# Inertia of Brushless DC Motor

1. ## Inertia of Brushless DC Motor

Experts! My sincere apologies if this post is misplaced for this category. I didn't find any other appropriate one to post it in.

My understanding of Motors - AC or DC, Brushed or Brushless - is extremely limited. My query is related specifically to a Brushless DC Motor used in an Electric Vehicle.

As I understand, there is "self inertia" of a motor which determines the amount of minimum power to be supplied to the motor to get rotating, under a given load.

I am trying to find/compute this value/parameter, but am unable to. Is there anything in a motor's specification that can help me compute the same?

I was watching some videos to see how a BLDC motor is constructed and I could see that there's nothing else causing any friction other than the weight of the motor and/or probably the bearing of the axle. I might be completely wrong in my assumptions.

I need this to understand the minimum power requirements for the vehicle design. Hence any inputs/pointers on this would be greatly appreciated.

@moderator - please feel free to move this post to an appropriate category if this isn't the right one.

Regards,
Deepak

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2. ## Re: Inertia of Brushless DC Motor

Hi,

Inertia has nothing to do with continous power or starting power.
It only plays a role on accelerting/decelerating.

When there is no friction, then every (even very tiny) torque causes acceleration (and thus causes movement) even if the inertia is huge.

Inertia and friction are two independent things.

Klaus

3. ## Re: Inertia of Brushless DC Motor

Thanks @KlausST for clarification.

So to put it differently, how can I determine/compute probably from the specs or some other parameters, how much minimum current needs to be supplied to a motor for a given load? For the purpose of this discussion, the load on the motor is maximum rated load. In this case, how much current will be needed to get the motor in motion?

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4. ## Re: Inertia of Brushless DC Motor

Hi,

A BLDC datasheet should contain a value in A/Nm or Nm/A. This gives the relation between current and torque.
On maximum load the friction of the bearings should not matter much.
For sure there is some tolerance. I recommend to put at least 120% maybe 150% (just a guess) of current to safely get the motor into motion.
Do tests for this. At different temperatures.

Klaus

5. ## Re: Inertia of Brushless DC Motor

I could see that there's nothing else causing any friction other than the weight of the motor and/or probably the bearing of the axle. I might be completely wrong in my assumptions...
Consider a simple motor connected to a load. The load is the causative effect for friction but it is rarely described in this fashion.

An insufficient current through the motor will not cause any motion because the load is providing a static friction. In other words, the motor is producing torque but the load is not moving. You need to apply a minimum force before the load can move.

Once the load starts moving, the static friction disappears and dynamic friction comes into play. Usually, you need a greater toque to start but a slightly smaller torque will be sufficient to keep the load moving (dynamic friction is a different beast altogether).

For a vehicle, dynamic friction is caused by rolling friction at the bearings, air friction, friction at the contact surface etc.

Usually for a loaded motor, the inertia of the rotor is only a small part of the friction; a well made motor can start will a small current (the friction at the bearing is relatively small). But when you have to start the motor with load on, you need to apply far larger current to start.

Internal combustion engines (say a diesel engine) cannot start on load; we need to have a clutch for that. Electric motors do not need a clutch. But an electric motor need excess current to start on full load (say 20-50% extra) and the motor should be able to withstand the higher current for a shorter period.

AC motors (say motors you find in house fans) have poor starting torques; here the brushless motors excel. To start an AC motor on load, the motor must be overrated but then it will take less power when running normally. DC motors are best when you have lots of start and stops.

To compute the current, first determine the load and compare that with the rated power of the motor. Use 50-60% extra current to start but reduce that to 80-90% of the rated capacity once the motor picks up speed.

6. ## Re: Inertia of Brushless DC Motor

Thanks @KlausST and @c_mitra for clarifying. Things seem to be a little clearer now. I'll take it from here and continue with my design. I have to admit that this forum has helped me lot more than some of the other fora. I'm getting fonder of this forum. Hope I don't trouble you guys too much. Thanks again!

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7. ## Re: Inertia of Brushless DC Motor

The motor's greatest torque and best efficiency is at a certain speed. It is a combination of the right inertia and the right magnetic attraction.

The sooner you reach that optimum speed, the better it pulls the load (that is, more smoothly or with faster acceleration).

Thus your load should be light as you start out. The common way to do this is by gearing just like you start driving a car in first gear.

If you do not install gearing, and you must accelerate from a standstill under load, then you need to precisely manage how you energize coils (current and timing), so that the motor starts to turn slowly without stalling, and accelerates quickly.

8. ## Re: Inertia of Brushless DC Motor

Break down the problem to basic mechanical equations. In a first step consider vehicle mass, gear ratio, motor constants translating voltage to speed and current to torque. Friction can be probably neglected in a first order. Additionally put in aerodynamic drag for higher vehicle speed and weight force if you drive up-hill. In a more accurate calculation consider rotor resistance and gear efficiency.

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