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"I am going to switch to an IE4 motor, but my current motor fits the machine exactly; will the new motor match its feet, shaft and cable entry?" This is one of the most frequent questions we receive from businesses considering a switch to IE4, and the answer reassures most buyers. At HEM Motor, while supplying IE4 high-efficiency electric motors from Turkey stock, we know that the most critical issue in retrofit (replacing an existing motor with a new one) projects is mechanical compatibility. This guide explains, from the eyes of a buyer about to make a purchasing decision, whether frame dimensions are independent of efficiency class, how foot holes and shaft diameter match, and what to check when replacing an old motor one-to-one with an IE4. In this article we will not get into the energy gain that comes with efficiency or the legal mandate; we have left those angles to separate articles. Here the only subject is mechanical compatibility. When you act on the right information, the switch to IE4 usually becomes a planned motor change fast enough to fit into a maintenance window, requiring no modification to the machine.
IEC Frame Dimensions Are Independent of Efficiency Class
The cornerstone of the retrofit decision is this: in low-voltage asynchronous motors produced to the IEC standard, frame dimensions do not change according to efficiency class. In other words, the foot-hole spacing, shaft height and shaft diameter of a frame 132 motor are produced to the same standard value whether that motor is IE2, IE3 or IE4. From frame 63 to 355L, the frame size conforms to this standard dimension table. This is the most important technical fact that makes the switch to IE4 possible: because if the connection layout in your existing machine sits on the IEC standard, an IE4 motor of the same frame size can be fitted in its place one-to-one mechanically.
The key concept here is "shaft height". Shaft height is the vertical distance from the motor's mounting surface to the shaft centre, and it determines the frame number; for example, frame 132 means 132 mm shaft height. IEC has also standardised the foot-hole spacings, shaft diameter and shaft length corresponding to this height. Therefore two motors with the same frame number connect to the machine in the same way, even if their efficiency classes differ. We detailed why frame size and power matching is important in our article on frame size and power matching.
Shaft height and foot-hole spacing
In foot-mounted (B3) mounting, two parameters ensure direct seating on the machine: foot-hole spacing and shaft height. The IEC standard specifies these hole spacings (longitudinal and transverse) for each frame number. If your old motor is the same frame number, the foot holes of the IE4 motor sit on the same base holes; there is no need to drill extra holes or modify the base. Just watch one point: some old domestic or imported motors may have been produced to non-IEC special dimensions. For this reason, we recommend sending us the nameplate and dimensions of the old motor before replacement. We explained how to do a one-to-one match with the nameplate information in our article on avoid receiving the wrong motor.
Shaft diameter, shaft length and key dimensions
The second critical parameter of mechanical compatibility is the shaft diameter. IEC assigns a standard shaft diameter to each frame number; for example, a specific diameter for frame 90 and a larger diameter for frame 132. The shaft of an IE4 motor of the same frame size will have the same diameter and length as the shaft of your old motor; this means the coupling, pulley or gearbox connection seats without problems. The keyway is also machined to standard dimensions. We collected how to correctly order shaft diameter and key dimensions for coupling and pulley compatibility in our article on shaft diameter and key dimensions. Thanks to this compatibility in a retrofit, you can often reuse the existing coupling or pulley.
Replacing an Old Motor One-to-One with an IE4: Step by Step
The practical flow to complete a retrofit project without surprises is as follows. This sequence minimises both assembly errors and the risk of ordering the wrong motor in the field.
1. Collect the details of the existing motor
The first step is to extract completely the nameplate information and mounting type of the motor to be replaced: power (kW), speed (rpm), frame number, mounting type (B3 foot-mounted, B5 flanged, B35 combined), shaft diameter and voltage. Mounting type is especially important here, because it is not possible to replace a B5-flanged motor with a B14 flange; the flange diameters and hole patterns differ. You can find the details of flange type selection in our B5 vs B14 article.
2. Match an IE4 motor of the same frame and mounting type
With the collected information, we identify the IE4 motor of the same power, speed, frame and mounting type. Thanks to the IEC standard, an IE4 motor of the same frame number sits one-to-one on the old motor's foot/flange layout and shaft. Even a difference in brand is usually no obstacle, because the compatibility comes not from the efficiency class but from the IEC frame standard. We covered the practical method of replacing an old brand with its equivalent in our article on replacing an old brand motor one-to-one.
3. Clarify the connection and accessory details
Once frame and shaft compatibility is ensured, small but important details that can be overlooked remain: terminal box orientation, cable entry direction, an additional cooling fan if any, bearing type and the required protection class. Our standard production is IP55 protection and Class F insulation; for special environments we evaluate these. Clarifying these details at the quotation stage ensures no surprises on installation day. You can find which information is needed for a fast and accurate quote in our article on information to provide when requesting a quote.
Mechanical Compatibility by Mounting Type: B3, B5, B35
The most practical distinction that determines compatibility in a retrofit is the mounting type. In foot-mounted (B3) motors, compatibility is determined by foot-hole spacing and shaft dimension; an IE4 motor of the same frame size sits on the base one-to-one. In flanged (B5) motors, compatibility is determined by flange diameter and flange hole pattern; the large flange of an IE4 motor of the same frame size connects one-to-one to the pump or gearbox housing. In combined (B35) motors there is both foot and flange connection, and both sit on standard dimensions. So saying only "I am going to switch to IE4" is not enough before replacement; you need to correctly identify the mounting type of the existing motor. Since flanged connection is mostly used in pump applications, flange compatibility especially stands out in pump electric motors.
TEVMOT and the Mechanical Ease of IE4 Retrofitting
An important programme supporting the switch to high-efficiency motors in Turkey is TEVMOT (the Turkey Efficient Motors Programme). TEVMOT is a state-supported programme that encourages the conversion of motors in the 7.5–375 kW range to the IE4 efficiency class. This power range corresponds to the most widely used motor band in industry; in other words, for many businesses the switch to IE4 is both a supported and a mechanically easy step. The reason for the mechanical ease is precisely the main idea of this article: thanks to the IEC frame standard, an IE4 motor of the same frame size is fitted in place of the old motor one-to-one; the base, shaft and connection layout do not change. This turns the retrofit into a fast change that usually requires no modification to the machine.
An important note: in this article we have deliberately not gone into the energy payback period or the legal mandate timeline for the switch to IE4, because we detailed those angles in separate articles. For payback and consumption calculations, see our replacing an old motor with an IE4 article, and to see which power rating is mandatory from which date, see our IE3 and IE4 efficiency mandate article. Here our aim is only to clarify the comfort of mechanical compatibility. For businesses operating in the power band covered by TEVMOT, this mechanical ease makes the decision to benefit from the programme even more practical; because no machine modification is required, the transition is both fast and low-interruption.
The Benefit of Mechanical Compatibility to the Business in a Retrofit
The fact that the IEC frame standard is independent of efficiency class translates directly into time and cost advantages for your business. Because the most expensive and longest part of a retrofit project is usually not the motor itself, but the mechanical modification that must be made to the machine: base drilling, intermediate plate fabrication, shaft adapter machining, coupling replacement. When an IE4 motor of the same frame size sits one-to-one, all of these items disappear; only the labour of removing and refitting the motor remains. This shortens production downtime, and the transition is completed fast enough to fit into a planned maintenance window.
This mechanical ease lets you extend the switch to IE4 not only to large investment projects but also to normal wear replacements. In other words, when renewing a failed motor or one that has reached the end of its life, by choosing IE4 in the same frame size you both preserve mechanical compatibility and gradually move your fleet to high efficiency. To browse our wide product range, take a look at our high-efficiency electric motors category and, for the standard band, our IE4 electric motors category. You can find our other purchasing guides about the switch to IE4 in our IE4 electric motors blog category.
The Most Common Mechanical Mistake in a Retrofit
In our field experience, most retrofit mistakes stem not from frame compatibility but from overlooked small details. The most common mistake is misidentifying the mounting type: ordering only a B3 in place of a B35 motor, or ignoring the flange dimension. The second common mistake is failing to notice that the old motor has non-IEC special dimensions; in this case a standard IE4 motor may not seat directly and an intermediate base may be required. The third mistake is not checking the shaft output direction and terminal box position. When you clarify these three points before ordering, the switch to IE4 is almost always one-to-one and trouble-free; your installation team does not have to deal with anything other than removing and refitting the motor. Because our standard-structure industrial electric motors are produced to IEC dimensions, replacement on existing motors that sit on the IEC standard usually requires no extra work.
Dimensions to Watch in a Flanged (B5/B35) Retrofit
While foot-hole spacing determines compatibility in foot-mounted motors, flange dimensions are at the centre in flanged applications. In B5 large-flange motors that connect directly to pump, gearbox and fan housings, three dimensions are critical: flange outer diameter, bolt circle diameter and centre spigot diameter. The IEC standard specifies these three dimensions for each frame number; for this reason, the flange of an IE4 motor of the same frame size sits one-to-one on the surface to which the old motor was connected. Since B35 combined motors have both feet and a flange, both dimension groups must be checked, but because both are standard, compatibility is again preserved.
The most common confusion in a flanged retrofit is mixing up B5 and B14. B5 is the large flange and B14 is the small flange, and their bolt-hole patterns differ; one cannot be fitted in place of the other. For this reason, it is essential to clearly identify the flange type of the existing motor. Since pump applications mostly use B5 or B35, choosing the right flange type guarantees a one-to-one connection to the pump housing. You can make use of our other guides for practical information on selecting a pump motor replacement correctly from the nameplate.
Frequently Asked Questions
Is an IE4 motor larger than an IE3 or IE2 motor?
At the same power and frame number, the external mounting dimensions of an IE4 motor (foot-hole spacing, shaft height, shaft diameter) are the same as those of an IE3 or IE2, because these dimensions are determined by the IEC frame standard, not by the efficiency class. As the efficiency class rises, the internal design of the motor (winding, core length) improves, but this does not change the standard mounting dimensions. Therefore an IE4 motor of the same frame size sits on your machine in the same way as the old motor. The only exception is if the old motor was produced to non-IEC special dimensions; we clarify this in advance with the nameplate and dimension information.
My old motor is a different brand; will the IE4 motor still fit?
Yes. Mechanical compatibility comes not from the brand but from the IEC frame standard. If your old motor is on a frame size conforming to the IEC standard, an IE4 motor of the same frame and mounting type sits one-to-one on the base and shaft despite the brand difference. What matters is that the frame number, mounting type and shaft dimension match. We explained step by step how to replace an old brand with its equivalent in our article on replacing an old brand motor one-to-one.
Is it essential to stay on the same frame size, or can I buy a smaller IE4?
The general rule is to stay on the same frame size at the same power and speed, because mechanical compatibility depends on it. An IE4 motor of the same power is usually produced on the same frame size, so you should not expect, nor should you require, a reduction in size. Changing the frame size would change the foot-hole spacing and shaft dimension and therefore require modification to the machine, which is exactly what the retrofit aims to avoid. Our article on frame size and power matching guides correct power and frame matching. In short: since the aim of a retrofit is to leave the machine as it is and replace only the motor with a more efficient one, keeping the same power-speed-frame-mounting type quartet is the safest path.
Get a Quote
Are you looking for a motor that will sit one-to-one in place of your existing motor in the switch to IE4? Send us the nameplate, frame number and mounting type (B3 / B5 / B35) of the motor to be replaced; let us quickly supply, from Turkey stock, an IE4 high-efficiency electric motor in the same dimensions so you have no surprises during installation. Call us now on +90 (532) 345 49 86 or fill in the form on our contact us page; our technical team will get back to you with a one-to-one compatible solution.
