Is a 3kw controller okay for a 3kw motor

[Pranav]

Hi,
I am building a ebike with a BLDC motor rated at 3kw (nominal). I was thinking of a kelly 3kw KEB72301X controller, but Fany at Kelly controller tells me this will not be adequate and should go for the 4.5kw KEB72451X controller instead. But it costs nearly 20% more ($259 vs $219). She says for optimal performance the controller should be 1.5 to 2 times the power rating of the motor. I don't get this. Can someone explain why this is so? I am a bit of a noob . Is she right in suggesting the 4.5kw controller or is she just trying to up-sell. :?

The specs of the 3KW controller are
72V 3kW
•Peak Phase Current, 10 seconds: 150A.
•Continuous Phase Current Limit: 60A.

The motor is a Golden motor 3kW @ 72V

Thank you

 

[friendly1uk]

If you were going to run the motor at full power all day long the 3kW controller would be perfect.

To average 3kW all day long, you will sometimes be taking less than 3kW and sometimes more. This is why people will often fit a larger controller than the motors rating.

You couldn't sell such a bike, as someone would point it at a hill and kill it. Such a thing would need nannying. Some sort of thermal monitoring where the controller would back off as the motor warmed. With just 20% more power going to the motor than it likes, it won't burn up instantly, it would gradually build up heat quicker than it could loose it. Hence the correct control gear can be higher rated than the motor.

I dunno if it is the correct control gear. I just buy cheap stuff. I know my controllers have no idea of impending doom. Lots of people still take a 250w nominal motor and feed it from a 500w dumb controller. They just need to understand what they are doing, and get familiar with overheat times to know when enough is enough.


Sorry I could not be more concise.

 

[wesnewell]

Do the math. VA=W, so that controller at 72V and 150A will put out 10KW for 10 seconds and 4KW constant. Do you need more than that? Will the motor survive more than that?

 

[crea2k]

Bear in mind this all means nothing if your batteries cant provide the power, as you can have as powerful controller as you like, but if your battery C rating is too low it wont make any difference if you have the world most powerful controller as that's your weak link.

 

[riba2233]

First, I would strongly suggest against golden motors, they are really badly engineered. Buy motenergy instead. I have hpm5000 with kelly keb48201, so 2kw controller, and running 70 kg moped, and I'm satisfied with performance. But 3 kw would sometimes be more likely for uphills.

 

[Pranav]

First, I would strongly suggest against golden motors, they are really badly engineered. Buy motenergy instead. I have hpm5000 with kelly keb48201, so 2kw controller, and running 70 kg moped, and I'm satisfied with performance. But 3 kw would sometimes be more likely for uphills.

I've already paid up for the motor. Nothing I can do about it now. They aren't a pleasure to deal with for sure.
Isn't the HPM5000 a 5 KW motor? Are you running a 5kw motor with a 2kw controller?What's the top speed you manage on your setup?

 

[Pranav]

Do the math. VA=W, so that controller at 72V and 150A will put out 10KW for 10 seconds and 4KW constant. Do you need more than that? Will the motor survive more than that?

That is precisely what I did. 4kw is more than I need and don't intend to run it at that for long periods. I was a bit thrown off by the "ContinuousPhaseCurrent" label. In AC circuits I know Line Current = Phase current * Power Factor. For DC circuits, I assumed Phase Current = Line Current, but I am no engineer, and was afraid I might be wrong.

 

[Pranav]

Bear in mind this all means nothing if your batteries cant provide the power, as you can have as powerful controller as you like, but if your battery C rating is too low it wont make any difference if you have the world most powerful controller as that's your weak link.

I am using a 30Ah battery capable of 50A continuous (restricted by the 50A BMS rating)

 

[Pranav]

If you were going to run the motor at full power all day long the 3kW controller would be perfect.

To average 3kW all day long, you will sometimes be taking less than 3kW and sometimes more. This is why people will often fit a larger controller than the motors rating.

You couldn't sell such a bike, as someone would point it at a hill and kill it. Such a thing would need nannying. Some sort of thermal monitoring where the controller would back off as the motor warmed. With just 20% more power going to the motor than it likes, it won't burn up instantly, it would gradually build up heat quicker than it could loose it. Hence the correct control gear can be higher rated than the motor.

I dunno if it is the correct control gear. I just buy cheap stuff. I know my controllers have no idea of impending doom. Lots of people still take a 250w nominal motor and feed it from a 500w dumb controller. They just need to understand what they are doing, and get familiar with overheat times to know when enough is enough.


Sorry I could not be more concise.

No bother. You and the others have been a great help. I am sure now I am being upsold (if that's a word). Will be going for the 3KW controller from Kelly

 

[riba2233]

I've already paid up for the motor. Nothing I can do about it now. They aren't a pleasure to deal with for sure.
Isn't the HPM5000 a 5 KW motor? Are you running a 5kw motor with a 2kw controller?What's the top speed you manage on your setup?

Yes, but one reputable member on the forum said that it can't stand 5 kw continuously. Also, when I was buying there wasn't 3 kw model. I run motor cool, and I know many cases where motor got destroyed from excessive heat, so you have to be careful, they are very fragile and tend to destroy themselves. I'm also converting to a much smaller motor, so golden will be past for me.
Top speed is 55 kph, I could do more but not legally on the moped. It's the one in my profile picture.