How much can a motor draw?

I recently built my first ebike from a kit. The motor (Voilamart 1k rear hub kit) is listed as 1k watts on high and 750 on low. I installed a power monitor (Volts, Amps, Watts. WH). Watching the numbers as I ride I have seen readings over 1.2k watts while riding, even on the low power setting it surges well over 1k on first accelerating. At cruise the numbers are much more rational. Maybe 250w at 18mph, around 1.1k at 30mph.
I was under the assumption that the wattage rated was peak but apparently it is not. Is this power usage normal?
48v/20Ah Li battery.
1993 Trek 950 host.

BTW, if anyone is tracking power use I am averaging 33w/mile at full throttle (+/- 30mph) and about 18w/mile at 17/18mph.

WaltherDawg said:

I recently built my first ebike from a kit. The motor (Voilamart 1k rear hub kit) is listed as 1k watts on high and 750 on low. I installed a power monitor (Volts, Amps, Watts. WH). Watching the numbers as I ride I have seen readings over 1.2k watts while riding, even on the low power setting it surges well over 1k on first accelerating. At cruise the numbers are much more rational. Maybe 250w at 18mph, around 1.1k at 30mph.
I was under the assumption that the wattage rated was peak but apparently it is not. Is this power usage normal?
48v/20Ah Li battery.
1993 Trek 950 host.

BTW, if anyone is tracking power use I am averaging 33w/mile at full throttle (+/- 30mph) and about 18w/mile at 17/18mph.

Click to expand...

It's normal (the motor with draw more under acceleration or under load/going uphill). As long as your battery can support that level of current, there shouldn't be a problem. If it doesn't, and you draw that level continuously, then you could damage the battery.

I agree that it sounds normal. Also power usage monitored between the battery and motor controller includes whatever inefficiencies are lost as heat out of the controller.

Controllers have a block time delay, that can be set on some. For some time in acceleration from a stop, it will let the motor pull more current than the limit that is set. After this delay, it will limit the current as set.

Peak mechanical output power is not equal to peak electrical input power drawn by the controller...

Motors are not 100% efficient.

Assume 85% motor efficiency at most, and

1.2 kW electrical input x 85% efficiency = 1.0 kW mechanical output.

My guess is that peak rating is specified as mechanical output... 1.0kW (roughly 1.3 HP).

Matador

Regarding the titular question: "How much can a motor draw?"

The answer is "as much current as teh voltage across it allows, for the coil resistance in series with teh voltage".

That means, at stall, it can draw enough to set it (and probably the wires leading up to it) on fire, if there is no limiting by the controller.


Practically, most controllers have a current limit, but it's not precise for the generic stuff you get in these kits, and the "ratings" given in them are an "advertising guideline" rather than an actual limit or specification.

So what you see is normal.

If you put a bigger controller on there (or upped the current limit of the one you have) it would draw more power, and you can reach a point at which the motor can't survive that if you were to put it into a situation it draws that much continuously.

Exactly, in other words, if the motor don't turn, the load is infinite. Which means it makes heat instead of motion. The more you over load and slow down the motor, the motor it makes heat instead of motion.

What keeps that in control, is the controller. Otherwise the motor would pull infinite watts when the load is highest.

Based on your reported wattage, I bet you have a 22 amps limit on your controller. Which allows your motor to draw as much as 1200w at max load, when you get started. So your motor is being limited to 1200w. This can vary from kit to kit, depending on what the vendor wants to sell you " as a 1000w kit" Some have 1500w ( 30 amps at 48v) controllers.

But both the 22 amps an 30 amps controllers will reach a max speed of about 27-30 mph. On 48v. And guess how many watts that takes? typically about 1000w. So its reasonable to call both 1000w kits. Both pull at most, 1000w at full speed. But both pull more when the load is maxed out, as you start up.

If you bog down a motor, it will keep taking more amps in order to get up to it's designed top RPM. It will keep raising the amp-draw until it melts down.

Once you get the rpm's up, the amps drop back down, and the motor has the opportunity to cool off a bit.

If you were selling motors to the public, how would you rate a motors power? They want something short and easy to compare. Most ratings "seem to be" its continuous load capability.

Most motors that are rated like this can take an occasional 50% overload for maybe ten seconds, as long as its allowed to cool off for a few minutes at its factory rating load...

How much you might choose to let your motor draw is another question.

Many typical e bike hub motors are rated 500w, but double that is not much of a risk of overheating, hence the 22 -30 amps controllers used. If allowed to cool enough between bursts, the typical 1000w kit motor can handle 3000w. Takes about 30 min to melt one if you hammer it continuous.

And then there's the battery's limitations, as mentioned in the first reply. If the O.P. is still with us - you have to track that down in terms of the particular battery you got. Battery's discharge capacity varies, depending on what cells they used, but the limits are often well within the range that a motor+controller could draw.

If I understand this correctly the controller determines the current used, not the motor. If this is true how does the controller know when to lower the current? Is it a set rpm?

ETA; my battery is listed as 1k watt or less motor. I have no idea what the actual limit is. I assume the max output of the battery will come into play as speed and power goes up.

WaltherDawg said:

If I understand this correctly the controller determines the current used, not the motor. If this is true how does the controller know when to lower the current? Is it a set rpm?

ETA; my battery is listed as 1k watt or less motor. I have no idea what the actual limit is. I assume the max output of the battery will come into play as speed and power goes up.

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Kind of. Ever have one of those slushy/icee drinks? If you suck through the straw too fast, you start getting air, but if you slow down, the syrup settles to the bottom and you can continue drinking it. You are the motor, and you can try to get as much slushy as possible, but the straw (controller) and slushy (the battery) are limiting factors. If the straw is too small, then it limits the amount you can drink. But you also can drink faster than the rate the slushy syrup can settle to the bottom (voltage sag). You can use a bigger straw, but the slushy will still be the limiting factor. If you make the straw smaller, then that's the limit. If you drink too fast, for too long, you get a headache (overheat the motor).

WaltherDawg said:

If I understand this correctly the controller determines the current used, not the motor.

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Well, you've got three things in play, at the motor: voltage, speed, and load. The controller sets the voltage input to the motor, which expects to rotate at a speed determined by that voltage. If it's there already and there's no load, then amperage ("current") will be approximately 0. Or if there's a heavy load and the speed is below what the voltage calls for, then the amperes will be high. That does depend on the motor, until you reach the controller's limit.

Motor ratings are whatever the seller dreams up in order to sell more.


Sellers like Grintech give ratings that are reasonable and honest.
https://www.ebikes.ca/learn/power-ratings.html

If you go to the "Learn" menu in the red headline area, there is a ton more info there.

Motor ratings are bench tests. No one is testing them on the road. That is why most motors are under rated, big hubs especially.