A motor nameplate says "1500 rpm," but when you connect it to the machine and measure, the speed comes out at about 1440 rpm. This is not an error but the nature of asynchronous motors, and its name is slip. This article was written for businesses that will buy a motor: it clearly explains which speed to choose when ordering, how the actual speed affects your machine speed, and why the speed drops as the load increases. The right speed choice is the key to your machine running at the correct speed and to not buying the wrong motor. If your requirement is a three-phase motor, we can clarify the speed selection together.

First, the short answer: synchronous speed is the motor's "theoretical" speed; the actual speed is the speed you measure under load, and it is always slightly below the synchronous speed. The difference between them is called slip.

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What Is Synchronous Speed, and Where Does It Come From?

In an asynchronous motor, two things determine the speed: the grid frequency (50 Hz in Türkiye) and the motor's number of poles. Synchronous speed arises with this logic: 3000 rpm in a 2-pole motor, 1500 rpm in a 4-pole motor, 1000 rpm in a 6-pole motor, 750 rpm in an 8-pole motor. The "3000/1500/1000" values on the nameplate are these synchronous speeds. Our asynchronous motor pole selection article, in which we addressed the effect of the number of poles on speed and torque in detail, sets the foundation of the speed choice; on this page we do not repeat the entirety of pole selection but focus on slip and actual speed.

Why "Asynchronous"? The Physics of Slip

In an asynchronous (induction) motor, for the rotor to be able to turn, a speed difference between the rotating magnetic field and the rotor is mandatory. If the rotor reached the exact synchronous speed, it could not "cut" the magnetic field and the force that makes it turn would not arise. That is why the rotor always turns slightly behind the synchronous speed; this "lagging behind" is slip. The word "asynchronous" (not synchronous) in its name describes exactly this.

Slip Percentage and Actual Speed Calculation

Slip is generally expressed as a percentage. In a typical standard asynchronous motor, slip is on the order of about 4%. A concrete example: in a 4-pole motor with a synchronous speed of 1500 rpm, 4% slip means an actual speed of about 1440 rpm (1500 − 1500×0.04 ≈ 1440). With the same logic:

  • 2 poles, 3000 synchronous → actual speed typically ~2880–2900 rpm,
  • 4 poles, 1500 synchronous → actual speed ~1440–1460 rpm,
  • 6 poles, 1000 synchronous → actual speed ~960–975 rpm.

That is why, in the catalog and on the nameplate, the speed is sometimes written with the actual value under load, such as "1440 rpm." When ordering, knowing that both the synchronous (1500) and the approximate actual (1440) value describe the same motor prevents misunderstanding.

Why Does Speed Drop as Load Increases?

Slip is not a fixed number; it changes with the load. When the motor is idle, the slip is very small and the actual speed is very close to the synchronous speed. As the load increases, the rotor needs to "cut" the magnetic field more, that is, to slip more; because this difference must grow to produce the required torque. For this reason, the same motor turns at 1490 rpm at light load but drops to 1440 rpm at full load. This drop in speed is normal; however, an excessive and continuously increasing drop can indicate that the motor is being strained or that a fault is developing. To distinguish normal behavior from a fault, our electric motor failures article lists the symptoms.

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The Effect of Actual Speed on Machine Speed

Many machines use the motor speed directly or through a belt-pulley/reducer. Therefore, even a difference of 60 rpm can affect the result:

  • In pumps and fans, flow and pressure depend on speed; the actual speed determines the expected flow. That is why pump selection must be calculated with the actual speed, not the synchronous speed. For details, you can look at our centrifugal pump motor selection article.
  • In belt-pulley systems, the output speed is determined by the motor's actual speed and the pulley diameters; making the design from the actual speed gives the correct result.
  • In reducer applications, the value entering the reducer input is the actual speed; using 1440 instead of 1500 when calculating the output speed gives a result closer to reality.

Speed Selection When Ordering: A Practical Guide

When ordering a motor, three steps are enough on the speed side:

  • 1. Determine the output speed your machine needs. High speed (3000), general-purpose medium speed (1500), or high torque/low speed (1000/750)? Most industrial applications run at 1500 rpm.
  • 2. Calculate with the actual speed, not the synchronous speed. In pump/fan/reducer calculations, subtract ~4% slip and use the actual speed.
  • 3. Watch the pole and torque balance. At the same power, a low-speed (multi-pole) motor produces higher torque but is larger and more costly. Our asynchronous motor pole selection article clarifies this balance.

The HEM Motor asynchronous range is offered with 1000/1500/3000 rpm options, 100% copper winding and IP55 protection. You can examine our 3-Phase Electric Motors and AC Electric Motors products; you can see all asynchronous/AC options from our asynchronous / AC motors category, and the wide product family through our IE4 product category. For the engagement behavior on the starting side, our star-delta or soft starter in asynchronous motors article is also useful.

The Relationship Between Slip, Efficiency and Heating

Slip is not just a matter of speed; it is directly related to efficiency. The speed difference that slip creates in the rotor induces current in the rotor bars, and this current produces heat (rotor copper loss). The greater the slip, the more heat is released in the rotor. That is why, in well-designed motors with quality windings, slip is kept lower; lower slip means lower rotor loss and higher efficiency. As the efficiency class rises (IE3, IE4), designers try to reduce slip and losses.

The practical consequence of this is: a motor running with very high slip, that is, far below the synchronous speed, is constantly under overload; this both consumes more energy and shortens the motor's life by heating it. If the actual speed you measure is far below the approximate value on the nameplate, the motor may not be able to handle the load and you may need to step up to a higher power. So the actual speed is also an indicator of whether the motor is sized correctly. To avoid missing the correct power selection, you can look at our mistakes made when buying an electric motor article.

Should Speed Stay Fixed? Speed Control with a Frequency Drive

The speed of an asynchronous motor connected directly to the grid is fixed by the frequency (50 Hz) and the number of poles; only a small slip change occurs with the load. However, in many applications it is desired to be able to adjust the machine's speed: such as lowering a fan's flow, slowing a conveyor, or running a pump according to demand. In this case, the speed is adjusted with a frequency drive (VFD) without changing the motor. The drive controls the synchronous speed, and therefore the actual speed, by changing the frequency going to the motor.

When a frequency drive is used, the speed is no longer a fixed "1440" but a value that changes according to demand; this provides serious energy savings especially in variable-load pumps and fans. For drive selection and motor compatibility, our frequency drive (VFD) with asynchronous motor article offers a detailed guide. In classic drive-free, fixed-speed use, making the correct pole/speed selection at the ordering stage is the most important decision.

What Happens If the Speed Selection Is Wrong?

Making the speed selection wrong is a frequently encountered and costly mistake:

  • If too high a speed is selected: Excess speed reaches the machine; it must be corrected with the belt-pulley/reducer ratio, otherwise the process is disrupted, and problems such as cavitation in the pump and excessive flow in the fan arise.
  • If too low a speed is selected: The machine runs slowly and capacity drops; moreover, a low-speed motor at the same power is larger and more costly.
  • If the actual speed is ignored: Calculating with 1500 and running with 1440, the pump/fan does not deliver the expected flow; this small difference can make a difference in the process.

That is why speed selection is as important a parameter as power. To determine the correct pole/speed combination according to your machine's real requirement, it is enough to convey your need profile to us; you avoid buying the wrong motor from the start. Do not ignore the single-phase/three-phase and voltage-side selection either; we addressed this in our single-phase or three-phase motor selection article.

Calculating with Actual Speed in Reducers and Belt-Pulleys

In most industrial applications, the motor's actual speed is transmitted to the machine not directly but through a transmission element. At this point, working with the actual speed is the only way to capture the correct output speed.

If you use a reducer, the output speed is found by dividing the motor's actual speed by the reduction ratio. For example, if you connect a motor with an actual speed of ~1440 rpm to a worm gear reducer with a 1/30 ratio, the output speed is about 48 rpm (1440 ÷ 30). If you had made the calculation with 1500, you would find 50 rpm, but in reality you would obtain 48 rpm; this difference can be important in precise positioning and dosing applications. We addressed the IEC frame and flange compatibility in motor-reducer matching in our matching a motor to a worm gear reducer article, and the question of whether to buy it geared or separately in our geared motor or separate motor + reducer article.

In a belt-pulley system, the output speed depends on the motor's actual speed and the ratio of the driving and driven pulley diameters. Here too, putting the actual speed (~1440) into the calculation, not 1500, is the key to running the machine at the correct speed. We detailed pulley/shaft compatibility and the correct shaft diameter selection in our motor shaft diameter and key dimensions article. As you can see, a "small" slip percentage, when multiplied by transmission elements, can turn into a concrete difference on the output side; that is why the actual speed is not just a technical detail but a part of the correct purchase.

Reading the Nameplate Speed Correctly When Ordering

When ordering a motor, interpreting the speed information on the nameplate correctly is the last link in not buying the wrong motor. The speed on a motor nameplate is generally written with the actual value under load; that is, a nameplate that says "1440 rpm" describes a 4-pole motor with a 1500 synchronous speed. Likewise, a "2880" or "2900" value indicates a 2-pole (3000 synchronous) motor, and a "960" or "970" value indicates a 6-pole (1000 synchronous) motor. Knowing this matching eliminates the hesitation of "should I order 1500 or 1440?"; they are both the same motor.

To prevent confusion when ordering, the safest way is to state both the number of poles (or the synchronous speed) and the application together; for example, "4 poles, 1500 rpm, 7.5 kW, B3, conveyor drive." A request with this clarity ensures the correct motor arrives the first time. To avoid returns and delays due to the wrong speed/frame, we explained nameplate matching before ordering step by step in our prevent the wrong motor from arriving article. At any point you hesitate, it is enough to send us a photo of your existing motor's nameplate; we will verify all parameters, including the actual speed, for you.

Summary: Slip Is Not a Defect but Part of the Design

Let us pull together the logic underlying slip in an asynchronous motor, and therefore the ~1440 rpm actual speed instead of 1500. Slip is the speed difference that is mandatory for the rotor to be able to turn; without it the motor cannot produce torque. For this reason, the actual speed is always slightly below the synchronous speed, and the difference between them grows as the load increases. In a typical standard motor this difference is on the order of about 4%; that is, a 1500 synchronous speed corresponds in practice to an actual speed of ~1440 rpm. This is not a fault but the nature of the induction motor.

The lesson to draw from the purchasing perspective is: when calculating speed for your machine, use the actual speed, not the synchronous speed, because pump flow, fan capacity, belt-pulley and reducer output speed all depend on the actual speed. The correct pole/speed selection and the correct power are the key to your machine running at the speed and efficiency you expect. As HEM Motor, when you convey your application and the output speed you need to us, we recommend the motor with the correct speed and number of poles, also taking the actual speed into account; this way you procure a motor where your expectation and reality overlap.

Frequently Asked Questions

The nameplate says 1500 but I measured 1440; is the motor faulty?

No, this is completely normal. 1500 is the synchronous speed; the actual speed measured under load is ~1440 rpm due to about 4% slip. What you should really watch is a continuous and excessive drop in speed or the motor being strained; a stable value of 1440 is the sign of healthy operation.

Can I reduce slip for a higher speed?

Slip is part of the motor's operating principle; it cannot be completely eliminated. If a higher speed is needed, the correct solution is to select a 2-pole (3000 rpm) motor or to adjust the speed with a frequency drive. For the correct speed/pole choice, convey your need profile to us.

Should I make my pump calculation with 1500 or 1440?

You should do it with the actual speed, that is, about 1440 rpm. Since flow and head are proportional to the actual speed, a calculation made with the synchronous speed predicts a slightly higher flow than expected. For correct pump-motor matching, base it on the actual speed.

Get a Quote

To select the motor with the correct speed and number of poles for your machine, get in touch with us to obtain the right motor in the shortest time with a clear quote. Phone: +90 (532) 345 49 86 — or reach us through our contact us page. Our expert team helps you with the selection of power, speed, mounting type and efficiency class, and quickly clarifies stock availability and delivery time.