Selection Of Crane Duty Motors (Part 1)

Selection Of Crane Duty Motors (Photo By polyglot via Flickr)

Introduction

The operating conditions such as duty cycle, startup, temperature and operating environment are vital considerations in the motor efficiency and reliability.

It is absolutely essential to match the motors to their specified operating conditions for minimizing stresses on the motors and to get predetermined performance and life.

One of the areas, in which significant technical requirements are considered lightly or even neglected, is selection of motors for operating various types of cranes and hoists.

These motors are specifically termed as “Crane Duty Motors” and are supplied by all the manufacturers.

Effort is made in this article to discuss in brief the vital factors required to be considered invariably when selecting the motor for performing the crane duty.

Definitions Of Technical Terms

Some technical terms used frequently in intermittent duty drives and hoisting are defined as follows:

1. Duty

Operation of the motor at the declared load(s) including starting, electric braking, no load and rest and de-energised periods to which the motor is subjected, including their durations and sequence in time.

2. Cyclic duration factor

The ratio of the period of energisation/loading, including starting and electric braking, to the duration of the one complete duty cycle expressed as percentage.

Generally the values for the CDF used are 25%, 40%, 60% and 100%.

3. Starting

The process of energizing a motor to bring it up to rated speed from rest.

4. Jogging or inching

This is an incomplete start during which the motor does not attain more than 25% of the rated speed.

5. Electric braking

A system in which a braking action is applied to an electric motor by causing it to act as a generator.

6. DC Injection braking

A form of braking of an induction motor in which a separate dc supply is used to magnetize the motor.

7. Plug braking

A form of electric braking of an induction motor obtained by reversing the phase sequence of its any two lines.

Duty Type and Class Of Rating

Before going to the principle discussion, it is necessary to throw some light on two of the most important technical requirements to be considered for better understanding.

Duty cycle assigned to the motor is one of the important factors in selection and operation of motors for several reasons, as it depends on how often and howlong a motor operates.

In accordance with Indian Standard IS:12824-1989; Types of Duty and Classes of Rating Assigned to Rotating Electrical Machines. The motors are to be designed for the standard duty cycles as given in Table.1 below.
Table.1 – Types of duty for electric motors

Duty Type	Motor performance requirements
S1	Continuous running duty – The motor works at a constant load round the clock or runs for adequate time to reach thermal equilibrium.
S2	Short-time duty – The motor works at a constant load for a definite time, but not long enough to reach thermal equilibrium. The rest periods are long enough for the motor to cool down to the ambient temperature.
S3	Intermittent periodic duty – The motor works with a sequence of identical duty cycles comprising of period of running at a constant load and rest and de-energized period. Thermal equilibrium is never reached due to these periods being too short. Starting current has little effect on temperature rise.
S4	Intermittent periodic duty with starting – The motor works with a sequence of identical duty cycles, each cycle consisting of significant periodof starting, a period of running at a constant load and rest and de-energized period. Thermal equilibrium is never reached due to these periods being too short, but starting current affects temperature rise.
S5	Intermittent periodic duty with electric braking – The motor works with a sequence of identical duty cycles, each cycle consisting of a period of starting, a period of running at a constant load, a period of rapid electric braking and rest and de-energized period. Thermal equilibrium is never attained due to periods of operating, rest and de-energized state being too short
S6	Continuous operation periodic duty – The motor works with a sequence of identical duty cycles, each cycle consisting of a period of running at a constant load and a period of running at no load without rest and de-energized period. Thermal equilibrium is never reached due to operation period at no load is too short.
S7	Continuous operation periodic duty with electric braking – The motor works with a sequence of identical duty cycles, each cycle consisting of a period of starting, a period of running at a constant load and a period of electric braking. Thermal equilibrium is never reached, as rest and de-energized period is not there.
S8	Continuous operation with periodic related variations in load and speed – The motor works with a sequence of identical duty cycles, each cycle consisting of a period of running at a constant load corresponding to a definite speed of rotation, followed by one or more periods of running at other constant loads at different speed.Thermal equilibrium is never reached, as rest and de-energized period is not there.
S9	Duty with non-periodic load and speed variations – The motor works generally at a load and speed, which are varying non-periodically within permissible operating range including frequent application of overloads that may exceed the ratingof motor. Thermal equilibrium is never reached, as rest and de-energized period is not there.

Classes of rating assigned to the electric motors are as given in Table.2 below. It may be noted that while assigning classes of rating, the motors should invariably comply with the requirements of Indian Standard referred above.

Table.2 – Class of ratings for electric motors

Class of Rating	Philosophy for operation of motor
Maximum continuous rating	The motor may be operated continuously for unlimited period at the load and service conditions assigned by the manufacturer.
Short time rating	Starting at ambient temperature, the motor may be operated continuously for limited period at the load and service conditions assigned by the manufacturer.
Equivalent continuous duty	The motor may be operated at the load and service conditions assigned by the manufacturer for the test purposes until thermal equilibrium is attained. This is considered to be equivalent to one of the periodic duty defined in duty type S3 to S8 or to the duty type S9.
Periodic duty type rating	The motor may be operated for duty cycles at the load and service conditions assigned by the manufacturer. When applied to the motor, this class of rating corresponds to the periodic duty S3 and S6 types maintaining time of duty cycle 10 minutes and one of the cyclic duration factors (CDF) with values – 15, 25, 40 or 60 percent.
Non-periodic duty type rating	The motor may be operated non-periodically for dutycycles having varying loads over varying speed and service conditions, including overloads, assigned by the manufacturer. When applied to the motor, this class of rating corresponds to the non-periodic duty with non-periodic load and speed variations as per duty type S9.

Designation of Duty Types and Class of Ratings

The duty types and class of ratings designated to the motor should be indicated on the nameplate following the rated output as discussed hereunder.
1. If no designation is indicated following the rated output, then the motor should be considered suitable for maximum continuous rating, i.e. S1 duty.
2. For the duty type S2, the duration of duty should be indicated in minutes after S2. For example, “S2 60 minutes”.
3. For the duty type S3 and S6, indication of the cyclic duration factor (CDF) in percentage should follow S3 or S6. For example, “S3 15 %” or “S6 60%”.
4. For the duty type S4 and S5, the indication of S4 and S5 should be followed by indication of the CDF in percentage, the moment inertia of motor (J_M) and the moment of inertia of load (J_ext), both referred to the motor shaft. For example, S4 25 %, J_M= 0.15 km-m², J_ext= 0.7 km-m².
5. For the duty type S7, the indication of S7 should be followed by indication of the moment inertia of motor (J_M) and the moment of inertia of load (J_ext), both referred to the motor shaft. For example, S7, J_M= 0.15 km-m², J_ext= 0.7 km-m².
6. For the duty type S8, the indication of S8 should be followed by indication of the moment of inertian of the motor (J_M) and the moment of inertia of the load (J_ext), both referred to the motor shaft,together with tee load, speed and cyclic duration factor for each speed condition.

For example:S8 J_M = 0.4 kg-m², J_ext= 4 kg-m²

16 kW, 740 rpm 30 percent
40 kW, 1460 rpm 30 percent
25 kW, 980 rpm 40 percent

Need of specifying Duty Type and Class of Rating

For the majority of applications, the motors assigned with duty types S1, S2, S3, and S6 would be found appropriate respectively for continuous rating, short time rating, intermittent ratings or continuous rating with intermittent loading.

However for special duties, such as cranes, the motors with equivalent continuous rating, short time rating or intermittent rating would be required.

In order to get the motor that meets the requirements for special duties, it is essential that the vendor is furnished all the data of required duty by the purchaser.

In case the data is insufficient, there should be an agreement by discussion between the purchaser and the vendor for the type of motor. If the duty type and class or rating are not specified or incorrect values are specified, the vendor may arrive at the motor rating and select a suitable motor to met the require duty from his standard list of ratings and the purchaser may not get the correct motor and cause problems during actual service.

Moreover, when a motor is designed for cyclic duty for crane, it should also be possible to subject the said motor for testing for equivalent continuous, short time or intermittent rating such that the motor would satisfactorily meet the actual duty requirements whilst complying with the specified conditions.

Will be continued in 2 days…

References:

Efficient Electric Motor Systems Handbook, by Todd Litmann
IS:12824-1989; Types of Duty and Classes of Rating Assigned to Rotating Electrical Machines
The Technical Literature of Indian Motor Manufacturers