As far as my practical knowledge allows, any good company make motor can work safely for a few minutes, if not for hours at an over load of 125 -130 %. And I have not seen any manufacturer giving the safe range of over loads and  times for which the motor can work with the over loads safely. If your peak load is 180 kW and if you are very sure that the over load remains for only a few seconds, I vote for a 150 kW motor. Moreover the induction motors are more robust in handling the over loads compared to dc motors.

The choice of correct size and power of an electric motor is directly linked to which of its mechanical parameters change mostly. The power equals speed multiplied by torque. If one has the constant speed and varying torque then it let us to choose the motor with the required rated speed (that is constant) and the needed torque. This case was described by K. Prakash Kumar in details (but in my opinion the most  induction motors manufactured now are capable to endure overloads up to 170-200% (in terms of torque) for a few minutes). But more often we have case when both speed and torque vary in some ranges. This case allows much more options for discussing. For one, if the highest values of speed  relate to the lowest values of torque one could choose the less rate power motor by using work under lower torques with higher values of frequency supplying the stator winding. This matter depends on the frequency converter, if any.     

I think the answer given by Alexander Pugachev is more precise. Size of motor is depends on its continuous rating and not on peak loading.

I want to incorporate an electric motor for a series hybrid vehicle with proper power rating such that the vehicle performance requirement is met. From drive cycle(velocity vs time graph of the vehicle) and vehicle dynamics, I have calculated that the maximum power requirement for the vehicle to follow that particular drive cycle is 145kW for the first 1000s and then it shoots upto 180kW for the next 800s. The average power requirement for the drive cycle was around 60kW. The main problem that is faced while selecting a motor for a series hybrid vehicle is its weight. So for this scenario i want to downsize the motor in such a way that the performance is met as well as the motor is not too bulky. Which criteria or parameters should be looked upon for selecting the motor.

Dear Swagatha,

As far I understand, your vehicle needs almost 145 kW during initial acceleration and 180 kW during later acceleration. During the free running, it requires almost 60 kW.

If my understanding is correct, then,

Instead of single motor, I suggest two motors coupled together, each of say 70 kW. During high power operation, both will be operated and during free running one of them can be switched. But there is a problem. The switched OFF motor acts as a load on the other working motor. That should not be a problem.

As far as the size of the motors is concerned, a 3-ph motor will be smaller than an ac series motor of the same capacity. At lower speeds, (frequency control of the motor, if adopted)  3-ph induction motor gives fairly high torque, almost equal to ac series motor. Well, the type of motor is your choice, depending on the type of supply, speed control method and like.

Dear Swagatha 

Motor sizing is not a simple process to be packed in minutes, hours or day. What you need is a wide speed or torque capability of your desired motor which might encounter various transients as well. Today, commercial design software packages such as MotorSolve or Speed are not able to even return a design which guarantees a proper performance at transient start ups or flux weakening operations. I believe you have to accept a level of trade-off and degradation in motor performance when it switches between two widely different points including various torques or speed.  If you want to have a en efficient design like what you are looking for you have to, for sure, do many FEM-based analysis of the motor at different operating points by changing motor geometry variables and considering inverter limitations such as maximum voltage, utilization factor and MTPA capability. So, I would recommend you to first take a loot at Prof. Tim Miller's book which is about motor design process. Then, if you have enough time, computational power and proper software, it is better start sampling different design possibilities, say 100 to 200 samples, with respect to motor geometry variations. Then, you will definitely be able to decide which design might fulfill you requirements. It is a hundred percent thing that you would achieve what you are looking for without doing such analysis. You might only be able to  improve your design a bit without doing such analysis. 

As a general comment, I would recommend considering the following terms in your design:

1- Maximum available torque at steady state and short time transient operations

2- Constant power speed range (CPSR)

3- Efficiency at 20%, 50% and 100% load rate speed and CPSR

Let me know, if you need to know how to start with it

Thank you Mr. K prakash kumar. Actually I had in mind to use two electric motors instead of one. In series hybrid vehicle, there are already 2 electric machines available-one traction motor and one generator. Adding more components may lead to power loss. SO I am not sure which one is more beneficial- using one over sized traction motor or use two smaller electric motors? Also I would like to know if I use two electric motors, do I need to take the power rating of the motors exactly same? What if I take two motors with different characteristics?

Thank you Mr. Vahid Ghorbanian for your feedback. I am not exactly looking in depth for optimising the electric motor. Basically I am trying to design a Series hybrid vehicle where I need to incorporate a traction motor that will drive the vehicle instead of an engine. For that I need to select a motor based on the power requirements of the vehicle. And I need to do that without over weighting the vehicle by using a properly sized traction motor. Like I said before most of the electric motors selected are based on the maximum load requirement. But the maximum load occur for a very few seconds. Hence this makes the electric motor to run at part load. I need to know how the electric motor efficiency is affected if it does not work in peak load?

Also if I select a motor with PF 1.5 or 2, then I can over load the motor 1.5 to 2 times its rated power(Hope I am correct!). Hence will it be beneficial that I select a motor with low power rating and high PF?

I am from mechanical engineering background and hence I am not quite familiar with electric machines..

Hello,  Swagata !

In general, the motor to be chosen has to satisfy the next requirements: its start-up torque has to exceed the start-up one from mechanical part, its maximum torque has to exceed the maximum torque from mechanical part, and, lastly, power losses of an electric motor over the duty cycle has to be less than the rated one (in opposite case, it leads to overheating). If you use the frequency converter for feed and control your motor, you can forget about first two requirements - the proper selected frequency converter decides these problems. But in terms of the third item it is still unclear if your motor satisfied the requirements.

To be honest, there are a number work in this field (I do not mean any optimization task, precise evaluation of overheating, etc.). From my point of view, it's good to start from explanations of basic conceptions (p. 27) and equations of equivalent units (pp. 29,29).

I hope my answer was useful, I'm trying to help you and improving my English. Best wishes!

Overloading the motor 2 times.. I don't think is a wise idea because it repeats for every cycle, I mean every stop and start of the vehicle. I vote for 1.5 times overloading and decide the motor capacity accordingly, I mean twin motor set up. That works good practically. 

It can carry the intermittent overload provided the time is short when it is continuously rated.However,there is build up of temperature that may inimical to the insulation.Thus,the motor should be selected based on the temperature rise

you can use from this link for motor sizing calculater

http://www.robotshop.com/blog/en/drive-motor-sizing-tool-9698

Dear Swagata,

I personally encounter the problem you have mentioned in the oilfield artificial lifting (SuckerRod). For a 100 hp nemaD motor the current consumption per load cycle vary between 65A and 145A in a few seconds.

The motor sizing determine for optimal performance would be as follows:

I wish the above info. serve your needs.

Best regards,

Khaled Rady