Do you really know your Motor?

(Part 1 / 3)

In electric vehicles, Motor is a very important part of powertrain. It is equivalent to Engine in the conventional fuel vehicles. Traditionally Engine design was a long term and complicated process and had to undergo various tests. But in EV we have seen that Motors are not taken that seriously and not equivalent efforts are being put for Motor.

Lot of Electric Vehicle startups and OEMs have started working on new EV models now a days. And most of the engineers working on it have no prior experience or sufficient working knowledge (I said most engineers, not all engineers - so no offense please).

Engineers just go by datasheet, check Average power, Peak power, Torque curves, few other things like bearings, connection, wire size, IP rating, mounting and choose the motor for the vehicle. And for prototype this might be OK (though it is not right way), but for production vehicles it is not as simple as this.

So let us understand what we need to consider while selecting motor. I am not going to talk about what is motor, which types, different technologies etc. There is lot of material available for this on Internet which you can go through. I will just talk about summary of what you may need to take care while you are selecting motor.

• Torque Characteristics @ RPM Band - Same power and RPM rating motors always have different Torque characteristics over the given RPM range. So compare the Torque Vs RPM Vs Power graphs to find the most efficient motor for the required RPM Band. (RPM band is derived from the speed in which you vehicle will be driven for >60% of the time. This band you need to derive from your defined Drive cycle). Do not go by peak efficiency mentioned in the Datasheet or Curves. The operating point of the given motor efficiency may not be your vehicle operating point. So check for RPM band as per your vehicle drive cycle.
• Lower RPM Torque and Efficiency - Almost all motor manufacturers do not give the Torque/Power curve for lower RPMs. So ask for it, or test for it. This is important, because in City you will have many start / stop conditions and your motor may mostly drive in Lower RPMs. It is also very important for you to get your acceleration time and acceleration power curves.
• Torque Ripple - None of the motors used in EV gives you fixed torque in same operating conditions (like the curve you see in the datasheet). Based on quality of motor construction, gaps, magnets, winding accuracy, age of the motor etc, the Torque values may have ripples as high as 10%. The ripple will have very adverse effect on your driving comfort, speed and range. So check this parameter thoroughly for different loading conditions.
• Cogging Torque – This is also one of the cause of Torque ripple - periodic disturbances in torque, and it results in noise vibration. This has to be taken care while motor designing. The cogging torque results from the combined influence of the interaction of each individual magnet with each individual tooth. So for selecting any motor, this value need to be taken in to consideration. The low quality motors have very high Cogging resulting in higher ripple and noise.
• Torque Vs Temperature Curves - Every motor has some characteristics change based on Temperature. Motor winding resistance, Magnetic flux density and some other parameters changes as per Temperature, resulting in changed performance. Try your motor on vehicle for full throttle on full gradient and check the motor temperature. You might be surprised see how hot it is. And believe me, in summer this situation is going to be worst. So check you motor power, torque, efficiency characteristics at high temperature.
• Regeneration Peaks - Regeneration is good or bad, depending on how you are controlling it. It is very important to check motor Regeneration waveforms to understand peak voltage and current being generated. This power need to be passed through Motor controller and then to Battery. And the peaks and not taken care properly, it may end up stressing your motor controller and battery.
• Sudden Deceleration - Is this important? Yes it is. Going inside the pothole, driving on rough roads, going over some object or similar conditions may result in very high current peaks. And your system must be able to handle these at any point of time. Moreover your motor cabling should be good enough to handle all these conditions.
• Acceleration / Deacceleration Endurance - All motors are stressed only in 2 conditions - Acceleration and Deacceleration. Once you attain the speed, they run in normal operating condition (which we don’t care actually). Bearing, End ring, Shaft, Rotor, Cage etc. mechanical components also undergo lot of stress during these conditions. So decide how many Acceleration/Deacceleration endurance cycles your motor should have based on vehicle warranty and life cycle (Warranty and part life are always different), and get it tested.

There are many such points to be considered for choosing or designing your Electric Vehicle motor, which most of the people do not consider (again I said Most of the people :-)).

Give your feedbacks and comments. See you back in second part of this topic.

Thank you,

Jaywant Mahajan ([email protected])