FREQUENTLY ASKED QUESTIONS
What information do I need to provide in order for you to help me select an appropriate motor solution for my application?
How do I calculate my load torque?
How do I calculate the rated speed or the speed at a specific torque for a motor or gearmotor?
How do you calculate the HP of a motor?
1.What information do I need to provide in order for you to help me select an appropriate motor solution for my application?
The absolute minimum information required to properly select a motor solution is the supply voltage, available continuous and peak current, load torque, speed at load, and desired motor technology (brush or brushless). It is also helpful to have a description of the application, duty cycle (time on, time off), ambient temperature, and motor size requirements (maximum diameter, length, and weight). Other application details that can be important include radial or axial loads, environmental conditions (exposure to water, dust, humidity extremes, special atmospheres), feedback requirements (encoder resolution), EMI/RFI suppression requirements, and brake requirements. For positioning applications, load inertia, angular displacement, move time, and friction torque can be provided instead of load torque and speed at load.
2. How do I calculate my load torque?
The torque requirement for an application can be calculated, measured directly using a torque measuring device (torque watch), or measured indirectly using a DC motor. When using a motor, measure the current draw of the motor under load, and calculate the torque using the equation below:
T = (I - INL) x (KT x N x h)
I = Current
T = Torque
KT = Torque Constant
N = Gear Ratio (Equals 1 if there is no gearbox) h = Gearbox Efficiency (Equals 1 if there is no gearbox)
INL = No-Load Current
Please be aware that this equation approximates the true load torque and does not take thermal conditions into consideration. The results are reasonably close and suitable for most purposes.
3. How do I calculate the rated speed or the speed at a specific torque for a motor or gearmotor?
The output speed for a motor or gearmotor is given by the equation below:
w = [VS - (I x Rmt)] / (KE x N)
w = Speed
VS = Supply Voltage
I = Current (Obtained from previous equation) Rmt = Motor Terminal Resistance
KE = Back-EMF Constant
N = Gear Ratio (Equals 1 if there is no gearbox)
Maximum continuous (rated) current can be obtained from the previous equation and used as the current value to obtain the speed at the maximum continuous (rated) torque.
Please be aware that this equation approximates the motor speed and does not take thermal conditions into consideration. The results are reasonably close and suitable for most purposes.
1. How can we keep the motor temperature to a minimum?
Temperature rise can be minimized by:
1. Keeping the ambient temperature as low as possible.
2. Circulating air around the motor.
3. Making sure the motor is not insulated.
4. Mounting the motor to a good heatsink.
Additionally, selecting a more powerful motor should decrease the temperature rise if it is an important consideration.
2. Is there an optimum load to motor inertia ratio? Do I need to match the inertia ratios like my stepper systems?
The load to motor inertia ratios of a servo system do not need to be matched. If a system is matched 1:1, it would take half of the power to move the motor alone. The advantage of keeping an inertia ratio lower is the ability of the system to reject disturbances, but a system with a higher ratio of load to motor has a higher inherent bandwidth given the same load. This is advantageous for systems requiring quick response. You cannot, however go too high without some consequences. Dynamics are the most important part; so coupling of the motor to the load is critical. The higher the ratio, the stiffer the coupling must be. It is much easier to tune a system with perhaps a 4:1 load to motor inertia ratio then a system with a 20:1 given a stiff coupling and the same load.
1. Do I have to run a motor at the rated voltage?
No. The voltage shown for the available motor windings is listed as a reference voltage, and it is just that. Certain motor parameters, including no-load speed, peak torque, and peak current are voltage dependent. Therefore, a voltage must be chosen in order to show these values. The choice is subjective to a degree, but provides a convenient way of comparing different motor windings.
2. Are your motors and gearmotors reversible?
Yes. All of our brush-commutated units are reversible. Simply reversing the polarity to the motor terminals or leads will reverse the output direction of our brush-commutated units.
4.Can I buy replacement brushes, gears, etc?
We can typically offer replacement components such as brushes or gears for our products. Please be aware that orders for component parts such as brushes and gears are subject to minimum quantity requirements (the exact requirement can be ascertained by contacting our sales department).
Please also be aware that while it is generally technically possible to replace the brushes or brush assemblies, it is not always practical. By the time brushes have finally worn out, significant brush-dust has typically accumulated within the motor. Additionally, the commutator, bearings, and gears have also experienced some degree of wear. These conditions are often impractical to correct and will typically contribute to a reduced time to failure than was experienced with the original brush set.