When you start an asynchronous motor, at the first instant it draws a current from the grid far above the rated current; this inrush (starting) current rapidly heats the winding until the motor reaches speed. On a single start this heating is not a problem; but if the motor stops and starts many times per hour, the heat accumulated at each start rises faster than the motor can cool down, and the winding reaches dangerous temperatures. That is exactly why every motor has a starts-per-hour limit. When this limit is exceeded, the motor can burn out purely because of frequent starting, even if there is no problem on the load side. In frequently stop-start applications such as elevators, door automation, presses, cranes, mixers and compressors, this subject directly determines motor life. In this guide we cover step by step why the inrush current heats the winding, the permitted starts-per-hour value, the effect of inertia (GD²), intermittent duty types (S4/S5), thermal protection and the selection of the correct motor and starting method in frequent stop-start duty. The aim is to select the right motor on the first attempt to prevent early failure in a frequently operating application.

Inrush current and winding heating in a frequently stop-start asynchronous motor

Which Applications Are Affected by Frequent Stop-Start?

The starts-per-hour limit is rarely a problem for a pump or fan running continuously at the same speed, because these motors may start only a few times a day. The real risk is in applications that by nature constantly stop and start. Elevators and escalators, door and barrier automation, presses, crane and hoist lifting systems, packaging and filling machines, mixers, compressors and conveyor feed systems top this group. In these applications the motor may start hundreds of times per shift. We covered conveyor drive motor selection in our heavy-duty conveyor drive motor article and motor supply for packaging machines in our corrugated cardboard and box factory motors content. The continuous load profile of a compressor motor can be found in our compressor motor replacement article.

Why Does the Inrush Current Heat the Winding?

When an asynchronous motor is at rest its rotor is stationary; at this moment the motor behaves almost like a short-circuited transformer and draws a current several times the rated current from the grid. This high current causes rapid heat generation in the winding. As the motor reaches speed the current drops to the rated value and the heating returns to normal. The problem is that the motor cannot shed this heat before the next start. In frequent stop-start each start adds a new heat pulse to the winding and the temperature rises step by step. We covered the source of inrush current and ways to reduce it in detail in our inrush (starting) current (LRA) article. The relationship between rated torque and starting torque can be found in our rated torque and starting torque content.

Heat Accumulation and Cooling Time

The heat the motor produces at each start is shed by cooling during running and during the stop. If the time between starts is not enough for the motor to cool, the heat accumulates. For this reason the starts-per-hour limit depends not only on how many times it starts but also on the running and stop times between starts. While a continuously running motor reaches thermal equilibrium, a frequently stop-start motor may never settle into equilibrium. We covered this difference of duty types in our duty type (S1-S6) selection article and the temperature rise and rise class in our temperature rise class (Delta T 80K) content.

The Role of Cooling and Insulation Class

How fast the motor can cool depends on the cooling method. A standard motor cooled by a fan always rotating in the same direction does not cool much while stopped. In frequent stop-start applications, better cooling or a higher insulation class increases the heat margin. We covered cooling methods in our cooling methods IC411 and IC416 article and the effect of winding insulation class on life in our winding and insulation class (F/H) content.

Permitted Starts-per-Hour (Z Value)

Motor manufacturers state the permitted no-load starts per hour for each motor (usually shown as Z₀). This value shows how many times per hour the motor can safely start unloaded. When a load is connected and inertia increases, the permitted number of starts decreases noticeably. So the starts-per-hour value is a property not only of the motor but of the motor-load system. In correct motor selection this value must be compared with the real stop-start frequency of the application. We covered load ratio and correct sizing in our motor load ratio and correct sizing article and torque classes in our torque classes (Design N/H) and starting torque content.

The Effect of Inertia (GD²)

During start the motor must accelerate the moment of inertia (GD² or J) of its own rotor and the connected load. The greater the inertia, the longer it takes to reach speed and the longer the high inrush current flows. This means more heat. High-inertia loads (large fans, flywheel systems, heavy pulleys) lower the permitted starts-per-hour value. We covered motor selection in shock and high-inertia loads in our motor selection in shock loads: flywheel and inertia article and low-speed direct drive in our low-speed high-pole motors content.

Cyclic Duration Factor (ED%) and Running Time

An important concept in intermittent duty is the cyclic duration factor (ED%): the ratio of the time the motor runs in a cycle to the total cycle time. For example, if the motor runs for 15 seconds of every minute and stops for 45 seconds, the ED is about 25 percent. A low ED gives the motor more time to cool; a high ED narrows the heat margin. Correct motor selection must consider both the start count and the ED value together. Selecting a motor smaller than needed on a frequently stop-start line quickly leads to failure; we covered correct sizing in our 7 most common mistakes when buying an electric motor article and the early failure causes of a motor in our motor life and 7 causes of early failure content.

Motor selection with PTC thermistor protection and soft starter in a frequent stop-start application

Intermittent Duty Types: S4 and S5

While a continuously running application is S1 duty, frequently stop-start applications are defined by intermittent duty types. S4 is intermittent duty including the thermal effect of starting; S5 is the duty type that adds electrical braking to this. In these duty types the motor is sized for a certain start frequency and load ratio (cyclic duration factor). Putting a motor selected for S1 into a frequently stop-start application is one of the most common causes of early failure. We covered the purchasing impact of duty types in our duty type (S1-S6) selection article and dual-speed applications in our dual-speed (Dahlander) motors content.

Braking and Stopping

In many frequently stop-start applications the motor must not only start fast but also stop fast. DC braking or dynamic braking stops the motor quickly, but this too produces heat and increases the thermal load. We covered braking methods in our braking: DC and dynamic braking article and brake motor supply in our brake motor supply content. Brake motor selection in crane and lifting applications can be found in our crane and hoist lifting motors article.

Thermal Protection: PTC and PT100

In frequent stop-start, monitoring the winding temperature is the most reliable way to protect the motor from burning out. A PTC thermistor, embedded in the winding, rapidly increases its resistance at a certain temperature threshold and the protection relay stops the motor. PT100 provides continuous temperature measurement. Since the thermal overload relay cannot always correctly sense the cumulative heating created by frequent starting, in-winding temperature protection is strongly recommended in these applications. We covered temperature monitoring in our protection with PT100 and PTC thermistor article and general protection selection in our protection: thermal, relay and fuse selection content.

Protection Breaker and Relay Setting

The motor protection circuit breaker (MPCB) and thermal relay setting must be done carefully in frequent-start applications. Too sensitive a setting trips needlessly on normal inrush current; too loose a setting leaves the motor unprotected. We covered correct setting in our motor protection circuit breaker (MPCB) selection and setting article and cable and fuse selection by rated current in our rated current: cable, fuse and contactor selection content.

Correct Starting in Frequent Stop-Start

One of the most effective ways to reduce the thermal load of frequent starting is to change the starting method. A soft starter or a frequency drive (VFD) limits the inrush current, reducing both the heat pulse and the mechanical shock. The VFD also provides controlled acceleration and deceleration, effectively increasing the starts-per-hour capacity. We covered starting methods in our star-delta or soft starter article and soft starter compatibility in our soft starter compatibility content. Operation with a VFD can be found in our frequency drive (VFD) with asynchronous motor article. For our product range you can visit our homepage.

Correct Motor and Pole Selection

In a frequent stop-start application the correct motor should be selected in a suitable duty type (S4/S5), with sufficient heat margin and, if needed, in a pole count suited to lower inertia. The pole count affects both the speed and the starting behaviour. We covered pole selection in our 2, 4, 6 poles which for which job article and motor supply in frequently operating applications such as elevators and escalators in our elevator and escalator motor supply content.

Correct Sizing of the Soft Starter

A soft starter not only limits the inrush current in frequent stop-start applications; it also heats up itself due to frequent starting. For this reason the starter must be sized according to the hourly start count and the start duration of the application; otherwise it is the starter itself, not the motor, that fails. We covered correct sizing in our soft starter compatibility article and the inrush current problem in generator-fed facilities in our motor selection on generator-run sites content. That protection devices must also be selected to suit this frequency in frequent starting can be found in our protection devices purchase article.

Frequently Asked Questions

My motor stops and starts frequently, why is it heating up?

At each start the motor draws an inrush current far above the rated current, and this current rapidly heats the winding. A single start is not a problem, but when starts become frequent and the motor cannot cool between them, heat accumulates. This accumulation can raise the winding temperature above the permitted limit and burn out the motor even if there is no problem on the load side. The solution is to select a motor in a duty type suited to frequent stop-start (S4/S5), use a soft starter or VFD and add in-winding temperature protection (PTC/PT100).

How many starts per hour are safe?

This depends on the motor size, duty type, cooling and especially the inertia of the connected load. The manufacturer states the permitted no-load starts per hour (Z value) for the motor; as load and inertia increase this number falls. For a correct decision the real stop-start frequency of the application must be compared with the permitted value of the motor. In an application requiring frequent starting, it is essential to select the motor in a duty type that meets this frequency and to add soft starting if needed.

Does a VFD increase the starts-per-hour limit?

Yes, indirectly it does. The frequency drive accelerates the motor from zero speed in a controlled way, keeping the inrush current at the rated-current level; thus the heat pulse created by each start is greatly reduced. This allows the motor to stop and start much more frequently. The VFD also manages braking heat through controlled deceleration. Still, the motor low-speed cooling and duty type should be considered; in very frequent starting an externally cooled motor should be preferred if needed.

Get a Quote

Let us determine together the motor in the correct duty type (S4/S5), with sufficient heat margin and matched with a suitable starting method for your frequently stop-start application. Share your hourly start count, load inertia and operating profile; we will quickly offer you a suitable solution. To request a quote, reach us through our contact page or call now: +90 (532) 345 49 86.

Frequent Stop-Start Motor Selection Checklist

  • Has the real hourly start count (starts/hour) of the application been determined?
  • Are the connected load inertia (GD²/J) and acceleration time known?
  • Has the duty type been selected suitable for frequent stop-start (S4/S5)?
  • Has the permitted starts-per-hour value of the motor been compared with the application?
  • Are the insulation class and cooling sufficient for the heat margin?
  • If there is a cooling problem at continuous low speed, has external cooling been evaluated?
  • Has in-winding temperature protection (PTC/PT100) been added?
  • Has the inrush current been limited with a soft starter or VFD?
  • If braking is needed, has a brake motor or a suitable braking method been chosen?
  • Has the protection breaker/relay setting been made according to frequent starting?