Correct selection of an electric motor for replacement is a task in which an error costs more than the machine itself: incorrectly selected power leads to overheating of the windings, inappropriate dimensions lead to alteration of the frame and couplings, and mismatch of the IP protection class leads to failure after several months of operation. In this article, Elektromotors engineers from Tashkent analyze the selection criteria in detail, provide a table of correspondence for the popular AIR, AIRS and ADA series, list common mistakes and formulate compatibility rules.
Where does the selection of a replacement electric motor begin
The first and main rule is never select an engine based only on power. The nameplate of an old motor contains at least seven parameters, and all of them must be taken into account. Before ordering a new unit, collect the following information:
- rated power, kW;
- synchronous and rated rotation speed, rpm;
- voltage and connection diagram (star/delta);
- rated current and cosφ;
- execution according to installation method (IM);
- overall and connecting dimensions (rotation axis height, shaft length, flange diameter);
- IP protection class and climatic version;
- operating mode S1...S9.
If the nameplate is lost, restore the parameters according to the technical documentation of the equipment or according to the results of electric motor diagnostics. In the practice of our workshop in Tashkent, every fifth selection error is due to the fact that the customer copied the data from a damaged plate and missed one digit.
Power and synchronized speed: a bond that cannot be broken
Power is selected taking into account the actual load, and not “with a reserve”. Increasing the power by 30-40% leads to a drop in cosφ and efficiency; an underloaded engine heats up no less than an overloaded one, because it operates at a non-optimal performance point.
Synchronous revolutions are determined by the number of pole pairs and the network frequency of 50 Hz. Standard range: 3000, 1500, 1000, 750 rpm. Changing the engine from 1500 rpm to 3000 rpm without recalculating the transmission is a gross mistake: the torque on the shaft will drop by half, and the fan or pump will go wrong.
| Number of poles | Synchronous speed | Rated speed (under load) | Typical application |
|---|---|---|---|
| 2 | 3000 rpm | 2850-2950 | pumps, compressors, grinders |
| 4 | 1500 rpm | 1420-1470 | fans, conveyors, machines |
| 6 | 1000 rpm | 950-980 | mixers, mills, presses |
| 8 | 750 rpm | 710-735 | low-speed gearboxes, crane mechanisms |
IM dimensions and mounting methods B3, B5, B14
Installation design according to GOST 2479 is coded with the letters IM. In practice, three options are most often encountered:
- IM1001 (B3) - on feet, horizontal shaft, classic design for frame installations;
- IM3001 (B5) - with a large flange without feet, centered along the collar, used on gearboxes and pumps;
- IM3601 (B14) - with a small flange, for low-power pumps and fans.
When replacing a foot-mounted motor with a flanged or combined one (B35), it is necessary to take into account that the mounting points, the diameter of the mounting collar and the length of the shaft will change. The height of the rotation axis (h) is selected strictly according to the old one: h = 80, 90, 100, 112, 132, 160 mm and further along the row. This is the figure by which it is easiest to compare asynchronous motors of different series.
IP protection class and operating conditions
Protection class IP55 - standard for general industrial conditions: dust does not penetrate in dangerous quantities, protection from water jets. For workshops with an aggressive environment or for outdoor installations, take IP56-IP66. For crane spans in the metallurgical shops of Tashkent, tropical T2 design and impregnation of the windings with moisture-resistant varnish are additionally required. If you plan to install the engine outdoors, under a canopy, be sure to require a version with a protective casing and thermal protection.
Climatic modification UHL4 is suitable for heated premises, and U2 is suitable for unheated warehouses and outdoor installation. This point is often missed, and after a season the winding picks up moisture and the bearings begin to rust.
Conformity table for the AIR, AIRS and ADA series
In post-Soviet workshops, engines of the old 4A, A, AIR series are still working, and when replacing, you need to understand what they are being converted to. Below is a simplified correspondence table for an engine with h=132, 7.5 kW, 1500 rpm.
| Old series | Modern analogue of AIR | Analog of AIRS (with increased slip) | Analog of ADA (increased reliability) |
|---|---|---|---|
| 4А132S4 | AIR132S4 | AIRS132S4 | ADA132S4 |
| 4А132M4 | AIR132M4 | AIRS132M4 | ADA132M4 |
| A132S4 | AIR132S4 | AIRS132S4 | ADA132S4 |
| AIR132S6 | AIR132S6 | AIRS132S6 | ADA132S6 |
The AIRS series is used where frequent starts and shock loads are needed (cranes, crushers), and the ADA series is used for critical drives with an increased service life. If the workshop is running an old machine and it is easier to restore it, contact winding rewinding without throwing away the body.
Typical mistakes when selecting
- Replacement by power without taking into account speed. Engine 5.5 kW 3000 rpm and 5.5 kW 1500 rpm are different machines with different torque and dimensions.
- Ignoring the height of the rotation axis. An engine with h=132 mm will not replace a machine with h=112 mm without an adapter frame.
- Incorrect shaft diameter. The pulleys and couplings will not fit, the shaft will need to be grooved, which reduces the service life.
- Replacement of B3 with B5 without modification of the seat.Flange version requires precise centering along the shoulder.
- Purchase of an IP44 motor for a dusty workshop.The winding will become clogged with graphite and burn out in six months.
- Ignoring the operating mode. S1 (long-term) will not withstand frequent starts of S3-S5, a motor with increased slip is needed.
Compatibility tips and final check
Before installing a new motor, be sure to check: axle height, shaft diameter and length, key type, flange size, center-to-center distance of mounting holes, direction of rotation, connection diagram and cable exit point. If the motor is mounted on a gearbox, check the torque and coupling type compatibility. For crane installations, consult with an engineer for repair of crane electric motors - the moment of inertia of the rotor is critical there.
After installing a new motor, it is recommended to carry out balancing the rotor assembled with the coupling and run-in under a load of 70-80% of the nominal value for 4-6 hours. This will reveal hidden alignment and bearing problems before reaching permanent operation.
When to repair and when to replace
Replacement is not always justified. If the housing, stator and shaft are in good condition, rewinding costs 2-3 times less than a new motor. It makes sense to replace it if the frame is destroyed, there is serious overheating with charring of the insulation, multiple previous repairs, or if the series is obsolete. In Tashkent, we regularly restore engines made in the 1980s - their magnetic core steel is of a higher quality than modern ones, and after rewinding they continue to work for decades.
Do you need help with selection or repair? Elektromotors engineers in Tashkent will provide free advice on choosing an engine, diagnose the old motor and suggest the optimal solution. Contact us or order selection and repair service online.