Motor dimensioning

When motors for crane drives are dimensioned (high?inertia drives), two criteria must be taken into account:

• The required maximum torque (starting torque)
• The rated output (thermal motor capacity)

When you check the torque, you check whether the motor can generate the required maximum torque (e.g. for acceleration). The maximum permissible torque is greater than the rated torque and is often specified as a multiple of the rated torque. An adequate safety margin from the stalling torque must be maintained.

When the output is dimensioned, the rated motor output is adjusted in accordance with the effective power requirements of the drive. The rated motor output depends on the motor temperature which, in turn, is influenced by the operating mode and the thermal behavior of the motor. The rating data of a motor differs for the various operating modes in accordance with EN 60034?1. The data is usually specified for one or more of the following operating modes:

• Continuous duty S1 (also corresponds to intermittent duty S3?100 %)
• Short?time duty S2
• Intermittent duty S3

Intermittent duties S4 and S5 vary to such an extent that accurate data can only be provided when certain additional conditions have been clearly defined.

The operating modes are defined in accordance with EN 60034?1.

Continuous duty (S1)

Definition

Operation with a constant load state, the duration of which is sufficient to attain thermal equilibrium.

Explanation

The operating time t_e of the motor must be greater than 3 ? T_L to ensure that thermal equilibrium is attained. The rated motor output for continuous duty must be designed such that the final temperature ?_e matches the permissible winding temperature. Start?up is deliberately discounted under the assumption that a single high?inertia start will not achieve the final temperature. The length of the subsequent idle time is insignificant. Caution is advised, however, when high?inertia starting is carried out on a warm machine or when a machine is started up several times in succession. Certain restrictions may apply or advice from a third party should be sought.

Short?time duty (S2)

Definition

Operation with a constant load state that, however, does not last long enough to attain thermal equilibrium, followed by idle time that lasts until the machine temperature differs from the coolant temperature by no more than 2 K.

Explanation

The operating time t_e must be less than 3 ? T_L to ensure that the theoretical final temperature is not reached. The rated motor output and the operating time are harmonized in such a way that the maximum winding temperature ?_max does not exceed the permissible values. Here, too, start?up is deliberately discounted because it is assumed that the machine starts up cold and the start?up procedure is short with respect to the operating time t_e.

The rated motor output for short?time duty can be higher than for continuous duty, although the permissible operating time must also be specified. The shorter the operating time, the higher the rated output of the machine. Operating times of 10, 30, 60, and 90 minutes are recommended.

The subsequent idle time must be sufficiently long to ensure that the machine can cool back down to the ambient temperature (i.e. t_P is greater than or equal to 3 ? T_St) because otherwise the maximum temperature will be exceeded the next time a similar duty cycle is carried out.

Intermittent duty without the effect of the start?up process (S3)

Definition

Operation that involves a sequence of similar duty cycles, each with a constant?load period and idle time, whereby the starting current does not have a noticeable effect on the temperature rise (the duty cycle duration is generally short enough to ensure that thermal equilibrium is not attained).

Explanation

The operating time t_e must be less than 3 ? T_L to ensure that the theoretical final temperature ?_e is not reached. The subsequent idle time t_p, however, is also less than 3 ? T_St, which means that the ambient temperature is no longer reached. A mean steady?state value ?_mean develops around which the temperature varies, but is below the theoretical final temperature ?_e.

The rated motor output during intermittent duty is greater than during continuous duty. The time constants T_L and T_St may be different. This influences the rated output during intermittent duty and is taken into account in the S3 motor tables.

To determine the most suitable motor, therefore, a knowledge of the operating and idle times is required in addition to the required output during the operating time. They are specified by the duty cycle duration (total time) and the relative ON duration in % of the cycle duration. If no data has been provided for the duty cycle duration, 10 minutes apply in accordance with DIN EN 60034?1. The S3 motor tables are based on this. Values of 15, 25, 40, and 60 % are recommended for the cyclic duration factor.

Effect of varying duty cycle durations

The S3 rated output is designed in such a way that the temperature peaks ?_max match the permissible values with a 10 minute duty cycle duration (see "a" in diagram below). Shorter duty cycle durations are not critical because lower temperature peaks occur at the same mean winding temperature ?_mean (see "b" in diagram below). Since longer duty cycle durations result in higher temperature peaks (see "c" in diagram below) which, in turn, reduce the service life of the insulation, advice from a third party should be sought in this case.

In S3 duty, the start?up processes are not discounted; the relevant standard assumes that they do not have any significant influence on the temperature rise. Any number of duty cycles can be carried out per hour provided that this standard is fulfilled.

Intermittent duty with effect of the start?up process (S4)
Intermittent duty with effect of the start?up process and electrical braking (S5)

Definition

Operation that involves a sequence of similar duty cycles, each with a noticeable start?up time, a constant?load period, a period of rapid electrical braking (with S5), and idle time.

Intermittent duty S4

Intermittent duty S5

Explanation

These operating modes closely resemble S3 duty, except that the temperature rise caused by start?up and, in some cases, electrical braking are also included. This additional power loss depends on the acceleration torque and the time in which this occurs; in other words, it depends on the linear and rotating masses to be accelerated (kinetic energy). The masses that are moved, therefore, must be known. These are based on the moment of inertia referred to the motor shaft. How often and over what period of time the masses are subject to acceleration and braking procedures must also be known.

The more duty cycles performed by the crane drives every hour (e.g. short traveling distances or low hoisting heights), the greater the importance of the acceleration work for motor dimensioning purposes.

To accurately dimension a motor for duty cycles S4 and S5, therefore, the following information is required in addition to the steady?state output:

• Cyclic duration factor (CDF)
• External moment of inertia
• Acceleration or acceleration torque
• Accelerating time
• Number of working cycles per hour

General performance specifications for motors in S4/S5 duty are not possible because they always vary depending on the specific conditions under which the driven machine is operating (external moment of inertia) and the operating mode (working cycles, ON duration). Crane drives do not have a constant load across several working cycles but instead have a collective load.

Calculating the rms value, ON duration

Actual duty can also be converted to a thermally equivalent S3 mode by means of "rms value calculation", which means that the S3 motor tables can be used again.

A torque diagram (duty cycle diagram) must be available when the calculation is performed (see diagrams below).

The value (assumed to be constant throughout the operating time) that would generate the same temperature rise as the actual torque is defined as the rms torque. The ON duration is the sum of operating times with respect to the total duty cycle duration.

If the individual traveling duty cycles are not the same (e.g. due to different loads or distances), all the different traveling cycles must be included in the rms value and ON duration calculation until they repeat themselves.

Differences in thermal behavior when the motor is running and when it is at a standstill are already taken into account with respect to the ON durations in the S3 tables. For this reason, M_rms must be calculated with respect to the operating time t_e and not to the duty cycle duration t_S.

To ensure that the rms value can be defined with sufficient accuracy, however, the operating phases during which the motor is not cooled as efficiently must be taken into account (e.g. during correction runs at low speeds and with naturally cooled motors).

Effects such as these can only be taken into account by the motor manufacturer.

The operating conditions for hoist and traversing gears also vary enormously:

• The external moment of inertia with respect to the motor moment of inertia is usually greater in traversing gears than in hoist gears; in other words, the acceleration and braking work are more relevant for traversing gears than for hoists even when the number of duty cycles performed and the ON duration are the same.
• The steady?state torque (load torque) with respect to the rated motor torque is usually greater in hoists than in traversing gears (traveling resistance).
• The torque diagram for traversing gears does not take the direction of travel into account (without wind forces). The effect of the payload is minimal with high traversing gear weights; i.e. with handling cranes, it is repeated after each travel movement (after the second travel movement at the latest).
• The torque diagram for hoists is largely dependent on the load. The motor torques when the same load is hoisted and lowered are different (efficiency) and, in the case of handling cranes, a traveling duty cycle with a load is usually followed by a traveling duty cycle with empty load tackle (collective loading; see also FEM, Section I, Calculation Principles for Cranes); i.e. the cycle required for calculating the rms value is repeated after the fourth travel movement at the earliest.

Torque diagram

Typical torque diagram for a gantry across one conveying cycle

Typical torque diagram for hoist across one conveying cycle