QS138 90H continuous power at 48V?

[GijsW]

Hi everyone,

I was wondering if anybody could offer me some insight in the maximum continuous power at 48V for the QS138 90H.
The planned purpose is a boat's propulsion.
I'm looking for a motor which can supply 6-7 kW continuous from a 16S (51.2V) LiFePO4 pack (940Ah).
It would be in an enclosed space of max 45C (110F), with 2 fans mounted on top for cooling.
Do you think the regular QS138 90H could do it? Or should I move to a 165 or a water cooled 138?
Could also do a QS180, but then I'd have to make a custom adapter for a belt pulley.

I can't seem to find much info on continuous power levels, especially at 48V.

Thanks in advance!

 

[dominik h]

QS rates the motor 4 kW peak at 72V.
I do not think it will survive running 5 hours with 7kW at 48V, which needs much higher phase currents as with 72V and that produces more heat.

I would go for a watercooled motor. As it is a boat you will always have cold water to get rid of the heat in the motor.

This is not a bike with a small battery that will be empty before the motor is catching fire.

 

[GijsW]

Thanks for your reply!
They rate them as "Rated power: 4000W (max continue 7500W)", which is confusing.
Their own data shows almost 11kW in peak power, and I know there are people doing 25-30 kW bursts.

One guy claims it does 8kW continuous (tested on bench), but at a higher voltage
Is the efficiency at 48V so much lower than 72V? Didn't think it would be such a big effect, but you might be right.

I'm might just go with the 180 then.
Watercooling doesn't make a lot of sense I think after all. Watercooling with salt/dirty water is always a hassle.
For about 200 euros more than the QS138 watercooled, I got a QS180 air cooled (which has plenty power) + belt adapter.

I'd have to spend that amount of money on watercooling stuff at least, so bottom line it's about the same cost.
But no clogged filters to pay attention to, no frost proofing in winter, no pumps which can break...

 

[dominik h]

Sorry meant 4kw continious at 72V.
I know that they can run higher peaks.

You have a battery that can power the motor for 10hours at rated power.
Most of the bikes can power this motor for an hour or less.

I have a 10kW rated hub motor in my scooter with a 7,7kWh battery. During acceleration I let the controller give it up to 30kw. The motor windings reaches 130C after a 50km trip with 15km of running at 20kW. I can easily burn my motor if I would run it for 20 minutes with 20kW.
If a manufacturer says 4kW continious, that means that is the power the motor can run all day long.

If you reduce voltage by 33% then you have to raise current by 33% to get the same power.
In electric systems, temperature is calculated with I²xT [current² x time]
So the motor will produce nearly 80% more heat with your reduced voltage at the same output power level of 4kW.

QS 180 will be a better choice.
Also the QS165 with gearbox could be a good choice

 

[GijsW]

All right, I was under the impression the motor efficiency wouldn't drop that fast for a lower voltage.
I understand copper losses would increase by 80%, but that is only a part of the total losses (inductive losses for example would go down).

Also, QSmotors is notorious for under stating their rated powers, so I thought the 138 could maybe get to the 6-7 kW, especially since I'll have fans, and this guy stated it does 8kW continuous at a bench without overheating.

But since the 180 isn't much more expensive, I'll probably go that route.
Thanks!