/690 V nameplate) — if a motor with a 230/400 V nameplate is connected to a star-delta panel, the windings are damaged within minutes. This detail is the error we most frequently correct in screw compressor motor orders. In inverter-driven (VSD) screw compressors, the motor must have an insulation structure suited to running with a drive; when requesting a replacement motor, be sure to state that the compressor is inverter-driven. In direct-coupled systems, renewing the coupling rubber and setting the alignment with a laser or dial gauge is the final step that determines the bearing life of the new motor. Since screw compressor motors run 4,000–8,000 hours a year, switching to the IE4 class on replacement visibly lowers the total cost of ownership; the in-stock IE4 efficient motor series is arranged from 0.55 kW to 355 kW according to this need.
Start-Stop Endurance: How Many Starts Per Hour Are Safe?
Every start produces, in the motor winding, a current on the order of several times the rated current and the associated heat. The motor expels this heat with its fan while running; but a motor that stops and starts at short intervals finds no time to expel the heat. In reciprocating compressors, if the pressure switch range is set too narrow, or if the tank capacity is too small for the consumption, the motor starts dozens of times an hour and the complaint "the new motor burned out too" arises. During replacement, the following three checks extend the life of the new motor: widening the upper-lower setting range of the pressure switch, reducing unnecessary starts by eliminating air leaks, and increasing the tank volume if necessary. In screw compressors, because the load-unload cycle keeps the motor turning, the number of starts is low; here the point to watch is that the motor consumes unnecessary energy if the idle-running ratio is very high. For similar start-stop problems in refrigeration applications and the joint selection of the fan-compressor motor pair, you can also look at our cold storage fan and compressor motors guide.
Supply Process: Lead Time, Stock and Commissioning Support
In compressor motor replacement, the criterion for choosing a supplier is not the thickness of the catalogue but the reality of the stock at the other end of the phone. At HEM Motor the process works like this: you send the nameplate photo and the shaft dimensions; the technical team does the matching the same day and reports the stock status with a definite delivery date on a frame-by-frame basis. In standard frames (from 71 to 315), IE3 and IE4 motors are dispatched from the factory and warehouse stock in Turkiye; the difference of being a manufacturer is that special-shaft or special-flange requests not in stock can be solved on the production line without waiting for imports. After shipment, the commissioning stage is not left alone: engineering support by phone is provided on the connection diagram, terminal bridging according to the starting type, direction-of-rotation check and first-start current measurement. For facilities with more than one compressor, a plan is drawn up to keep one spare motor in common frames; in this way the waiting time at the next breakdown drops to zero.
For compressor manufacturers and overhaul workshops, an OEM working model is also applied: the motor type to be used in series production is determined together once, the shaft-flange configuration is frozen, and a call-off shipment programme is set up according to the annual quantity. In this way the manufacturer pulls the motor according to the production plan rather than stocking it; the price is fixed periodically and the same configuration arrives in every batch. For overhaul workshops too, keeping the five or six most frequently used frames ready on the workshop shelf forms the infrastructure of the "delivery tomorrow" promise they give their customers.
Frequently Asked Questions
The old motor's nameplate is completely erased; can you still find the right motor?
Yes. The make and model of the compressor, a frame photo of the motor, the foot hole spacing and the shaft diameter-length measurement are enough for us. The frame size is identified from the foot dimensions, and the power and speed are worked back from the type plate of the compressor manufacturer. In doubtful cases, if there are two possibilities, the thermal relay setting value in the panel tells which power is correct.
My screw compressor's motor has a 400/690 V nameplate; can a 230/400 V motor be fitted in its place?
If there is a star-delta starter, no. On a 400 V grid, star-delta starting can only be done with a motor whose delta connection is 400 V (that is, with a 400/690 V nameplate). If a 230/400 V motor is connected to this panel, it sees overvoltage in the delta step and the windings are damaged in a short time. In small compressors with direct-on-line starting, a 230/400 V motor is used without trouble with a 400 V star connection.
Would it be safer to choose the motor one size up in power?
In most cases, no. The compressor manufacturer designed the screw block or crank mechanism for a particular power; a larger motor both creates incompatibility with the protection elements in the panel and runs inefficiently at low load. The correct approach is to preserve the nameplate power and to provide endurance not by increasing power but through the correct efficiency class, the correct regime and correct commissioning.
Get a Quote
If your compressor has stopped, do not lose time: send us a photo of the motor nameplate and the shaft dimensions, let us do the matching and report the stock and lead-time information the same day. Our IE3/IE4 motors for reciprocating and screw compressors are dispatched from Turkiye stock; engineering support during commissioning is on us. You can call us on +90 (532) 345 49 86 or send your request through our contact us page.
