# I need some help getting input current and voltage



## Siwastaja (Aug 1, 2012)

Decide your pack voltage and max RPM needed for full-torque operation. Divide the pack voltage by 1.4 (approx) and you have your voltage at that rpm. It scales linearly down with the frequency (rpm). Calculate the electrical frequency for that rpm too. To do that, you need to know the pole count. If you are doing an induction motor, go with 4-pole and use this cheat table:
50 Hz = 1500 rpm
60 Hz = 1800 rpm
100 Hz = 3000 rpm...... and so on. Easy, huh? (This is a synchronous speed, the real rpm is a bit less, but the difference is meaningless in this context.)

(After designing it like this, your motor _can_ still rev to even higher rpms in so called "field weakening", which is a constant power region with diminishing torque, simply because you cannot increase the voltage anymore.)

So, using our AC induction motor rewinding as an example, it went like this:

320 VDC battery voltage -> divide by 1.4
230 VAC line-to-line voltage.
Frequency for this voltage: we decided 150 Hz -> will do 4500 rpm synchronous speed (it is a 4-pole motor)
Now you can see that if it's 230 VAC at 150 Hz, it will be 92 VAC at 60 Hz and 77 VAC at 50 Hz. This is now easy to compare to the original nameplate which says: 380 VAC at 50 Hz. Yes, what we did was exactly drop the voltage to 1/5 of the original by rewinding.

In AC motor, it is also voltage what drives the current. And just like DC, you need more voltage with higher revs. So, by scaling the voltage linearly with the frequency, you have the same current going to the motor, but more voltage gives you linearly more power on higher revs. So, the same torque -> current controls torque, voltage controls rpm.

If you step aside from the linear volts per Hz curve by applying more voltage than "needed" for the frequency, you force extra current to flow, giving a boost in torque. And the opposite is also true; lower than expected voltage reduces torque. Ultimately, 0 volts will coast freely.

Just some thoughts, probably not very clearly presented, but hope this helps.


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## major (Apr 4, 2008)

acosgrove said:


> Hi there,
> I'm working on specifying an AC motor for my '99 Miata EV conversion. I have run the performance numbers for the IC engine, and now I'm trying to match an electric motor to those numbers. My main questions are:
> 
> 
> ...


Hi acos,

You should start out working with motor power. You can always adjust gearing to get the proper RPM/torque relationship. Figure the vehicle and duty requirements for sustained power which is likely top speed into a headwind up a slight incline fully loaded. Use that for the motor "rated power" which is typically given as continuous or one hour. Vehicle propulsion motors need to be capable of overload for acceleration and steep hills. Most motors used in EVs have a 2 or 3 times overload capability but often at reduced RPM. From this you can size the motor. Typically, for the power, the higher RPM motors are smaller.

Once you find a motor, or several choices, that will determine the voltage and current requirements. Obviously the available batteries and controllers will influence in which voltage ranges you look for your motor.

There are resources on this web site and others which can assist you.

Regards,

major


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## PStechPaul (May 1, 2012)

The power and torque figures for an ICE are usually peak values, while AC motors are typically specified for continuous, and they are capable of at least 2x-3x for a few minutes. So as a general rule of thumb, an equivalent AC motor would be one with 1/2 to 1/3 the ICE peak power. If you will be using the original drivetrain, figure that the ICE motor normally cruises at about 1500-2500 RPM, so a 4 pole 1800 RPM motor will do that and will have twice the torque of a two pole 3600 RPM motor of the same HP. With a standard VFD you can get full 230 VAC with a 300V battery pack, and you can overclock the motor 2x to 3600 RPM where it will have twice the power but the same torque as the 3600 RPM 2 pole motor. 

Depending on various factors, a fairly small EV such as a Miata would probably need 20 HP continuous to cruise at highway speeds on fairly level roads. A 30-40 HP motor will give good performance if it fits. If you want highest efficiency, choose a motor rated for 90% or better, and designed for inverter duty. Typical energy consumption is 250-350 Wh/mile, so you can figure your pack capacity to get the range you want.

I made an EV calculator using JavaScript that can help with these calculations:
http://enginuitysystems.com/EVCalculator.htm

My experience is mostly with smaller motors and vehicles such as an electric tractor. I was able to run it at 24 VDC and 15 amps, or about 1/2 HP from the battery. See my youtube videos for my Craftsman Rider Mower Project: http://www.youtube.com/PaulAndMuttley


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