Peak and continuous power

[2 self-build ecars later]

I am planning to convert my mid-sized tractor to electric. I will use six 12 volt lead acid batteries in series, to get 72 volts.
I have ordered a PMS 72vdc 7.5kW motor. I need an inverter which has not been easy to find. I have located one for a PMS motor with correct voltage, 10000W peak power but only 3800W continuous power.
I have done some research but no satisfactory answers so far.
Will that continuous power be sufficient? Should I find out from the supplier what continuous power rating the motor has?

 

[JackFlorey]

A Grin phaserunner should give you 3800W continuous and 11kw peak, assuming 72V. At those powers you will want to be VERY generous with a heatsink. Should run you around $350.

Is that enough? Depends how you use the tractor. If you have the gearing to get it to very low speeds at reasonable motor RPMs then it should be able to handle most jobs around a large property.

 

[neptronix]

35A batt x 72v = 2520w continuous ( given that you bolster the heatsink a massive amount )
Nowhere near enough, i think you're thinking the 90A phase amps = battery amps, but battery amps max is around 35A


How many horsepower did the original engine have?

 

[2 self-build ecars later]

35A batt x 72v = 2520w continuous ( given that you bolster the heatsink a massive amount )
Nowhere near enough, i think you're thinking the 90A phase amps = battery amps, but battery amps max is around 35A


How many horsepower did the original engine have?

Batteries are each 170Ah. Interested to know where you get 35A from? Gas engine was 28hp. Electric motor is 7.5kW equivalent to little over 10hp.
Given efficiency of electric motor, I would have thought 10hp motor would be enough, because the tractor will be used primarily for slashing grass in a paddock.

 

[bananu7]

170Ah means the batteries could theoretically provide a current of one ampere for 170 hours. (1 Amp-hour = 1 Amp * 1 hour).
35A max current rating means exactly that.

Ah and Wh are units of capacity (respectively in terms of just current and total energy stored), while maximum current rating is directly related to the maximum power you can draw from the battery in a given time window.

Those are two completely different, yet often mixed parameters.

 

[2 self-build ecars later]

Batteries are each 170Ah. Interested to know where you get 35A from? Gas engine was 28hp. Electric motor is 7.5kW equivalent to little over 10hp.
Given efficiency of electric motor, I would have thought 10hp motor would be enough, because the tractor will be used primarily for slashing grass in a paddock.

And I should have added run time is likely to be about an hour and 20 minutes at any time. Meaning about 9300W over that period, which should be enough to safely power the 7500W motor? Have I got that right?

 

[bananu7]

There are two things you have to fulfill: battery power (or current rating) and battery capacity. The capacity of 6x170Ah batteries is enough for your planned use, but you need to make sure their continuous current (power) rating is high enough to sustain such a load during discharge.

 

[AdR]

1hp=.746Kw
Rounding up: 28hp X .75Kw=21Kw in motor equivalent power.
21Kw÷72V=292 battery amps for continuous power.
So your battery need to be able to sustain 292 amps@72V minimum for however long you need continuous power

Note: Peak power is usually rated for less than 10 seconds of run time.
Continuous power is usually rated for upto 3 hrs of continuous run time.

 

[E-HP]

I am planning to convert my mid-sized tractor to electric. I will use six 12 volt lead acid batteries in series, to get 72 volts.

Batteries are each 170Ah.

And I should have added run time is likely to be about an hour and 20 minutes at any time.

Assumption: 6 - 12V lead acid batteries rated at 170Ah, connected in series. So 72V with 170Ah

Lead acid battery capacity rating is based on a 0.05C discharge rate (20 hour discharge). Discharging at a higher rate decreases capacity due to losses/heat. They can discharge at a much higher rate for very short periods (e.g. starting your car), but not continuously. You can't assume the 170Ah is available when discharging in an hour and 20 minutes. IF you used the entire 170Ah of energy in that time, it would be 0.75C or 15 times as fast as 0.05C. You may damage the batteries in the process.

I have ordered a PMS 72vdc 7.5kW motor. Should I find out from the supplier what continuous power rating the motor has?

Usually what is advertised is the continuous rating. Is there a reason you think that it may be something else?

At 72V, the 7.5kW motor requires 104A continuous. You may or may not be running it at that level, but if you did, you would be demanding 0.6C from the batteries. That's 61 times the 0.01C discharge used to determine the battery's capacity, so expect less run time.

Have you considered LiFePO4 batteries instead. It's the safe lithium alternative and capable of the discharge rates your rig will require.

Using lead acid, you won't need to worry about peak power, but continuous may not be a good idea, and may kill your batteries sooner that later.

Not sure what you mean by inverter. Maybe the controller? I'll wait for you to confirm that you are married to the lead acid battery idea before suggesting a controller.

 

[2 self-build ecars later]

Assumption: 6 - 12V lead acid batteries rated at 170Ah, connected in series. So 72V with 170Ah

Lead acid battery capacity rating is based on a 0.05C discharge rate (20 hour discharge). Discharging at a higher rate decreases capacity due to losses/heat. They can discharge at a much higher rate for very short periods (e.g. starting your car), but not continuously. You can't assume the 170Ah is available when discharging in an hour and 20 minutes. IF you used the entire 170Ah of energy in that time, it would be 0.75C or 15 times as fast as 0.05C. You may damage the batteries in the process.


Usually what is advertised is the continuous rating. Is there a reason you think that it may be something else?

At 72V, the 7.5kW motor requires 104A continuous. You may or may not be running it at that level, but if you did, you would be demanding 0.6C from the batteries. That's 61 times the 0.01C discharge used to determine the battery's capacity, so expect less run time.

Have you considered LiFePO4 batteries instead. It's the safe lithium alternative and capable of the discharge rates your rig will require.

Using lead acid, you won't need to worry about peak power, but continuous may not be a good idea, and may kill your batteries sooner that later.

Not sure what you mean by inverter. Maybe the controller? I'll wait for you to confirm that you are married to the lead acid battery idea before suggesting a controller.

Thanks. I already have the lead acid batteries. As for a controller, I'm hoping it will have internal inverter capacity, meaning a separate inverter will be unnecessary. The motor uses a reluctance sensor which seems to narrow the field of suitable controllers according to the suppliers I have contacted.

 

[bananu7]

Virtually all controllers used for the builds here for three phase BLDC motors have internal inverters. This is because they are meant to operate from a battery. VSCs meant for wall three-phase operation won't have them, as they're not needed in such case.

 

[2 self-build ecars later]

Virtually all controllers used for the builds here for three phase BLDC motors have internal inverters. This is because they are meant to operate from a battery. VSCs meant for wall three-phase operation won't have them, as they're not needed in such case.

Yes, good not having to source an inverter as well. But I've still drawn a blank so far with controller suppliers when I say my motor has a VR sensor.

 

[2 self-build ecars later]

Yes, good not having to source an inverter as well. But I've still drawn a blank so far with controller suppliers when I say my motor has a VR sensor.

Can anyone direct me please to a supplier who stocks a controller compatible with my 72vdc 7.5kW PMSM with a resolver (VR) sensor?

 

[2 self-build ecars later]

Can anyone direct me please to a supplier who stocks a controller compatible with my 72vdc 7.5kW PMSM with a resolver (VR) sensor?

E-HP, I am reconsidering battery type in light of the good points you make about lead acid.
I have found one said to be 72v 105Ah LifePO4 200A 14.72kW.
How does that sound for the usage I am proposing, ie about 1 hour 20 minute run time?

 

[j bjork]

Can anyone direct me please to a supplier who stocks a controller compatible with my 72vdc 7.5kW PMSM with a resolver (VR) sensor?

3shul and sevcon are the ones I come to think of. Maybe curtis too?

 

[2 self-build ecars later]

3shul and sevcon are the ones I come to think of. Maybe curtis too?

Thank you. I will check them out.

 

[E-HP]

E-HP, I am reconsidering battery type in light of the good points you make about lead acid.
I have found one said to be 72v 105Ah LifePO4 200A 14.72kW.
How does that sound for the usage I am proposing, ie about 1 hour 20 minute run time?

Might be barely enough. If you ran full throttle under load the whole time, 7.5kW @104A, you’d get an hour runtime, but I doubt that would be the case unless you are towing one thing big.most of the time you’ll probably use a lot less once it’s rolling. What kind of work does the tractor need to perform?

 

[2 self-build ecars later]

Might be barely enough. If you ran full throttle under load the whole time, 7.5kW @104A, you’d get an hour runtime, but I doubt that would be the case unless you are towing one thing big.most of the time you’ll probably use a lot less once it’s rolling. What kind of work does the tractor need to perform?

Only grass slashing, at slow speed, never full throttle. Most times 1.25 or 1 hour 20 minutes run time. LiFePO4 expensive here. Saw good price on 105Ah one but should I be looking for a higher Ah one to give me greater margin?