It continues to amaze me how people think that they can take an electric golf cart (with a series-wound motor in it) that is running well and make it go faster by putting a higher ampere rated controller in it. Unless the cart is somehow exceeding the limits of the existing controller, putting a “bigger and better” controller in it won’t make it go any faster. The speed that the cart will run is determined by the motor. Not the controller (if everything is working like it should). Once again, I’m talking about a cart with a SERIES motor and controller.
Think of a series controller as a “switch”. It can only be turned on to a maximum of 100%. A motor speed controller uses a technique called pulse width modulation to do its “switching”. The process of pulse width modulation can only go to 100% duty cycle (which is the same as being totally “on”), and no farther. It is like a light bulb (the motor) and a switch (the controller). You can’t make the light bulb get brighter by getting a bigger switch (unless the switch isn’t doing its job to begin with). Once the switch supplies full power to the light bulb, the brightness is determined by the light bulb and the source of power. If the existing controller is going to 100%, that’s all you get. You can tell if your controller is going to 100% if you put a meter on M- and watch it at full throttle. If it is going to 100% duty cycle, the voltage at M- will go completely to 0 volts (the same as B-). At that point, motor is connected directly between B+ and B- and the speed is up to the motor.
Now if you go to a different motor that has the ability to go faster (by drawing more current from the battery pack than the old one was designed to), then you very well may need a higher rated controller to be able to handle the additional current (without damaging the controller). Also, if you have increased the tire size (changed the gear ration from what it was originally), then your controller may be internally hemorrhaging, trying to supply the current needed due to the increased load. It may even be pushed into a self-induced state in which it limits the current due to overload or temperature. In that case, a “bigger and better” controller is justified. A newer controller may increase the torque available (even with the stock motor) because of improved switching speed (gets to 100% pulse width modulation faster), but not the top speed. Shunt controllers are a totally different story as the voltage and current used by the separately excited field coil can be adjusted (mapped) for many different combinations.
Here is another way to look at it: Let’s say that we had a simple circuit that consisted of a 12 volt battery connected to a simple rheostat (variable resistor) in series with a 12 volt light bulb. As the rheostat is adjusted for less resistance, more current flows through the switch to light the bulb. Once the rheostat reaches its maximum position, its resistance reaches zero ohms, and the bulb lights as bright as it is designed to with 12 volts applied. Now if we had a rheostat that was twice a big, it wouldn’t light the bulb any brighter. That’s because it couldn’t go any lower than zero ohms, no matter how big it is. Once the entire 12 volts is applied to the bulb (no resistance in series with it), that’s all you get. That is the way it works with a golf cart motor speed controller. Once it has provided the entire energy source (36 or 48 volts) to the motor, it’s up to the motor to do what it is designed to.
So, why would I ever need a “bigger and better” motor speed controller? There are lots of good reasons. Going back to our light bulb example, let’s talk about the current that the bulb is using when it connected to the whole 12 volts (the rheostat has reached 0 ohms). The bulb has a wattage rating (probably printed on its base) that determines its brightness. Let’s say it is a 6 watt bulb.
To do some analysis, we’ll use a few basic formulas based on “ohm’s law”:
E = I x R
I = E / R
R = E / I
P = I X E
I = P / E
E = P / I
In the above, E is voltage in volts, I is current in amps, and R is resistance in ohms, and P is power in watts.
Using I = P / E, our 6 watt bulb would draw .5 amps from the 12 volt battery (.5 amps = 6 watts / 12 volts).
But what if we wanted a brighter light? We could replace the bulb with a 12 watt bulb. Here is where the rating of the rheostat becomes very important. In the case of a golf cart, the rheostat represents the motor speed controller. So, my new bulb would require twice as much current (1 amp = 12 watts / 12 volts). If my rheostat can’t handle that much current, then I will probably damage it (at least eventually).
Golf cart motor speed controllers have a rating that needs to be observed also. Let’s say that you replaced your stock motor with a new higher speed and torque unit. What about your “poor” motor speed controller. It could do several things. One thing it could do is “fry” its internal MOSFET transistors that provide the switching. As I said before, the way that a controller provides less resistance to the current that the motor draws (kind of like our rheostat) is by a method called pulse width modulation. We could spend hours on that subject, but for our purposes here, we’ll just pretend that the motor speed controller is a virtual variable resistor. The controller “reads” the position of the throttle and then responds by increasing its pulse width modulation so as to provide the additional current to flow to the motor. Most of the controllers have some sort of protection built into them to help keep them from destroying themselves if they are asked to deliver more current than they were designed to handle, but from experience I can tell you, that a controller that is consistently asked to work too hard, will have problems. Some of them will actually shut down altogether to protect themselves when they detect an “over current” situation. Others will sense that they are getting too hot so they either shut themselves down or limit the current to something they can handle. However they react, it’s not a good thing to push one that hard. So, just like when we went to the 12 watt bulb and needed a better rheostat, when we start requiring more current than the original controller was designed for, we need a different controller (bigger and better) with a higher current rating.
Most suppliers of motors and controllers offer “package” deals that include not only an upgraded motor but a controller that will handle it.
So why wouldn’t it be very easy to determine the current drawn by the motor from the battery pack with just a little math? After all, we know that if we select a 3.5 HP motor that turns 3000 RPM and we know the battery pack voltage is 36 volts, we can figure out the current to be drawn. One HP is equal to 746 (actually 745.7) watts and we have 3.5 HP, so that is 2609.949 watts. If P = I X E, then we have 2609.949 = I X 36. So, I = 2609.949 watts divided by 36 volts = 72.5 amps. So, all I need is a controller capable of delivering 72.5 amps and I am good to go. Right? WRONG!
The calculation that we just made assumes that motor is running along with a load requiring 3.5 hp to function at its specified 3000 RPM and a full 36 volts applied. But golf carts seldom work that way. When we first start the golf cart moving, our motor speed controller is intentionally limiting the voltage (and therefore the current) to the motor so that the cart can take off gradually and not snap our neck. As a matter of fact, the motor is actually “stalled”, meaning that when the voltage is first applied, the motors shaft (which is part of the armature) is not spinning. The amount of current drawn by a DC motor in its stalled state is very high, because the only thing limiting the current flow through the circuit (as far as the motor goes) is the DC resistance of the field and armature coils. As the RPM of the motor starts to rise, a phenomenal event takes place called EMF. The letters stand for electro-motive force and as the motor spins, its armature produces a BACK EMF that opposes the battery voltage and therefore reduces the amount of current required by the motor to do its work. The value of the EMF is directly proportional to the RPM of the armature. But, before the motor gets going, the EMF is 0 volts, so it doesn’t affect the current flow at all. Therefore, the combined resistance of the field and armature coils can be used to calculate approximate the current flow. Of course, the motor speed controller is limiting the voltage at the start, but let’s pretend that you just put the “pedal to the metal” so that there was no other current limiting and the motor received the entire amount of voltage available from the battery pack all at once. Now, we need a value for the combined DC resistance of the field and armature coils, and that is a problem. I’ve never seen a motor manufacturer publish that information. Both of these coils are made with very heavy wire and offer very little resistance in the stalled state due to the missing EMF. I can, however, tell you from experience, that if you take an ohmmeter and measure them for DC resistance the two of them together will only be a few tenths of an ohm. I’ve measured dozens of them. For the rest of this discussion, we will use .5 ohms for the combination’s resistance.
So now, if we calculate the stall current for our motor, we get the following:
Our 3.5 hp motor equals 3.5 X 746 = 2611 watts
2611 watts / 36 volts = 73 amps (rounded)
So, our 3.5 hp motor in its stalled state, will draw about 73 amps.
Granted, we don’t usually push a golf cart to the point of actually stalling the motor (I hope you don’t), but we are always pushing the motor towards it limits. Especially if you are using one in hilly terrain or pulling a trailer with it or if you have increased its rolling resistance with ”knobby” tires. Larger diameter tires also affect the carts overall gear ratio to the road, and heavy accessories like back seats add up also. But, by using the stall current to help select the size of the controller, we are sort of looking at the “worst case” scenario.
When selecting the controller, however, you wouldn’t select one (in our example) that was only capable of delivering 73 amps. Most of the suppliers match up a motor and controller combination that gives the controller lots of room to work. The controller would usually be rated at 2.5 to 4 times what you would come up with for a stall current.
In order to demonstrate this, I found a couple of popular combinations that are sold by a very reputable supplier and got the information they had available for them.
The first one had a 9 peak hp motor (no other information about the motor) and a 400 amp controller. Using our .5 ohm rule of the thumb for the combination resistance of the field and armature coils, I calculated the stall current as follows:
9 hp X 746 = 6714 watts
6714 watts / 48 volts =139.87 amps
As you can see, they have given themselves plenty of room with the 400 amp controller. If we divide 400 by 139.87 we get a ratio of 2.86 times the stall current as we figured it.
The next one that I picked had a 12.3 peak hp motor and a 500 amp controller. Once again, I used the .5 ohm rule and this is how it came out:
12.3 hp X 746 = 9175.8 watts
9175.8watts /48 volts = 191.16 amps
You can see that they have upped the amperage of the controller to still maintain a ratio of 2.62.
Another thing that makes picking the right motor and controller combination on your own, is the way that the motors and controllers are rated. To try to demonstrate this, I went to the internet again and looked up a few.
As far as the motor goes, most all of the motors are given in “peak” horsepower. Peak meaning the most horsepower that the motor can possibly deliver. They usually don’t tell you at what RPM. Some do give you an operating Horsepower (instead of peak) at a specific RPM, but in either case (operating or peak) you don’t know how that will match up with the actual operating curve of the golf cart.
As far as the motor speed controller goes, it gets even more confusing. I dug very deeply into the specifications of a very popular (and very good) brand of controllers and discovered that what the label says and what the unit will actually do are a little different. The controller I looked up was rated at 400 amps. But going into the specifications a little deeper I discovered that the amount of current that it was rated for actually depended (and they really all do) on the length of time that the controller was “asked” to supply it. The particular model that I picked was rated for 400 amps, but only for a maximum of 2 minutes. The rating went to 300 amps for a 3 minute period and down to 140 on a continuous period. That may seem a little frightening at first, but EMF TO THE RESCUE! When we really draw large amounts of current from the controller is when the motor hasn’t gotten up speed. As the armature starts to spin, the EMF comes up very quickly to reduce the amount of current flowing through the motor. The specification went on to read (in the fine print), that the rating was only for 50% pulse width modulation. That means the throttle is only at about half depressed.
When you buy a motor and controller combination, they usually try to help take a bunch of the work out of it for you. A good reputable distributor has already had someone (hopefully a good engineer) look into all of these things and put together packages that match certain conditions. They usually ask you what circumstances the cart will be subjected to: like flat ground or hilly, normal tires or oversized, “lifted” or not and how much weight has been added to the original configuration. This is good, because they have hopefully figured out the math for you, and have undoubtedly sold some of these combinations and have some feedback on how well they worked out. They even, normally, give you some estimates about how fast the cart will run with the new combination and what kind of torque increase you will experience.
Ron Staley has published the following books, and you can get more information about them by just clicking on each title below:
Electric Golf Cart Repair 101 (and a half)
Techniques, Tips, Tools and Tales
Gas Golf Cart Repair 101 (and a half)
Techniques, Tips, Tools and Tales
4-Stroke Golf Cart Engines Explored
What Makes Them Tick
By an old Slot Machine Mechanic