Can someone explain how a 350W motor can be limited to 250W?

[barclay]

I'm still plumped on the sofa, pregnant and trying to digest turkey, goose and too much Christmas pud amongst other things. I've seen a Speedict video which shows a Bosch 250W motor outputting 600W and am confused about what all this means?

Can some explain to a numpty?

Cheers,

Fat Barclay.

 

[Geebee]

The rated wattage as in 250w is just a continuous rating the manufacturer states to make the bike legal in the UK, in reality most 250w motors will draw anywhere up to 1kw.
My Tonaro is a legal 250w bike but will actually take over 600w if required.

That is one of the reasons for the disparity in climbing ability of various bikes even though they maybe all rated at 250 watts, in reality their wattage will vary greatly.

 

[amigafan2003]

Speedict shows peak amp draw @ the battery, but the motor is rated at 250w continuous*.

*The continuous current rating of the motor is determined as the maximum amount of power (Power = I2*RL) the motor can continuously dissipate without exceeding its temperature rating.

 

[Geebee]

From the motors I have played with and seen the rating is pulled out of fresh air to match the legal requirements.

 

[morphix]

The rated wattage as in 250w is just a continuous rating the manufacturer states to make the bike legal in the UK, in reality most 250w motors will draw anywhere up to 1kw.
My Tonaro is a legal 250w bike but will actually take over 600w if required.

That is one of the reasons for the disparity in climbing ability of various bikes even though they maybe all rated at 250 watts, in reality their wattage will vary greatly.

I've tried to understand this too. It is really the controller which determines the legality of the motor power output? You see controllers rated at 250W and I assume that means the controller is configured to limit power output to the motor as well as speed? Yet it's common knowledge as stated here motors (even those sold as 250-350W wired to 250W controllers) can be measured at much higher watts when under extreme loads, for instance when climbing a hill?

Can someone give an explanation in layman terms of how this technically works? Eg: is it a case of the controller is designed to output < 250W under a nominal load/normal riding conditions.. but when those conditions change and the motor requires more energy to maintain the legal 15.5mph speed, it just draws more energy, and so for that moment, the motor is technically no longer operating to legal specification? Or is there no restriction as such, and it's simply a case that to maintain 15.5mph on a reasonably flat surface doesn't normally require more than 250W anyway?

Regardless of how or why a motor might exceed the 250W power, it's surely how it operates MOST of the time under nominal power/normal riding conditions, that determines legal compliance?

 

[Jeremy]

First off, the motor continuous rated power is nothing like the same as the actual power that the motor can deliver, and depends almost entirely on the way that the motor power test has been defined. Maximum acceptable motor internal temperature is the limiting factor, and this is very dependent on the conditions under which the test is conducted. For example, changing the airflow over the motor (to simulate being on a moving bike, rather than a test bench) will significantly change the test result, as will changing the temperature of the air around the motor whilst it is being tested, as both of these factors determine how well the outside of the motor can lose heat. The chosen maximum power rpm and torque also makes a massive difference - a small motor may well be able to deliver far more power continuously at high rpm than it would if loaded to a lower rpm, so the test procedure really determines the rating more than anything else.

In practice this means that the motor rated power is a fairly meaningless number unless it is accompanied with a well-defined test procedure that tells you all the test conditions from which the power figure was derived.

The other big factor is that the motor doesn't ever determine the rated power of the bike in practice. All ebike motors can deliver far more power than the UK or EU legal limit and it is the motor controller that sets the power level. Permanent magnet motors are power hogs and will draw as much current as they can when heavily loaded, such as when climbing a steep hill. They will quite happily draw far more current than is safe and will burn themselves out, unless the controller intervenes to limit the current (and hence power) to a safe level. Much of the time, at low speed, high load, the motor current will be substantially greater than the battery current (perhaps two to three times greater), making determining the current that's actually causing the motor to warm up internally difficult to estimate and near-impossible to measure.

Motor power can also be confusing in that the power limits in the regulations (both the UK 200 W and EU 250 W figures) are the motor mechanical power output in watts, that is the product of motor shaft torque and rpm. The electrical power input (also in watts) will always be higher, in fact at the motor maximum power point it will be double the mechanical power output (as motor maximum power is at the 50% efficiency point for a PM motor, although this may well be beyond the ability of the motor to deliver for more than a short time).

Therefore it is quite reasonable to expect to see a 500 W peak electrical power input for a small motor that can only deliver a 250 W peak mechanical power output. In practice even the smallest hub motors seem capable of delivering around 350 W continuous mechanical output power at typical UK air temperatures, so peaks of 700 W for even a small hub motor like the Q100 are reasonable and much greater peak figures are often seen on bigger motors.

In practice, many EU ebikes (the 250 W ones) deliver far more than 250 W, because one of the commonly used test methods in the EU regulations tests the motor power using an indirect measurement. Ebike acceleration time over 20 metres is measured and a calculation used to estimate motor output power. The method allows a motor power output of 500 to 600 W to be measured as 250 W if the controller power application rate is suitably adjusted, meaning that many ebikes that are defined as 250 W under the EU regulations really have a much greater motor output power.

 

[jackhandy]

I was following this fine, until "motor current can be up to twice the battery current"

Could you explain that gently, please jeremy?

The rest makes sense, even to me

 

[morphix]

I was following this fine, until "motor current can be up to twice the battery current"

I'm puzzled by that too. That reads as though the current output from the motor increases when the motor is operating and can be greater than the current input? If so, is that due to the physics/electrical effect of the motor generating energy, like a generator does?

 

[amigafan2003]

I was following this fine, until

If you draw 600watts from the battery, @ 75% efficiency the motor will only be providing 450watts of motion, the other 150watts is wasted as heat. At low rpms, high load efficiency can drop off to 50% - hence "motor current can be up to twice the battery current".

Another thing to consider is phase current.

Phase current can be variable on some contollers. For example I'm running phase @ 1.5x battery current. Stock controllers seem set at 2.5x battery current. Some like running upto 3.5x.

 

[morphix]

If you draw 600watts from the battery, @ 75% efficiency the motor will only be providing 450watts of motion, the other 150watts is wasted as heat. At low rpms, high load efficiency can drop off to 50% - hence "motor current can be up to twice the battery current".

Another thing to consider is phase current.

Phase current can be variable on some contollers. For example I'm running phase @ 1.5x battery current. Stock controllers seem set at 2.5x battery current. Some like running upto 3.5x.

*morphix looks at Jackhandy with a shared glazed expression*

 

[Jeremy]

I was following this fine, until "motor current can be up to twice the battery current"

Could you explain that gently, please jeremy?

The rest makes sense, even to me

I'll try..............

The controller essentially works as what's called a switched mode voltage converter, in this case a "buck converter", meaning that the average output voltage is usually lower than the input voltage.

The input voltage is pretty constant, it's the voltage from the battery. The controller uses the inductance of the motor as an energy store and by switching the input voltage on and off quickly can effectively convert voltage to current. It works a bit like a very efficient variable transformer, where a high voltage at a low current can be converted into a lower voltage at a higher current.

Because energy is pretty much totally conserved across the controller, if, say, the input voltage is 36V and the input current is 10A, giving an input power of 360W, then the controller output power will also be about 360 W (less some very small losses within the controller). So, if the motor is running at half its maximum speed at this power, meaning that the motor effective voltage is 18V (36v / 2) then the motor current has to be 360W / 18V = 20A. If the motor is loaded right down, to say 30% of it's maximum speed, with the same 360W of power into the controller, then the motor current would rise further to 360W / (36V x 30%) = 33.3A

This is one reason why motors heat up a lot more when under a high torque load, such as when climbing hills. As the motor slows down, the controller current limit kicks in to limit battery current, but the motor current continues to increase the slower the motor runs. You can quite easily get two or three times the battery current flowing in the motor under these conditions. Because the heat dissipated inside the motor is proportional to the square of the motor current (not the battery current) this means that the motor can get hot pretty quickly when heavily loaded, even though the battery current seems to show that all should be OK.

 

[mountainsport]

I was following this fine, until "motor current can be up to twice the battery current"

Could you explain that gently, please jeremy?

The rest makes sense, even to me

I just got up to "First off"

Mountainsport.

 

[mountainsport]

I'll try..............

The controller essentially works as what's called a switched mode voltage converter, in this case a "buck converter", meaning that the average output voltage is usually lower than the input voltage.

The input voltage is pretty constant, it's the voltage from the battery. The controller uses the inductance of the motor as an energy store and by switching the input voltage on and off quickly can effectively convert voltage to current. It works a bit like a very efficient variable transformer, where a high voltage at a low current can be converted into a lower voltage at a higher current.

Because energy is pretty much totally conserved across the controller, if, say, the input voltage is 36V and the input current is 10A, giving an input power of 360W, then the controller output power will also be about 360 W (less some very small losses within the controller). So, if the motor is running at half its maximum speed at this power, meaning that the motor effective voltage is 18V (36v / 2) then the motor current has to be 360W / 18V = 20A. If the motor is loaded right down, to say 30% of it's maximum speed, with the same 360W of power into the controller, then the motor current would rise further to 360W / (36V x 30%) = 33.3A

This is one reason why motors heat up a lot more when under a high torque load, such as when climbing hills. As the motor slows down, the controller current limit kicks in to limit battery current, but the motor current continues to increase the slower the motor runs. You can quite easily get two or three times the battery current flowing in the motor under these conditions. Because the heat dissipated inside the motor is proportional to the square of the motor current (not the battery current) this means that the motor can get hot pretty quickly when heavily loaded, even though the battery current seems to show that all should be OK.

Hi jeremy,

In other words we can't never win here in terms of efficiency,basically when a larger motor is under extreme loads,does this mean that the quicker the heat dissipates from the motor will make this more safer and efficient,(performs better during the winter longer to overheat),at the same the battery is at it's lower efficiency point,(due to cold weather). What is then the case during the summer period,when at his time the battery is now at it's highest performance rate and the motor is much slower to dissipates it's heat

I know these three valuable components, (BATTERY,CONTROLLER AND MOTOR),all do work in sequence,but under varaible condition which of the three components in your opinion tops the efficiency ranking rate?

Mountainsport. I hope that you all EAT and DRANK well xxxxxxxxxxxxxxxxxxx

 

[jackhandy]

Oooer!

So things can get somewhat more than tepid in the motor, without showing anything amiss at the battery meter....?

Would a meter connected at the motor then show a lower voltage than one at the battery, in this situation, or have I only got hold halfway up the stick? Meaning a brushed motor, as I know you can't read voltage accross field wires - I think..

Thanks for your patience...

 

[Jeremy]

Hi jeremy,

In other words we can't never win here in terms of efficiency,basically when a larger motor is under extreme loads,does this mean that the quicker the heat dissipates from the motor will make this more safer and efficient,(performs better during the winter longer to overheat),at the same the battery is at it's lower efficiency point,(due to cold weather). What is then the case during the summer period,when at his time the battery is now at it's highest performance rate and the motor is much slower to dissipates it's heat

Getting best efficiency isn't that easy, particularly with a hub motor and its fixed gear ratio. The biggest single factor is the motor current, the lower this is the better the efficiency. There are two ways to lower the motor current:

1) Use a motor that needs a higher voltage to run at the speed you need. This means that for a given power the motor current will be lower, so giving better efficiency. Hub motors are available in a wide range of rpm per volt configurations, so picking the right one for the wheel size you have and the speed you want, with the highest practical battery voltage you can use helps.

2) Alter the gear ratio between the motor and the wheel to allow the motor to turn faster. Because power is rpm x torque, you can get a higher power by keeping the motor torque low and increasing the motor rpm. Within limits this keeps the motor current low, so improves efficiency, especially if you have variable gearing to allow the motor to spin within its most efficient range no matter what the bike speed.

Certainly keeping the motor cool improves efficiency, as the resistance in the motor windings increases quite a bit with temperature. The lower the motor resistance the more efficiently it will run.

For reliability is also helps to keep the motor as cool as possible, as the magnets and the Hall sensors are both susceptible to damage from high temperature. Generally the magnets or Hall sensors will fail before the windings themselves suffer enough damage to the wire insulation to fail.

I know these three valuable components, (BATTERY,CONTROLLER AND MOTOR),all do work in sequence,but under varaible condition which of the three components in your opinion tops the efficiency ranking rate?

Mountainsport. I hope that you all EAT and DRANK well xxxxxxxxxxxxxxxxxxx

The motor has the greatest losses, usually by a fair bit. Even the best ebike motor will struggle to peak at better than 90% efficiency and most are around 80 to 85% as a rule. Heavily or lightly loaded the motor efficiency will be a lot lower than this figure.

The controller efficiency is usually pretty good, typically around 98 to 99% if the controller is running within its design current limit. Modding controllers for greater current, without changing the internal components for ones with lower losses, will make this a bit worse, sometimes by enough to cause overheating damage.

Battery efficiency should be pretty high, typically 98 to 99% or better. The exception is some of the batteries available that use low maximum discharge rate cells intended for use in laptops. These can have a high internal resistance and cause a substantial drop in efficiency. For example, a 10 Ah, 36V nominal pack made from low internal resistance cells could be as much as 99.5% efficient during a 10A discharge, the same pack made from high internal resistance cells would only be around 92% efficient during a 10A discharge.

So, the two areas likely to give the most benefit when looking for better efficiency (which translates to better range and potentially also improved reliability) are really the motor and battery pack. Picking the motor that is the best match to the speed you want, together with the sort of terrain you normally ride, will make the biggest difference, followed by picking the battery that will best fit the bike for the performance you need and has the lowest internal resistance.

It's always a compromise though, as the best motor might be too big and heavy for your needs and so might the best battery pack.

 

[Jeremy]

Oooer!

So things can get somewhat more than tepid in the motor, without showing anything amiss at the battery meter....?

Would a meter connected at the motor then show a lower voltage than one at the battery, in this situation, or have I only got hold halfway up the stick? Meaning a brushed motor, as I know you can't read voltage accross field wires - I think..

Thanks for your patience...

Yes, the motor voltage is almost always lower than the battery voltage. The motor voltage will be approximately the same as the throttle setting, unless the controller current limit is operating. Generally the controller will be limiting the current whenever the bike is accelerating or going up a hill, and under those conditions the motor voltage will be a fair bit lower than the throttle position would indicate.

50% throttle with the bike running gently along on level ground means that the motor is probably running at around 50% of the battery voltage and around twice the battery current.

Measuring motor phase current is tricky, as you need to measure the current in at least two of the three motor wires and do some calculations to derive the true current. With a brushed motor you can, in theory, just measure the motor current, but because it won't be DC, but a rather complex waveform with a large DC component and ordinary ammeter won't give a true value, either.

 

[morphix]

Certainly keeping the motor cool improves efficiency, as the resistance in the motor windings increases quite a bit with temperature. The lower the motor resistance the more efficiently it will run.

For reliability is also helps to keep the motor as cool as possible, as the magnets and the Hall sensors are both susceptible to damage from high temperature. Generally the magnets or Hall sensors will fail before the windings themselves suffer enough damage to the wire insulation to fail.
.

Is this why the Speedict has a temperature sensor and gives temp readouts? I still haven't figured how that works or how/where it's fitted yet? Or is that sensor to go on the battery?

 

[Jeremy]

Is this why the Speedict has a temperature sensor and gives temp readouts? I still haven't figured how that works or how/where it's fitted yet? Or is that sensor to go on the battery?

TBH I'm not up on the Speedict at all, as I don't possess a mobile phone, let alone one that would run this software. My guess is that it's intended to monitor motor temperature, as several hub motors now have a motor temperature sensor, or the wiring internally to allow one to be fitted.

If the battery has a low internal resistance, then there's no need for temperature monitoring, as the temperature rise during even high current discharge will be very tiny.

 

[Deleted member 4366]

Is this why the Speedict has a temperature sensor and gives temp readouts? I still haven't figured how that works or how/where it's fitted yet? Or is that sensor to go on the battery?

You can fit the temperature sensor wherever you want. The controllers often have their own temperature protection, so that leaves the motor and battery, but it's not easy to get a temperature sensor into a hub-motor without difficult dismantling and wiring. Of course the temperature sensor isn't an option if you want to use the PAS and speed sensors, which are potentially more useful.

 

[mountainsport]

I'll try..............

The controller essentially works as what's called a switched mode voltage converter, in this case a "buck converter", meaning that the average output voltage is usually lower than the input voltage.

The input voltage is pretty constant, it's the voltage from the battery. The controller uses the inductance of the motor as an energy store and by switching the input voltage on and off quickly can effectively convert voltage to current. It works a bit like a very efficient variable transformer, where a high voltage at a low current can be converted into a lower voltage at a higher current.

Because energy is pretty much totally conserved across the controller, if, say, the input voltage is 36V and the input current is 10A, giving an input power of 360W, then the controller output power will also be about 360 W (less some very small losses within the controller). So, if the motor is running at half its maximum speed at this power, meaning that the motor effective voltage is 18V (36v / 2) then the motor current has to be 360W / 18V = 20A. If the motor is loaded right down, to say 30% of it's maximum speed, with the same 360W of power into the controller, then the motor current would rise further to 360W / (36V x 30%) = 33.3A

This is one reason why motors heat up a lot more when under a high torque load, such as when climbing hills. As the motor slows down, the controller current limit kicks in to limit battery current, but the motor current continues to increase the slower the motor runs. You can quite easily get two or three times the battery current flowing in the motor under these conditions. Because the heat dissipated inside the motor is proportional to the square of the motor current (not the battery current) this means that the motor can get hot pretty quickly when heavily loaded, even though the battery current seems to show that all should be OK.

Thanks Jeremy for that valid information,i can not agree anymore with you about the size and weight of any particular motor which is a very important factor when it measures up to a high efficiency rating.

On youtube there is a bloke by the name of scott from High Powered Cycles,show casing one of his bikes called Black Lightning with a 500w motor which pushes up to 38-40 mph therefore much smaller less weight more efficient.

So yes Jeremy size does really matter or does it really?

Mountainsport. Thanks for your time again xxxxxxxxxxxxxxxxxxxxxxxxxxx