Why Electric motors are inefficient at low revs?

Hello!

Everywhere efficiency is elaborated they always mention that electric motors have low efficiency at low revolutions or power. For example Tesla announced that they added a second motor to the model S which improved it's efficiency.
I was wondering what are the root causes of this apparently general inefficiency. Is it the inverters or other driver electronics? Or the motors themselves?

Here are two examples:

http://www.yasamotors.com/wp-content/uploads/2014/07/Datasheet-YASA-750_en-ID-15637.pdf


http://www.neweagle.net/support/wiki/docs/Datasheets/UQM/PP150.pdf


The YASA motor is less than 60% efficient under 250 RPM compared to 92%+ near its peak.

The efficiency which a PMSM motor produces a given amount of torque at, doesn't really vary that much, until the parasitic losses start to dominate...

Lower RPM means lower voltage. Lower voltage means higher amperage. Higher amperage means more heat loss, as well as higher resistance. Higher resistance means more power to push through. And your electric bike embraces the vicious cycle.

Here's a simple thought experiment that skips over some stuff but gives the overall picture.

Imagine that, starting from zero RPM, you apply a given current to the motor windings and keep that current constant as the motor accelerates. Roughly speaking, a constant current gives you a constant torque, and it also gives you a constant electrical loss due to the resistance of the windings.

Also keep in mind that mechanical power is proportional to torque times RPM.

So at zero RPM, mechanical power is zero, regardless of how much torque you have. Meanwhile you have electrical loss proportional to the square of current. So efficiency is zero.

Now the motor accelerates. Since you're holding the current constant, the torque remains the same, as does the electrical loss. But since RPM is no longer zero, you now have some mechanical power. But you're starting from zero, so power is still low while loss is constant, so efficiency is still low.

Keep accelerating, and the power increases with RPMs while the loss remains constant, so efficiency keeps getting better.

Since you always have the same nonzero electrical loss, you can never get to 100% efficiency, even in this simple-minded model. In reality, mechanical friction and magnetic eddy current losses increase at higher RPMs, so this is why efficiency drops after reaching a peak rather than just plateauing at high RPMs.

Dauntless said:

Lower RPM means lower voltage. Lower voltage means higher amperage. Higher amperage means more heat loss, as well as higher resistance. Higher resistance means more power to push through. And your electric bike embraces the vicious cycle.

Click to expand...

Ummm, I think lower voltage means proportionally lower current.

cycborg said:

Here's a simple thought experiment that skips over some stuff but gives the overall picture.

Imagine that, starting from zero RPM, you apply a given current to the motor windings and keep that current constant as the motor accelerates. Roughly speaking, a constant current gives you a constant torque, and it also gives you a constant electrical loss due to the resistance of the windings.

Also keep in mind that mechanical power is proportional to torque times RPM.

So at zero RPM, mechanical power is zero, regardless of how much torque you have. Meanwhile you have electrical loss proportional to the square of current. So efficiency is zero.

Now the motor accelerates. Since you're holding the current constant, the torque remains the same, as does the electrical loss. But since RPM is no longer zero, you now have some mechanical power. But you're starting from zero, so power is still low while loss is constant, so efficiency is still low.

Keep accelerating, and the power increases with RPMs while the loss remains constant, so efficiency keeps getting better.

Since you always have the same nonzero electrical loss, you can never get to 100% efficiency, even in this simple-minded model. In reality, mechanical friction and magnetic eddy current losses increase at higher RPMs, so this is why efficiency drops after reaching a peak rather than just plateauing at high RPMs.

Click to expand...

Except that on the diagram the efficiency was much less dependant on the torque(so current) than RPM. If you look at 500 RPM on the YASA image and you compare the efficiency at 400 and 100 Nm they're almost the same, with four times the current it should be much less by logic. The little drifference might be what you described, but there's something a lot more significant it seems.

avada said:

Dauntless said:

Lower RPM means lower voltage. Lower voltage means higher amperage. Higher amperage means more heat loss, as well as higher resistance. Higher resistance means more power to push through. And your electric bike embraces the vicious cycle.

Click to expand...

Ummm, I think lower voltage means proportionally lower current.

Click to expand...

A motor isn't just a resistor, it's a resistor in series with a generator. When the motor is spinning, it generates "back EMF" which is a voltage that opposes the applied voltage. The back EMF is the voltage that Dauntless is talking about, not the applied voltage. At zero RPM the back EMF is zero, so the windings see all of the applied voltage, with correspondingly high current. At higher RPM the back EMF increases, so if the applied voltage is the same, the voltage across the windings is smaller, so less current.

Keeping it simple...

Copper losses (resistance losses) are the penalty for torque.

Parasitic losses (iron losses etc.) are the penalty for speed (angular velocity).

The point of maximum efficiency is where the copper losses are at parity with the parasitic losses.

Mechanical power transfer only can occur if you provide motion.

Take the world's most efficient motor, stall the rotor, you now have a way to input current and torque against the stalled rotor, but since it's not moving your power output is Zero.

Motors actually only increase is the amount of losses as they increase in RPM. This is a rule for all motors.

Why one sees efficiency climb as the motors losses increase is due to the core losses beginning to balance the copper loss for making the torque. (As Miles more elegantly said above).

Look at your drive train from a loss perspective and the shape of the efficiency curve will stop seeming so mysterious.

Miles said:

Keeping it simple...

Copper losses (resistance losses) are the penalty for torque.

Parasitic losses (iron losses etc.) are the penalty for speed (angular velocity).

The point of maximum efficiency is where the copper losses are at parity with the parasitic losses.

Click to expand...

Okay. So a little bit more simplification is required. Does this mean that the losses in the windings are inherent to the inductor coils and are insurmountable? Meaning is it impossible to create a motor that's much closer to its peak efficiency at low revolutions? (Obvioiusly excluding superconductors and such )

You could design a motor to be efficient at low revs. So what you're talking about is low speed efficiency of motors designed for high speed power output. I think the primary factor is the winding resistive losses. Motor coils which were designed for the high voltage at high speed are now used for low speed so have a turns count which require low voltage. Depending on the load, currents will be the same as high speed. So the voltage drop in the coil is a higher percentage of the applied voltage at low RPM vs high RPM.

You'll also note that efficiency suffers at low loads. Both low speed and low load operation occur at low power so low efficiency isn't a nearly the concern it is at high power. There are certain motor types and/or control types which can mitigate this effect to some degree.

Ten efficiency points at 700W output is 70W and means less than 1% of a 150kW output (1500W). The motor designer is unlikely to sacrifice any high power performance to bolster the low power point unless it was criteria established from the start. With the motors you're viewing, I doubt this was the case.

You also mention the inverters. And yes. Devices which have a loss component which is fixed with respect to load or frequency will make the inverter less efficient at low load or low speed. You'll see this with the switching devices.

Major makes a good point. It's all in the geometry. In the extremely high efficiency direct drive wind turbine designs, they are targeting something like 25RPM peak motor speed, yet they have some pretty awesome efficiency like ~97% IIRC.

However, they are still 0% efficiency at 0rpm. The geometry, may enable them to make 10,000ft-lbs of torque from 1w of input power, but with no rotation that's still 0% efficiency, even if no other system in the world could do a better job at using as little of energy as possible to provide any amount of force, even of you make motor geometry to make 1,000,000ft-lbs if torque from only 1nW (nano-watt), it's still called 0% efficiency if it's only providing a torque with no motion. I would call 1,000,000, ft of torque for 1nW a fantastic solution if I needed to control 1,000,000ft-lbs of torque with ultra low power for some reason, yet it would still be 0% efficient on a motor chart at stall/low speed.

You can think of it as a trait inherent to the nature of efficiency in drive. For example, even when a dragsters engine is at 10,000rpm and he drops the clutch, all the torque transfer of all that momentum and torque production is dumping into a stationary wheel and vehicle initially, and hence it's also a 0% efficiency system until the moment drivetrain gains motion. At that moment of motion, both gas or electric have an efficiency just a whisker above 0%. As the clutch starts out slipping from a stop in a dragster, it begins ramping up its efficiency from 0% just like the electric motor. However, it can't escape that whatever the change in RPM across the clutch * torque being transferred is all loss. It doesn't stop being the dominate engine power consumer until sometime like half way down the track in the fastest dragsters.

For example, if you took any of the motors from your graph, and spun them to the peak efficiency RPM, and then slipped a clutch to engage your stationary wheel, you still have 0% drivetrain efficiency, but now your also wasting the bulk of your HP at that moment into heating the clutch friction material and not moving you forward. Simply applying the current to the stalled rotor is very often only a tiny power consumption.

major said:

So what you're talking about is low speed efficiency of motors designed for high speed power output.

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More like a motor that needs fairly low/high power and RPM, where you need to design it for a broad range. Like one for an average car. You can go highways speeds at 130 Km/h. or you can drive around in a tight urban location with 20-40 km/h (or crawl in a traffic jam). Both ends are significant. Most probably drive less on highways but it drains a lot of power comparatively, while driving at low speeds you loose because you do it more. So doesn't matter much where's the peak if efficiency drops a lot if you're not very near it.
Well, it looks like we need superconductors/perfect conductors.

I did some simulations to give an idea of the relationship between speed, peak eta and power out at peak eta.

[pre]RPM Peak Eta Power Out

15,000 96.95% 19,540 W
12,000 96.78% 14,400 W
10,000 96.72% 10,010 W
8,000 96.65% 6,313 W
6,000 96.54% 3,837 W
5,000 96.43% 2,852 W
4,000 96.27% 1,946 W
3,500 96.14% 1,504 W
3,000 95.97% 1,118 W
2,500 95.71% 824 W
2,000 95.36% 585 W
1,500 94.89% 387 W
1,000 94.13% 200 W
750 93.52% 117 W
500 92.45% 56.5 W
250 89.90% 17.3 W
150 87.53% 7.8 W[/pre]

So, not much variation of efficiency with speed there.....

Miles said:

I did some simulations to give an idea of the relationship between speed, peak eta and power out at peak eta.

[pre]RPM Peak Eta Power Out

15,000 96.95% 19,540 W
12,000 96.78% 14,400 W
10,000 96.72% 10,010 W
8,000 96.65% 6,313 W
6,000 96.54% 3,837 W
5,000 96.43% 2,852 W
4,000 96.27% 1,946 W
3,500 96.14% 1,504 W
3,000 95.97% 1,118 W
2,500 95.71% 824 W
2,000 95.36% 585 W
1,500 94.89% 387 W
1,000 94.13% 200 W
750 93.52% 117 W
500 92.45% 56.5 W
250 89.90% 17.3 W[/pre]

So, not much variation of efficiency with speed there.....

Click to expand...

Also, if you look at the Watts loss for each RPM value it will range from 596W at 15k to 1.75W at 250RPM. The actual power loss is insignificant at the lower speed even though the efficiency appears to take a big hit. You would have to travel great distance at ridiculously low speed in an EV to justify paying attention to low speed efficiency (design-wise), IMO.

So the answer to the question in the thread title is "they're not"

Another educational and interesting thread

Here's the last increment:

[pre]150rpm 87.53% 7.8 W[/pre]

So, we have peak eta over a hundredfold speed range.

major said:

Also, if you look at the Watts loss for each RPM value it will range from 596W at 15k to 1.75W at 250RPM. The actual power loss is insignificant at the lower speed even though the efficiency appears to take a big hit. You would have to travel great distance at ridiculously low speed in an EV to justify paying attention to low speed efficiency (design-wise), IMO.

Click to expand...

I disagree. If you put something like the YASA 750 in a car with a direct drive configuration. (Which they did for the E4) you get like 70-78% efficiency at 500 RPM which mean about 60 km/h which is actually more than the typical speed limit within a city. But if you live in a city like most do, you wont be even going 50km/h half the time because all the starting and stopping. If yo go with thirty the efficiency will be 60% or less. Yes highway speeds are a power hog. But lots of people scarcely go on highways.

Punx0r said:

So the answer to the question in the thread title is "they're not"

Another educational and interesting thread

Click to expand...

No one ever said. As a matter of fact everybody said the opposite.

Yes Luke points it out and tries to hamer it home.
Its this simple. HP (KW) IN VS HP(KW) out!!! At 0 rpm you will always have zero HP (0kw) but you are putting some power in but because nothing is moving even though there is torque there is no motion making HP(kW) 0!!! All the power you put in is tuning into heat!

avada said:

major said:

Also, if you look at the Watts loss for each RPM value it will range from 596W at 15k to 1.75W at 250RPM. The actual power loss is insignificant at the lower speed even though the efficiency appears to take a big hit. You would have to travel great distance at ridiculously low speed in an EV to justify paying attention to low speed efficiency (design-wise), IMO.

Click to expand...

I disagree. If you put something like the YASA 750 in a car with a direct drive configuration. (Which they did for the E4) you get like 70-78% efficiency at 500 RPM which mean about 60 km/h which is actually more than the typical speed limit within a city. But if you live in a city like most do, you wont be even going 50km/h half the time because all the starting and stopping. If yo go with thirty the efficiency will be 60% or less. Yes highway speeds are a power hog. But lots of people scarcely go on highways.

Click to expand...

If that is the case, choose a better motor for your EV.

It's not that they are inefficient at low rpm. It's that they're inefficient during the early stages of acceleration, since as pointed out above they begin at 0% efficiency at 0rpm. In real world use it's difficult to ride very slowly at a steady state rpm, especially in bumpy conditions, so you end up with lots of small accelerations that drive the overall efficiency way down. To make matters worse while riding very slowly, the controller is trying to imitate a very low voltage, so the width of the ON pulses of PWM are very short and phase currents are much higher than they would be if the battery voltage was low enough to drive the motor naturally at that rpm. Those spiking phase currents cause increased losses in both the controller and motor.

Would it matter if the motor was rewound for a much lower rpm?

Like taking a 100rpm per volt motor and rewinding to 3rpm per volt. Assuming the copper fill was the same would the motor be as efficient if they were both fed 100V with one hitting 10,000rpm vs the other only 300?

Bluefang said:

Would it matter if the motor was rewound for a much lower rpm?

Click to expand...

No. Not if the only difference was the no. of winding turns vs the voltage.

What fails to be truly understood or easily explained in the power /force /work equations is where electric motors excel with torque production at the low RPMs. The work equations suggest that when I stand still holding a 100 pound bag of cement at my belly, I am doing no work. This is not at all true as I am dying to maintain the forces to keep this mass from falling back to earth in just a few moments. Yes it is not work as defined as in pounds lifted in additional feet of elevation, but much force to maintain muscular tensions and skeletal stability against the acceleration of gravity.

It is this type of force, torque that matters most when stalling a vehicles motor/motion. The case where the 125cc ice moto is stuck in the mud hole up to the axle, and you have lots of power on tap, but still can not get it to break loose from the grip of the mud unless you dump the clutch at 14000 rpms and burn it up a bit. The electric version with half the power and twice the low speed torque climbs out as if it did not see the problem. Same with starting on a steep hill. Torque matters, and key to what is needed for changes in speed. This is missed many times in discussions when talking about power and efficiency, but a critical component to paint a fuller picture and not be somewhat under defined as it would relate to motion. With respect to these other critical forces, I am not certain that the electric motor efficiency numbers truly take a hit in real life terms at low RPM's.

speedmd said:

What fails to be truly understood or easily explained in the power /force /work equations is where electric motors excel with torque production at the low RPMs. The work equations suggest that when I stand still holding a 100 pound bag of cement at my belly, I am doing no work. This is not at all true as I am dying to maintain the forces to keep this mass from falling back to earth in just a few moments. Yes it is not work as defined as in pounds lifted in additional feet of elevation, but much force to maintain muscular tensions and skeletal stability against the acceleration of gravity.

Click to expand...

You make a good point.

The low efficiency is in how the dyno measures the HP or KW output of the motor.
I think as long as you understand this its a mute point but there is something to be said for 100ft/lbs at 0 rpm its is work in some cases and in some cases it is not work So some cases it is not 0% efficient and some cases it is 0%.

IE a machine designed to clamp and hold something the arm holding something is doing its work by squeezing so its efficiently doing its job.
But a bike you plan to use for transportation is applying 100 ft/lbs to the wheel with out moving the wheel it is 0% efficient, but if its job is to just hold you up an incline on the road and its applying 100 ftlbs to hold you at the spot without moving then it is doing work and is not 0% efficient.