How they work

Custom output shaft options increase design flexibility.
Select figure to enlarge.

Permanent magnet direct current (dc) motors convert electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly; the other by an electrical current flowing in the motor windings. Pushing against each other, the fields produce a torque that tends to rotate the rotor. As the rotor turns, the current in the windings is synchronously switched (commutated) to maintain continuous torque output.

Brush dc motors can be operated over a wide range of voltages, speeds, and loads. Output power is a product of speed and torque; input power is a product of applied voltage and motor current.

General sizing procedures

The first step in motor selection is deciding whether or not you need a gearbox. This will typically depend on the maximum required load speed. If speeds are below 1,000 rpm, it’s a good idea to use a gearmotor. If speeds exceed 1,000 rpm, a motor by itself should work fine.

To select a gearbox, start by considering the torque requirements of your application. Gearboxes are usually rated by their maximum allowable output (load) torque. Once you’ve chosen a specific type, you need to select the appropriate ratio. The easiest way to do this is to divide the maximum acceptable (gearbox) input speed by the maximum desired output (load) speed, then select the closest available ratio. Acceptable gearbox input speeds vary, but are typically on the order of 6,000 rpm.

The next step is to calculate motor speed and torque requirements using the following equations:

W_M = W_L × N and T_M = T_L/(N × n)

where W_M = motor output speed
W_L = load speed
N = gear ratio
T_M = motor output torque
T_L = load torque
n = gearbox efficiency

Once the motor requirements have been determined, choose a motor type and frame size capable of producing the required motor torque. For continuous operation, select a motor with a continuous torque rating greater than that of the required torque. For intermittent operation with a sufficiently short on-time, select a motor with a continuous torque greater than that of the rms value of the required torque.

Motor makers specify continuous torque based on certain operating conditions, including ambient temperature (often 25 or 40°C) and thermal resistance (heat sink, etc.) Be sure to read the fine print when comparing continuous torque ratings as they may need to be adjusted if these assumptions do not match your actual operating conditions.

After choosing a frame size, you need to specify the proper windings. Voltage and torque are generally known; speed and current need to be determined. The best winding choice will be that which comes closest to providing the desired speed and current draw given the supply voltage and load torque. The governing motor equations to determine speed and current follow:

W = (V_s - I × R_mt)/K_E and I = T_L/K_T + I_NL

where W = speed
V_s = supply voltage
I = current
R_mt = motor terminal resistance
K_E = back emf constant
T = load torque
K_T = torque constant
I_NL = no-load current

While these equations are suitable for most applications, it is important to realize that they are only the basic formula and do not account for thermal considerations. Motor heating will alter some of the parameters in these calculations, including resistance, torque constant, and back-emf constant. Accounting for these effects adds more complexity to the process.

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