What motor?

Got a nameplate pic?

 

No nameplate, sorry. It was a build I saw and was told it was a 120V motor using a 144V pack. For sure lead acid, looked like car batteries. The motor was DC, old, cylindrical, maybe brushed. The more I thought later the more questions I wished I’d asked. Can we just do a hypothetical?

 

I suggest you do some more reading on here and get out of paying attention to the 1960's way to build an electric vehicle.

 

Yes, asking questions is a total waste of time. Thank you professor.
Anyone else know why you would run a pack with higher voltage than the motor?

 

Anyone else know why you would run a pack with higher voltage than the motor?

Even if the motor will never be supplied with full pack voltage, a higher pack voltage might make sense because it might be the most viable arrangement of available battery modules. It also means lower current in the high voltage battery to controller link, but that's not likely to be a major factor.

 

This would be a good one for the There are No Stupid Questions thread. Here is a general answer.

Motor voltage on the nameplate is a rating related to the windings and the top speed that it can operate.

When a motor rotates it creates a back emf in the windings that opposes the direction of the applied voltage.

The controller regulates the current (and voltage) sent to the motor to reach the commanded torque and speed.

As you go faster the back emf increases, which requires an even higher controller output voltage to overcome it.

So if you want to drive your 120 V rated motor at top speed, then you need more than 120V input to the controller in order to have the headroom necessary to drive current into the windings at that top speed.

Plus as the battery voltage decreases (fuel gauge running low), you want margin in your Pack such that you can still operate at full throttle. So even when nearly empty, the pack voltage needs to be high enough to still satisfy the top speed conditions. So a 144V pack when Full might be 120 or less when Empty, depending upon the type of cells, etc

So no they seldom match, but the specific answer depends upon your motor and controller and battery.

 

Major, back in the day recommended a max of 160 volts for a 48v rated motor. The issue is gaps on the commutator getting filled with brush carbon or being insufficient width for avoiding comm arcing, a thing that Tesseract named Zorching which fubars the motor

 

Thanks for those answers. So there IS a good reason to run a pack higher than motor voltage.
To go a step further, my example was for a DC motor.
How about an AC motor? Same kind of effect? How much voltage “headspace” is recommended? (ex: 400V pack for a 360V motor). Just generally, I can see it all depends on specifics.

I dug this up:
“Is there any back EMF in induction motor?
Induced EMF in the rotor of the Induction motor can be called as Back EMF, which is analogous to back EMF of the DC motor. However there is a difference in the effect of back emfs of Induction motor and DC motor. In DC motor at the starting the back emf is zero, while in Induction machine Back emf is maximum.”

Thank you.

 

Going to be a bit hard to exceed 300 volts and still stuff that into a car using a homebuilt, commonly available parts

 

You have to spec the performance requirements first. Then the vehicle.

You're going about this bass ackwards with the 400V question and are wasting your time with pointless hypotheticals that have no constraints or context.

 

Either add to the discussion or stay off the board. Stop cluttering up the place with such inane remarks. Hypotheticals are worth discussing. You don’t want to play, don’t play. Not everyone has your depth of knowledge.

 

Dc motors are often over volted. The motor and the controller must both be able to handle the voltage. I imagine the same is true of AC motors.
later floyd

 

It's not true of AC motors, or even DC motors. You can undervolt a Leaf AC motor by 50% if it's for a city car. 400V is insane for some applications or battery module configs.

That's why this whole hypothetical nonsense is a total waste of his time. You have to spec the performance and mission FIRST. Defining and constraining the problem is Design 101.

Without doing that, every answer to these questions can be correct...or wrong.

 

The thing to watch with any drive system (DC or AC) is to make sure that you do not exceed the "Absolute Maximum Voltage" rating of the controller/inverter (in the controller/inverter Specification)
To work this out in your system, your "Absolute Maximum Voltage' will be the voltage that your main pack reaches on a full charge. eg. for a 120V nominal LiFePO4 pack, the maximum charge voltage will be around 132V, or for a 144V nominal LiFePO4 pack, the maximum charge voltage will be around 160V.

The 120V DC motor will work on a 144V pack as long as you do not exceed the "Absolute Maximum Voltage" rating of the controller.