In last month’s article, I wrote about Power Programming Basics and the common adjustments that can be made to the programming of a power wheelchair to affect the drive-ability of such a chair for a particular user. Over the next few months, I would like to focus on the “gross anatomy” of a power wheelchair base to help in de-mystifying power wheelchairs for those who may not feel comfortable working with power wheelchairs. A power base includes the following: wheels, casters, motors, batteries, and the controller.
The position of the drive wheel – that is, whether the drive base is rear-wheel, mid-wheel, or front-wheel drive – has an effect on the overall performance of a power chair under different conditions. This is such a large topic that I will hold it for a future Clinical Corner article. Similarly, the topic of the controller and electronics is another vast topic that is best saved for a future Clinical Corner article.
This month, let’s focus on the other main components of the power base – the batteries and the motors.
A power wheelchair uses two batteries that are connected in series. The batteries provide the electrical energy to operate the power wheelchair. If we compare a power wheelchair to a car, the batteries of a power wheelchair would be most similar to the gas tank of a car – both provide the “juice” needed to operate the motors. When there is an insufficient level of power in the batteries or gas in the gas tank, the power wheelchair or the motorized vehicle, respectively, will not operate.
Like gas tanks, batteries come in different sizes. Two common sizes of wheelchair batteries are Group 22NF and Group 24. Group 22NF batteries are smaller and lighter than Group 24 batteries. This helps to decrease the overall weight and width of the power wheelchair; however, less power is available in the Group 22NF batteries to operate the power wheelchair. Group 22NF batteries are a good choice for use on basic power wheelchairs that will be used in indoor environments.
The opposite is true of Group 24 batteries, which are heavier and larger than their Group 22NF counterparts, making the overall weight and width of the power wheelchair heavier and wider when such batteries are used. The benefit of the Group 24 batteries is in providing more power or “juice” to operate the power chair. Group 24 batteries are a good choice for power wheelchairs that will be used in outdoor environments and/or that have other systems, such as power tilt system, operating off the power of the battery.
I am often asked how long will a battery hold it’s charge or how far can a user go on a battery charge. That is a difficult question to answer! So many factors go into the draw on the battery, such as weight of the user and power chair, type of motor, temperature and terrain of the environment in which the chair is being used, programming parameters (e.g. per cent of power available), and operation of other systems through the batteries.
It is more difficult to compare a power wheelchair to a car when looking at the motor. While a car has one engine or motor to operate both drive wheels (or all drive wheels in the case of a four-wheel drive vehicle), a power wheelchair has two motors. Each motor operates a drive wheel on either side of the power wheelchair. The motors draw power from the batteries and move the drive wheels.
Just as there are choices in batteries, there are choices in motors available. Some motors may be two-pole or four-pole. The number of poles refers to the number of sets of brushes and magnets. So, a two-pole motor will have two sets of brushes and magnets and a four-pole motor will have four sets of brushes and magnets within the motor. What this means is that electricity from the battery will enter the motor (via the control module) and outputs at two sites for a two-pole motor and at four sites for a four-pole motor.
There are advantage and disadvantages to these different types of motors. A two-pole motor has fewer brushes and draws less power from the batteries than a four-pole motor does. Two-pole motors pull more smoothly in very slow operation than four-pole motors do, making two-pole motors a good choice for cramped, indoor environments. If a brush on a two-pole motor fails, however, the wheelchair will become inoperable as there would be insufficient ability to power the wheelchair in a straight line without both poles working. (Note: Motors on power wheelchairs can be disengaged to allow for manual pushing of the wheelchair if needed.) Two-pole motors are not suitable for power wheelchair users who require heavy duty components because of their weight or who will be travelling on uneven terrain.
So, what are the advantages and disadvantages of a four-pole motor? Well, a four-pole motor, with its four sets of brushes and magnets, will still allow the operation of the power wheelchair if one of the brushes fails to work properly. In addition, a four-pole motor has more torque than a two-pole motor, and as such, can take on uneven, outdoor terrain and heavier weight capacities. This performance ability requires more draw from the batteries, resulting in the batteries draining faster with four-pole motors than compared to two-pole motors. While a two-pole motor will operate well at very low speeds, the same cannot be said of a four-pole motor. A four-pole motor will have a rougher operation than a two-pole motor at very low speeds. This is why it is so important to understand the environments in which a person will be using the power wheelchair and the activities that will be performed in order to ensure the right fit between the person and the assistive technology chosen.
Next month, we will look at how the controller works to provide instructions to the motors to move the power wheelchair in the desired manner.
Until then, please provide your comments, questions and suggestions regarding Clinical Corner on my blog at www.clinical-corner.com. I look forward to hearing from you!
Sheilagh Sherman, OT Reg. (Ont.)
Sunrise Medical Canada
Note: The content of this article is not meant to be prescriptive; rather, it is meant as a general resource for clinicians to then use clinical reasoning skills to determine optimal seating and mobility solutions for individual clients.