# DC motor / voltage switching question



## dmac257 (Jun 30, 2010)

I have been reading the various threads and a lot of the specific motors discussed mean nothing to me at this point but I think this is a general question that might clear up some misconceptions on my part.

From the thread on motor selection at the low end of the cost is brushed DC motors and I looked at the data for the three motors shown in the table. One motor is rated at 12v-72v 20kW continuous/35kWpeak and this confuses me a little. My understanding is that DC motor speed is a function of the voltage applied.. and power is a function of the Amperage.

If this is correct then on start from standstill .. you apply voltage and the amperage goes way up to provide power to rotate motor and push car. Are you applying full battery voltage of 72v at startup or does voltage ramp up with a controller from 12v up to max drive volatage (I assume controller ramp would be needed to limit starting current to something that will not melt the motor) but here is where I am confused. IF the starting voltage is lower to start (say 12v at some point early in accerleation) is the amperage allowed to be higher than full voltage amperage because of peak 35kW .. ie full voltage is 72v so peak power at full voltage is 486a .. but controller ramps voltage to start so is current allowed to go to 2916a at 12v or is the controller actually limiting the current BY varying the voltage.

And it would seem that you need power / current at start up but don't need the high current after you are at crusing speed, so COULD you have your drive batteries set in three banks of 24V and have contactors to provide switching between two configurations .. parallel for acceleration amperage and then "shift" at crusing speed to series for 72v to lower the amperage draw from the batteries? Or is this thinking wrong for other reasons? I am not saying that I WOULD want to do this just testing my understanding.


----------



## major (Apr 4, 2008)

dmac257 said:


> I have been reading the various threads and a lot of the specific motors discussed mean nothing to me at this point but I think this is a general question that might clear up some misconceptions on my part.
> 
> From the thread on motor selection at the low end of the cost is brushed DC motors and I looked at the data for the three motors shown in the table. One motor is rated at 12v-72v 20kW continuous/35kWpeak and this confuses me a little. My understanding is that DC motor speed is a function of the voltage applied.. and power is a function of the Amperage.
> 
> ...


Hi dmac,

I think the source of your confusion lies in the difference between torque and power. Acceleration from a standstill requires torque to launch the vehicle, not power. Afterall, power is the product of torque and speed. By definition, at standstill, there is zero speed, so torque times zero RPM is zero power. Acceleration is defined by Sir Isaac Newton as F=ma. Translating it to the rotation system, acceleration is the torque divided by the polar inertia (mass). To accelerate your vehicle (especially at low speeds), it requires torque, not power.

Fortunately the electric motor is very well suited for this. It produces the most torque at zero RPM, unlike an ICE. In the normal DC system, the motor controller will start out a hard acceleration in current limit. This is accomplished by voltage reduction. So at standstill, you floor it. The controller will output its current limit, say 500A. At the start, this may only be a few volts to the motor. At that instant, the motor produces torque, say 90 lb.ft., but there is actually no mechanical power output from the motor. Once the vehicle starts to move and the motor rotates, then mechanical power will be delivered by the motor. 

The first period of the acceleration occurs in current limit, 500A. As the vehicle gains speed, the controller steadily increases voltage to the motor. All the while, the motor torque stays constant at 90 lb.ft. All the while, the motor RPM is increasing as well as motor power. Finally, the motor controller will hit 100%, passing full battery voltage to the motor. This will be peak power for the system. Full battery voltage to the motor and maximum current.

The vehicle will continue to accelerate, but at a reduced rate because the current will start to decline. The voltage to the motor can go no higher. The
motor torque will reduce. And motor power will reduce. Finally, the motor output torque will fall to a value just equal to overcome the forces working against it (friction and windage) and the vehicle will achieve an equilibrium speed because no more torque is available for acceleration.

The acceleration event is far from a constant power situation.

Cheers,

major


----------



## major (Apr 4, 2008)

dmac257 said:


> And it would seem that you need power / current at start up but don't need the high current after you are at crusing speed, so COULD you have your drive batteries set in three banks of 24V and have contactors to provide switching between two configurations .. parallel for acceleration amperage and then "shift" at crusing speed to series for 72v to lower the amperage draw from the batteries? Or is this thinking wrong for other reasons? I am not saying that I WOULD want to do this just testing my understanding.


Hi dmac,

As far as battery switching goes, once you understand the operation of the PWM controller, I think you'll see the light. Battery switching and resistor starting was used way back when PWM was not available. It is a legitimate method for starting and crude speed control. But was abandoned years ago when the cost of PWM came down and the performance/durability advantage was realized. The last EV to use this method was the Citicar back in the 1980's.

major


----------



## dmac257 (Jun 30, 2010)

major said:


> Hi dmac,
> The first period of the acceleration occurs in current limit, 500A. As the vehicle gains speed, the controller steadily increases voltage to the motor. All the while, the motor torque stays constant at 90 lb.ft. All the while, the motor RPM is increasing as well as motor power. Finally, the motor controller will hit 100%, passing full battery voltage to the motor. This will be peak power for the system. Full battery voltage to the motor and maximum current.
> 
> The vehicle will continue to accelerate, but at a reduced rate because the current will start to decline. The voltage to the motor can go no higher. The
> motor torque will reduce. And motor power will reduce. Finally, the motor output torque will fall to a value just equal to overcome the forces working against it (friction and windage) and the vehicle will achieve an equilibrium speed because no more torque is available for acceleration.


Ok .. I think I follow you through that pretty well .. so the PWM will limit the current throughout the whole process. So is the limit the peak Power values for the motor ratings? For the motor in the table rated at 12v-72v 20kW continuous/35kWpeak would this be 35kW/72v= 486a .. so limit of PWM would be set at like 486a or slightly less to get max torque on startup. PWM ramps up the voltage keeping the 486a steady till reach 72v and then acceleration slows while amps decrease to equilibrium.

At this point would the motor voltage be held at 72v and speed be maxed out (assuming road conditions are flat etc).. and if you go up a hill or something .. voltage can't go up so motor starts to draw more amperage unless the torque required to maintain speed is beyond the PWM limits (which would cause the car to slow down untill no inertia left and it would stop .. or car gets over hill and starts down again)

Hopefully this is correct,
dmac257


----------



## dmac257 (Jun 30, 2010)

major said:


> Hi dmac,
> 
> As far as battery switching goes, once you understand the operation of the PWM controller, I think you'll see the light. Battery switching and resistor starting was used way back when PWM was not available. It is a legitimate method for starting and crude speed control. But was abandoned years ago when the cost of PWM came down and the performance/durability advantage was realized. The last EV to use this method was the Citicar back in the 1980's.
> 
> major


so basically I will never really need to get more than the limit set by the PWM as that will "protect" the motor from too much current. and if I understand correctly the total voltage of my drive bank will determine the MAX speed of the motor .. if I am only going 60% of that value the PWM will not be showing full voltage to the motor? So I would have ONE series bank of batteries to produce 72V and as long as the batteries are CAPABLE of supplying current at the PWM limit setting I will get max torque allowed for motor. If the batteries only rated for 300 amps then that is all I would get unless I have a Capacitor bank accross the battery to supply acceleration the extra current? (but that would be farther in the future of my EV quest)

dmac257


----------



## nitbolta (Jul 4, 2010)

I really prefer brushless motors, since they might give high RPM with torque. They are expensive though.


----------

