IEC Frame 132–355 Three-Phase Electric Motors: Replacement, Base and Shaft Height on Large Frames

In IEC frames between 132 and 355, the frame number also gives the height from the foot base to the shaft centre in millimetres. What makes replacement difficult in this range is not reading the code; it is that the motor gets heavier, the base is fixed to concrete or a steel skid, and the alignment margin becomes smaller. Because the same power can fall into a completely different frame depending on speed, the price of a wrong choice on a large motor is tearing out and rebuilding the base.

This article focuses on replacement in large frames: the power–speed–frame relationship, the base, coupling alignment, lifting and terminal box orientation. The power list for all frames is on the IEC frame–power table page; we do not repeat that list here.

Same power, different speed, different frame

When selecting at high power, first ask this question: what speed does the machine require? What really determines the frame is the torque requirement. A 1,000 rpm motor delivers the same power with roughly three times the torque of a 3,000 rpm motor; that torque needs a larger rotor and a larger frame.

The table below shows how the frame changes with the number of poles at five power steps selected from HEM Motor product records:

Two practical conclusions follow from the table. First, at 37 kW moving from 2 poles to 6 poles takes the frame from 200 to 250; in other words, a change of speed also changes the shaft height. Second, at 75 kW and above, 2-pole and 4-pole models often share the same frame number. In that case, even though the frame looks the same, the shaft end and bearing arrangement may differ according to the number of poles; compare the technical drawing before fitting a 2-pole motor in place of a 4-pole one as "the same frame".

What do the letters S, M and L change on a large frame?

Frame 315 comes in three different lengths marked S, M and L. In the HEM Motor list at 4 poles, 315S-4 is 110 kW, 315M-4 is 132 kW and 315L1-4 is 160 kW. The shaft height is the same for all three; what changes is the frame length and, with it, the longitudinal spacing of the foot holes.

A common situation on site is this: the old motor is 315S, the new requirement is one power step higher and a 315M arrives. The coupling side stays in place, but the rear foot holes do not match the base. New holes must be drilled in the base or an adapter plate fitted. If you are planning a power increase, check on the drawing before ordering whether it creates base work. You can review suitable HEM Motor models and current prices in the online store.

The base: the same frame number is only half the job

On large motors the base is usually a steel skid or a plate anchored onto a concrete plinth. If the frame number matches, the shaft axis comes to the same height; even so, the following points are checked separately:

• Flatness of the foot surfaces: If one of the four feet is left unsupported, the frame flexes as the bolt is tightened. This "soft foot" condition comes back later as vibration and bearing heating; each foot is checked with feeler gauges before tightening.
• Shim pack: The shims under the old motor are not transferred to the new motor as they are. Each motor has a different foot height tolerance; the shims are rearranged according to the alignment measurement.
• Anchor and bolt condition: If a base that has run for years has cracks, loose anchors or damaged threads, the new motor will suffer the same vibration.
• Longitudinal position: The shaft end of the new motor may sit further forward or back than the old one. The distance between coupling hubs must stay within the range given by the coupling manufacturer.

Coupling alignment: tighter tolerance on a large motor

At high power the torque carried by the shaft is high; even a small misalignment wears coupling elements, bearings and seals quickly. Alignment is done on two axes: parallel offset and angular misalignment between the shafts. It is measured with a dial indicator or a laser alignment tool, and the motor side is corrected with shims and side adjustment bolts.

There are two more details to watch. On machines that run hot, such as pumps or fans, thermal expansion can raise the shaft axis; if the machine manufacturer gives an allowance for cold alignment, follow it. Also, alignment is checked one last time with the pipe and duct connections attached; connection strain can shift the motor. For first start-up steps, see the motor commissioning article.

Lifting eye and transport

Large cast iron frames usually have a lifting eye (eye bolt) on top of the motor. This eye is designed only to lift the motor itself. Lifting the motor by its eye together with a pulley, coupling or machine part attached to it puts loads on the eye and frame that they were not designed for.

On large frames with two lifting eyes the chain angle matters; the chains should work as vertically as possible, and a spreader beam should be used if needed. Check that the eye bolt thread is fully engaged and that the ring sits against the frame. When the motor is lowered onto the base, the feet are not dragged; burrs on the foot surface spoil the alignment.

Terminal box orientation and cable entry

In standard horizontal mounting, the terminal box is usually on top of the frame. On large motors the cable is thick and its bending radius wide, so the direction from which the cable arrives matters. If the box was on the side of the old motor or the cable tray comes down from a particular direction, specify the box position of the new motor when ordering.

There are designs in which the box can be rotated to change the cable entry direction; this is also something to ask about when ordering. The cable cross-section is calculated by the project or panel engineer according to motor current, cable length, installation method and ambient temperature. Most HEM Motor cast iron models in frame 132 and above are wound for 400/690 V (Δ/Y); on a 400 V supply they are connected in delta and are suitable for star-delta starting.

Hypothetical example: a power increase on frame 315

Imagine a plant with a 1,500 rpm, 110 kW fan motor in frame 315S, which is to be increased to 132 kW to raise the fan's airflow. In the HEM Motor list, the equivalent for 132 kW at 4 poles is 315M-4: rated 1,488 rpm, 224 A rated current at 400 V, 847.2 Nm rated torque.

The shaft height stays the same, but the 315M frame is longer than the 315S; the base needs adapting for the rear foot holes. Because the current increases, the cable, switchgear and protection settings are recalculated as well. This decision is made after confirming with the fan manufacturer that the fan really requires the new power.

Conclusion: before ordering a large frame

On frames 132–355 the right choice is made when power, speed, frame letter and mounting type are read together. Compare dimensions not by guesswork but using the technical drawings. All cast iron frame models are listed on the cast iron frame electric motors page.

Example models: 180L-4 (22 kW, 1,470 rpm), 280M-4 (90 kW, 1,486 rpm), 315M-4 (132 kW, 1,488 rpm) and 355M1-6 (160 kW, 990 rpm). Review suitable models and current prices on the buy electric motors page. Send a photo of your existing motor's nameplate, its speed and mounting details via the nameplate submission page and request a quote for the right model.

Frequently Asked Questions

Can a frame 315 motor be fitted in place of a frame 280 motor?

The shaft axis rises and the foot pattern changes; so the base and coupling side need rework. Stay with the same frame number if possible; if unavoidable, an intermediate base is prepared from the drawing.

Can 2-pole and 4-pole motors in the same frame replace each other?

The shaft height is the same, but because the speed differs, the machine may not accept it. On large frames the shaft end may also differ by number of poles; confirm from the technical drawing.

Can I reuse the old motor's shims on the new motor?

They can be used as a starting point, but the final shim arrangement is set by the alignment measurement. A soft foot check must also be done on each foot.

Can I change the terminal box position after ordering?

Box position and cable entry direction depend on the motor's design; changing them afterwards is not always possible. Stating the direction the cable comes from when ordering is the safest way.